Coal gasification and combustion of LCV gas

Coal gasification and combustion of LCV gas

Bioresource Technology 65 (1998) 105- 115 0 1998 Elsevier Science Ltd. All rights resewed Printed in Great Britain 0960-8524/98 $19.00 ELSEVIER PII:S...

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Bioresource Technology 65 (1998) 105- 115 0 1998 Elsevier Science Ltd. All rights resewed Printed in Great Britain 0960-8524/98 $19.00 ELSEVIER

PII:SO960-8524(98)00007-8

COAL GASIFICATION AND COMBUSTION OF LCV GAS P. D. J. Hoppesteyn,

W. de Jong, J. Andries & K. R. G. Hein

Delf University of Technolog)!, Department of Mechanical Engineering and Marine Technology; Laboratory ,for Thermal Power Engineering, Mekelweg 2, 2628 CD, Del& The Netherlands

(Received

24 August 1997; revised version received

12 December

gasification,

15 December

1997)

material. Until a few years ago this was also the maximum inlet temperature of an expansion turbine as incorporated in the PFBC system. Owing to material improvements in modern gas turbines, the maximum turbine inlet temperature has increased to 1427°C (DeCorso et al., 1996). One way to increase the efficiency of a PFBC plant, and hence to reduce the CO;! emission, is to increase the maximum process temperature. By modifying the pressurized fluidized bed combustion system into a pressurized fluidized bed gasification system (PFBG), with separate combustion in a pressurized combustor of the produced low calorific-value (LCV) fuel gas, the maximum process temperature can be increased to about 1400°C. Another way to reduce the COZ emission is to use biomass or coal/biomass blends. Again pressurized fluidized bed gasification appears to be an attractive route (Faaij, 1997). To assess the technical feasibility of such a scheme, fueled with coal and/or biomass, a 1.6 MW PFBG with a separate pressurized combustor has been designed and built at Delft University of Technology. Experiments with this test rig, using coal, biomass and coal/biomass mixtures have been planned. In order to gain insight into the processes taking place in this test rig, preliminary experiments were performed using a 2.5 kW atmospheric fluidized bed gasifier (AFBG) with a separate atmospheric combustor. During the experiments described in this paper the AFBG was fueled with coal. In the near future biomass (crushed miscanthus, straw and wood) will be used. Coal contains sulphur comounds, mainly in the form of organic sulphur as well as inorganic sulphur, e.g. pyrite (FeS,). During combustion of the coal the sulphur is liberated and oxidized to SO,. In order to prevent high SO* emissions, limestone or dolomite is added to the fluidized bed, which reacts during the combustion process with SO2 to form gypsum and COZ. During gasification of coal most of the sulphur compounds react to H2S and COS. To prevent the formation of SO, during subsequent combustion, these components can also be reduced

Abstract Experiments were petiormed on the gasification of bituminous coal with air and steam in a 2.5 kW bed. Thermal conversion atmosphen’c fluidized eficiencies, the retention of .sulphur and the conversion of fuel nitrogen to NH, were determined for various oxygen-to-carbon ratios (equivalence ratios between 0.58 and 0*82), with and without limestone in the bed. The conversion ejjiciency increased with increasing equivalence ratio, while the conversion of fuel-nitrogen to NH, decreased with increasing equivalence ratio. The sulphur conversion without the addition of limestone increased with increasing equivalence ratio. As expected, the presence of limestone in the bed led to increased sulphur retention, more so at the high and low ends than at intermediate values of the range of equivalence ratios. In addition, limestone adversely affected thermal conversion ejjiciency and increased the conversion of fuel-nitrogen. The low calorific value (LCV) fuel gas, produced by the AFBG, was combusted in a cyclone combustor at atmospheric pressure. Stable combustion of the LCV fuel gas was possible down to a calorific value of I.57 MJlmi. The combustion efficiency in the atmospheric topping combustor varied between 98.72 and 99.96%. Nearly 60% of the NH, in the fuel gas was converted to NO in the combustor: 0 1998 Elsevier Science Ltd. All rights reserved Key words: Combustion,

1997; accepted

LCV fuel gas,

sulphur retention.

