CSIRO EXPLORATION AND MINING
Waste Coal and Ventilation Air, Hybrid Rotating Kiln Gas Turbine
Electric Power Generator
Proof of Technology
Patrick J. Glynn, Michael N. Wendt
CSIRO Australia
PO Box 883 Kenmore QLD 4069
Australia
Abstract
Greenhouse gas emissions from coal mines in Australia constitute approximately 6.7% of
the total Australian emissions. Major sources included underground fugitive emissions of
methane (~43%) and spontaneous combustion of waste coal (~11%). The favoured
technique for mitigation of methane emissions by the Australian coal mining industry is
its use for the generation of electricity. CSIRO has constructed a 1.2 MW pilot plant
which uses the co-firing of waste coal and waste gas to mitigate both underground
fugitive and waste coal spontaneous combustion emissions. The gas and coal are burnt in
a rotating kiln, which provides sufficient residence time at high temperature to burn low
concentration ventilation air. The flow rate of waste coal can be varied to compensate for
fluctuations in the methane concentration in the ventilation air; maintaining constant exit
gas temperature. Greenhouse benefits can be maximised by sizing the plant to use all the
mine ventilation air and also consuming all the rejects. The flexibility of the system
allows the proportion of waste gas to waste coal to be changed to suit the mine. At gassy
mines, the availability of drainage gas allows a design with efficiencies approaching that
of modern combined cycle power plants whilst still deriving almost two-thirds of its
energy from high ash waste coal. A commercial system is now available that utilises
steam turbine technology, with an increase in the flow of ventilation air and reduced
waste coal throughput. This plant is sized at 10MW and will be constructed at the United
Coal Mine in NSW Australia, with 4 more plants to be constructed in the Changqiang
province of China.
1
Outline of Problem
The Australian coal mining industry contributes approximately 6.5% of Australia's greenhouse
gas production while producing 1.9% of GDP and hence the coal mining industry's greenhouse
intensity is high. Current estimates indicate that approximately 75% of the greenhouse gas
emissions attributed to the coal mining industry are in the form of fugitive emissions of methane.
Based on a 20-year impact, methane produces a 21 times greater heating effect per ton than
carbon dioxide. Treating the methane emission represents the best opportunity for the coal
mining industry to reduce emissions. In a carbon trading/tax environment, mitigation of methane
could provide mines with a significant financial benefit.
In an underground mine, approximately 2/3rd of the methane is contained in the ventilation air.
Ventilation air is very difficult to utilise for combustion or to flare because the concentration of
the methane is low, percentage of methane is highly variable and the flow rates are very high. In
addition, the gas contains major flows of water droplets, coal and stone dust. Considerable
innovation is required to utilise this resource and Australia has been at the forefront in the
development of technologies to tackle this problem. This report details the design and testing of a
new technology that promises to mitigate a large proportion of the greenhouse gas emissions from
an underground coal mine, including the ventilation air. This technology involves the
combustion of the ventilation air and drainage gas together with waste coal, specifically coarse
reject, from the mine in a rotating kiln. It is proposed that heat from combustion is to be
converted into electricity by the use of an externally fired gas turbine. Co-firing of the waste coal
with ventilation air ensures a sufficiently high temperature to ensure oxidation of the low
concentration methane. In addition, the stockpile of coal allows the methane concentration
variation to be managed and ensures a constant energy flow exiting the kiln for use in power
generation. The reason for using a kiln, instead of other waste coal burning technologies, is to
study the feasibility of converting the coarse reject material into a lightweight expanded
aggregate which is a potentially valuable by product.
To provide a pathway for the commercial application of the results of this project and to assist
with meeting the cost of the research, a company has been formed, ComEnergy. ComEnergy is a
joint venture partnership between the Liquatech Turbine Company (LTC), the Sustainable Energy
Development Agency of New South Wales (SEDA) and the Commonwealth Scientific and
Industrial Research Organisation (CSIRO) of Australia. During the course of this project, the
consortium has built a 200 kW externally fired turbine system which can harvest the heat from
the combustion of waste coal and ventilation air. This report details the design and construction
of such a facility and its test with waste coal from various NSW mine sites.
