UNIVERSITY OF NAIROBI
INVESTIGATION ON THE PRODUCTION POTENTIAL OF ENERGY CONTENT FROM SOLID WASTE
BY
RUNGURUA NDERITU JOHN
F16/2326/2009
A research project submitted in partial fulfillment of the requirements for the award of degree of
Bachelor of Science degree in the Department of Civil and Construction Engineering School of
Engineering
MAY, 2015
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DECLARATION
I, RUNGURUA NDERITU JOHN, hereby declare that this project is my original work and has not been
presented for a degree in any other university.
Signed………………………………
Date………………………………………………..
RUNGURUA NDERITU JOHN
DECLARATION OF SUPERVISOR
This research project has been submitted for examination with my approval as a University Supervisor in
the Department of Civil and Construction Engineering.
Signed…………………………………Date………………………………………………..
DR.P.K NDIBA
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ACKNOWLEDGEMENT
May the Almighty God receive all the glory and honour for it is Him who has taken me this far. Indeed no
eye has seen, no ear has heard and no heart has perceived what God has in store for His children whom
He loves. (1 Corinthians 2:9)Ebenezer!
I am greatly indebted to my supervisor DR.P.K. NDIBA under whose counsel, guidance and advice I have
successfully completed my project.
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Abstract
Daily human activities inevitably generated and accumulated Solid waste. The
waste may bring about severe environmental degradation unless an appropriate
solid waste management system is in place. The solid waste management method
sought should deal with the waste efficiently. It will be more advantageous if not
beneficial if energy can be harnessed through a variety of processes such as
incineration, pyrolysis, gasification etc. In the design of these solid waste
management processes, it is necessary to estimate the energy content of
municipal solid waste in order to achieve the required optimal system
performance.
This project evaluated the energy content and viability of energy derived from
solid waste from Dandora dumpsite in Nairobi. Heat energy was calculated from
municipal solid waste composition and their heating values per unit mass. The
potential electricity production of waste is calculated using the net calorific value
of 1400 k-Cal/kg at efficiency of 80%.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS……………………………………………………………………………………………..………….1
DEDICATION…………………………………………………………………………………………………………………………2
ABSTRACT………………………………………………………………………………………………………………………….…3
Table of contents………………………………………………………………………………………………………… ….…4
LIST OF TABLES………………………………………………………………………………………………………….……… ...7
LIST OF PLATES……………………………………………………………………………………………………………………..8
ABBREVIATIONS, ACRONYMS AND MEASUREMENTS……………………………………………..……… ……9
CHAPTER 1………………………………………………………………………………..…………………………..….…….….. 10
INTRODUCTION…………………………………………………………………………………………………………………….10
1.1 Background information………………………………………………..………………………………………………………10
1.2 PROBLEM STATEMENT…………………………………………………………………………………….….………......11
1.3Objective of the Project………………………………………………………………………………………………………… 12
CHAPTER 2……………………………………..………………………………………………………………..........…………... 13
LITERATURE REVIEW……………………………………………………………………………………………………………... 13
2.1 Definitions……………………………………………………………………………………………………………...................13
2.2 Types of Solid Wastes……………………………………………….…………………………………………………………....13
2.2.1 Municipal Solid Waste(MWS)…………………………………………………………………………………………..... 13
2.3 Waste Collection and Transportation……………………………………….……………………………………………..15
2.4 Solid Waste Management……………………………………………………………………………………………........... 15
2.5 Methods of Solid Waste Management………………………………………………..……………………………….….16
2.5.1 Composting / Biological Treatment……………………………………………………………………………………… 16
2.5.2 Landfilling………………………………………………………………………………………………………………… ………. 17
2.5.3 Incineration of Solid Waste…………………………………………………………………….…………………… ………18
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2.5.4 Comparison of Incineration with Other Methods of Waste Management….……….…………… 18
2.5.5 Basic Objectives of Incineration………………………………………..……………………………………………….19
2.5.6 Description of Incineration………………………………………………………………………………………………. 19
2.6 Combustion Conditions………………………………………….…………………………………………………………... 23
2.7 Energy Recovery…………………………………………………………………………………………………………………. 23
2.7.1 Assessment of Energy Recovery Potential……………………………………………….………………………..24
2.8 Environmental Impacts……………………………………………………………………..………………………………....24
2.8.1 Air Emissions………………………………………………………………………………………………………..…………….24
2.8.2 Control of Air Emission………………………………………………………………………………………….…………...28
2.9 Disposal of Ash……………………………………………………………………………………………………………………...29
2.9.1 Processing…………………………………………………………………………………………………………………………..30
2.9.2 Treatment…………………………………………………………………………………………………………………………..30
2.10 Economic Costs of Thermal Treatment………………………………………………………………………………..31
2.11 Advantages and Disadvantages of Incineration……………………………………………………………………31
CHAPTER 3……………………………………………………………………………………………………………………………. 32
RESEARCH METHODOLOGY…………………………………………………………………………………………………….32
3.1 Location and Description of Area of Study…………………………………………………………….……………..32
3.2 Sources of Solid Waste on Dandora dumpsite…………………………………………………………………….. 32
3.3 Management of Solid Waste in Dandora area…………………………………………………………………….. 32
3.4 Types and Sources of Data………………………………………………………………………………………………….. 37
3.5 Laboratory Analysis…………………………………………………………………………………..……………………….. 37
CHAPTER 4……………………………………………………………………………………………………………………..……….38
RESULTS AND DATA ANALSIS…………………………………………………………………………………………………. 38
4.1 Composition of the Solid Waste…………………………………………………………………………………..……….. 38
4.2 Moisture Content of theWaste……………………………………………………………………….……………………...39
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4.3 Heat content of the Waste……………………………………………………………..………………………………….. 39
4.4 Potential Electricity Production from the Waste……………………………………………..………………….. 45
4.5 Total Energy from dandora dumpsite…………………………………………………………….……………………..46
4.6 DISCUSSION…………………………………………………………………………………………….…………………………….48
CHAPTER 5………………………………………………………………………………..…………………………………………..50
CONCLUSION AND RECOMMENDATIONS……………………………………………….……………………….…….50
5.1 Conclusion………………………………………………………………………………..……………………………………………50
5.2 Recommendation……………………………………………………………………………………………………..……………50
REFERENCES……………………………………………………………………………………………………………….………….52
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LIST OF TABLES
Table 2.1 Higher (Gross)Heat values (HHV)of selected materials (Kiser ,J,V,L et al,1992)
………………………………………………………………………………………………………………………………….13
Table 2.2 Typical proximate Analysis of selected combustible components of
Municipal solid waste Niessen 1997…………………………………………………………………………………14
Table 4.1 Waste Composition of the sample…………………………………………………………………….30
Table 4.2 proximate Analysis of components of MSW(%weight); Tchobanoglous et al,:
1993)……………………………………………………………………………………………………………………………………32
Table 4.3 wet and adjusted Dry weight of the sample……………………………………………………………..33
Table 4.4 Energy content of the combustibles……………………………………………………………………….35
Table4.5 population growth rate in Kenya(CIA world factbook,jan,2011)…………………………………….36
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LIST OF PLATES
Plate 3.1.1 plastic waste……………………………………………………………………………………………………….34
Plate 3.1.2 metal waste………………………………………………………………………………………………………..34
Plate 3.1.3 glass waste………………………………………………………………………………………………………….35
Plate 3.1.4 food waste………………………………………………………………………………………………………….35
Plate 3.1.5 paper waste………………………………………………………………………………………………………..36
Plate 3.1.6 ash and dust waste……………………………………………………………………………………………..36
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ABBREVIATIONS, ACRONYMS AND MEASUREMENTS
ᵒC
Degree Celsius
BTU
British thermal unit
EfW
Energy from Waste
Hawf
ash and water free calorific value
Hinf
Lower (inferior) calorific value
Hsup
upper (superior) calorific value
HHV
Higher Heating Value
IWM
Integrated Waste Management
Kj
Kilojoule
Kcal
Kilocalories
lb.