INTRODUCTION Coal-fired power plants based on pressurized fluidized-bed combustion (PFBC) have reached the stage of commercialization.. Compared to conventional coal-fired power plants, PFBC systems have the advantage of low NO, and SO* emission. The efficiency of such a power plant is limited by the maximum process temperature and this is about 95O”C, due to sintering problems of the bed 105

106

I!D. J. Hoppesteyn, W. de Jong, J. Andries, K. R. G. Hein

to gypsum and CaS by adding limestone or dolomite to the coal. However, the introduction of these Ca compounds into the fluidized bed will affect the thermal conversion efficiency of the gasification process, as will the oxygen-to-carbon ratio. These effects have been investigated experimentally in the 25 kW AFBG. In addition, experimental evidence has been obtained on subsequent combustion of the product gas in the atmospheric combustor. METHODS Experimental facility

The 25 kW atmospheric test facility consisted of a fluidized bed, a gas clean-up section and a topping combustor. Fig. 1 shows a schematic drawing of the test facility.

When limestone (particle diameter between 0.8 and 1.4 mm) was added, it was premixed with the coal. Steam was fed into a plenum chamber just below the bottom of the distribution plate, where it mixed with the main part of the fluidization air. The temperature of the steam-air mixture in this chamber was kept at 300°C to prevent the possibility of condensation of steam. The distribution plate consisted of a porous, sintered-metal plate. The bed consisted of quartz sand particles (diameter 710-850 pm) cooled by an annular, external heat exchanger. By adjusting the air flow through the heat exchanger, the bed temperature could be varied independent of the other process parameters. Steam was used as an additional gasifying agent to produce CO and Hz, according to the following reactions (Schobert, 1990):

Atmospheric fluidized bed gasifier

C+H,O+-+CO+H, AHF98K,1 atm= 119 kJ/mol

The atmospheric fluidized bed reactor consisted of a bed section and a freeboard section. The bed section had an internal diameter of 0.07 m and a length of 0.27 m. The freeboard section had an internal diameter of 0.05 m and a length of 1.68 m. The fluidization air was partly supplied through an air distribution plate (60%) and partly through the coal (40%) inlet pipe. Coal, with a particle diameter of between O-6 and 2-Omm, was fed into the bed at a constant rate through a radial fuel-inlet pipe situated just above the air distribution plate.

(Carbon-steam

CO+H,OctCO,+H,

reaction)

AH:98K,’atm= -41 kJ/mol

(Water-gas shift reaction)

(R2)

Carbon monoxide and hydrogen are the main combustible components in coal-derived LCV fuel gases.

,.+

STACK

STACK LcvFmLGw To CQlecfsToR

1. 2. 3.

(RI)

Steam-fir

chamber

Fluidited Bed Freeboard

5. Ash Hopper 6. Ceramic Filter 7. as Cooler 8. Gas Otyet Fig. 1. AFBG test facility.

9.

Gas Volume Meter 10. Gas otnnp 11. Gas Cooler E'uolFeed System 13. Digital Balance 14. Fluidized Bed Cooler 15. Ignition Valve

107

Coal gasification and combustion of LCV gas

Gas clean-up section Particles were removed from the gas flow by a cyclone and a ceramic fibre filler, then collected and analysed for carbon content. The gases were cooled to about SC, so that the water vapour concentration in the LCV fuel gas was reduced to about 0.5 ~01%. An electrical heater preheated the fuel gases to 130°C before they entered the combustor. Atmospheric comhustor The higher heating value of the fuel is a very important design variable for application of the gas in combustion systems. Most industrial gas turbines are designed for operation on natural gas, with higher heating values of 31-47 MJ/m$ According to Meier et al. (1986) standard fuel gas systems can handle gas with a higher heating value as low as 26.0 MJ/m:, after minor modifications to the fuel injector orifices and other fuel gas control-system parts. When multiple fuel-control components are used in parallel, fuel gas with a heating value down to 13.0 MJ/mz can be used. Such gases are usually classified as being of medium calorific value. A fuel gas is classified as low calorific when its higher heating value is below 7 MJ/mz. Such fuels present special difficulties for combustion due to the low combustion temperature and small combustion velocity, with flame stability problems and low combustion efficiency (Lefebvre, 1983; Chomiak et al., 1989). After passing a cyclone and a ceramic filter, the LCV fuel gas produced by the AFBG was combusted in an atmospheric combustor, as shown in Fig. 2. The combustor was based on the cyclone

