Operating Experience with Air Blown 200 ton/day Entrained Flow Coal Gasifier
P. O.Box 9035, 6800 ET Arnhem, the Netherlands
Abstract
In the present paper some applications are discussed of the TOPHAT cycle2 which comprises a Joule/Brayton cycle featuring quasi-isothermal compression of the air by saturating the air with water after each compression stage and a recuperator after the gas turbine for heating the compressed air. The first application illustrates the advantages of combining the TOPHAT cycle with the Optimized Gasification Combined Cycle (OGCC)1 in which a coal-water slurry is used as a quench medium after a gasifier. Both this scheme and a similar scheme comprising dry coal pressurization result in efficiencies of about 5 2 % with a slight advantage for the OGCC option.
Further it is shown that a coal fired TOPHAT cycle including particulate removal at 900 °C (1 650 °F) and a hot gas expander can result in a simple power station without any need for steam and with efficiencies in the range of 4 5 - 5 0 % . Such a scheme is even more advantageous for power production from wet hydrocarbon feedstocks as biomass, peat, lignites and Orimulsion®. The efficiency for biomass is > 4 5 % which is significantly higher than for gasification based power stations using feedstocks with such a high water content.
Introduction
Most advanced coal fired power stations which have been proposed during the past t w o decades were based on integrated combined cycles featuring gas turbines with ever higher inlet temperatures. This has resulted in a number of large demo plants which have a few important things in common:
• The capital cost in $/kW is well above that of modern conventional coal fired power stations whereas the efficiency of these plants is not or only slightly higher. Even for fully commercial gasification based power plants using the same principles as currently applied in the demonstration plants it is unrealistic that the somewhat higher efficiency will warrant the additional capital expense.
• i.'c.:n . se for the high capital cost is the . . .rut no; of the major process/equipment items was specifically built for a gasification based power plant. All coal gasifiers were originally built for making synthesis gas, virtually
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all treating processes were adopted from the refining industry and in the gas turbines the same cycle was used as in aircraft turbines, that is without intercooling in the compressor and without reheat in the turbine. Moreover the fact that a combined cycle was used added to the higher cost of these plants.
• Using the Carnot criterium: n = 1 - TL/TH it may be concluded that too little attention has been given to lower TLovv. The only attempt to cope with this omission have been the HAT cycles in their various embodiments.
• Despite the emphasis during the past years on exergy too little attention has been given to upgrade the low level heat in the gas turbine exhaust gases to that of the gas turbine inlet by applying recuperators.
• The operability of the plants is often difficult due to a.o. too much integration of the various units. This implies that the plants are most suitable for base load operation and large capacities and it is very questionable whether such plants will be required in the future.
Some of the attempts to cope with the above problems and challenges for that matter have already been discussed 1,2. In the present paper these topics have been further worked out.
Coal Gasification based Power Stations The combination of OGCC and TOPHAT
A simplified block scheme for such a plant is given in Figure 1. In a two-stage dry coal feed Destec type gasifier coal is gasified at 32 bar (460 psi) in a first stage with oxygen/steam under slagging conditions at 1 550 °C (2820 °F) and the remaining char is gasified with steam in a non-slagging second stage at 1050 °C (1920 °F). The hot gases leaving the gasifier are quenched with a coal-water slurry of 90 °C (194 °F) to 300 °C (570 °F) after which the dry coal is separated from the gas and transported with nitrogen to the gasifier. The sulphur removal is either accomplished by a hot gas treating such as the KEMA warm gas treating process or by flue gas desulphurization.
In this and all other coal gasification based systems the stack gas temperature is such that there is sufficient heat left for raising the medium pressure steam for gasification, gas treating and -where required- for drying of the coal.
The fuel gas and the humidified air leaving the TOPHAT compressor (two stage with humidification after each stage) are both heated to 550 °C (1020 °F) in a recuperator before being combusted at 1350 °C (2460 °F) in the gas turbine. The exhaust gas f r o n tl gas turbine is routed through the re...r at-.' anc .ed for drying the coal, process steam generation and coal slurry preheating, etc. before being routed to the stack. The efficiency of the above scheme is 52.3 % based on the LHV of the coal.
