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Power Generation Technologies for Low-Rank Coals.

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Dev. Chem. Eng. Mineral Process., 7(5/6),pp.483-500. 1999.
Power Generation Technologies for
Low-Rank Coals
D. J. Brockway
Cooperative Research Centrefor w Technologiesfor Power
Generationfrom Low-Rank Coal, Mulgrave, Victoria 31 70, Australia
World electricity production is expected to double over the next twenty-fwe years [I].
Dependence on fossil fuels for electricity production will continue to dominate
through this period and well into thefuture. Currently, coal is the fuel for about 30%
of total installed power generation capacity and the coal share of the electricity
market is expected to remain essentially stea4 over the next 25 years (Figure I).
Nevertheless, the amount of coal consumed for power generation will increase
substantially (Table 1) [I]. It is expected that world coal-fired power production will
grow at an average rate of 3% per annum. The forecast demand growth for power
generation will require an investment of the order of US$3 trillion with about
US$I trillion in coal-fired power generation. The increase in demand is attributed to
growth in world population by 50 percent over the next 25 years [2] and by heavy
demand growth in the Asia Pacific-rim nations.
In Australia, coal is the principal fuel for power generation providing 82 percent
of electricity production. Two third of the electricity generated >om coal in
Australia relies on high-rank coals (bituminous coals) and one-third on low-rank
coals (brown coals and lignite).
While the rate of growth of renewable energy and biomass technologies is
expected to be substantial in the next few decades, it is apparent that these
technologies will provide only a relatively small proportion of Australia’s total
energy nee&. Coal will remain the principal fuel for electricity generation, in
Australia and worldwide,for many decades to come.
In order to minimise the impact of coalfiredpower generation on the environment
there is a need to develop and install advanced cycle power generation Jystems when
it is economically viable to do so.
D.J. Brockway
Figure 1. Percentage Distribution of Fuels for Power Generation in 1995 and 2020.
Installed in
Net increase
Growth rate
86 1
43 5
Type of
iW - Gigawatts-electrical
Table 1. Installed Generating Capacity in 1995 and Projectionsfor 2020 [I].
Power generation technologiesfor low-rank coals
Electricity Production from Low-Rank Coal
The major use of low-rank coal in Australia is in electricity production; however, the
properties of the coal provide particular challenges for its use[3]. Low-rank coals are
characterised by high moisture content, low ash-melting temperature and often highly
fouling nature of the ash.
Victorian deposits in the Latrobe Valley have been used for many years for
power generation and generally have high moisture contents (60 - 70%), low ash (1 3% db) and low total sulphur (< 0.5% db). Sodium in ash is usually low in coals
currently utilised in the Latrobe Valley (generally <5%).
Victorian low-rank coal
from the Anglesea region is also used for power generation and is characterised by
somewhat lower moisture (46%), slightly higher ash (4% db) and higher sulphur (2 3% db). While the South Australian coal (Leigh Creek) which is presently utilised for
power generation has a moisture content of <38%, other deposits generally have
higher moisture (50 - 61%), higher ash contents (generally <20% db) and higher
sulphur (generally <5% db). Sodium in ash is often relatively high (3 to 16%). The
relatively high sodium in ash militates against the use of many SA coals for power
generation using conventional pulverised fuel fired boilers because of the problems
associated with the fouling propensity of ash. Western Australian sub-bituminous
coals currently used for electricity production have moisture contents of about 26%
with ash of 6 to 7% (db) and low sulphur (<O.6% db).
As a result of the high moisture content of the coal and its propensity for
spontaneous combustion when dry, Victorian low-rank coals are used in power
stations adjacent to the open cut mines. The coal is delivered by conveyor from
each mine into a bunker where it is stored for short periods (generally less than 24
hours) before being conveyed to the power station for combustion in pulverised fuel
@f,fired boilers. The coal is fired in its moist state and all the moisture passes
through the boiler, necessitating large and relatively expensive furnaces.
Due to the site of the South Australian coal (Leigh Creek) in an arid region and
the need for cooling water for the power station, the coal is transported about 300
km by rail from the open cut mine to the power station at Port Augusta.