In the current project it was desired to test a number of coals originating from four different
collieries. The collieries chosen were BHP Billiton’s Appin Colliery, South Coal’s Metropolitan
Colliery, Anglo Coal’s Dartbrook Colliery and Xstrata’s United Colliery. Coal from each of
these mines was generously supplied by the mine at no cost to the project. To reduce the cost and
energy consumption of pre-processing the waste coal material, it was decided to test the coals
from each mine in the unprocessed state as received from the mine. The only processing was
screening out of material with a size greater than 50mm. A small qualitative study was
performed on individual aggregate to study the sintering and bubbling processes.
The facility was built over a period of several years and included approximately six months of
commissioning. To allow simplification of the operation of the system and to keep the project
cost within a reasonable limit, it was decided to only construct only the primary heat exchanger
and omit the heat recovery heat exchangers required for efficient system operation. This allowed
the demonstration of the operation of the system but without an absolute confirmation of the
efficiency of the system. The results from the full trials with the use of waste coal resulted in the
following major observations.

As the team modified a second-hand aero-derivative turbine to demonstrate the system
function, a very conservative system was used in the ducting of the cooling air to the
turbine. This ducting, combined with the desire to test both heat exchangers in parallel,
lead to a significant pressure drop between the turbine compressor and turbine expander.
This pressure drop prevented the turbine from reaching its full operational speed.
However, the results from the operation of the turbine, topping combustor, heat
exchangers and safety systems gave the team sufficient confidence for the scaling of the
prototype to a 10MW prototype system. In a larger system, the ability to match the
design of the kiln to the turbine, knowledge of the correct amount of cooling flow
together with the optimisation of the heat exchanger and ducting systems should provide
for a better result.

The results showed that although it was possible to burn the low calorific (8-12MJ/KG)
waste material from each of the mines in the kiln, large amounts of additional fuel, in the
form of LPG, were required to maintain combustion and to achieve complete carbon
burnout. The ideal waste coal calorific value would be minimum 14.5MJ/KG, this is
achievable by blending the washeries fines with waste coal, we have burned such a blend
in the rotating kiln with very good results. Although drainage gas from a gassy mine
could be used in the place of LPG to assist with the burnout of the coal, this limits the
number of mines to which the technology could be applied. In the original plan for the
operation of this project, it was assumed that the operation of the kiln had already been
demonstrated by a company Thermal Technologies, who had completed this
demonstration for BHP on coal from its Peak Downs operation.

One of the primary reasons for choosing the kiln technology to test the coal was the
possibility of converting the waste into a valuable lightweight aggregate. However, the
high levels of silicon and aluminium within the coals tested, and in Australian coals in
general, results in a high melting point for the ash. In order to form the aggregate, the
softening point of the aggregate needs to be exceeded to ensure sintering of the various
molten particles. However, given the high melting points, and the need to have a high
temperature to maintain effective heat transfer, means the flame temperature must reach a
temperature where significant levels of oxides of nitrogen (NOx) are produced. This,
coupled with the already high levels of nitrogen which are a characteristic of Australian
Coals, means a NOx emission level which far exceeds the legislated levels. Tests, during
which the maximum temperature was limited, showed a significant reduction in NOx
levels. As expected however, these tests showed minimal amounts of lightweight
aggregate production. Of the four coals tested, only the waste material from Dartbrook,
which has a low ash melting point, was a candidate for aggregate production.
In conclusion it was found that, while the testing of an externally fired turbines produced
significant results to allow the development of a much larger commercial system, a significant
amount of work is still needed with regard to the design and operation of the coal burning system.
2
Technological Solutions
It is now widely acknowledged that anthropogenic emissions of carbon dioxide, and other gases
such as methane, have had and will continue to have a significant effect on climate change. Since
the Kyoto Protocol, the pressure to stabilise or reduce CO2 levels has increased. In particular,
coal mining and its use for steel production and power generation are targeted with increasing
pressure to shift to other fuels such as natural gas or to pursue alternative sources of energy.
However, given the low price of coal and its dominance of the electricity and steel production
markets, major changes are unlikely to happen quickly. However, the emissions from mines are
likely to cause an increasing pressure from the community to prevent the start-up of new mines
and restrict the operation of existing collieries.
