MAKERERE
UNIVERSITY
COLLEGE OF NATURAL SCIENCE
SCHOOL OF PHYSICAL SCIENCE
DEPARTMENT OF CHEMISTRY
BSC.INDUSTRIAL CHEMISTRY
COURSE UNIT:
ENERGY TECHNOLOGY
COURSE CODE:
ICH 2116
TOPIC:
BIOMASS CONVERSION TECHNOLOGY AND BIOMASS COMBUSTION
PRESENTED BY:
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GROUP TWO
MEMBERS
NAMES
NSEREKO SHAFIK
MALE
NSUBUGA MARVIN
JONATHAN
NSUBUGA DICKSON
BATENGA MOUREEN
NAMIIRO MARIAT
MUKHWANA FELIX
NAMIIRO JOY KIYEGA
APIYO SHARON ODONG
KATUMBA JUSTUS
MURUNGI EDRIN
KAMUKUMBA
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REG. NO
21/U/12304/Ps
21/U/19421/Ps
21/U/1540
21/U/10189/Ps
21/U/0364
21/U/07217/Ps
21/U/1383
21/U/12464/Ps
21/U/0611
19/U/20718
SIGNATURE
Table of content
Contents
BIOMASS CONVERSION TECHNOLOGY .............................................................................................. 6
COMBUSTION ........................................................................................................................................ 6
GASIFICATION....................................................................................................................................... 7
PYROLYSIS: ............................................................................................................................................ 7
LIQUEFACTION: .................................................................................................................................... 7
BIOMASS COMBUSTION ..................................................................................................................... 7
SOURCES OF BIOMASS FUELS........................................................................................................... 8
VIRGIN WOODS (DRY): .................................................................................................................... 8
ENERGY CROPS (DRY):.................................................................................................................... 8
AGRICULTURAL RESIDUES (WET AND DRY): ........................................................................... 8
INDUSTRIAL RESIDUES (WET AND DRY): .................................................................................. 8
CHARACTERISTICS OF BIOMASS FUELS ........................................................................................ 8
CALORIFIC VALUE (CV) .................................................................................................................. 8
NET CALORIFIC VALUE (NCV) ...................................................................................................... 8
GROSS CALORIFIC VALUE (GCV) ................................................................................................. 9
MOISTURE CONTENT (MC)............................................................................................................. 9
BULKY DENSITY ............................................................................................................................... 9
ENERGY DENSITY .......................................................................................................................... 10
ASH CONTENT ................................................................................................................................. 10
CHEMICAL CONTENT .................................................................................................................... 10
PRINCIPLES OF BIOMASS COMBUSTION ...................................................................................... 10
1.
Preheating phase: ........................................................................................................................ 10
2.
Gaseous phase: ............................................................................................................................ 10
3.
Solid phase .................................................................................................................................. 10
USES OF COMBUSTION ..................................................................................................................... 11
ADVANTAGES OF BIOMASS COMBUSTION ................................................................................. 11
DISADVANTAGES OF BIOMASS COMBUSTION ........................................................................... 11
COMBUSTION EQUIPMENT .................................................................................................................. 12
DOMESTIC COMBUSTION EQUIPMENT. ........................................................................................ 12
INDUSTRIAL COMBUSTION SYSTEMS .......................................................................................... 12
COMPONENTS OF BIOMASS COMBUSTION SYSTEM................................................................. 12
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Fuel transfer equipment ...................................................................................................................... 12
Fuel feed equipment ............................................................................................................................ 12
Combustion grate ................................................................................................................................ 12
Refractory material ............................................................................................................................. 12
Heat exchangers .................................................................................................................................. 12
Ash extraction equipment ................................................................................................................... 12
Control equipment .............................................................................................................................. 12
Exhaust gas treatment equipment ....................................................................................................... 12
Flue (chimney) .................................................................................................................................... 13
Ignition Equipment ............................................................................................................................. 13
Expansion tank .................................................................................................................................... 13
Fire protection equipment ................................................................................................................... 13
COMBUSTORS ..................................................................................................................................... 13
DIRECT COMBUSTORS ...................................................................................................................... 13