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The combination of TOPHAT compression and a gasifier with dry coal pressurization
The block flow scheme of this plant is similar to the previous one and is depicted in Figure 2. The coaJ is dried in a conventional dryer, pressurized in lockhoppers or another coal 'pump' and then routed to a gasifier which operates under similar conditions as in the previous case. The hot gases leaving the gasifier are quenched with water of 200 °C (390 °F). The efficiency of this scheme is 51.8 %. A possible advantage of the present scheme is that it can be operated without the danger of any tar formation. A disadvantage is that as long as lockhoppers have to be used for pressurization of dry coal the pressurization is more complicated than with a slurry system.
Using a dry coal feed pressurizing system it is possible to completely avoid tar formation with the result that in principle the filtering of the coal could be carried out at a temperature of 500 °C (930 °F). However, by doing away with the steam cycle and thus with the expensive syngas cooler it is more advantageous to lower also the filtering temperature in this case to 300 °C (570 °F) because then more heat is extracted in the recuperator which has a beneficial effect on the station efficiency.
The above scheme has also been studied for a combination of a cold (TOPHAT pressurized) nitrogen quench and a water quench but this resulted in a lower station efficiency due to the required compression energy for the nitrogen. The atmospheric nitrogen can still be usefully employed though buy using it for diluting the inlet air of the air compressor of the gas turbine. Doing this results in combustion air with an about one percent lower oxygen content as well as in a lower air inlet temperature.
The lower oxygen content reduces the stoichiometric adiabatic flame temperature and hence the thermal N0X emission and the lower air temperature results in a lower compressor duty and hence in a higher station efficiency. This use of the surplus nitrogen from the Air Separation Unit is also applicable to the OGCC case described above.
Highly integrated coal gasification based power plants
It is of course possible to obtain higher efficiencies of up to 5 5 % with coal gasification based power plants by applying higher pressures, more isothermal compression, reheat gas turbines, etc. Flow schemes with these features have been calculated but it is believed that these have only theoretical value because they are very complex and hence lead to expensive plants which are difficult to operate.
Coal fired TOPHAT cycle
The coal fired TOPHAT cycle was specifically developed with the purpose of obtaining an efficient, economical and clean coal fired power station. A block flow scheme of
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a coal fired TOPHAT cycle is depicted in Figure 3. Air is compressed in one, two or three stages and cooled by direct injection of water after each stage. The air is then heated in a recuperator before being used as combustion air and optionally also as quench air. The combustion takes place at a low pressure of 8 bara (120 psia). The combustor may comprise a PFBC, PF firing or a slagging combustor followed by a quench. During or after the combustion the gases are desulphurized with limestone and/or dolomite. After injecting an alkali getter the gases are filtered at 900 °C (1650
°F) and routed to a hot gas expander. The gas expander is rather simple as no blade cooling is required. The hot gases leaving the expander pass through the recuperator, and optionally through a heat exchanger preheating the water which is injected during compression, before being sent to the stack.
The resulting efficiencies for the various cases which were studied are given in the table below.
Station efficiencies for coal and biomass fired TOPHAT cycles.
Because of losses and the own power consumption 2 - 4 percentage points should be substracted to obtain realistic figures.
Temperature injection water,
°C/°F =>
COAL Injection stages 0
1
2 3 6 BIOMASS
2
25/80
43.8 47.3 48.5 49.5
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Boiling point at prevailing pressure in the
compressor
45.1 47.7 48.7
200/390
45.5 48.0
48.9 I 49.6 I
The data in the above figure show that most benefits of the water injection are already obtained after 2 or 3 in;"'"-'' *n stage-' This implies that injection inside the ~^ essor is not per se required to reap the benefits of the TOPHAT cycle. The effect of the temperature of the water injected after each compressor stage is largest in case only
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one compressor stage is applied but becomes appreciably smaller in case of more injection stages. Nevertheless even in the case of three injection stages the effect is still about half a percentage point in case water of 200 °C (390 °F) is injected instead of water of ambient temperature (25 °C, 80 °F).