While the principal worldwide focus for greenhouse gas is on CO2 emissions, methane is also
recognized as a gas with a large effect on radiative forcing. In fact, it has been estimated that
methane has contributed about 20% of the incremental radiative forcing {OECD 1995}. Based on
a 100-year impact, methane produces a 21 times greater heating effect per ton than carbon
dioxide. However, unlike CO2, it can be utilised as a fuel source, and thus offers the potential for
not only significant greenhouse gas emission reduction but for these projects to be undertaken
without cost and with an income stream to the mine.
Australia's coal mining industry is a large contributor to the economy of Australia accounting for
approximately 1.9% of GDP. The coal mining industry also contributes, at best estimate, 6.5% of
Australia's greenhouse gas production. Per dollar of wealth produced, the coal mining industry's
greenhouse intensity is high. Current estimates indicate that 75% of the greenhouse gas
emissions attributed to the coal mining industry are in the form of fugitive emissions of methane
from coal mining. Of these approximately 43% is from fugitive emissions of predominately
methane from underground mines and 21% from fugitive emissions of open cut mines. Other
significant sources include the contribution from open cut energy at 21%, from spontaneous
combustion of waste coal at 11% and 4% from underground mining energy consumption
{Williams et al [1996]). Treating the methane and spontaneous combustion emissions represents
the best opportunity for the coal mining industry to reduce emissions. If carbon trading is
introduced the penalty for fugitive emissions could have a dramatic impact on the profitability of
coal mining ventures.
The use of Coal Mine Methane (CMM) in Australia, however, is hampered by geography and the
nature of its supply, namely;
 Fluctuating flow rate
 Fluctuating concentration
 Low concentration
 Relatively small quantity
 Long distance to industry
 Relatively small towns nearby
 Lack of coal seam methane pipeline infrastructure
 Large supply of cheap energy - coal
 No economic penalty for release to environment
Burning the methane emitted by the mines and converting the gas to CO2 and water vapour
reduces the global heating effect on the environment by a factor of approximately 7. This
reduction is a product of the Global Warming Potential of methane being 21 times that of carbon
dioxide and because the mass of CO2 produced is 2.75 times that of the methane. CMM has other
environmental advantages as it has no ash, contains almost no sulphur and because there is no
nitrogen in the combustible portion of the gas. Substitution of CMM for coal in a number of
applications results not only in a reduction in methane released to the environment but also in
significant reduction in SO2, NOx, CO2 and particulate emissions. Co-firing CMM with coal can
produce additional NOx emissions by non-selective reduction to N2. Australia is presently
operating successful and environmentally beneficial technologies for controlling CMM emissions
(reciprocating gas engines, Vocsidiser).
While these instillations are progressing and are going some way to reducing the impact of the
fugitive emissions from the mine, they are predominately focussed on the high concentration and
high quality drainage gas emanating from the mine. Unfortunately, this gas only represents
approximately 1/3rd of the fugitive emissions from underground coal mines. The remaining 2/3 rd
of the emissions come from the methane contained within the ventilation air. The gas is forced,
by a legislated safety requirement, to be diluted to a concentration below 1.25 %v/v. This gas,
which is usually in the sewer return from a longwall, is then mixed with other ventilation gas to
be vented to the surface. Large fluctuations in concentration are present in the gas which is
mostly related to atmospheric pressure and longwall advance rate. The concentration is usually
between 0.3 %v/v and 0.7 %v/v when it exits the mine. Although the flow rate of the gas is
approximately constant, this concentration can fluctuate significantly by up to 100% over a single
day. While variations in concentration and flow rate of drainage gas can easily be
accommodated, the dilute nature of ventilation air means that any flare or utilisation process
which attempts to use this gas must also be able to cope with these large fluctuations. A sample
of these fluctuations is given below in Figure 1 over a one year period.
1.0
0.9
0.8
CH4, %
0.7
0.6
0.5
0.4
0.3
0.2
0.1
30/12
16/12
02/12
18/11
04/11
21/10
07/10
23/09
09/09
26/08
12/08
29/07
15/07
01/07
17/06
03/06
20/05
06/05
22/04
08/04
25/03
11/03
26/02
12/02
29/01
15/01
01/01
0.0
Date
Figure 1: Variation in Methane concentration in a coal mine with time
One technique that can be implemented at facilities to compensate for variability in concentration
is the use of supplementary fuel. The most convenient form is natural gas from a nearby pipeline.