Pounds
LHV
Lower Heating Value
MSW
Municipal Solid Waste
NCV
Net Calorific Value
PCB
Polychlorinated biphenols
RDF
Refuse Derived Fuel
SWM
Solid Waste Management
WtE
Waste-to- Energy
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CHAPTER 1
INTRODUCTION
1.1 BACKGROUND INFORMATION
Solid water management is the process of control, collection, storage, and
disposal of solid waste. The process includes the separation of waste materials
and the processing, treatment, and recovery of some of the waste. Solid waste
management also involves safe transport of this waste, and its ultimate disposal.
Solid waste management is one of the vital services conducted by local
governments.
Urbanization in many towns has resulted in high growth rate, which give rise to
the problem of solid waste collection and disposal. The wastes resulting from
daily human activities must eventually reach a designated area such as a
dumpsite. When waste is left to accumulate unmonitored at the dumpsite, it
leads to pollution of the environment which may be in form of unpleasant smell
as well as leading to outbreak of diseases as was evidenced in 2004 when an
epidemic of avian flu broke out and origins was credited to the dumping site. The
uncensored continual disposal of wastes in the site creates unfavorable living
conditions to the residents as well.
The Dandora dumpsite is destination of 850 tons of solid waste generated daily by
around 3.5 million inhabitants of Nairobi. Being a low income residential area,
poor collection and disposal is rampantly creating a serious issue that needs to be
addressed. These large volumes of solid waste could serve as raw material for
energy production.
Solid waste contains both organic and inorganic matter. The latent energy present
in the organic matter can be harnessed for gainful utilization through adoption of
suitable waste processing and treatment technologies. The recovery of energy
from wastes also other benefits including reduction of the total quantity of
wastes disposed, reduced demand for land for land filling, forgoing costs of
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transportation of waste to far away landfill sites and ease in environmental
pollution
While every effort should be made to minimize generation, recycle, and reuse
waste materials, the option of energy recovery from wastes should be properly
examined and wherever viable should be reincorporated in the overall scheme of
waste management.
1.2. PROBLEM STATEMENT
Economic growth and development are accompanied by the generation of large
amounts of wastes that must be re-used in some way or disposed off in landfills.
The generation of wastes can be reduced to some extent by improved design of
product and packaging materials and by increasing intensity of service per unit
mass of material used. Solid waste disposal and energy conservation are not
usually considered to be related. However, a great deal of research has recently
been focused on energy recovery from solid waste.
The energy sector in Kenya is largely dominated by petroleum and electricity, with
wood fuel providing the basic energy needs of the rural communities, urban poor,
and the informal sector. An analysis of the national energy shows heavy
dependency on wood fuel and other biomass that account for 68% of the total
energy consumption (petroleum 22%, electricity 9%, others account for 1%).
Electricity access in Kenya is low despite the government’s ambitious target to
increase electricity connectivity from the current 15% to at least 65% by the year
2022. Due to increased poverty, there is a significant shift to non-traded
traditional biomass fuels. The proportion of households consuming biomass has
risen to 83% from 73% in 1980.
Kenya has an installed capacity of 1.48 GW. Whilst about 57% is hydro power,
about 32% is thermal and the rest comprises geothermal and emergency thermal
power. Solar PV and Wind power play a minor role contributing less than 1%.
However, hydropower has ranged from 38-76% of the generation mix due to poor
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rainfall. Thermal energy sources have been used to make up for these shortfalls,
varying between 16-33% of the mix.
As a rapidly developing country, there is an increase in demand for energy that
needs to be addressed hence paramount that alternative sources of energy be
sought out to curtail this situation.
A potential source of energy in Kenya is incineration of solid waste which
produces a relatively high energy through thermal treatment methods. The
treatment uses solid waste beneficially as well reduces its volume.
There is need for energy sources that promote energy independence, avoid fossil
fuel use and reduce greenhouse emissions. Research shows that for every ton of
waste processed at a waste-to-energy facility, a nominal one ton of carbon
dioxide is prevented from entering the atmosphere (L.P. Joseph et al 1975 use
standard referencing
1.3 Objective of the project
The overall objective of the project is to investigate viability of energy recovery
from solid waste from Dandora dump site, Nairobi. Specific objectives are to:
1. The moisture content of the samples
2. The heat energy of the samples
3. The potency of the energy recovered from the solid waste
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CHAPTER 2
LITERATURE REVIEW
2.1 Definitions
Human activities generate waste materials that are often discarded because they are considered
useless. These wastes are normally solids, and the word waste suggests that the material is
useless and unwanted. However, many of these wastes materials can be reused, and thus they can
become a resource for industrial production or energy generation, if managed properly.
2.2 Types of Solid Wastes
Wastes can be classified into physical forms (solids, liquids, gaseous); original use (packaging
wastes, food wastes, etc.), material (glass, paper, etc.), origin (domestic, commercial,
agricultural, industrial, etc.), physical properties (combustible, compostable, recyclable); orsafety
(hazardous, non-hazardous).
2.2.1 Municipal Solid Wastes (MSW)
MSW is a waste arising from residential and commercial activities. With increasing urbanization
and industrialization, millions of tones of MSW are generated. Domestic wastes by nature is one
of the hardest wastes to manage effectively due to its diverse range of materials mixed together.
Different types of MSW include:
i. Domestic waste
Wastes from house hold activities, including food preparation and leftovers, cleaning, fuel
burning, old clothes and furniture, obsolete utensils and equipments, packaging, newsprint, and
garden wastes.
In low-income countries, domestic waste is dominated by food and ash. Middle-and higherincome countries have a larger proportion of paper, plastic, metal, glass, discarded items, and
hazardous matter due to advanced industrialization.
ii. Commercial Waste
Waste from shops, offices, restaurants, hotels , and similar commercial establishments typically
consisting of packaging materials, offices supplies, and food waste and bearing a close
resemblance to domestic waste.
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In low-income countries, food markets may contribute a large proportion of the commercial
waste. Commercial waste may include hazardous components such as contaminated packaging
materials.
iii. Institutional Waste
Waste from schools, hospitals, clinics, government offices, military bases, and so on is termed as
institutional wastes. Institutional waste is similar to both domestic and commercial waste,
although there are generally more packaging materials than food waste. Hospitals and clinical
wastes include potentially infections and hazardous materials. It is important to separate the
hazardous and non-hazardous components to reduce health risks.
iv. Industrial Waste
The composition of industrial waste depends on the kind of industries involved. Basically,
industrial waste include components similar to domestic and commercial source waste, including
wastes from kitchen and canteens, packaging materials, plastics, papers, and metal items. Some
production processes, however, utilize or generate hazardous(chemicals or infectious)
substances. Disposal routes for hazardous wastes are usually different from those for nonhazardous wastes and depend on the composition of actual waste type.
v. Street Sweepings
Wastes swept from the streets is dominated by dust and soil together with varying amounts of
paper, metal, and other litter from the streets.
vi. Construction and Demolition Wastes
The composition of the construction wastes depend on the type of building materials, but
typically includes soils, stones, brick, concrete and ceramic materials, wood, packaging materials
and the like.
2.3 Waste Collection and Transportation
Collecting municipal waste and transporting it to disposal sites is crucial activity in Solid Waste
Management. Even though the government collects waste, private companies have bee formed
which have their major businesses he collection and transport of solid waste. Such companies are
responsible for collecting majority of commercial, institutional and industrial waste.
2.4 Solid Waste Management
Waste is inevitable product of the society; therefore it should be managed effectively. Solid
waste management will include production of less and an effective system to waste produce.
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Effective solid waste management systems need to ensure human health and safety. In addition
to these, it should also be both environmentally and economically sustainable.
1.
Environmentally sustainable:
The management must reduce as much as possible the environmental impacts of waste
management, including energy consumption, pollution of land, air and water and lots of amenity.
2.
Economically sustainable
The management must operate at a cost acceptable to the community, which includes private
citizens, businesses and government. The costs of operating an effective solid waste system will
depend on local infrastructure, but ideally should be little or no more than existing local waste
management costs.
There are different ways of managing solid waste including:
1.
2.
3.
4.
5.