D.J. Brockway
Advanced Coal-Fired Power Generation Cycles
A number of advanced technologies are in various stages of development worldwide
to improve thermal efficiency of coal-fired power plant and reduce the emission
levels of CO,, NO,, and SOxin a cost competitive manner. The technologies include
Supercritical Pulverised Fuel Fired Boilers (SCPF), Circulating Fluid Bed
Combustion (CFBC), Pressurised Fluid Bed Combustion (PFBC), Integrated
Gasification Combined Cycle (IGCC) and Advanced Pressurised Fluid Bed
Combustion (APFBC) cycles. Most developments of these advanced technologies are
focussed on the use of high-rank coals.
The efficiency of power generation using low-rank coal in conventional pf-fred
boiler plant is significantly influenced by the high coal moisture content of the coal.
The efficiency obtained from the modem boiler plant at Loy Yang Power Station is
about 29% on a higher heating value basis (HHV) for coal with a moisture content of
62%. The comparable efficiency for conventional boiler plant fuelled with high-rank
(black) coals is about 37%.
Supercritical PF Boilers
Supercritical pulverised fuel (SCPF) fired boiler technology is a development of
conventional pf fired boilers that has been the dominant technology for coal-fired
power generation for many years.
The SCPF technology uses higher steam
temperatures and pressures in the boiler to increase efficiency and thereby reduce coal
consumption. Application of SCPF boilers has been made possible by the use of
advanced alloys which can withstand the operating conditions and the technology is
now commercially available.
Circulating Fluid Bed Combustion
The Circulating Fluid Bed Combustion (CFBC) technology uses combustion air to
fluidise and circulate pulverised coal with recirculated ash, char and sorbents such as
Power generation technologiesfor low-rank coals
limestone. The bed material is circulated through an atmospheric pressure boiler and
heat exchanger which are used to remove heat from the system and moderate the
temperature in the fluid bed. Combustion gas is removed from the system via a
cyclone and used to raise steam for a steam turbine.
The technology is commercially available and it is very tolerant to fuel types.
For application to coals, CFBC is usually limited to high fouling coals and/or those
with high concentrations of sulphur.
Pressurised Fluid Bed Combustion
The Pressurised Fluid Bed Combustion (PFBC) technology utilises a bubbling bed
with combustion carried out under pressure of 10 to 15 bar. Since combustion is
carried out under pressure the flue gas can be expanded through a gas turbine to
generate power and the turbine exit gas is used to raise steam for a steam turbine.
Again limestone may be added to the bed to remove sulphur from combustion
gas. The flue gas is cleaned in cyclones to remove most of the dust before passing
through the gas turbine.
A limitation of the technology is that it requires a
“ruggidised” turbine which has a relatively low efficiency compared to modem gas
turbines. The technology is now commercially available and several small plants are
in operation with at least two large plant under construction.
Recent development work is aimed at incorporating circulating fluid beds to
achieve higher air velocities and improve combustion conditions.
Integrated Gasification Combined Cycle
The Integrated Gasification Combined Cycle technology (IGCC) gasifies coal and the
product fuel gas is cooled, cleaned and fited in a gas turbine. Maximum efficiency is
achieved by firing at high inlet temperatures (<1300°C) with an advanced gas turbine.
The exit gas is utilised in a combined cycle to raise steam for a steam turbine. There
are a number of variations of the IGCC technology and these are markedly different
for high and low-rank coals.
D.J. Brockway
For high-rank coals the IGCC technology generally involves entrained flow
gasification with oxygen at temperatures >14OO0C and slagging conditions such that
slag runs down reactor surfaces to protect the reactor from the very high
temperatures. Pressures of 225 bar are used.
For low-rank coals the IGCC technology generally involves the High
Temperature Winkler process which comprises an air-blown bubbling fluid bed
gasifier at temperature <lOOO°C and again with pressure 225 bar[4,5].