concept, which was considered particularly suitable for poor-quality fuels such as LCV fuel gas because of the intensive mixing of fuel and air and the resulting long residence time needed for complete oxidation of CO. Despite these advantages, cyclone combustors are not widely used nowadays, because the long residence times at high temperatures result in high NO, emissions. For a preliminary investigation such as the present one, however, the simplicity of its design and its robustness in operation make the cyclone burner an obvious choice. The fuel gases and the combustion air entered the combustion chamber via two tangential inlets near the bottom of the chamber (Fig. 2). The volume flows of these gases were measured using a rotameter. The inlet temperatures of the fuel gas and the combustion air were measured by CromelAlumel thermocouples. The fuel was ignited by a propane-fired Bunsen burner, located at the central axis on the inlet side of the combustor. The combustion of the gas took place in a long annular flame front close to the combustor wall. The flue gases left the combustor at the top. The combustor was thermally insulated with a lo-cm ceramic wool layer. Instrumentation Temperatures were measured by Chromel-Alumel thermocouples located in the steam supply line, just below the distribution plate, in the bed, at the top of the freeboard and at the in- and outlets of the combustor and of the cooling air. In addition the was axial temperature profile in the combustor traversing axially at measured by a thermocouple

WBL GAS

INLET

Fig. 2. Atmospheric

topping combustor.

108

I! D. J. Hoppesteyn, W de Jong, J. Andries, K. R. G. Hein

2 cm from the wall. All temperatures measurements were performed in duplicate and the average error in all temperature measurements was +4”C. Solid mass flows (coal and limestone) were measured by a balance; gas flows ‘(air and steam to the AFBG, air to the air cooler, fuel gas and air to the combustor) by rotameters. Gas sampling points were located downstream of the freeboard, before the inlet and in the outlet of the combustor. The gases were analysed by two analysis systems: a system for on-line measurement of 02, COZ, CO, NO and SO2 with a paramagnetic and four infra-red analysers, and a system for off-line measurement of CO*, CO, NO, NO*, N20, NH3, SOZ, H2S, COS, CH4 and C2H4 by a Fourier Transform-Infra Red (FT-IR) spectrometer, and of HZ, N2, 02, CO, COZ, HZ& C,H, (xc 6) by a gas chromatograph. Each system had its own sampling lines, vacuum pump, filters and coolers. In both systems the gases were extracted from a sample point by a vacuum pump and cleaned by a ceramicfibre filter upstream of the pump. The sample line, the filter and the vacuum pump were heated to 150°C. The gases which flowed to the on-line system were cooled to about 5°C and the dried gases transported to the analysers. The gases which flowed to the FT-IR and gas chromatograph were kept at 150°C although it was also possible to cool this gas to about SC, remove the water vapour from the gas and reheat it to 150°C on its way to the spectrometer. An advantage of this alternative method for conditioning the gas was that components which dissolved easily in condensed water vapour, e.g. ammonia and sulphur dioxide, could be analysed accurately with the FT-IR spectrometer. The relative error of all gas analysis measurements was &3%.