MOVING GRATE SYSTEMS ............................................................................................................... 13
MODE OF OPERATION OF MOVING GRATE SYSTEMS .............................................................. 14
ADVANTAGES OF MOVING GRATE SYSTEMS......................................................................... 14
DISADVANTAGES OF MOVING GRATE SYSTEMS .................................................................. 15
PLANE GRATE SYSTEMS .................................................................................................................. 15
MODE OF OPERATION OF THE UNDERFED TYPE ....................................................................... 15
ADVANTAGES OF PLANE GRATE SYSTEMS ............................................................................ 16
DISADVANTAGES OF PLANE GRATE SYSTEMS ...................................................................... 16
STOKER BURNER SYSTEMS ............................................................................................................. 16
MODE OF OPERATION OF STOKER BURNER SYSTEMS ............................................................ 16
ADVANTAGES OF THE STOKER BURNER SYSTEMS .............................................................. 17
DISADVANTAGES OF THE STOKER BURNER SYSTEMS ....................................................... 17
FACTOR TO CONSIDER WHEN CHOOSING A COMBUSTION SYSTEM ....................................... 17
INDUSTRIAL BOILER OPERATIONS ................................................................................................... 17
DIFFERENT TYPES OF BOILERS ...................................................................................................... 18
BIOMASS HOT WATER BOILERS ..................................................................................................... 18
LOW-TEMPERATURE HOT WATER (LTHW).............................................................................. 18
HOT TEMPERATURE HOT WATER (HTHW) .............................................................................. 18
BIOMASS STEAM BOILERS............................................................................................................... 18
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MATERIAL AND ENERGY BALANCE IN A BOILER ......................................................................... 18
MATERIAL BALANCES ...................................................................................................................... 18
ENERGY BALANCES .......................................................................................................................... 19
BASIC PRINCIPLES OF MASS AND ENERGY BALANCE ............................................................. 19
MATERIAL AND ENERGY BALANCE ............................................................................................. 21
BOILER HEAT BALANCE................................................................................................................... 21
UNIT OPERATIONS AND UNIT PROCESS ........................................................................................... 22
UNIT OPERATION ............................................................................................................................... 22
UNIT PROCESS ..................................................................................................................................... 22
EXAMPLES OF UNIT PROCESSES ................................................................................................ 22
CLASSIFICATIONS OF UNIT OPERATIONS................................................................................ 22
MECHANICAL UNIT OPERATION ................................................................................................ 22
MASS TRANSFER PROCESSES ..................................................................................................... 23
HEAT TRANSFER OPERATIONS ................................................................................................... 23
DIFFERENCES BETWEEN UNIT OPERATIONS AND UNIT PROCESSES. .................................. 23
REFERENCES ........................................................................................................................................... 23
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BIOMASS CONVERSION TECHNOLOGY
Biomass can be converted into three main products; two related to energy and power /heat
generation and transportation fuels and one as a chemical feedstock. Amongst the options for
energy conversions that form the basis of this study only those that produce the fuel suitable for
use in a S.I.G.E (Spark ignition gas engine) are the main focus of this study.
Conversion of biomass to energy is undertaken using two major processes, Thermo-chemical and
biochemical or biological. The third technology for producing energy from biomass is
mechanical extraction (esterification) e.g. rapeseed methyl ester
Within the thermo-chemical process, four main conversion routes for biomass conversion are
involved which are;
COMBUSTION
Is the study of chemically reacting flows with rapid, highly exothermic reactions. In this case,
combusted fuel is used to generate heat which is then used to raise steam, which operates in a
standard steam generator set as the prime mover to produce electricity. (Prieto, n.d.) On a small
scale, steam is inefficient due to the high temperatures and pressure required. Replacing water as
the working medium with an organic compound with a lower boiling point allows greater
efficiency at lower temperatures
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GASIFICATION:
This is a process of converting the input fuel to a gas mixture -a synthetic gas or syngas -by
reacting it at high temperatures with controlled amounts of oxygen or steam. (Prieto, n.d.) The
syngas is typically combusted in the reciprocating engine (but could also potentially be used in a
gas turbine). However, this process is inherently more complex than simple combustion
PYROLYSIS:
This is a thermal degradation process that occurs in the absence of oxygen which produces a
variety of products that can be used for power generation and is typically combusted in
reciprocating engines (but could also potentially be used in a gas turbine)
LIQUEFACTION:
Is the conversion of biomass into a stable liquid hydrocarbon using low temperatures and high
pressures
Biochemical conversion involves two process options which include anaerobic digestion which
is the production of biogas (a mixture of mainly methane and CO2) and fermentation which is the
production of ethanol from starch crops e.g. sugarcane
BIOMASS COMBUSTION
Biomass combustion simply means the burning of organic matter or materials
It is interdisciplinary in nature comprising thermodynamics, chemical kinetics, fluid mechanics,
d transport phenomena.