Advanced Pressurised Fluid Bed Combustion
The technology with the highest efficiency for power generation from coal is the
Advanced Pressurised Fluid Bed Combustion (APFBC) cycle (Figure 2).
technology involves pressurised partial gasification (carbonisation) of the coal and
pressurised combustion of the resulting char. The cycle produces fuel gas from the
carboniser and hot flue gas from the char combustor. These product gases are mixed
and burnt with additional air in a gas combustor. The final combustion-product gas is
thereby raised to a high temperature and expanded through a gas turbine. Power is
generated from both the gas turbine in combined cycle and a high pressure steam
APFBC has key advantages over IGCC in that the higher efficiency is achieved
with 100% carbon conversion to energy under less severe process conditions
(<lOOO°C for high-rank coal and substantially lower for low-rank coal, pressure 15
bar). The lower temperatures in APFBC mean that the demand for gas cooling for
clean-up purposes is less than in IGCC while the total fuel and flue gases can be
combined and raised to a high turbine inlet temperature. This enables a greater
proportion of energy to be generated from the gas turbine (Brayton cycle) which is
inherently more efficient than the combined cycle steam turbine (Rankine cycle).
The APFBC technology also has the market advantage that the technology is
basically the same for low-rank and high-rank coals, albeit with the need to integrate
a drying process for the former.
Power generation technologies for low-rank coals
APFBC is the least developed of the advanced cycles and will not be
commercially available for perhaps five years.
Pressurised Gas
Carboniser Clean-up Combustor Turbine
Water Steam
Figure 2: Advanced Pressurised Fluid Bed Combustion
Efficiencies of Advanced Power Generation Cycles Utilising Low-
Rank Coals
The overall efficiencies of advanced power cycles for high-rank coals have been
evaluated by a number of organisations with possibly the most comprehensive study
undertaken by the International Energy Agency[6]. CRC Power Generation has
completed a comprehensive series of process evaluations for the utilisation of lowrank coals in advanced power technologies[7,8]. The evaluations considered the
requirement to incorporate a coal drying process into the technologies and other
issues specific to the utilisation of low-rank coals for power generation.
Table 2 and Figure 3 provide a summary of IEA and other data for overall
efficiencies of high-rank coal power generation technologies together with similar
D.J. Brockway
data for low-rank coal (assuming 62% moisture). The values quoted are estimates of
overall efficiency at a level that is considered to be reasonable for the “upper
boundary” for each technology. Efficiencies quoted in the literature cover a range of
values. The data is expressed in terms of Higher Heating Value (HHV) because this
allows meaningful comparison between efficiencies for high and low-rank coals in
similar technologies. However, in the USA and Europe it is common to express
efficiencies in terms of Lower Heating Value (LLV): for high-rank coals, the LHV is
generally about 2% above the HHV.
Low-Rank Coal
40 (41)
PF Boiler
In Table 2 all technologies for low-rank coal (except pf boiler) incorporate an
integrated coal drier (Steam Fluid Bed Drier) for 62% moisture coal. IGCC for lowrank coal gives 40% efficiency with 9 1% carbon conversion in gasifier and remaining
char burnt in CFBC. For 100% carbon conversion in gasifier, efficiency increases to
For low-rank coals and assuming complete carbon conversion in a gasifier, the
process evaluations ,carried out by CRC Power Generation [7,8] indicate in general
the following ranking in terms of the efficiencies from the technologies.
consistency, in all but one case (IGCC with hot gas drying) an integrated SFBD with
heating steam extracted from the steam turbine is assumed.
(ICCC) <
(Hot gas drying)
Power generation technologiesfor low-rank coals
Assuming 100% carbon conversion, the process evaluations showed that
efficiencies as high as 42% (HHV) can be achieved from technologies utilising coal
gasification. However, it is known that carbon conversion is not complete in the High
Temperature WinkIer (HTW) fluid bed type of gasifier generally proposed for low-
rank coals. Comprehensive research undertaken in Germany by a consortium led by
Rheinishe Westfalische Elektrizitatswerk (RWE) developed a technology based on
HTW that is expected to achieve about 91 percent carbon conversion in the gasifier.
As a consequence RWE proposed a cycle in which the char is burnt in a separate
boiler at atmospheric pressure [4].