Material recycling
Biological treatment
Thermal treatment(incineration with solid and without energy recovery)
Compositing
Land filling
2.5 Methods of solid waste management:
2.5.1 Composting/ biological treatment
Composting is the biological decomposition of the biodegradable organic fraction of MSW under
controlled conditions to state sufficiently stable for nuisance-free storage and handling and for
safe use in land applications (Goluekeet al, 1995; Golueke, 1972; Diaz et al, 1993). Biological
treatment involves using naturally occurring micro-organisms to decompose the biodegradable
components of waste. Aerobic organisms require molecular oxygen to use external electron
acceptors in respiratory metabolism; this results in rapid growth rates and high cell yields.
Anaerobic metabolism occurs in the absence of oxygen and does not involve an external electron
acceptor. This fermentative metabolism is a less effective energy producing process than aerobic
respiration and therefore results in lower growth rates and cell yields.
If left to go to completion, biological processes results in the production of gases (mainly carbon
dioxide and water vapor from aerobic processes and carbon dioxide and methane from anaerobic
processes) plus a mineralized residue. Normally the process is interrupted when the residue still
contains organic materials, though in a more stable form, comprising a compost-like material.
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There are many advantages to composting. First, it would reduce the amount of waste requiring
ultimate disposal, extending the life of landfills. When doe correctly, the end result becomes a
useful product, capable of being used at the household or farm level to augment soil nutrient
levels and increase organic matter in soil, increasing soil stability. If the product is of high
quality and markets exist, the product can be sold. Environmentally, the process by which
composting organic waste is preferable to landfill processes. In a landfill, bacteria break down
organics anaerobically in absence of oxygen, resulting in the release of methane gas. When
properly composted, the organic matter is decomposed using an aerobic respiration process,
which produces no methane by-product.
In reference to incineration, there are parameters that will limits composting as a disposal
method. Nearly 20%-30% by weight, of municipal waste consists of non-compostable, nonrecyclable waste hence another method has to be sought to dispose this waste most probably land
filling. While one incineration plant can serve a large population, composting is limited to size of
community it can serve. Land which is suitable for composting plants may be difficult to acquire
in large metropolitan areas. Some wastes are better incinerated than composted. Wastes arising
from construction e.g. timber will take a lot of time to decompose; instead it will produce a high
calorific value in an incineration.
2.5.1.2 Advantages and Disadvantages of Composting/ Biological Treatment
Composting of the organic fraction of the waste leads to numerous advantages in the well overall
waste treatment process:
1. Reduction of the amount of waste that has to be incinerated or put in landfills and
therefore reduction of incinerator ash to be disposed off.
2. In general lower costs than incineration, although treatment costs are very sophisticated,
completely enclosed composting systems are now near those for incineration.
3. Recycling of humus and nutrients into the soil.
4. Protecting and improving the microbiological diversity and quality of cultivated soils.
5. Beneficial role of composting micro-organisms in crop protection, in as much as they
compete with plant pathogens.
6. Beneficial role of compost micro- organisms in biodegradation of toxic compounds and
pollutants
If composting is not carried out properly, it can also have some disadvantages:
1. The most common complaint about composting instillation are odour nuisance, that’s
why the tendency goes to completely enclosed systems where the outlet air is treated in a
bio-filter before being emitted. The best way, though, to prevent malodor generation is a
composting process with a high degradation rate, in order to remove the putrescible
substances as quickly as possible.
2. Dispersion of potentially pathogenic and allergenic micro-organism.
3. Soil pollution is the heavy metal content of the compost is too high.
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4. Ground water pollution if composting is carried out on a surface that is not made up
properly or where the runoff water is not collected.
2.5.2 Land filling
Land filling is the term used to describe the process by which solid waste residuals are placed in
a land fill. A sanitary land fill refers to an engineer facility for the disposal of MSW designed
and operated to minimize public health and environmental impacts. Landfills for individual
waste constituents such as combustion ash, asbestos, and other similar wastes are known as
mono fills. Landfills for the disposal of hazardous wastes are called secure landfills. Those
places where waste is dumped on or into the ground is no organized manner are called
uncontrolled land disposal sites or waste dumps.
Solid wastes placed in sanitary land fill undergo a number of simultaneous biological, chemical
and physical changes. The most important biological occurring in landfills are those relate to the
conversion of the organic material in MSW, leading to the evolution of landfill gases and,
eventually, leach ate. Important chemical reaction that occur within the landfill include
dissolution and suspension of landfill materials and biological conversion products in the liquid
percolating through the waste, evaporation and vaporization of chemical organic compounds and
water into the evolving landfill gas, sorption of volatile and semi-volatile organic compounds
into the land filled material, dehalogenation and decomposition of organic compounds, and
oxidation-reduction reactions affecting metals and the solubility of metal salts. Among the more
important physical changes in landfills is the settlement caused by consolidation and
decomposition of land filled material.
Concerns with the land filling of solid waste are related to the following:
a) The uncontrolled release of landfill gases might migrate off-site and cause odour and
other potentially dangerous conditions.
b) The impact of the uncontrolled discharge of landfill gases on the greenhouse effect in the
atmosphere.
c) The uncontrolled release of leachate that might migrate to underlying groundwater or to
surface streams.
d) The breeding and harboring of disease vectors in improperly managed landfills.
e) The health and environmental impacts associated with the release of the trace gases found
in landfills arising from the hazardous materials that often placed in landfills in the past.
The goal for the designed and operation of a landfill is to eliminate or maximize the impacts
associated with the above concerns.
2.5.3 Incineration of Solid Waste
The concerns raised by land filling and composting led to consideration of incineration as a
superior solid waste management method. Some of their shortcomings are solved by incineration
and are discussed below.
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Incineration is a controlled combustion process for reducing solid, liquid, or gaseous combustible
wastes primarily to carbon dioxide, water vapor, other gases, and relatively small, noncombustible residue that can be further processed or land-filled in an environmentally acceptable
manner. Incineration and other high temperature waste treatment systems are known as thermal
treatment of solid waste. Incineration with energy recovery is one of those methods referred to as
waste to energy (WtE) where energy is produced in the process. Other methods include
gasification, plasma arc gasification, pyrolysis and anaerobic digestion.
2.5.4 Comparison of incineration with Other Methods of Waste Management
Both incineration and land filling are widely used around the world as MSW disposal strategies
and their benefits
and hazards have been documented. A basic comparison between these technologies indicates
that incinerators
fare much better than landfills in the following categories: emission of greenhouse gasses
(Solano et al., 2002;USEPA, 2006a), discharge of toxic leachate (Jones-Lee, 1993; USEPA,
1996; USDOE, 2000; Keck and Seitz, 2002), duration of the impact (Elias-castellas,2005),
materials recovered (landfills have none), power generation (USEPA,2006a), creation of jobs
(USDOT,2000; CEPA, 2003)and land consumption (Renova, 2000).Public perception is an area
where incinerators fare far worse than landfills, even though landfills also suffer from the not-inmy-back-yard syndrome (McCarthy, 2004).
2.5.5 Basic Objectives of Incineration
i. Volume reduction
Depending on the composition of the waste, incinerating MSW reduces the solid waste by 90%
on average. The weight of solid waste to be dealt with is reduced by around 70%. This has both
environmental and economic advantages since there is less demand for final disposal to landfill;
as well as reduces costs and environmental impacts due to transport if a distant landfill is used.
ii. Stabilization of waste
The output of incineration (ash) is more inert than the input (waste), due to the oxidation of the
organic component of the waste stream. This leads to reduced landfill management problems
since the organic fraction is responsible for landfill gas and leachate production.
iii. Recovery of energy from waste
Energy recovered from burning waste is used to generate steam for use in on-site electricity
generation or export to local factories and district heating schemes. Combined heat and power
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plants increase the efficiency of power recovery by producing electricity as well as utilizing the
residual heat. Burning solid waste can replace use of fossil fuels for energy generation.
iv. Sanitation of waste.
Incineration of MSW will also ensure destruction of pathogens prior to final disposal in a
landfill especially for clinical waste.