Thermodynamics allows us to do the bookkeeping on how much chemical energy is converted
to thermal energy in such a process and to determine the thermal and compositional properties of
the products when equilibrium is reached (apply first, second and third laws)
Chemical kinetics is needed to prescribe the paths and rates through which reactions take place.
Fluid mechanics helps to understand the chemical reactions occurring in a flowing medium.
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Transport phenomena help to understand the heat transfer between different regions
(conduction, convection, and radiation)
SOURCES OF BIOMASS FUELS
Biomass is a form of stored solar energy and is available in several different forms.
There is a wide range of sources of biomass fuels that can be broadly defined in terms of “wet”
and “dry” sources and under those two broad categories.
The sources can be classified into four;
VIRGIN WOODS (DRY):
is untreated, free of chemicals and furnishes, it comes from a variety of sources e.g. forestry is
the primary source with other sources being arboricultural arisings (tree surgery wastes) and coproducts from wood processing facilities such as sawmills, and furniture factories.
ENERGY CROPS (DRY):
Are crops grown specifically for the production of energy? These include woody energy crops
(short rotation forestry, willow eucalyptus, poplar), grassy energy crops (miscanthus and hemp),
sugar crops (sugar beets), starch crops (wheat, barley, maize /corn)
AGRICULTURAL RESIDUES (WET AND DRY):
Are from animals and crops. From animals, we get animal wastes (urine, dung). From crops, we
get maize corn, etc.
INDUSTRIAL RESIDUES (WET AND DRY):
Include sewage sludges, residues from sawmills, construction, industrial pallets, chipboards, etc.
CHARACTERISTICS OF BIOMASS FUELS
Characteristics Biomass fuels have a range of characteristics that affect their performance and
also the type of biomass heating equipment they can be used in. Some of the most important
factors are listed below and a table presenting the most common fuels and their associated
characteristics is given at the end of this section
CALORIFIC VALUE (CV)
It indicates the heating potential of a fuel and is a measure of its energy content. (Fournel et al.,
2015) It is defined as the amount of heat released from a specific unit of fuel by its complete
combustion. Biomass fuel CVs are conventionally expressed as MJ/kg. The calorific value of a
fuel is expressed either as Gross Calorific Value (GCV – also sometimes known as Higher
Heating Value (HHV)), or Net Calorific Value (NCV – also sometimes known as Lower Heating
Value (LHV)).
NET CALORIFIC VALUE (NCV)
This is the quantity of heat given off by the complete combustion of a unit of fuel when the water
vapor produced remains as a vapor and the heat of vaporization is not recovered. This can be
calculated by subtracting the heat of vaporization of the water produced from the GCV. The
NCV is more widely used in the UK than the GCV.
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GROSS CALORIFIC VALUE (GCV)
This is the quantity of heat liberated by the complete combustion of a unit of fuel when the water
vapor produced is condensed, and the heat of vaporization is recovered. The water is condensed
by bringing the products of combustion (flue gases) below 100ºC (as in a condensing plant). This
generally does not apply to biomass as the flue gases cannot be cooled below c.130ºC, and hence
the water vapor cannot be condensed.
The key determinant of biomass material’s calorific value is the inherent moisture content (MC)
The greater the MC the less energy is contained within the fuel.
MOISTURE CONTENT (MC)
This is expressed as a percentage, measured either on a ‘wet’ or ‘dry’ basis. Wood suppliers (for
example) typically use the wet-basis method because it gives a clearer indication of the water
content in timber.
The wet basis calculation expresses the moisture content as a percentage of the mass of the
material including any moisture. In the formula below ‘oven-dry mass is defined as the mass of
biomass that has had all the moisture driven out:
Wet basis MC = (Fresh mass-Oven dry mass)/ (Fresh mass) x 100 (%)
The dry basis calculation expresses the moisture content as a percentage of the oven dry mass:
Dry basis MC = (Fresh mass-Oven dry mass)/ (Oven dry mass) x 100 (%)
A higher MC implies a lower calorific value as each unit mass of fuel contains less oven-dry
biomass – which is the part of the fuel that undergoes combustion to release heat. The effect is
more noticeable for most biomass heating systems where the water vapor in the combustion
products cannot be condensed. This is because the moisture in the fuel also has to be vaporized
before combustion can occur and this requires energy input that cannot be recovered later.
The majority of the biomass industry uses a wet basis when discussing biomass fuels.