Table 1. Ultimate and proximate analysis of the coal

Species

Composition raw (wt%)

C H N 0 S Moisture Ash Volatiles HHV (MJ/kg)

74.37 5.01 1.64 8.54 1.75 5.29 3.39 35.20 30.72

Operating procedures for each experiments were as follows. First the bed was heated to 650°C by burning propane with air. Next the propane was shut off and the air flow adjusted to achieve stable combustion of coal. After a stable period in the combustion mode (3, > l), part of the fluidization air was replaced by steam, so that the equivalence ratio (1) decreased below 1 and the AFBG began to operate in the gasification mode. After preliminary mass balance and off-line gas analysis measurements at different locations, the combustor was started up with propane and air. When the temperature in the combustor reached about 700°C cleaned, dried and preheated fuel gas from the AFBG was admitted to the combustor and the propane flow discontinued. Air flow to the combustor was then adjusted until the equivalence ratio reached 1.2. After stabilization each experiment continued over a stable period for at least 4 h and was ended by stopping the coal feed and replacing process air and steam by nitrogen, thus quenching the fluidized bed with inert gas. After each run the bed content, the cyclone catch and the filtration products were collected and weighed and the char contents were determined. RESULTS

AND DISCUSSION

Experiments

Two series of experiments were carried out, one without and the other with addition of limestone (Middleton, 97% CaC03). In the latter series the Ca/S ratio was maintained at 2.3, the value foreseen for the 1.6 MW test rig, by premixing an amount of limestone with the coal. Bituminous coal from the Kiveton Park mine (UK) was used throughout (Table 1). Each series consisted of six experiments, performed at equivalence ratios between 0.58 and 082. The set-point values of the equivalence ratio were adjusted by varying the coal flow to the fluidized bed. The set point values of the bed temperature and fluidization velocity were 850°C ( f4”C, with one case of undershoot to 824°C) and 0.7 m/s &5% (with one case of undershoot to 0.69 for all experiments. These m/s), respectively, set-point values were maintained by adjusting the air flow through the air cooler and the air/steam mixture flow to the bed.

The main results believed to be of general interest are presented in Figs 3-7 for the AFBG and in Figs 8-10 for the atmospheric combustor, and discussed below. All data points in these figures represent time averaged values over at least 4 h of stable operation. AFBG

Thermal conversion efficiency and fuel gas composition As the equivalence ratio decreases and the process atmosphere shifts from oxidizing to reducing, the product gas will grow richer in H2 at the expense of the CO component. This trend is illustrated in Figs 3a and 3b, while its beneficial effect on the conversion efficiency and HHV is borne out by Fig. 4. Owing to the far higher HHV of H2 and its hydrocarbon compounds as compared to CO, this beneficial effect occurs in spite of the slight decrease in total carbon conversion with decreasing equiva-

109

Coal gasification and combustion of LCVgas of entrained

bed gasifiers operating around 1200°C. Recycling the cyclone catch to the fluidized bed will increase the residence time of the carbon particles in the fluidized bed, thereby increasing the carbon conversion. Kurkela & Stahlberg (1992) observed such an increase in the carbon conversion of about 11% (from 80-85% to 91-96%). Fig. 5 also shows that the addition of limestone had a negative effect on the carbon conversion. While it is known that limestone and dolomite are catalytically active species (Kurkela & Stihlberg, 1992) the hypothesis that the catalytic effect on the Boudouard reaction (2CO-+C+CO,) in the freeboard and cyclone regions results in a higher production of C and CO2 and a smaller production of CO requires further investigations.

lence ratio shown in Fig. 5 (which is based on the amount of unreacted carbon remaining in the bed and caught by the cyclone and the ceramic filter). This trend was also observed by Kurkela & Stihlberg (1992) in the fluidized bed gasification of peat and brown coal, in a small-scale pressurized, fluidized bed gasifier. As the equivalence ratio decreases, heterogeneous gasification reactions (i.e. reactions of solid carbon with 0,). According to Moulijn & Kapteijn (1993, the reactivities of HZ0 and CO;! are orders of magnitudes smaller than the reactivity of 02, leading to a decreasing carbon conversion efficiency with decreasing equivalence ratio (at the same temperature and residence time). Relatively low carbon conversion rates are inherent in the low temperatures in fluidized bed gasifiers. Kurkula et al. (1995) observed a slight improvement of the carbon conversion for coal gasification at a bed temperature of lOOO”C, but conversion rates remained well below those typical