2.5.6 Description of incineration
Incineration is fundamentally a form of chemical processing, involving the rapid oxidation of
materials. The combustion involves several stages in which the waste is dried (moisture
evaporated) as it enters the furnace, the organic compounds are volatilized, and the volatile
compounds are ignited in the presence of oxygen.
Detailed combustion characteristics are needed for the calculation of heat balance for the
incinerator. The most important characteristics are thee higher heating value, moisture content
are percent of inert material in the waste.
a)
Heat value
Once ignited, the ability of waste to sustain a combustion process supplementary fuel depends on
number of physical and chemical parameters, of which the lower calorific value (Hinf) is the
most important. The minimum required lower calorific value for a controlled incineration also
depends on the furnace design. Low-grade fuels require a design that minimizes heat loss and
allows the waste to dry before ignition.
During incineration, water vapour’s from the combustion process and the moisture content of the
fuel disperse with the fuel gases. The energy content of the water vapors accounts for the
difference between a fuel’s upper and the lower calorific values.
The upper calorific value (Hsup) of a fuel may, according to DNA 51900, be defined as the
energy content released per unit weight through total combustion of the fuel. The temperature
of the fuel before combustion and of the residues (including condensed water vapors) after
combustion must be 250C, and the air pressure 1 atmosphere. The combustion must result in
complete oxidation of all carbon and sulphur to carbon- and sulphur dioxide respectively,
whereas no oxidation of nitrogen must take place. The lower calorific value differs from the
upper calorific value by the heat of condensation of the combined water vapors, which comes
from the fuel’s moisture content and the hydrogen released through combustion.
The ash and water free calorific value (Hawf) expresses the lower calorific value of the
combustible fraction (ignition loss of dry sample). A major consideration for the selection or
design of an incineration is the fuel value of the materials to be burned. The fuel value is
described in terms of the gross heat value or the higher heat value (HHV) and the net heat value
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also called lower heat value (LHV). The heat values of various materials in MSW are listed in
table 2.1.
In addition to the heat value, other important fuel properties are moisture content, the
combustible material content, and ash content. fuel with a high heat value (greater than
5000kJ/kg), low moisture content ( less than 50%), and low ash content (less than 60%) can be
burned without additional fuel, whereas material with a low heat value, high moisture content,
and high ash content require supplementary fuel. An analysis of waste to determine its heat
value, moisture content and ash content is called a proximate analysis. The proximate analysis of
MSW and selected compounds found in municipal waste are given in table 2.2 below.
An element or ultimate analysis determines the chemical composition of the combustible portion
of the waste in terms of the ash content and the chemical elements, carbon, hydrogen, oxygen
nitrogen, sulphur and chlorine.
In the absence of calorimeter data, the heat value of combustible material may be estimated using
equation developed using element analysis or waste composition. Hazome et al (1997) developed
an empirical equation for refuse fitting elemental composition to HHV data;
HHV=0.339 (C) + 1.44 (H) -0.139 (O) + 0.105 (S) MJ/Kg
(HHV=145.7 (C) + 619 (H) – 59.8 (O) + 45.1 (S) BTU/Ib.
Where HHV is higher heat value and (C) ,(H),(O) and (S) are weight percents of carbon,
hydrogen, oxygen and sulphur respectively.
Table 2.1 Higher (Gross) Heat Values (HHV) of selected materials (Kiser, J.V.L et al,)
Type of waste
Heat value
(kJ/Kg)
11,600-12,100
Heat value
BTU/Ib
5,000-5,200
13,300-14,400
5,700-6,200
16,700-18,600
7,200-8000
9,800-16,300
4,200-7000
4,200-18,100
1,800-7,800
22,100-41,900
9,500-18,000
Mixed MSW
Refuse-derived fuel
Paper
Yard
Food
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Plastics
10,900-16,300
4,700-7000
27,900-32,600
12,000-14,000
16,000
6,900
20,900-33,700
9,000-14,500
40,500-44,200
17,410-18,990
44,600-45,900
19,170-19,750
Wood
Rubber waste
Lignite coal
Bituminous coal
#6fuel oil
#2home heating fuel
The table shows that wood has nearly the same heating value per unit mass as paper, while yard
and food wastes contain less energy because of their high moisture content. For example, food
wastes contain about 78% moisture. Thus, high- moisture food wastes contain enough heat to
burn autogenously (i.e. without fuel addition) but cannot generate much electricity.
Table 2.2 Typical proximate Analysis of Selected Combustible components of
Municipal solid waste, Niessen 1997.
Component
Moisture
(%)
Paper
10.24
Magazines
4.11
Yard
waste 75.24
(grass)
Food waste
78.29
Polyethylene
0.20
Wood
and 20.00
bark
Rubber
1.20
Leather
10.00
Volatile matter Fixed carbon Ash
(%)
(%)
(%)
75.94
8.44
5.38
66.39
7.03
22.47
18.64
4.50
1.63
17.10
98.54
67.89
3.55
0.07
11.31
1.06
1.19
0.80
83.98
68.46
4.94
12.49
9.88
9.10
b) Moisture content.
22
Moisture lowers the fuel value .As the moisture increases ,there is less combustible materials .per
unit mass, In addition ,a significant amount of gross heat energy is used to heat and evaporate the
moisture in the waste : the heat of vaporization of water is 2275 kj/kg. Thus there is need to
reclaim this energy rather than lose or lower heat value (LHV), which represents the energy that
the can be captured from the combustion of waste is mused .LHV is calculated from the equation
( Hogan et al., 19554):
LHV = HHV (in MJ/Kg)- 0.0244(w + 9H) MJ/KG
Where W is the mass percent of moisture, and H is the percent of weight (or mass) of hydrogen
in the dry waste.
c) Combustion air.
Another consideration for design of an incinerator is quantity of air that must be supplied to
achieve the complete combustion of waste according to the reaction:
CaHbOcCLdFeNfSg + [a+b – (c+d+e-f)
aCO2 + [(b-d-e)/2] H2O+dHCL+eHF+fNO+gSO2
Frequently, fluorine, chlorine, nitrogen and sulphur are present in small amounts only and are
omitted in calculations.
2.6 Combustion Conditions.
Efficient combustion of waste requires sufficiently high combustion temperatures, adequate
retention time of the wastes in the combustion chamber, and turbulence to expose unburned
surfaces. When waste is feed into an incinerator, they are heated by contact with hot gases,
preheated air or radiation from the furnace walls. The heat dries the waste, and then thermally
decompose them to produce volatile matter which is generally combustible and ignitable.
Many incinerators provide turbulence by burning the fuel on sloping grates that move in some
fashion to agitate the waste .The movement causes the waste to tumble forward through the
combustion chamber. The grate system allows ash to fall through the grates as the refuse is
transported from the point of induction to a final collection bin.
Primary air is fed through the waste from below the grate. This air supports the combustion of
waste and also cools the grates. Secondary air is blown into the chamber from nozzles above the
wastes in order to promote the mixing and burning of the combustion gases.
Tertiary air cools flue gases before the reach the air pollution control equipment.
The temperature in the incinerator is maintained by controlling the feed rate of the wastes and
primary and secondary air supplies. The amount of air that must be supplied to ensure complete
combustion exceeds the stoichiometric amount predicted from the equations describing the
burning process. Efficient burning requires 100% excess air (i.e as twice as much as the
stoichiometric amount) Supplied as primary air .This air is introduced from below the grate. The
23
addition of high pressure secondary air into the combustion chamber serves to control
temperatures and promotes the mixing f gases; incinerators are operated at a temperature above
750oC, but below 1000oC to prevent ash from melting and plugging the grates.
2.7Energy Recovery
The burning of solid waste produces only about 20% as much energy, on per weight basis ,as
that resulting from burning of fossil fuels ( RCEP ,1993) Revenues from sales of the energy can
significantly reduce the cost of incinerations .The primary energy products generated depend on
the design of the facility, the fuel and the requirements of the energy buyer. An estimate of
amount of steam that is generated can be found using the rules of thumb that 3kg of steam are
produced from 1kg of refuse in a water wall incinerator and 2.2 kg of steam are generated from
1kg of refuse in a modular incinerator (Hecht,1983)
Steam and hot water are the easiest to generate, but transporting these products requires laying
pipelines to the buyer’s facility. This construction requires a sizeable capital cost outlay that may
be justified if a long term arrangement can be instituted with a suitable buyer.