BULKY DENSITY
This is a measure of the mass of many particles of the material divided by the volume they
occupy; the volume includes the space between particles. The higher the bulk density, the more
mass of fuel exists in a given volume.
For example
Wood pellets (c.660kg/m3) have a higher bulk density than wood chips (c.250kg/m3). Bulk
density, unlike density, is not intrinsic to a material; for example, the same piece of wood could
have different bulk densities if processed into logs, pellets, or woodchips. Moisture content also
affects bulk density as each particle has a greater mass but does not occupy more space.
This is an important point because fuels with higher moisture contents will have greater masses
and, therefore, have lower bulk densities. With higher moisture content comes lower energy
density, and therefore the volume of fuel required for a given amount of heat will be larger.
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ENERGY DENSITY
This is a measure of the energy contained within a unit of fuel. Energy density is conventionally
expressed in MJ/m3. It can be derived by multiplying the calorific value (MJ/kg) by bulk density
(kg/m3).
Energy density is an important variable that will help users understand volumetric fuel
consumption rates, the size of fuel storage required, the number of deliveries required, and the
total annual quantity of fuel required.
Energy density = CV x Bulk density (MJ/m3) (MJ/kg) (kg/m3)
ASH CONTENT
Although the amount of ash produced is partly dependent on the type and performance of the
biomass plant it is being used in, it is also an inherent fuel property that is specified as a fuel
characteristic.
CHEMICAL CONTENT
It is natural for biomass to contain low levels of mineral salts and another trace ‘non-biomass’
material, taken up from the soil or air during growth. The presence of these salts and other
elements in ‘virgin’ biomass fuels does not normally cause any significant issues, but it does
partly determine the level of gaseous/particulate emissions, ash, and slagging (also known as
‘clinkering’). If, for instance, an annual crop is being used (e.g. straw) then more care is required,
as these can have higher levels of alkaline metal salts. Also, if amounts of sand are present in the
fuel, this can result in glass formation during combustion
PRINCIPLES OF BIOMASS COMBUSTION
Combustion, or burning, is a complex sequence of exothermic (heat-generating) chemical
reactions between a fuel and an oxidant.
Biomass combustion takes place in three distinct but overlapping phases (similar to other solid
fuels), the extent and nature of which are different for each type of biomass fuel:
1. Preheating phase:
Moisture in the unburned fuel is driven off and the fuel is heated up to its ‘flash point’ and then
its ‘fire point’ (the temperature at which it will continue to burn after ignition for at least five
seconds).
2. Gaseous phase:
A mixture of flammable gases is given off (volatized) by the solid fuel and is ignited. Energy is
transferred from chemical energy into heat and light (flames).
3. Solid phase:
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The rate of release of flammable gases from the solid fuel is too low to maintain a flame and the
‘charred’ fuel glows and then only Smulders.
USES OF COMBUSTION
 Used for space heating of buildings.
 Used in hot water production.
 Used in steam production.
 Generation of electric power.
ADVANTAGES OF BIOMASS COMBUSTION
 Significant carbon saving. Biomass combustion can play a major role in reducing an
organization’s carbon footprint. To reduce the overall carbon emission and improve
environmental performance carrying out biomass combustion could help do
 Operation cost saving. The cost of biomass fuels is typically lower than the fossil fuels
being displaced and biomass combustion can therefore provide attractive operational cost
savings.
 Reduced fuel price volatility. Security of energy supply is a recusant concern for fossil
fuels, geopolitical instabilities in oil and gas-producing regions can threaten availability
and lead to unexpected price changes. While biomass fuels can be subjected to changes in
price over time, these are not extreme as those of fossil fuels and biomass can be
accessed locally.
 Wider sustainable development benefits. Fuels used typically with biomass combustion
tend to have diverse and localized fuel supply chains. This has a positive benefit along
this supply chain such as improving the biodiversity of existing woodlands and providing
opportunities for rural employment and economic diversification.
 Resources diverted from landfill. Using certain biomass resources as fuels can divert
them from becoming waste and being sent to landfills.
 Reduced exposure to climate change-related legislation. Biomass fuels do not register
part of an organization’s overall carbon emissions and by this, it reduces the emission of
greenhouse gases and combustion of fossil fuels.
DISADVANTAGES OF BIOMASS COMBUSTION
 Fuels tend to vary in their specifications and quality, and obtaining biomass of a required
or desired standard can sometimes be challenging.
 Source of fuel may be difficult in certain areas.
 Climatic changes such as drought due to deforestation.