In the case of a low-rank coal, fluid bed gasification cycle with an atmospheric
pressure boiler, the efficiency is lower than for IGCC with 100% carbon conversion
in the gasifier. For a char yield of about 22% (dry coal basis) the efficiency is
reduced to about 39%. This efficiency is only marginally greater than that expected
from the simpler PFBC cycle fed with coal predried in an integrated Steam Fluid Bed
Drier (SFBD). In the likely real case where the char yield is 9 percent in the IGCC
systems [4] and (say) 22 percent in the two hybrid systems (which do not aim to
achieve complete gasification of coal), the efficiencies (with atmospheric char
combustion coupled to IGCC)are:
(Hot gas drying)
The Advanced F’ressurised Fluid Bed Combustion (APFBC) cycle has the potential to
provide power from high moisture (>60%) coal at an efficiency of 44% (HHV).
D.J. Brockway
Figure 3: Comparison of Eflciencies :Low and High-Rank Coals
CRC Power Generation APFBC Technology
CRC Power Generation has now developed what is believed to be a unique concept,
which maintains a relatively high efficiency for the APFBC technology when applied
to high moisture coals. The key processes in the Centre’s concept for APFBC
drying of high moisture coal in a fully integrated technology;
removal of alkali vapours !?om gas ahead of the gas turbine;
combustion of product gas to give a turbine inlet temperature approaching the
maximum allowable limit.
Power generation technologiesfor low-rank coals
The CRC Power Generation APFBC technology will give 100% carbon
Opportunities and Challenges for the Use of Low-Rank Coal for
Power Generation
While there is no doubt that coal-fired power generation world-wide will continue to
rely heavily upon high-rank coal, it is arguable that those nations with large reserves
of low-rank coal will seek to expand the proportion of power generated using this
fuel. These nations include Germany, Russia, US, Poland, Czech Republic, Greece,
Turkey, Australia, China, Romania, Canada, Bulgaria, India, Thailand, Hungary,
Spain and Indonesia (in order of current production). It should be noted that the list
includes China, India, Indonesia and Thailand which are among the list of nations
who have the greatest demand to increase power generation capacity.
Low-rank coals contain high moisture and cannot be economically transported or
exported without costly processing. Low-rank coals are generally used for power
generation at the mine mouth. It is logical for countries with both high-rank and lowrank coals to seek to export the former and use the latter for local power production.
Most importantly, low-rank coals are often easy and cheap to mine. Finally, as
reserves of high-rank coals dwindle (either locally or internationally) and prices rise,
the proportion of low-rank coal used for power production is expected to rise. It has
been estimated that proven world reserves of low-rank coal are sufficient to fuel
2,100 power stations of lOOOMW capacity for 30 years[9]. In Australia alone, at
current usage rates there is sufficient economically accessible reserves of low-rank
coals for about 1000 years[ lo].
Strategic Advantage of Low-Rank Coal For Power Generation In
The strategic advantage of low-rank coal for power generation, particularly in
Victoria, essentially relates to the very low cost of the fuel.
D.J. Brockway
Considering the high moisture content of low-rank coals, the efficiency of power
production from competing fuels and the need to express costs on a comparable basis
it can be shown that the fuel costs for electricity production from Victorian brown
coals, Australian black coals and natural gas are of the order of $3MWh, $1 1- 15MWh
and >$20MWh respectively.
The cost of fuel is, of course, only one element in the total cost of power
production - the other principal components being capital cost and operations and
maintenance of the power station. The fuel cost does, however, starkly show the
strategic advantage of low-rank coal in power production.
Strategic Threat to the Use of Low-Rank Coal for Power
The high moisture content of low-rank coals and the resultant thermal inefficiency of
electricity production in pf-fired power stations give rise to the strategic threat to
power generation from low-rank coal. The strategic threat is Greenhouse Gas (GHG)
Factors Affecting Commercial Application Of Advanced Cycles
The decision for commercial application of a particular power generation technology
is influenced by a number of factors including cost, availability and environmental
These factors are:
maturity of the technology with particular reference to its ability to deliver power
and earn income. This is often a central consideration for banks and financial
institutions who are increasingly becoming involved in decisions on large power
plant developments.
availability, reliability and ease of maintenance are critical factors to ensure that
plant remains on-line as far as possible in order to continue to generate revenue.