Selling the steam requires the identification of buyers who have demands that match steam
production .Seasonal variation may occur in the demand for steam for heating and cooling that
do not match waste generation. The primary buyers of steam for steam are industries, institutions
and central heating district that provide services to multiple buildings .Government buildings or
universities tend to be the best candidates for energy buyers because they can readily guarantee
the purchase of the product over a long time period.
Electricity is more marketable than steam .Transportation is not limited to short distances as is
the case for steam ,and the demand is likely to be less seasonal.
2.7.1Assessment of Energy Recovery Potential
A rough assessment of energy from MSW through different treatment methods can be made
from knowledge of its calorific value and organic fraction, as shown below.
In thermo-chemical conversion all of the organic matter, biodegradable as well as non –
biodegradable, contributes to the energy output:
Total waste quantity: W tons.
Net Calorific Value: NCV k-cal /kg.
Energy recovery potential (kWh) = NCV x W x 1000/860= 1.16 x NCV x W
Power Generation potential (kW) = 1.16 x NCV x W/24 =0.048 x NCV x W
Conversion Efficiency =25%
Net power generation potential (Kw) -0.012 x NCV x W
If NCV =1400k-cal/kg, then
Net power generation potential (kW) = 14.4 x W
In bio-chemical conversion, only the biodegradable fraction of the organic matter can contribute
to the energy output.
Total waste quantity: W (tons)
24
Total Organic /Volatile solids VS =50%, say
Organic bio –degradable fraction: approx 66% of VS =0.33 x W.
Typical Digestion efficiency =60%
Typical bio-gas Yield: B (M3) =0.80 m3/kg of VS destroyed.
= 0.80 x 0.60 x 0.33 x W x 1000 =158.4 x W
Calorific value of bio-gas = 5000kcal/m3 (typical)
Energy recovery potential (kWh) =B x 5000/860 =921 x W
Power generation potential (Kw) = 921 x W/24 -38.4 x W
Typical Conversion Efficiency =30%
Net power generation potential (kW) = 11.5 xW
In general, 100 tons of raw MSW with 50-60% organic matter can generate about 1-1.5 Mega
Watt power, depending upon the waste characteristics.
2.8 Environmental Impacts:
2.8.1 Air Emissions.
The quantity and composition of air emissions depend upon the composition of the refuse, this
design of the incinerators and the completeness of combustion .The a major products o
incineration of municipal waste are carbon dioxide, water sulfur dioxide, nitrogen oxides
particulates and smaller amounts of toxic chemicals such as polychlorinated biphenyls (PCBs),
dioxins and heavy metals (e.g cadmium chromium, lead and mercury). These undesirable
materials may be introduced if sludge from wastewater treatment facilities or industrial waste are
incinerated.
i.Carbon dioxide (CO2)
Carbon dioxide is one of the main products of the incineration process. The other main product is
water. In low concentrations CO2 has no short-term toxic or irritating effects; it is abundant in
the atmosphere, necessary for plant life and is not considered a pollutant (Tchobanoglous bet al ;
1993). Never the less, due to the high increase in global concentrations of CO2, it has been
recognized as one of the gases responsible for global warming (IPCC, 1966).
ii .carbon monoxide (CO)
Carbon monoxide is formed by incomplete combustion of carbon due to lack of oxygen to
complete oxidation to CO2. This gas is very toxic; it reacts with hemoglobin in the blood causing
a decrease of available oxygen to the organisms. The lack of oxygen produces headaches, nausea
and eventually death. Carbon monoxide in the flue gas is used to monitor the incomplete
combustion of other emission, such as unburned hydrocarbons and to provide information on the
combustion performance.
iii.Hydrochloric acid (HCl)
25
Hydrochloric acid results from the high concentration of chlorine-containing materials in MSW
(some type of plastic polyvinyl). Chlorine easily dissolves in water to form HCl. Its presence in
the gaseous stream may increase the acidity of local rain and groundwater, which can damage
exposed unprotected metal surface, erode buildings and may affect the mobilization of heavy
metals in soils (Clyton, 1991).
iv. Hydrogen fluoride (HF)
Hydrogen fluoride is more toxic and corrosive than HCl, although its presence in the emission
from MSW incinerators occurs in much smaller quantities. It is formed due to the presence of
trace amounts of fluorine in the waste.
v.Sulfur oxides (SOx)
The emission of SOx is a direct result of the oxidation of sulfur present in MSW, but other
conditions such as the type of incinerator used and its operating conditions may also influence
Mass-Burn production although to a lesser degree. Approximately 90% of Sox emissions are as
sox emissions are as SO2 and 10% are as SO3. In the atmosphere most of the SO2 is transformed
into SO3 (Benitez, 1995).
SO2 may lead to the production of H2SO3 (sulphurous and sulphuric acids, respectively) in the
atmosphere increasing the acidity of rain. Its effects on human beings depend on concentration.
At high concentrations, it can produce eye, nose and throat irritation and other respiratory
problems. Particulates that carry Sox on their surface intensify harmful respiratory effects, as this
particulate matter can penetrate deep into the lungs (Benitez, 1995).
vi. Nitrogen oxides (NOx)
Nitrogen oxides are predominantly formed during the incineration process; however they oxidize
to NO2 in the atmosphere. NOx is formed from two main sources: thermal NOX and fuel NOx .
In thermal formation the oxygen and nitrogen in the air react, the free oxygen atoms produced in
the flames by dissociation of O2 or by radical attack the nitrogen molecules and being a chain
reaction. Fuel NOx production is formed during reactions between oxygen and nitrogen fuels.
These reactions are highly sensitive to temperature, and are also important because they
participate in several processes in the atmospheric chemistry. They are precursors of the
formation of ozone (O3) and peroxyacetal nitrate (PAN): photochemical oxidants known as
smog and which contribute to acid rain formation.
vii. Particulates
Particulates are formed during the combustion processes by several mechanisms. The turbulence
in the combustion chamber may carry some ash into the exhaust flow. Other inorganic materials
present in the waste volatize at combustion temperatures. This material condenses downstream to
26
form particles or deposits on ash particles. Organic materials can also be emitted through
pyrolitic reactions near the fuel bed. This material can also be carried away and condense
downstream. The main components of fly ash are chemically inert silica but it may also contain
toxic metals and some toxic organic substances (Benitez, 1995).
viii. Heavy metals (Hg, Cd, Pb, Zn, Cn , Ni, Cr)
Municipal solid waste contains heavy metals and non-metallic compounds in the combustible
and incombustible fractions. During the incineration process, metals may vaporize directly or
form and chlorides in the high temperatures in the combustion zone. They condense over other
particles and leave the incineration process in the fuel gas (tchobanoglouset al, 1993).
ix. Dioxins and furans
Polychlorinated dibenzo-p- dioxins and polychlorinated dibenzofurans (PCDD/PCDF) have been
detected in the emissions from Municipal Solid Waste incinerators (Olieel al, 1997).they
immediately become a major issue in the debate over the place of incineration within the
municipal waste management strategy. Dioxins can be formed in all combustion processes where
carbon, oxygen and chlorine are present, although the processes by which they are formed during
incineration are not completely understood or agreed upon.
Dioxin is the generic name given to a family of over 200 chlorinated organic compounds. Their
molecular structure is very similar; 12 carbon atoms form two benzene rings, which are
connected by two oxygen atoms. They differ from each other by the number of chlorine atoms
and their spiritual arrangements. Furans are similar to dioxins, but the difference relies on the
location of the chlorine atoms, which are situated at positions 1-4 and 6-9.there are 75 dioxins
isomers and 135 furan isomers and all are collectively named `dioxins`.
The concerns over dioxins and furans is due to a number of animals studies that have shown that
for some species they are highly toxic at a very low levels of exposure (tosine, 1983; Oakland,
1988).the extrapolation of these animal data of humans, albeit contentious, has helped these
compounds acquire their notoriety. Nevertheless, they are known adverse health effects on
human such as chloracne, which was clearly documented in the Sevenso incident (Porteous,
1994).