 Biomass fuel contains moisture content which affects its energy content (the calorific
value)
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 Expensive during installation on an industrial basic
 Health-related problems like lung cancer.
COMBUSTION EQUIPMENT
These are apparatuses that are used for burning biomass fuels.
These are categorized into;
DOMESTIC COMBUSTION EQUIPMENT.
These include;
•
Fireplaces
•
Stoves -use wood or coal. There are two types; radiant which transfers heat through a hot
surface and circulating which transfers heat through vents
•
Furnaces
INDUSTRIAL COMBUSTION SYSTEMS
– These are mainly used in industries and include Boilers and combustors
COMPONENTS OF BIOMASS COMBUSTION SYSTEM
Fuel transfer equipment
This transfers fuel from where it is stored to the plant
Fuel feed equipment
The system for transferring fuel into the plant at the required rate
Combustion grate
The main point at which combustion starts that is to say moving grate and plane grate
Refractory material
Also known as fire bricks, designed to reflect heat onto the grate to drive off moisture from the
fuel and maintain optimum combustion temperature
Heat exchangers
These means of transferring the heat in the hot combustion gases to the medium (water) – e.g.
via ‘fire tubes’ with a ‘water jacket
Ash extraction equipment
Transfer the ash into an external receptacle which can be emptied manually.
Control equipment
Used for controlling output via the fuel feed rate and air levels.
Exhaust gas treatment equipment
Used to minimize emissions of such things as particulate matter and fly ash from the plant’s
combustion chamber.
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Different levels of emissions abatement equipment are available from relatively simple, singlestage cyclones to multiple stages involving bag filters and other devices.
Flue gas fan(s) some plants need a flue gas fan or induced draft fan to draw the flue gases from
the combustion chamber and through the plant heat exchanger. The flue gas fan discharges to the
chimney.
Flue (chimney)
The chimney stack has two functions: it draws the flue gases through the plant and disperses the
gases to the atmosphere at a safe level.
Ignition Equipment
Plants may be ignited automatically using a hot air gun (smaller systems) or electrically ignited
gas pilot (larger systems).
Expansion tank
Not part of the main plant itself but a key component of a system to allow the natural expansion
of the water in a heating system as it gets hot – in sealed systems, the ‘expansion vessel’ (a small
pressurized container) accommodates the extra volume.
Fire protection equipment
Used to prevent fire from the combustion chamber moving back into the fuel store. Can be a
water ‘dousing’ approach or some form of automatic shut-off gate(s) on the feed mechanism
systems offering varying levels of fire protection.
COMBUSTORS
This is an enclosed chamber where the fuel is burnt by heating it and adding oxygen (air) in the
right amount and proportion.
Combustors are categorized into two categories;
DIRECT COMBUSTORS
These are devices in which the fuel is heated, dried, and combusted all in one compartment.
This is the most common type of combustor and there are several varieties.
Industrial combustion Plants/systems are usually classified by their type of grate based on the
method of heat exchange, fuel type, and level of automation, the main types being:




moving grate,
plane grate,
batch-fired,
and stoker.
MOVING GRATE SYSTEMS
Moving grate (also known as step-grate or inclined grate)
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MODE OF OPERATION OF MOVING GRATE SYSTEMS
Fuel is delivered onto a series of inclined or flat panels of fire bars which move in a sequence so
that the fuel travels slowly (shuffles) down the grate towards the far end of the combustion
chamber.
The fuel dries and then combusts as it moves down the grate (primary air is supplied under the
grate). Gases are emitted, and char burns out.
Once the wood has combusted, the remaining ash falls off the lower end of the grate and is
removed mechanically into the ash pan or ash bin. They are generally more common at the
higher output range (300kW-1MW+).
ADVANTAGES OF MOVING GRATE SYSTEMS
Wide tolerance of fuel type, moisture content (up to 60%), and particle size.
As a result of wide fuel tolerance, cheaper fuel may be procured, helping to offset the higher
capital cost.
The positive movement of fuel down the grate avoids clinkering and blockages.
Well-regimented combustion leads to high efficiency
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DISADVANTAGES OF MOVING GRATE SYSTEMS
Relatively large fuel inventory in the plant leads to a slow response to load swings, although
modulating controls improve controllability.
Large amounts of refractory (heat reflective) material on wet wood plants can result in a long
warm-up time from very low to full load (up to 2 hours).
Prolonged low-load mode operation can result in higher maintenance costs and reduced
efficiency as a result of tarring of heat exchangers and condensing gases.