Although this is a satisfactory solution to the problem the high cost of natural gas in Australia and
the low disposal price of electricity will likely force the project to become financially unviable.
A low cost, highly plentiful supplementary energy source that is available at mine sites is the
waste coal stream that results from the cleaning of coal. Spontaneous combustion of this material
presents its own significant source of emissions to the environment with estimates that it
represents 11% of the total coal mining greenhouse emissions {Carras et al [2001]}. It is the use
of this resource and the reduction of these emissions that prompted the creation of this project.
To research the possibilities of combining ventilation air from underground coal mines and waste
coal, an Australian company, the Liquatech Turbine Company (LTC), the Sustainable Energy
Development Agency of New South Wales (SEDA) and the Commonwealth Scientific and
Industrial Research Organisation (CSIRO) of Australia have formed a joint venture company,
ComEnergy to develop a new innovative technology which aims to combust dilute methane from
ventilation air and waste coal to generate electricity. This project used the funding from ACARP
as seed funding to leverage the total project by over 400%.
The consortium has built a 200 kW externally fired turbine system which can harvest the heat
from the combustion of waste coal and ventilation air. Although waste coal can be burnt in a
number of combustion systems, the group decided to trial a new system which not only promises
to successfully burn waste coal, mine ventilation air and drainage gas effectively but has the
potential to convert the reject ash into a valuable by-product with sufficient value to be
transported away form the mine site. A photograph of the heat exchanger portion of system is
shown in
Figure 2, and a photograph of the turbine part of the system is shown in Figure 3.
Figure 2: Photograph of the system showing the primary heat exchanger and stack.
Figure 3: Photograph of the turbine section of the system
The experimental program used raw waste coal taken from four Australian underground coal
mines. The combustion system employs a co-flowing rotating kiln attached to an indirect fired
turbine generating power from the waste flue gas. The coarse reject coal waste is used directly in
the form that is taken from the coal processing plant with no additional processes involved. The
plan is to burn coal burnt together with the ventilation air in the kiln to generate heat. The ash is
removed at the exit of the kiln and the hot flue gas pass through a high temperature heat
exchanger to transfer its thermal energy content to the turbine system which inturn converts the
heat into electricity. The relatively long residence times and high temperature of the flue gas
within the kiln allows for the combustion of very low concentration mine methane. All waste
methane from inseam and post drainage can also be oxidised in the kiln and converted to
electricity.
The essential part of the process is that not only can large quantities of low concentration
ventilation air be handled by the kiln, but the coal stockpile allows the coal flow rate to be varied
to balance the methane input. An alternative technology to using the externally fired turbine
system is the use of a steam boiler and steam turbine. Although this has the advantage of using
conventional plant with well proven and tested designs, the lack of makeup water and cooling
water to condense the steam may present a barrier to its application in some of the more remote
mine sites. In addition the potential exists for the turbine system to generate electricity at very
high efficiency, thus further reducing the emissions of the mine through offsetting conventional
electricity-production
Coal fuelled Externally-Fired Turbines
Coal burning externally fired turbines were first built in the late 1950’s and 60’s in Ravensburg,
Coburg, Oberhaussen and Haus Aden (Figure 4) in Germany and Kashira in Russia {Keller &
Frutschi 1966}. The Japanese also produced a plant at Kokan as shown in the photgraph in Figure
5. The Haus Aden plant also burnt drained mine gas with the coal in its 6MW unit. In these
facilities the maximum temperature was limited to 700C and hence the efficiency was limited to
between 25 and 37%.
Most of these systems used closed cycle turbines with helium as the
working fluid. In the closed cycle system the gas from the exhaust is collected, cooled and
reinjected into the intake of the system. The use of closed cycle technologies generally requires
the design of a custom designed turbine system which is well beyond the scope of what could be
achieved in this project. The advantages of the closed cycle are in the ability to run at a much
higher pressure and to use helium gas, both of which lead to a reduction in the size of the heat
exchanger system.