Even though the relationship between dioxins and human cancer at everyday levels of exposure
is still under debate and not convincingly demonstrated, research by a group of scientists from
the World Health Organization`s (WHO) International Agencies for Research on Cancer (IARC)
concluded that there is enough evidence to classify one dioxin (2,3,7,8-TCDD) as a known
carcinogen.
27
Three pathways were proposed to explain the presence of dioxins in incinerator emission (Hut
zinger et al, 1985):
1. Dioxins are present in the incoming feed and are completely destroyed or transformed
during combustion.
2. Dioxins are produced from related chlorinated precursors such as polychlorinated
biphenols (PBC), chlorinated phenols and chlorinated benzenes.
3. Dioxins are formed via de novo synthesis from chemically unrelated compounds such
as a polyvinyl chloride (PVC) and other chlorocarbons, or are formed by the burning
of non-chlorinated organic matter such as polystyrene, cellulose, lignin, coal and
particulate carbon in the presence of chlorine donors.
It has now been verified that dioxins are present in all emissions, fuel gases, bottom ashes, fuel
ashes and scrubber water. Although each of the three pathways described above do not occur in
large scale incinerators, pathways 2 and 3 are more significant than 1.
2.8.2 Control of Air Emission
Air pollution control systems in incinerators must confirm to current national and state standards.
Common air emission control systems are described below;
a) Settling Chambers
A settling chamber is used as a primary collector or first stage collector to remove large particles
that may clog filters or foul the equipment. A settling chamber is a enlargement in the fuel gas
path to slow the speed of the gas to allow gravitational sedimentation to occur. Only particles
with diameters larger than 50 x 10-6m are removed efficiently. Because many particles have
diameters smaller than this, a settling chamber alone does not provide efficient removal.
b) Cyclones
Because cyclones have low capital and operating costs, they are commonly used for particulate
removal. Fuel gases enter a cylindrical chamber tangentially and swirl around the chamber. As
the fuel gases move in a circular path around the cylinder, inertia carries particulates to the wall,
where they move downward in a hopper. Particles of size 10 x 10-6m longer within this system
c)
Electronic precipitators
Electrostatic precipitators are used for the removal of particulates. The fuel gases pass through an
array of wires and plates. The wires are maintained at a large positive or negative electrical
voltage with respect to the plates. The voltage difference produces charged ions and electrons
that are attached to the surface of particulates. These charged particulates are then attracted to the
28
plates or wires and collected. Particulars in the size range 0.1 x 10-6m to 50 x 10-6 m are
effectively removed.
d) Fabric filters
the fuel gases are directed through ducts with fabric bags attached to the ends. The bag filters the
particulates from the air as it passes through them. The bags are occasionally shaken to release
particles to a collection bin below. Filters efficiency remove particle with diameter lager than 5 x
10 -6 m. removal of smaller particles is possible by using a more tightly woven fabric. Because
the filter is fabric, the flue gases must be cooled before introducing to the bags or acid
condensation may form on the internal surfaces.
e) Wet scrubbers.
Wet scrubbers are used for removing particulate matter from the gas stream. The common types
include spray towers, packed towers, cyclone scrubbers, jet scrubbers, venture scrubbers and
mechanical scrubbers. The wet scrubber works through a process in which fine liquid and
aerosol particles are intermingled. Large solid particles are captured by impingement and smaller
particles by diffusion into the droplets. High collection efficiency is possible, but this may
require a high relative speed between the droplets and the particles and this requires large
amounts of energy. Effective removal of particles down to a size of 1 x 10-6 m is possible.
f) Semi – wet and dry scrubbers
Semi – wet and dry scrubbers control acidic gases (e.g. SO2 and HCL) by neutralization. The
flue gases pass through an alkaline mist of calcium or sodium based slurry. The droplets
neutralize the acids as they evaporate. The large dried particles settle on the floor and the smaller
particles exit with the flue gas to be collected in a bag house or in an electrostatic precipitator.
The nu-reacted alkaline particles can further neutralize acidic gases as they pass by. Dry
scrubbers can neutralize 90% or more of the acid but little effect on particulate emission.
g) Selective catalytic reduction
Selective catalytic reduction (SCR) is a pollution control system capable of removing 70% or
more of the NO emissions. Ammonia (NH3) is injected into the flue gas immediately before the
gas enters a catalytic chamber. The catalyst enables these reactions to occur at lower
temperatures. The temperatures of the exhaust gases must be above 233oc to operate the catalyst
efficiently. Although this condition must be met with a waste to energy facilities which produce
steam, at facilities producing electricity the gases must pass through economizers and heat
exchanger and exit at temperature lower than this. The flue gases need to be reheated and then
cooled.
2.9 disposal of ash
29
An incinerator produces two distinct types of ashes: bottom ash and fly ash. Bottom ash is the
residue left from the burned waste and often contains partially burned materials. Bottom ash
accounts for about 90% of the volume of ash that is generated. Fly ash consists of the particulates
removed from the flue gas.
Ash consists mostly of metal oxides, when water leaches through the ash, the soluble metal oxide
are not soluble or mobile. Ash is usually not deposited in a landfill with municipal solid waste
because the leachate from MSW tends to be acidic and could release heavy metal ions.
2.9.1. Processing
Ash residues can be processed at the waste to energy facility to reduce the rate of release of
contaminants into the environment; facilitate disposal; improve the quality of the residues;
remove valuable, useful or harmful materials; and to prepare portions of the ash for beneficial
use. The residue can be treated by washing, chemical treatment, or the use of additives and
specific chemicals in order to retain remove or immobilize potentially toxic compounds
Ferrous metal can be separated from ash residues by solid or electromagnets. As much as 15% of
the bottom ash from mass-burn facilities is ferrous which can be extracted magnetically. The
quality of the ferrous metal is measured by the amount of contamination with combustibles and
fine ash materials. Tumbling of the ferrous product in a trammel can separate contamination to
improve quality, as is washing with water.
Screening processes can remove unwanted oversize and undersize components and separate ash
into usable products, including aggregate for use in construction.
Washing process can provide clean aggregate materials and ferrous metals for beneficial use, and
remove the cementations fly ash. The wash water can be processed and re-circulated.
The blow-downstream must be treated or evaporated to remove /recover and render harmless the
dissolved metals and salts (Hasselriis et al, 1991; Exner et al., 1989).
2.9.2 Treatment
Various methods or treatment may be used to reduce the amount of leachable metals and salt
concentrations, and thus the render the ash more environmentally acceptable, as well as improve
the chemical and physical stability and durability of product so that it can be used for a variety of
purposes. Treatment methods include ferrous separation and compaction and various methods
that modify release rates by chemical and physical charges, including solidification, stabilization
and encapsulation ,addition of Portland cement, phosphate, waste pozzolans and bituminous
materials; washing and chemicals treatment, thermal treatment, and verification.
30
After ferrous separation and screening, bottom ash residues can be used for fill and road base
under certain conditions.
2.10 Economic Coast of Thermal Treatment
An economic assessment of the refuse-fuel system must include a consideration of the costs
absorbed by both the public agencies and utilities involved. The major economic elements to be
considered include:
(a) Capital and operating cost associated with processing raw solid waste and delivering it
the utility in question. It is assumed that these costs would be absorbed by the public
agency.
(b) Cost required installing burning port, pneumatic feeders, and possibly fuelling storage
facilities. These costs would normally be absorbed by the utility.
(c) The value of material recovered in the refuse processing operation, most valuable being
ferrous and nonferrous metals. This income would be absorbed by the operator of the
refuse processing operation.
(d) Power plant operating cost allowance due to increase ash handling and other differentials
cost.
(e) The equivalent cost of the prepared refuse. Base on a heating value of 5000 BTU/Ib, the
prepared refuse would contain approximately10 million BTU/ton.
(f) The cost of land filling material which cannot be reclaimed, or used as supplementary
fuel.
2.11 Advantages and Disadvantages of Incineration
The merits of incineration include:
(a) The volume and weight of the waste are reduced to a fraction of their original size.