More complex design and bulky components
PLANE GRATE SYSTEMS
Plane grate biomass plants can feature either underfed or side-fed combustion chambers. The
main difference to the moving grate system is that the combustion bed is smaller.(Frounfelker &
Hausfeld, 1983)
Plane grate systems are suitable for fuel with moisture content below 35% e.g. Joinery waste this
is because the fuel is fed directly into the combustion chamber by the fuel feed mechanism,
rather than being dried first as in the case of the inclined grate plants.
MODE OF OPERATION OF THE UNDERFED TYPE
Fuel is fed by an auger into the base of an inverted cone or trough, where it wells up into the
combustion chamber, spreading out to the sides.
Primary air is supplied below the fuel, and secondary air is above. Underfed stokers are usually
supplied as part of a complete plant package; however, they may be constructed as a separate
unit from the plant itself (i.e. the heat exchange element of the system where the heat in the hot
combustion gases is transferred to water in the plant). In such instances, the heat exchange unit
will be of an open-bottom construction that sits on top of the stoker unit. On larger units, the
stoker may be situated within the combustion chamber of a shell-and-tube plant.
Ash is created on all sides of the combustion zone, and its removal from the combustion bed
relies on simple displacement due to the emergence of new fuel in the center of the combustor.
The removal of ash from the bottom of the combustion chamber is sometimes by manual
intervention, but it is more common to have the ash augured from the bottom of the combustion
bed to an external ash bin
For the side-fed grates, the grate is vibrated regularly to shake ash through holes within it, and it
tips periodically to allow the removal of larger non-combustible items. In terms of their ability to
tolerate moisture content in fuels, side-fed units offer an intermediate option between underfed
and moving grate systems.
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ADVANTAGES OF PLANE GRATE SYSTEMS
A smaller combustion area and less refractory material mean that these types of plants have a
smaller spatial footprint.
Commonly dual fuel plants, therefore providing flexibility of operation41.
In total capital cost terms (£/kW, installed) they are cheaper than the moving grate systems due to
the simpler design and exclusion of refractory material
DISADVANTAGES OF PLANE GRATE SYSTEMS
Due to the smaller combustion bed, the plant requires lower fuel moisture content – typically 2035% – rising to 40% if the plant has some refractory material lining the combustion chamber.
Due to the smaller combustion bed and lower moisture content tolerances, these systems require
consistently good quality fuel. As a result, they are best suited to applications where site owners
are confident of securing good quality fuel (<35% MC).
STOKER BURNER SYSTEMS
They are often cheaper than planes and moving grate systems.
MODE OF OPERATION OF STOKER BURNER SYSTEMS
Biomass fuel is augured into a burner head, which has a special cast iron liner to reflect heat onto
the fuel. Air is introduced by a small fan, which passes around the outside of the cast-iron liner,
thus heating up. The air then enters the fuel space via small holes, some below the fuel to provide
primary air, and some above to provide secondary air. The burner head produces a vigorous
flame in the combustion chamber, and the resultant hot combustion gases pass into the heat
exchanger unit.
Ash from the burner head is pushed away and into the ash pan by the incoming fuel. De-ashing is
almost always manual, especially on smaller units.
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ADVANTAGES OF THE STOKER BURNER SYSTEMS
Small fuel inventory makes for a relatively rapid response to load swings.
The heat generated on slumber (when there is no heating requirement and the unit is simply
maintaining ignition with as little heat output as possible) is very low.
Often lower cost than plane grate or moving grate systems.
DISADVANTAGES OF THE STOKER BURNER SYSTEMS
The fuel must be fairly dry: preferably <30%, never more than 35%.
The fuel particle size and moisture must be consistent: the small, intense combustion zone is
easily disrupted.
In the lowest cost, smaller units, no separate provision for primary and secondary air supply
exists, limiting the opportunity for fine-tuning to the needs of varying fuel
FACTOR TO CONSIDER WHEN CHOOSING A COMBUSTION SYSTEM
Initial cost, the equipment should not be expensive
Fuel compatibility, the fuels should be compatible with the combustor
Safety, a combustor should be constructed in a way that it is safe to operate
Durability, it should have a long lifespan
Maintenance costs should have a low maintenance cost
INDUSTRIAL BOILER OPERATIONS
The industrial boiler is used to produce steam or hot water space and process heating and for the
generation of mechanical power (Hasanuzzaman et al., 2012)
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DIFFERENT TYPES OF BOILERS
These are categorized according to the different characteristics best suited for their unique
purposes. And these include the following;
BIOMASS HOT WATER BOILERS
The biomass boiler burns the biomass cleanly; the heat is transferred to the hot water for heating
and the flue gas is cleaned of particulate.