Figure 4: 6 MW Closed Cycle Gas turbine running on Coal and Mine Gas at Haus Aden Mine in
Germany 1963. (Source: Keller & Frutschi 1966)
Figure 5: Japanese Closed cycle turbine system at Kokan (Source: Keller & Frutschi 1966)
Most modern developments in this type of system have used an open cycle, externally fired
turbine as this allows for the modification of off-the-shelf stationary gas turbine, or aeroderivative systems. New developments in heat exchanger materials and bottoming cycles have
increased the maximum potential efficiency of the cycle. The US Department of Energy has an
externally fired coal and gas turbine program called Combustion 2000. The DOE system has
predicted efficiencies ranging from 47% for a relatively simple system to 55% for a Cascaded
Humid Air Turbine system {Foster Wheeler 1996, United Technologies 1996}. Preliminary tests
on the radiant air heater section have already produced continuous hot air temperatures of greater
than 1100C. A layout of DOE’s proposed 55% efficient Cascaded Humid Air turbine layout of
the system in shown below in Figure 6.
Figure 6: Schematic of DOE Combustion 2000 Coal/Gas Hybrid system using Intercooled
Cascaded Humid Air Turbine (source: Ruth and Smoose 1999)
The flow rate of waste material from the processing plant is comparable in flow rate to that
required to use all the ventilation air and therefore the power plant could be co-located with the
processing plant to minimise handling costs. It is anticipated that some form of stockpile will be
needed, however, to cope with variation in the flow rates. Although the system is designed to use
coarse reject, waste heat from the system could be used to dry tailings which could either be
burnt, or preferentially, added to the exported product. Other mine site uses for waste heat
include the heat driven desalination of wastewater.
Although a number of waste coal burning coal technologies are possible, ComEnergy has chosen
to explore a new system that uses a kiln coupled to an externally-fired gas turbine. In this system,
the combustion chamber of the gas turbine is replaced by a heat exchanger to recover the heat of
combustion. Externally-fired turbines are significantly simpler than steam systems because the
working fluid is air and hence no water treatment systems, condensers, cooling towers or high
pressure fluid handling systems are required. The disadvantage is that although the heat
exchanger for the gas turbine operates at a significantly lower pressure than a steam turbine
boiler, the metal temperature is much higher. Because of the pressure difference between the
inside and outside of the tubes, creep of the metal is the primary limiting factor for the lifetime of
the heat exchanger. Corrosion of the tube also presents a significant problem. Overseas research
efforts in the US and Germany have concentrated on the testing of advanced steel alloy and
ceramic based tubes. A new heat transfer system developed by CSIRO utilising regenerator
technology is proposed to overcome the difficulties with existing metal heat exchangers at high
temperatures.
Because the combustion is external and the heat is taken into the turbine by a heat exchanger, no
coal combustion products such as corrosive gasses, ash or condensable vapours pass through the
turbine system. The result is a low complexity system which promises efficient, simple and dry
operation on a small scale in remote locations. It is also anticipated that an externally fired turbine
using gas and coal could produce electricity with less CO2 production per kW than a conventional
pulverised coal fired steam plant of similar size, thus further reducing greenhouse emissions due
to offsets.
The system, shown in a simplified schematic in Figure 7, has two separate fluid circuits in a
similar manner to a steam turbine. The right hand side of the system contains the combustion of
coal and mine gas in the kiln and the left hand side represents the power generation via the
turbine. Ventilation air is preheated by a recuperator before it flows at low speed through the kiln
together with waste coal. The combustion products then pass through a high temperature heat
exchanger (HTHx) and back through the recuperator. Although shown as separate units, it is
possible to combine the HTHx and kiln recuperator into a single unit as the same flue gas flows
through each. The ash, which is heated to its softening point, agglomerates in the bottom of the
kiln and is discharged separately. Any airborne ash or condensable vapours are completely
isolated from the gas turbine and only the energy is transferred to the turbine by the HTHx. In the
turbine air circuit, clean air is compressed by the gas turbine and preheated by its own exhaust gas
recuperator. The HTHx raises the air temperature to approximately 900C before the gas is
expanded via the turbine to make electricity. Gas temperatures of 900C, and higher, can be
achieved by “off-the-shelf” commercially available air heaters [Stahl, 2001]. Significant gains in
efficiency are to be made if innovative higher temperature heat exchangers can be developed to
increase the maximum temperature throughout the cycle. This is a priority area for future
research and is the primary area of research for overseas bodies interested in such a system.