(b) Waste reduced is immediately; it does not require long-term residence in a landfill on
holding pond.
(c) Waste can be incinerated on-site, without having to be carted to a distant area.
31
(d) Air discharged can be effectively controlled for minimal impact on the atmospheric
environment.
(e) The ash residue is usually sterile.
(f) Technology exists to completely destroy even the most hazardous of materials in an
complete and effective manner.
(g) Incineration required a relatively small disposal area, compared to the land area required
for conventional landfill disposal.
(h) By using heat-recovery technique the cost of operating can often be reduced or offset
through the use or sale of energy.
CHAPTER 3
3.0
RESEARCH METHODOLOGY
The study entailed collecting a sample of solid waste from the field. The sample is
separated according to composition and its moisture content determined in the laboratory.
The amount of energy of the sample of waste was then calculated. The sample was
collected at Dandora dumpsite in Nairobi.
3.1
Location and Description of the Area Of Study
Dandora is an Eastern Suburb in Nairobi, Kenya. It is part of the Embakasi Division in
Nairobi County. Dandora Estate was established in 1977 with partial financing by the
32
World Bank in order to offer higher standards of housing. However, the estate has turned
into a high density slum as well as Nairobi’s Principal dumping site. The 30 acre site,
which is one of the largest in Africa, was once a quarry. That the City Council of Nairobi
sought to use temporarily, but is still in use even after being declared full, the slum
consists of an informal settlement ranging from mud, iron sheets to stones. The type of
settlement in the area gives and idea of how solid waste management is undertaken in the
area.
3.2
Sources of Solid Waste in Dandora dump site
The Dandora dump site is the destination of about 850 Tonnes of solid waste generated
daily by around 3.5 Million inhabitants of Nairobi City. Solid waste that ends up in the
dump site include household waste which will comprise of food left over’s, vegetables,
fruits, textiles, paper bags; construction waste comprising of brick, timber e.t.c and
wastes from commercial and industrial activities; the plates below shows the different
types of wastes from the dumpsite.
3.3
Management of Solid Waste in Dandora area
Solid waste in Dandora area is managed by the City Council of Nairobi, though the
services are not adequate, waste can be seen by the road side and also dumped in open
spaces, residents claim that though they pay a fee of collection the City Council does not
give them good services. The City Council via Youth Programmes provides collection
paper bags at every household for collecting the wastes generated
Plate 3.4 plastic waste.
33
Plate 3.5 metal waste
Plate 3.6 glass waste
34
\
Plate 3.7 food waste
Plate3.8 paper waste
35
Plate3.9 ash and dust waste
3.4
Types and Sources of Data
36
Data was collected from the field using the direct observation method. Areas of disposal
of solid waste, the type and composition of waste recorded, pictures were taken randomly
on those disposal areas and the snapshots kept. Two samples of wastes heading to the
dumpsite were collected. The two samples were mixed to make one sample of 14.6Kgs
which was used for analysis.
3.5
Laboratory Analysis
A sample of solid waste was collected and brought to the laboratory for analysis. The
waste was then separated according to its type, measured and recorded. The moisture
content of the waste was also determined. Different methods of calculating amount of
energy (both electricity and heat) from the waste were used to estimate the amount of
energy that can be generated from the sample of solid waste.
CHAPTER 4
37
4 RESULTS AND DATA ANALYSIS
4.1 composition of the solid waste
Table 4.1below shows an approximate composition of solid waste by weight of the sample collected
from the study area.
Table 4.1 waste composition of the sample
Waste
Weight (kg)
Percentage Weight
Food
3.1
29.5
Paper
1.2
11.4
Plastics
1.5
14.3
Wood (includes leaves banana 2.2
stalks & leaves, etc)
20.9
Glass
0.2
1.9
Metal
0.5
4.8
Textiles
1.2
11.4
Other (includes dirt, ashes)
0.6
5.7
Total
10.5
100
38
4.2 Moisture Content of the Waste
Moisture lowers the energy content of municipal waste because some heat energy has to be used to
heat and evaporate the moisture in the waste. It was thus important to determine the moisture
content of the sample. The moisture content of solid waste is usually expressed in one of two ways;
1. In the wet – weight method of measurement , the moisture in a sample is expressed as a
percentage of the wet weight of material
2. Dry-weight method, it is expressed as a % of the dry weight of the material.
Wet- weight Moisture content is expressed as follows
M = w - d × 100……………………..eqn 1
W
Where: M= wet- weight moisture content,%
w =initial mass of sample as delivered, kg
d = mass of sample after drying, kg
In determining the moisture content of the waste, the wet – weight method was used. A sample of
2.13kg of waste dried in an oven at approximately 1090C for 24hours. The following results were
obtained.
Initial mass of sample, w
= 2.13kg
Mass of sample after drying, d
= 1.44kg
Using the formula above, the moisture content of the waste was found to be
=
2.13-1.44
× 100
2.13
= 32.4%
This is just the average moisture content of the waste since different wastes have varying moisture
contents, e.g. the moisture content of food wastes is always more than 70% in most cases.
4.3 Heat Content of the waste
Tchobanoglous et al, 1993, proposed a table of Proximate Analysis Composition of various type of
combustible waste materials (Table 4.2), i.e. % moisture, volatile matter (principally hydrocarbons),
fixed carbon and non-combustible, non- volatile ash. The experimentally
Determined heats of combustion of each waste material are also shown. The values are an average
for several sources of waste.
39
Table 4.2 proximate Analysis of Components of MSW (% weight); (Tchobanoglous et al., 1993)
Moisture
Volatile
matter
Fixed carbon
NonCombustible
Kj/Kg As
collected
10.2
75.9
8.4
5.4
15814
5.2
77.5
12.3
5.0
16380
2.0
95.8
2.0
2.0
32800
10.0
66.0
17.5
6.5
17445
1.2
83.9
4.9
9.9
25330
10.0
68.5
12.5
9.0
18515
20.0
68.1
11.3
0.6
15445
60.0
30.0
9.5
0.5
6050
70.0
21.4
3.6
5.0
5350
21
52
7
20
11630
Dry Combustibles
Paper
Cardboard
Mixed plastics
Textiles
Rubber
Leather
Wet combustibles
Wood
Yard Waste
Food Waste
Dry and Wet
The energy of the sample of waste collected was calculated using the table 4.2 above
Because the heat in table 4.2 is based on dry weight, the waste was adjusted for moisture content,
table 4.3 the dry weight, X was calculated using the equation below:
32.4 =w - x × 100
w
Where w= wet weight of a certain
X=dry weight of the waste and
32.4= moisture content of the entire waste calculated earlier
40
Table 4.3 Wet and Adjusted Dry Weight of the sample
Waste
Weight (kg)
Dry Weight (Kg)
Food
3.1
2.10
Paper
1.2
0.81
Plastics
1.5
1.01
Wood (includes leaves, banana 2.2
stalks & leaves, etc)
1.49
Glass
0.2
0.14
Metal
0.5
0.34
Textiles
1.2
0.81
Others (includes dirt, ashes,)
0.6
0.41
Total
10.5
7.11
From this table, the weight in the sample was found to be 10.5 – 7.11 = 3.39 kg
It is assumed that WTE plant provides steam to a standard power plant and that the exhaust gases leave
the boiler at 1200C and 135 kPa (20psi). Accordingly, the amount of heat wasted per of water in the
feed, as water vapour in the exhaust gases, is calculated to be 2636Kj/Kg.
41
The non-combustible materials in the feed, mainly glass and metals will end up mostly in the bottom
ash. If it is assumed that the ash leaves the grate at about 7000C and a reasonable value
42
For the specific heat of ash, the corresponding heat loss to inorganic materials fed with the
combustibles is estimated to be as follows:
i)
ii)
iii)
Glass and other siliceous materials :628kJ/kg
Iron :420 kJ/kg
Aluminium:1134kJ/kg
Considering that the iron/aluminium ratio in MSW is about 4, the mean heat loss per kg of metal is
estimated to be 544kJ/kg
From the above effects the energy of the waste can be expressed as follows:
Heat Energy = Heat value of – Heat loss due – Heat loss due to – Heat loss due to
of sample
Combustibles
water in feed glass in feed
metal in feed…………eqn 2
The energy of the combustibles is shown in Table 4.4 below; calculated using the equation:
Energy Value, kJ= Weight of waste, kg × Energy content, kJ/kg (Table 4.2)…………eqn 3
The energy content of ash and dirt was taken to be 7000 kJ/kg.