From the above data it can be concluded that the efficiency of this simple power station is only 2 - 3 percentage points lower than that of a gasification based power station in case 3 or more compression stages are applied. Further it can be concluded that this cycle has a similar efficiency as the most advanced PF steam cycle presently under construction in case 2 compression stages are applied. As the recuperator is rather simple because nowhere the metal temperatures exceed the 500 °C (930 °F) the capital cost of the coal fired TOPHAT cycle is expected to be much lower than for the gasification based schemes and the operation is much simpler.
The above scheme is especially attractive for wet feedstocks such as biomass (see Figure 4), peat, lignite, etc. because the remaining heat in the flue gases can be used to dry the feedstock. With biomass containing 5 0 % water the efficiency is 50 % ! An additional advantage of the indirectly fired TOPHAT cycle for biomass is that it can be built for relatively small power stations of 5-10 MWa which implies that biomass has to be transported over much smaller distances.
Further it is observed that for low pressure ratio Joule/Brayton cycles with low compressor outlet temperatures the recuperator can extract more heat from the stack gases resulting in relatively high station efficiencies considering the low gas turbine inlet temperature.
Coal pressurizing
The fact that the pressure of the coal fired TOPHAT combustor is only 4 - 8 bara (58 - 11 6 psia) makes coal pressurizing relatively easy and can make expensive lockhopper systems superfluous. An elegant system comprises a high bunker which is designed such that:
• the pressure in the combustor is lower than the corresponding head of the coai column in the bunker
• the upward velocity of the gases through the interstices of the coal bed in the bunker is equal or lower than the downward velocity of the coal bed in the bunker
• rm w p'ev. in the bunker.
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Another elegant option is to use a 'Stamet' pump for transporting the coal against pressure. The principle of this pump is that the coal is entrained by the sides of a rotating spool by the same effect which causes hang-ups in coal bunkers thus making a virtue of necessity.
Conclusions
• With modern gas turbines combined with two-stage TOPHAT air compressors coal gasification based power stations can be built with an efficiency of 52 % based on the LHV of the fuel. Due to the absence of a steam cycle the capital cost of such power stations is lower than for conventional coal gasification based combined cycle stations whereas the operation of the plant becomes simpler. Integration with the Air Separation Unit is not required.
• To obtain the efficiency of 52 % a two-stage gasifier of the Destec type is required using a dry coal feed. The dry coal feed can be obtained by pressurizing dry coal in e.g. lockhoppers but it is more elegant to pressurize a coal-water slurry and use this as a quench medium for the gas leaving the gasifier as applied in the OGCC. Moreover the OGCC option results in about a 0.5 percentage point higher station efficiency.
• The coal fired TOPHAT cycle comprises the almost direct firing of coal in a gas turbine. The hot combustion gases do pass through a filter though and moreover an alkali getter is required. Although the alkali problem will probably limit the gas turbine inlet temperatures to 900 °C (1650 °F) the efficiency of these power stations range from 46 - 48 % provided at least 2-3 compression stages are used. The low gas turbine inlet temperature has the advantage that no blade cooling is required.
• For wet feedstocks a directly fired TOPHAT cycle may be advantageous as it results in a high station efficiency and a simple plant. For biomass with a 50 % moisture content the efficiency amounts to 50 %. Moreover the absence of the steam cycle has in this case the additional advantage that the plant can be built for small duties of 5-10 MWe which is especially attractive for biomass because it can reduce the problems associated with the transport of this material with its very low energy density.
• Recapitulating it can be concluded that the TOPHAT cycle offers in all applications major advantages in terms of efficiency, capital expense and operability. In relation to this it is reminded that these advantages also apply to r -jtu. gas fired power stations as was illu.
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References
1. Thirteenth EPRI Conference on Gasification Power Plants. OGCC: Optimized Gasification Combined Cycle, 1994, Sheraton Palace Hotel, San Francisco.
2. EPRI Conference on New Power Generation Technology. TOPHAT and REX turbines, 1 995, ANA Hotel, San Francisco.
Coal fired TOPHAT cycle two stage air compression