Power generation technologiesfor low-rank coals
versatility and flexibility in operational performance, including thermal
efficiency under part load and start-up time, is important in today’s competitive
markets because of the need to follow load.
capital cost is very high for power plant and has a significant influence on the
price that power can be delivered to consumers. Factors influencing capital cost
include the type of technology to be used, relative plant size, whether factory
construction is possible, construction time, the number of possible suppliers,
whether plant suppliers have other major projects underway and the level of
engineering guarantees required from suppliers.
plant size consideration involves a balance between the need to match the
capacity of power plant with the projected demand growth; availability of critical
plant components of the required size; and economies that can accrue through
increased size of plant.
thermal efficiency is critical to reduce coal consumption with consequent savings
in fuel cost and greenhouse gas emissions.
fuel flexibility is important for plant that is provided with coal from a number of
sources. While not common in Australia, this is the norm for a number of plants
in other countries.
environmental performance and whether there is increased costs from installing
inherently “cleaner” technology.
Key Barrier to the Introduction of Clean Coal Technologies
The key barrier to the introduction of advanced cycles for power generation is the
high relative cost of demonstration and of the first few commercial plants (Figure 4).
Not only do these plants suffer from relatively high cost, they also invariably have
low availability in early years which severely influences generation of income.
Power generators and manufacturers are reluctant to fund demonstration plants
without government assistance and it is noticeable that the first major commercial
demonstration IGCC plants (Buggenun, Puertollano, Wabash River and Tampa) all
have varying levels of government support. These “first of a kind” commercial plants
are essential if a new technology is to be introduced but will almost certainly not be
installed without government support or incentives in some form.
Figure 4: Cost Curvefor Introduction of New Power Cycles
(Source - EPM)
Cost of Power
If advanced coal-fired power cycles are to be adopted, the levelised cost of power
sent out over the life of the power station must be competitive with other available
power generation technologies.
The projected cost of power for new installations of existing pf boiler technology
is itself a much vexed question. Due to the competitive market, pf power plant
suppliers have continued to slash prices and there is considerable uncertainty about
actual costs. These will only become known when a definite power project is
initiated. The problem is compounded for accurately estimating the cost of a mature
advanced cycle plant when these technologies are far from mature and still subject to
relatively high capital costs for early versions of each technology.
While there are any number of costs for power quoted in the literature, most
relate only to a particular technology without providing the basis on which the cost
Power generation technologiesfor low-rankcoals
was estimated, making a direct comparison to other technologies invalid.
significant exception is an evaluation of a number of advanced power technologies
for black coal fired power plant undertaken by the International Energy Agency[6]
and an earlier study by Australia’s Energy Research and Development Corporation
and the Electricity Supply Association of Australia[1 11.
In order to overcome this difficulty in relation to low-rank coal, CRC Power
Generation commissioned consulting engineers Sinclair Knight Merz (SKM) to
undertake a comparative cost study of a number of advanced technologies for low-
rank coal to ensure that the costs were estimated on exactly the same basis.
General conclusions that can be drawn from the SKM comparative cost
evaluation are:
the capital cost for a number of low-rank coal technologies range from about
$1600 to $2000/kW,
the levelised cost of power sent out from a “greenfields” development based on
low-rank coal is expected to be of the order of $40/MWh ($38 to $43/MWh)
the cost of power sent out from an APFBC power station is expected to be cost
competitive with power from other (mature) advanced cycles fired with low-rank
coal. (This outcome is generally consistent with the IEA evaluation for black
coal power generation, although the IEA study concluded that the cost of power
sent out from an APFBC plant would in fact be lower than that from any other
advanced technology.)
In relation to high-rank coals, the overall position is similar. The ERDC report
of 1992 is still considered to provide valid data. Making allowances for inflation,
capital cost and O&M reductions, it is reasonable to conclude that:
the capital cost for a number of technologies range from about $1200 to
the levelised cost of power sent out from a “greenfields” development is
expected to be 1 $40/MWh (depending on the cost of coal).
D.J. Brockway
International Markets: Advanced Technologies for Asia
and Pacific-Rim Nations
As indicated above, coal is expected to remain the principal fuel for power
generation worldwide into the foreseeable future. The Asia - Pacific Rim
nations require large growth in power generation to sustain their rapidly
growing economies and lift the standard of living of their populations. Many
of these nations (China, Indonesia, India, Thailand) have large reserves of lowrank coal, which they will, in all likelihood, use for power generation.