7

2oa.o

--

m.0

--

30 1soo.0

--

Nitrogen conversion The conversion of fuel nitrogen Fig. 6. High NH3 concentrations

to NH3 is shown in in the product gas

I

a 2

f

looo.0 --

wo.0

i

t 0.00

0.63

0.06

0.72

0.82

Equlvabnco Ratio t-1 3ooo.o ,

ac2Ha

(b)

ocH4 NH2 , ?? co

F 2soo.o i

0%

1 2000.0 t

10%

0.50

0.63

0.07

0.72

0.82

Equlvaknce Ratlo [-] Fig. 3. Higher

heating value (a) without limestone

addition,

(b) with limestone

addition.

110

P D. J. Hoppestqm, W de Jong, J. Andries, K. R. G. Hein

vi--

s

i B

-- 40

j:: i

zz

0

\

zo..

a

A

fl

-4

A

-4

0

--,Ol

2ooo -07 OAO

E

8

ro 030

0.60

0.80

0.70

0.00

1.00

Eq__N Fig. 4. Chemical

efficiency and thermal

should be avoided, because during the combustion of the LCV fuel gas most of the NH3 will be converted to NO. As the equivalence ratio decreased, i.e. as the bed atmosphere grew richer in hydrogen, the N-conversion increased in accordance with chemical equilibrium calculations. Limestone addition enhanced the conversion of fuel nitrogen to NH3 by about 20%. This has also been observed by Leppalahti (1993) who explained it by the fact that limestone and dolomite have a catalytic effect on the conversion of N-compounds to NH3. Sulphur conversion

In a fluidized-bed combustion process (equivalence ratio > l), it is convenient to add limestone or dolomite to reduce the emission of SOZ. However, in a fluidized bed gasification process (equivalence ratio cl), the fuel sulphur is not only converted to

f

conversion

SO, but also to H2S and COS, which will be oxidized to SO2 during subsequent combustion of the LCV fuel gas. Figs 7a and 7b shows the conversion of fuel sulphur to S02, H2S and COS as a function of the equivalence ratio without and with the addition of limestone, respectively. In the former case total conversion of fuel sulphur decreases with decreasing equivalence ratio, as a consequence of the corresponding strong decrease in SO2 formation due to the lack of oxygen at low equivalence ratios. The effects of limestone addition on sulphur retention follow from the calcination of this additive, while sulphur in the form of H2S is retained directly. The foregoing difference explains the occurrence of a maximum in the total sulphur emission (hence minimum in the total sulphur retention), observed in Fig. 7b for an equivalence ratio of 0.67. As the

wo--

= oao--

e

x h

s&o--

8

f

80.0 --

8

71.0 --

4

70.0-6&O+ 0.40

0.60

0.60

efficiency.

0.70

m-Fig. 5. Carbon conversion

0.80

-

t-1 to dry gas.

Coal gasification and combustion of LCVgas

0.0

4

111

I

0.40

0.60

0.60

0.70

1.00

Equhnlence RaUot-1 Fig. 6. Conversion

of fuel nitrogen

to NH,.

80.0

E

70.0

1 00.0 jj

So.0

B

40.0

x

30.0 20.0

10.0 0.0 030

0.63

0.60

0.72

0.68

EqUhml~

0.82

Ratio[-I

1w.o 90.0 -- @)

j;g

80.0--

imso2 ‘I

z

70.0 --

{

60.0

g

50.0

8 5

40.0

B 30.0 Q)

W.0 10.0 0.0 0.50

0.03

muFig. 7. Conversion

of fuel sulphur

0.72

0.07

m

(a) without limestone

0.82

t-l

addition,

(b) with limestone

addition.