Figure 7: Simplified schematic of system
An alternative power generation system could use a commercially available waste heat steam
turbine system. In this case the compressor would be replaced by a boiler feed pump, the
recuperator by a condenser / economiser and the gas turbine by a steam turbine. In the first
commercial application of the technology, ComEnergy is planning to proceed on this route. This
allows the first project to concentrate on the kiln combustion side of the project where the major
difficulties remain. It also allows for an easier estimation of the capital cost of the system.
As is the case of other fluidised bed systems, Limestone or other minerals may also be added to
the coal to convert any sulphur dioxide emissions to calcium sulphate. The long residence times
and large surface area of high temperature particles within the kiln allow combustion of very low
concentration mine methane. All waste methane from pre and post drainage can also converted to
energy in the kiln, with coal flow rate varied to balance the methane input as shown in Figure 8.
50% Ash, 6.6% H20, 14.5 MJ/kg,
Tin=450C, Tout=1200C
Tonne Waste Coal / hr for 200 m3/s
air
80
70
60
y = -18.541x + 63.695
50
40
30
20
10
0
0
0.5
1
1.5
2
2.5
Methane %
Figure 8: Variation of Waste Coal Flow rate with Methane Concentration
Coarse reject coal burning facilities have traditionally used fluidised bed combustors because they
perform well with low-grade fuel such as coarse reject. However, the present research is focused
around a co-flowing rotating kiln to burn the low-grade coal because the promise exists to convert
the ash into a Lightweight Expanded Aggregate (LEA), which is a potentially valuable byproduct. In the process described by Thermal Technologies, a small company that originated the
idea (Coert, 1999), the coarse reject coal waste is firstly ground and bonded into 6-8mm diameter
particles. A proprietary binder is used to bond the particles together to provide sufficient strength
during processing and to help with formation of the LEA. The particles are transported through
the kiln where the carbon is burnt to generate heat while the remaining ash is converted into the
LEA by a process of bloating and agglomeration. Agglomeration occurs when some of the
material fuses to bond together some of the particles. To achieve proper expansion, the raw
material should contain sufficient gas producing constituents and, on heating, pyroplasticity
should occur simultaneously with the formation of gas. The required formation of gas can be
bought about by various reactions; decomposition and combustion of sulphides and carbon
compounds from about 400C onwards, expulsion of water of hydration from clay minerals at
about 600C, splitting-off of CO2 from carbonates at about 900C, and reduction of iron oxides at
about 1100C causing expansion by liberating oxygen. In addition, the combustion of the coal
and volatiles produce CO2 and H2O. The product can be either open or closed celled in structure
and applications include; lightweight concrete and concrete blocks, clay brick filler material,
perlite and vermiculite substitute, insulation material, soil stabilisation and landscaping fill.
In the current project it is desired to test a number of coals originating from several collieries.
The collieries chosen were BHP Billiton’s Appin Colliery, South Coal’s Metropolitan Colliery,
Anglo Coal’s Dartbrook Colliery and Xstrata’s United Colliery. Coal from each of these mines
was generously supplied by the mine without cost to the project. The current economics of power
generation do not allow for significant pre-processing or materials handling of the waste coal
material. Hence, it was desired to test the coals from each mine in the unprocessed state as
received from the mine. However, if a market could be found for the LEA produced and the
value of the product exceeded the cost of production and transportation, then the development of
the LEA process should be a priority area for future research. A qualitative study on individual
aggregate was conducted to examine the melting and expansion process with one of the sample
coals.
Proof of Technology
The kiln has been in operation for many hundreds of hours, burning high grade coal (25MJ.KG),
waste coal (down to 8MJ/KG), wood waste (chippings and sawdust) and LPG gas. In operation
the rotating kiln proved to be simple to control with the ability to maintain a target temperature to
+ or – 250C mainly due to the high thermal inertia of the system.
A problem that became evident when burning high grade coal in the rotating kiln was the lack of
oxygen in the bottom half of the kiln leading to an ash with a high carbon content, this can be
overcome by introducing a secondary heated airstream into the kiln.
The Allison 250C-18 gas turbine was chosen because it had an external combustor that could be
easily removed to allow the turbine to be externally fired. An Allison turbine is designed to
operate with avgas (kerosene) as a fuel, this burns at approximately 17000C. We intended to use
methane gas in the top up combustor which burns at approximately 22000C, the temperature seen
by the turbine section was going to be above the temperature limits on the turbine blades.