They are categorized into two;
LOW-TEMPERATURE HOT WATER (LTHW)
They operate within a temperature of 121⁰c and the maximum allowable working pressure is
usually 2 bar. They are generally used for space heating in homes, offices, residential buildings
HOT TEMPERATURE HOT WATER (HTHW)
These operate at temperatures exceeding 177⁰c or more and their maximum operating pressure is
usually less than 20.7 bar. These are mainly used in industries
BIOMASS STEAM BOILERS
Any organic waste product can be a fuel for a biomass steam boiler, but generally, it is usually
wood waste from sawmills that are used for large-scale steam boilers. The most used source of
heat for sawmills and processing plants is steam.
Biomass steam boilers are significantly larger and more complex than standard gas-fired boilers,
however, the fuel for the system can be essentially free.
The biomass boiler burns the biomass cleanly, the heat is utilized to generate steam at high
pressure, and the flue gas is cleaned of particulate.
The steam is generally used for heating directly, however, larger systems can also generate
electricity by placing a steam turbine between the boiler and the heat load. Many customers may
not have a direct source of biomass on the property; however wood waste can generally be
sourced for a very low cost or even free of charge from local sawmills in your area.
MATERIAL AND ENERGY BALANCE IN A BOILER
Material quantities, as they pass through processing operations, can be described by material
balances. Such balances are statements on the conservation of mass. (Veverka & Madron, 1997)
Similarly, energy quantities can be described by energy balances, which are statements on the
conservation of energy. If there is no accumulation, what goes into a process must come out.
This is true for batch operation. It is equally true for continuous operation over any chosen time
interval.
Material and energy balances are very important in an industry.
MATERIAL BALANCES
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Are fundamental to the control of processing, particularly in the control of yields of the products.
The first material balances are determined in the exploratory stages of a new process, improved
during pilot plant experiments when the process is being planned and tested, checked out when
the plant is commissioned and then refined and maintained as a control instrument as production
continues. When any changes occur in the process, the material balances need to be determined
again.
ENERGY BALANCES
Are used in the examination of the various stages of a process, over the whole process, and even
extending over the total production system from the raw material to the finished product. The
increasing cost of energy has caused industries to examine means of reducing energy
consumption in processing.
Material and energy balances can be simple, and at times they can be very complicated, but the
basic approach is general. Experience in working with simpler systems such as individual unit
operations will develop the facility to extend the methods to the more complicated situations,
which do arise. The increasing availability of computers has meant that very complex mass and
energy balances can be set up and manipulated quite readily and therefore used in everyday
process management to maximize product yields and minimize costs.
BASIC PRINCIPLES OF MASS AND ENERGY BALANCE
If the unit operation, whatever its nature is seen as a whole it may be represented
diagrammatically as a box, as shown in Figure. 4. 1. The mass and energy going into the box
must balance with the mass and energy coming out.
The law of conservation of mass leads to what is called a mass or a material balance.
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Mass In = Mass Out + Mass Stored
Raw Materials = Products + Wastes + Stored Materials.
Mr. = ΣmP + ΣmW + ΣmS (where Σ (sigma) denotes the sum of all terms).
ΣmR = ΣmR1 + ΣmR2 + ΣmR3 = Total Raw Materials
ΣmP = Σmp1 + ΣmP2 + ΣmP3 = Total Products.
ΣmW= SmW1 + S mW2 + SmW3 = Total Waste Products
ΣmS = ΣmS1 + ΣmS2 + ΣmS3 = Total Stored Products.
If no chemical changes are occurring in the plant, the law of conservation of mass will apply also
to each component, so that for component A:
MA in entering materials = mA in the exit materials + mA stored in the plant.
For example, in a plant that is producing sugar, if the total quantity of sugar going into the plant
is not equaled by the total of the purified sugar and the sugar in the waste liquors, then there is
something wrong. Sugar is either being burned (chemically changed) or accumulating in the
plant or else it is going unnoticed down the drain somewhere.
In this case:
MA = (map + mAW + mAU)
Where mAU is the unknown loss and needs to be identified. So the material balance is now:
Raw Materials = Products + Waste Products + Stored Products + Losses where Losses are the
unidentified materials.
Just as mass is conserved, so is energy conserved in food-processing operations. The energy
coming into a unit operation can be balanced with the energy coming out and the energy stored.