The potential impact of cost-competitive advanced power generation
technologies for low-rank coal on slowing the rate of growth in world
greenhouse gas emissions will arguably be significant. The growth in
greenhouse gas emissions from Asia - Pacific Rim nations will swamp any
reductions made in developed nations and consequently the availability to
them of cost competitive but lower emission cycles are important. Such
technologies will provide substantial business opportunities for Australia in
Potential Influence of Advanced Coal-Fired Power Cycles
on Greenhouse Gas Emissions
The USDOE[ 121 has projected that widespread adoption of clean coal technologies
worldwide could more or less stabilise greenhouse gas emission from power
generation over the next century (Figure 5 ) . This scenario assumes:
1. Advanced coal-fired generation technologies supplant 25% of year 2000 coal
plant by year 2025,50% by 2050 and 75% by 2075, and
Advanced cycles achieved efficiencies of 40% in the period from year 2000 and
60% from 2025 (the latter assume integrated gasification - fuel cell systems).
Power generation technologiesfor low-rank coals
f 35000
.O 5000
Figure 5. Potential Impact of the Global Deployment of Clean Coal Technology on
Greenhouse Gas Emissions Reductions [I I].
This scenario may be optimistic but it does indicate that projected world growth
in coal utilisation for power generation could occur without the large increase in
greenhouse gas emissions that would naturally follow large power generation capacity
growth with pf boiler technology.
Coal forms one of Australia’s major energy resources and will continue to be the
dominant energy source for power generation into the foreseeable future.
New cost-competitive technologies for power generation with substantially
reduced GHG emission will be installed as current plant is replaced or new base-load
plant is required to provide for increased demand.
The author gratefully acknowledges the support for this work by the Cooperative
Research Centre for New Technologies for Power Generation from Low-Rank Coal,
which is funded in part by the Cooperative Research Centres Program of the
Commonwealth of Australia.
D.J. Brockway
1. Wok, R, World Congress of Chemical Engineering, San Diego, 88, July 1996.
2. ESAA, “Towards 2020 and a New Order in Energy Demand”, Electricity Supply
Magazine, voU7, p25, 1997.
3. Brockway, DJ, Ottrey, AL, and Higgins, RS, “Inorganic Constituents”, Chapter
11, The Science of Victorian Brown Coal, RA Durie (Ed), Butterworth
Heinemann, Oxford, 1991.
4. Schippers, K, Wischnewski, R, Keller, J, Herbert, PK, and Sendelbeck, G,
“KoBra Will Demonstrate High Temperature Winkler IGCC”, Modem Power
Systems, 4 1,February, 1993
5. Pleasance, GE and Johnson, TR, “Development of New Technology for Power
Generation from Low Rank Coals - IDGCC”, Australian Institute Of Energy
Seminar, June, 1996.
6. Maude, C, “Advanced Power Generation - A Comparative Study of Design
Options for Coal”, IEA Coal Research, Report No. IEACW55, London, March,
7. Bhattacharya, SP and McIntosh MJ, “Evaluation of Advanced Cycles for Use
with Low-Rank Coals”, Cooperative Research Centre For New Technologies For
Power Generation From Low-Rank Coal, Melbourne, Report No. 97005, 1997.
8. McIntosh MJ, “Assessment Of Advanced Cycles with High Moisture Content
Low Rank Coals”, APEC Experts Group on Clean Fossil Energy, Technical
Seminar, Reno, October, 1997.
9. HRL, Australian Mining, Vol. 88, p.25, 1996.
10. Brockway, DJ, “Clean Competitive Energy from Low-Rank Coal”, Australian
Academy of Technological Sciences and Engineering, 1997 Symposium,
Sydney, November, 1997.
11. ERDC, “Advanced Generation Options for the Australian Electricity Supply
Industry and their Impact on Greenhouse Gas Emissions. Summary Of Phases 1
And 2”, Report No 131, January, 1992.
12. Miller, CL, “Facilitating Investment in Clean Coal Technologies. The
Greenhouse Challenge, APEC Coal Trade and Investment Liberalisation and
Facilitation Workshop, Jakarta,August 1997.
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