Specific Energy content of combustibles (Tables 4.4) = 143304.8
10.5
= 13648.08kJ/kg
Specific Energy content of water = 3.39 × 2636 kJ/kg
10.5
= 851.10kJ/kg
Specific Energy content of glass =
0.2 × 628 kJ/kg
=11.96 kJ/kg
10.5
Specific Energy content of metal = 0.5 × 544 kJ/kg
= 25.90kJ/kg
10.5
From eqn 2, the specific energy of the waste is given by:
=13648.08 – 851.10 – 11.96 – 25.9
= 12759.12 kJ/kg
43
Table 4.4 Energy content of the combustibles
Waste
Weight (kg)
Energy content (kJ)
Food
3.1
16585
Paper
1.2
18976.80
Plastic
1.5
49200
Wood (includes leaves banana
stalks & leaves, etc)
2.2
33979
Textiles
1.2
20934
Other (includes dirt, ashes)
0.6
3630
Total
9.8
143304.8
4.4 Potential Electricity production from the waste
A rough estimate of the amount of electricity to be produced can be made using the net calorific
value (NCV). The calorific value of raw MSW averages around 1,200 to 1,400 k-Cal/kg.
Total waste quality: 10.5
tons
1000
Net Calorific Value: NCV k-cal/kg
Energy recovery potential (kWh) = NCV × 10.5 × 1000/860
1000
Power generation potential (kW) = 0.016 NCV × 1/24 =6.4922 ×10-4 × NCV
Conversion Efficiency = 25%
Net power generation (kW) = 1.623 × 10-4 × NCV
44
Taking NCV=1400 k-cal/kg, then
Net power generation potential (kW)=1.623 × 10-4 × 1400 =0.227kW
4.5 Total Energy from Dandora dumpsite
Dandora estate had a population of about 110,164 people in 2010
(www.nabuur.com/files/attach/2013/14) population growth rates in Kenya are as shown below:
Table 4.5 Population growth rates in Kenya (CIA World Facebook, Jan 2014)
2010
2011
2012
2013
2014
2.8
2.76
2.69
2.59
2.46
The average growth rate from 2007-2011, r = 2.8+2.7+2.69+2.59+2.46
5
=2.66%
t
The current population can be estimated as = (1+r) P
Where r = annual growth rate
t= time in years
p= population in 2010
= (1+2.66)5 × 110,164
100
= 125,616 people
A resident in Nairobi town produce approximately 0.6kg of waste in a day (Integrated Solid Waste
Management Plan for Nairobi – 1st Draft, 19 February 2010).Nairobi has a population of 3.363million
people.Hence computing the amount of waste produced in Nairobi for 1 year that ends up at
Dandora landfill;
3,363,000 × 0.6 × 365 = 736,497,000 kg
Assuming that all conditions remain constant as the waste sample collected, the amount of heat
energy that can be produced in a year by direct proportion will be:
45
=736,497,000 Kg × 12759.12
10.5 Kg
=8.95 × 1011 KJ/Kg
Assuming an efficiency of 80% since the incinerator efficiency is always less than unit, the available
specific heat energy will be
= 80% × 8.95 × 1011 =7.16 × 1011 KJ/Kg
The potential electricity production from the waste also by direct proportion will be:
=736,497,000kg × 0.227kW
10.5kg
= 15,922,363.71 kW
Assuming also an efficiency of 80% in the conversion process the available electric power will be:
= 80% × 15922363.71 =12,737,891 kW
The gross revenue generated in a year if the electric power will be solid @ say Ksh.48/kW ($0.60kW)
will be:
=Ksh.48 × 12,737,891
=Ksh. 611,418,766
46
4.6 DISCUSSION
From the point of view of combustion efficiency and maximization of energy recovery per ton
of MSW, it is clear that it would be preferable to separate wet materials. This will increase the
heating value of the material being burned and therefore will generate more energy per ton of
waste burned. Separation of non-combustibles in the waste (glass and metals) will also
increase heating value and energy generation per ton. However, the prime function of waste
combustion facilities is waste disposal. The removal of specific materials before combustion
requires either collection of separate streams at the source, or separation at the combustion
facility before burning. Either of these processes increases the collection, transportation and
processing costs with relatively little change in the energy generation per ton of original
waste. Thus, although heating value is increased, the net cost of energy from solid waste
increases.
A sample of 10.5kgs of municipal waste was collected and analyzed as shown in the above
sections of this chapter. The moisture content of the waste was found to be 32.4%.The
specific heat energy of the waste was calculated as 12,759.12kJ/kg, while the electric power
potential of the waste was estimated to be 0.227Kw.
The best way of managingmunicipalsolid wasteis by recoveringrecyclable materials. However,
even when the best available technology is applied for collecting, sorting and processing,
there still remains a large fraction of wastes that cannot be recycled. It is then advisable, both
from the economic and environmental viewpoints, to recover energy from the combustibles
fraction of MSW instead of consigning it to landfills.
The wide variety of materials covered by the notion of “waste” are increasingly perceived as a
resource to be used as efficiently as possible. This contributes to reducing the non-recoverable
fraction of wastes. If proper regulatory safe guards are in place and reliable decision support
tools such as life-cycle assessment gain in acceptance, this trend should lead to the natural
optimization of waste management. The global objective must remain to find the best use for
wastes while minimizing adverse effects on public health and the environment.
The estimation of the energy content of municipal solid waste can be of practical interest in the
design and operation of the related energy conversion systems. Model development for
accurate estimation of heating value is a necessity in order to not only save the time but also
decrease the cost in the design and operation of municipal solid waste based on engineering
application. In this project, the energy recovery was evaluated i.e. specific heat energy using
low and high heating values of waste and its composition and calorific value used for electricity
generation. From the project, it can be concluded that the latent energy of municipal waste can
be utilized for gainful uses using technologies like incineration, gasification, pyrolysis,e.t.c.Since
the Dandora dumpsite is not a sanitary landfill, the waste will just stay there and pollute the
47
environment,later producing greenhouse gases. When this waste is incinerated green energy,
which has no effect on the atmosphere is recovered.
The different technologies for recovering useful energy from Municipal solid wastes already
exist and are being extensively utilized in different countries for their multiple benefits. It is
necessary for the success of technologies to evolve an Integrated Waste Management system,
coupled with necessary legislative ad control measures. A detailed feasibility study needs to be
conducted In each, taking into account the available waste quantities and characteristics and
the assessment of the different waste disposal options. Suitable safeguarding and pollution
control measures further need to be incorporated in the design of each facility to fully comply
with the environmental regulations and safeguard public health. The waste-to-Energy facilities,
when set up with such consideration, can effectively bridge the gap between waste recycling,
composting and land filling.
48
CHAPTER 5
5 CONCLUSIONS AND RECOMMENDATION
5.1 Conclusion
1. The amount of specific heat energy of the sample was found to 12759.12 kJ/kg which for the
entire Dandora area is equivalent to 7.16×1011Kj/kg in a year.
2. The potential electricity production of the waste was found to be 0.227Kw while the electricity
production for the area for the whole year was estimated to be 12.74GWh (Gigawatt hours).
This power was estimated to fetch a gross revenue of Ksh.611, 418,766 in a year if an energy
waste recovery system waste system was to be set up.
5.2 Recommendation
The recommendations of this project are;
1. Future studies should do a cost benefit analysis of an energy recovery facility if put in the area
and establish its viability economically
2. It is not only energy that can be recovered from solid waste but also metal and ash.
Future studies should research on the same and their use
3. Gas emissions and other negative effects to the environment and their control measures
brought about by setting up an incinerator should also be researched in future.
4. Introduction of Garbology studies in institution of higher learning to pave way to research in
energy recovery.
49
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