Energy In = Energy Out + Energy Stored
ΣER = ΣEP + ΣEW + ΣEL + ΣES
Where ΣER = ER1 + ER2 + ER3 + ……. = Total Energy Entering
ΣEp = EP1 + EP2 + EP3 + ……. = Total Energy Leaving with Products
ΣEW = EW1 + EW2 + EW3 + … = Total Energy Leaving with Waste Materials
ΣEL = EL1 + EL2 + EL3 + ……. = Total Energy Lost to Surroundings
ΣES = ES1 + ES2 + ES3 + ……. = Total Energy Stored
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MATERIAL AND ENERGY BALANCE
Energy balances are often complicated because forms of energy can be interconverted, for
example, mechanical energy to heat energy, but overall the quantities must balance.
The material balances are required to be developed at various levels.
Overall material balance: this involves the input and output streams for the complete plant.
Section-wise material balances: this involves material and energy balances to be made for each
section/ department/ cost center. This would help to prioritize focus areas for efficiency
improvement.
Equipment-wise material balances: material balances, for key equipment would help assess
performance, which would in turn help identify and quantify energy and material avoidable
losses.
BOILER HEAT BALANCE
Boiler performance parameters such as efficiency and evaporation ratio decrease with time due
to poor combustion, surface fouling of heat transfer, poor operation, and bad maintenance. Even
for a new boiler, reasons such as poor fuel quality and water quality can lead to poor
performance of the boiler. Boiler heat balance can help in identifying heat loss that can or cannot
be avoided. Boiler efficiency tests can help in finding the deviation efficiency of the best boiler
efficiency and target the problem areas for corrective action.
The process of combustion in the boiler can be described in terms of a flow energy diagram. This
diagram illustrates graphically how the incoming energy from the fuel is converted into useful
flow energy and the flow of heat and energy loss. The thick arrow indicates the amount of energy
contained in each stream.
Figure 1: Diagram of Boiler Energy Balance
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Boiler heat balance is a balance of the total incoming energy to the boiler and that leaves the
boiler in a different form. The following figure provides a range of losses that occurred for steam
generation
UNIT OPERATIONS AND UNIT PROCESS
UNIT OPERATION
Is a basic step in a process that involves physical change during the process like separation,
filtration, and evaporation. (Rathoure et al., 2019)
UNIT PROCESS
Is a process in the chemical changes that takes place to the material present in a reaction.
EXAMPLES OF UNIT PROCESSES
Sulphonation
Nitration
Oxidation
Halogenation, etc.
CLASSIFICATIONS OF UNIT OPERATIONS
There are four classifications of unit operations;
Material handling, transportation, or fluid flow process
These involve; pumping, compression, and fluidization.
MECHANICAL UNIT OPERATION
These involve; size reduction, agitation, blending, etc.
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MASS TRANSFER PROCESSES
These involve; evaporation, distillation, absorption, leaching, etc.
HEAT TRANSFER OPERATIONS
These involve; conduction, convection, radiation, etc.
DIFFERENCES BETWEEN UNIT OPERATIONS AND UNIT PROCESSES.
Unit operations
Unit processes
Processes in which physical changes and not
chemical changes e.g. distillation, mixing.
Chemical changes take place only e.g.
decomposition, burning, etc.
REFERENCES
Fournel, S., Palacios, J. H., Morissette, R., Villeneuve, J., Godbout, S., Heitz, M., & Savoie, P.
(2015). Influence of biomass properties on the technical and environmental performance of
a multi-fuel boiler during on-farm combustion of energy crops. Applied Energy, 141, 247–
259.
Frounfelker, R., & Hausfeld, B. A. (1983). Survey of Foreign Systems for Incineration and
Energy Recovery. SYSTECH CORP XENIA OH.
Hasanuzzaman, M., Rahim, N. A., Hosenuzzaman, M., Saidur, R., Mahbubul, I. M., & Rashid,
M. M. (2012). Energy savings in the combustion based process heating in the industrial
sector. Renewable and Sustainable Energy Reviews, 16(7), 4527–4536.
Prieto, C. (n.d.). Air gasification 04/01628. Technology, 4, 1527.
Rathoure, A. K., Ram, B. L. G. P., & Aggarwal, S. G. (2019). Unit Operations in Chemical
Industries. International Journal of Environmental Chemistry, 5(2), 11-29p.
Veverka, V. V, & Madron, F. (1997). Material and energy balancing in the process industries:
From microscopic balances to large plants. Elsevier.
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