In view of this it was decided to limit the efficiency tests to less than 30 minutes to minimise
damage to the turbine blades.
Experimental Design
Three fuel sources were used in the experiment to provide the results for the Proof of Technology
POT trial. These were;
 Coal
 Liquefied Petroleum Gas (LPG), and
 Compressed Natural Gas (CNG)
The coal and LPG were burnt together in a co-flowing rotating kiln. The resultant hot gas
expelled from the kiln was then passed through three heat exchangers before it exited the system
via the stack. Clean air was firstly compressed by an Allison 250C-18 gas turbine then passed
through the heat exchanger to recover the heat from the coal and LPG burning. CNG was used in
a separate combustor to increase the temperature of the heated air to a sufficient temperature for
the operation of the turbine system. Exhaust gas that still contained >16% oxygen from the
turbine was used as the feed air for the kiln. The system is illustrated in the schematic in Figure
1.
Figure 1 – Layout of System
Experimental Runs
The test program for POT consists of two tests occurring on the 17 February, 2003 and March 3,
2003.
The results of these tests are presented in the following subsections.
There were three input fuel streams. The measurement of these flow streams is described below.
Coal
The screw feed system for the coal was calibrated so that an accurate flow rate of coal could be
calculated during the test. The hopper was loaded with sample coal and the screw feed motor was
set to rotate at a constant rate for a set period. The mass of coal transferred during this period was
measured. Several different speeds were set and a calibration of the coal flow rate was achieved.
LPG
The LPG flow rate was measured by observing the readings on the calibrated flow meter before
and after the experiment.
CNG
A flow meter was located on the fuel supply and this was used to provide the flow of CNG.
The power output of the turbine is taken from a measure of oil pressure from the internal torque
meter located inside the gas turbine and a water brake dynamometer. The torque meter is
normally used to give the pilot an indication of the power output of the turbine during the
operation of the helicopter. A calibration of this torquemeter to give the power output from the
turbine is given from the C-18 Allison Gas Turbine operation handbook and is reproduced below
in Figure 2.
Figure 2 – Torquemeter Calibration
First Experimental Run
The first test for the program was run at steady state on the 17 February, 2003. The length of the
test period was 20 minutes. The results of the energy flows are given below in Table 1.
Engine Efficiency = Energy Output / Energy Input x 100
Table 1 – Values recorded from February 17 2003 test.
Energy In
Value
Methane
Coal
LPG
3
15.9 m @ STP
24 kg
4.6 m3 @ STP
Conversion
Energy
33.57 MJ/m3
14 MJ/kg
95.76 MJ/m3
533.76 MJ
336 MJ
440.5MJ
1310.3 MJ in 20 min
1092 kW
Total
Energy Out
Pressure of Internal 70 PSIG
Dynamometer
Efficiency
Figure 2
180 kW
16.5%
Second Experimental Run
The second test for the program was run at steady state on the March 3, 2003. The length of the
test period was 30 minutes. The results of the energy flows are given below in Table 2.
Table 2 – Values recorded from Febuary 17 2003 test.
Energy In
Value
Methane
Coal
LPG
3
9.5 m @ STP
8 kg
3 m3 @ STP
Conversion
Energy
33.57 MJ/m3
14 MJ/kg
95.76 MJ/m3
319 MJ
112 MJ
287 MJ
718.3 MJ in 30 min
399kW
Total
Energy Out
Pressure of Internal 20 PSIG
Dynamometer
Efficiency
Figure 2
64 kW
16 %
Conclusions
The above results does not take into account the electrical energy necessary to operate the kiln
and ancillary fans, with all energy inputs included the overall efficiency of the system was 14.7%
This is very close to the design calculated figure of 15%, the reason for the low efficiency of the
system was the heat exchangers were designed to price rather than efficiency to keep the project
costs low.
We set out to demonstrate that we could build a power generation unit that would use the waste
products of a coal mine, produce enough energy to power a coal mine and put the excess back
into the grid to earn cash to pay for the investment in a reasonable time. We believe we have
achieved this and ComEnergy are going to install the first 10MW unit in the near future.
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