P D. J. Hoppesteyn, W de Jong, J. Andries, K. R. G. Hein

112

EquivaknceRatioAFBC3= 0.63,no iimosto~ addkion

~hP&fP/)~~~~~~&~

*cP~~cP&h Fig. 8. Conversion

of the main species in the combustor.

in the Appendix 1, a maximum of 0.72 had to be observed for the equivalence ratio in the AFBG in order to exceed the lower flammability limit of the fuel gas. No attempt was made to investigate flame stability. The sole aim of the present tests was the acquisition of data on conversion of the main fuel gas constituents: combustibles, sulphur compounds and nitrogen compounds, and on the formation of NO, from N2 present in the combustion air.

equivalence ratio increased beyond this value, the additional supply of o2 increased the retention of decreasing equivalence ratios H2S. Conversely, below 0.67 enhanced the predominance of direct sulphur retention by CaS formation. These effects, and the resulting minimum in sulphur retention as a function of equivalence ratio, were also found and commented upon by Hansen (1991). Atmospheric combustor Introduction

Conversion of combustibles

A value of l-15 for the air factor ;Z was maintained throughout the combustion experiments, after it was found to safeguard flame stability and to yield adequate conversion of CO. In this section on the combustor the term ‘air factor’ is used instead of with common ‘equivalence ratio’, in accordance practice in the gas turbine community. As explained

The conversion of the main species in the combustor is shown in Fig. 8 for an equivalence ratio of 0.63 in the AFBG without limestone addition. Oxygen is consumed during the combustion of CO, H2, CH4 and C2H4 to H20 and C02. The CO emission from the combustor varied between 20 and 656 ppm (15%

-a&.- E.R- 0.72,withoutlimestom .-H-‘E.R=O.66, with limostonr -El- - E.R=O.Q, with limestone 0

3

6

9

12

16

16

21

24

27

HoifJht from bottomATC [cm]

Fig. 9. Temperatures

in the combustor.

36

33

36

36

Coal gasification and combustion of LCV gas

loo 80 --

0 convumion RatJo(Uu ?? d schtnidli) mcoIl~eZltkcoddarhfod~g8s ACOIWOlSiOtlRdO~fUOlgw

a0 -E

113

70 -??

??

t

A

cl

%

>

40 --

d

20 --

A



20 --

A

10 -0,

I 0

I

::::

:

::

:

:

::::

i

So 100 150 200 250 300 360 400 450 500 SW 600 650 700 750 800 NH3 in fwl gas Ippm] Fig. 10.

Conversion of NH3 in the fuel gas to NO.

02, dry) while the conversion of the other components was almost complete, resulting in a high combustion efficiency (98.72-99.96%). Conversion of sulphur compounds

Hydrogen converted

sulphide (H2S) and COS were completely to SO2 in the combustor.

Conversion of nitrogen and its compounds

One of the most pollutants emitted by gas turbines is NO,. By source and formation pathway, these combustion-generated nitric oxides may be subdivided into thermal NO, formed by direct reactions of gaseous N2 and 02, fuel NO, formed from chemically bound N in the fuel, and prompt NO,, formed from gaseous N2 via intermediate products. Flame temperature is the main parameter in thermal NO, formation, while (molecular or ionic) oxygen concentration is of paramount importance to the other two pathways. Van Ree et al. (1995) report that, at present, (thermal) NO, levels of 15 ppm (15% 02, dry) are obtained in standard gas turbine combustors burning natural gas and using steam injection for NO, reduction. Very low NO emissions are also obtained with so called ‘dry low NO, burners’ in gas turbine combustors (Santos, 1993; Stambler, 1995), where a lean premixed flame is created with adiabatic flame temperatures low enough to depress the thermal NO mechanism. When LCV fuel gas is used instead of natural gas, thermal NO emissions are bound to decrease because of the lower adiabatic flame temperatures. Liu & Schmidli (1996) report thermal NO emissions of about 1 ppm when firing LCV blast furnace gas, while Kelsall & Laughlin (1995) reported thermal NO emissions typically below 5 ppm (15% 02, dry) when firing syngas. As shown in Fig. 9 all temperatures measured in the combustor were below SOO”C, confirming the relative unimportance of the thermal pathway as

compared to the fuel NO, formation. The only nitrogen compound measured in the fuel gas was NH3. In the combustor, where a conventional diffusion flame is created, without any attempt at NO, abatement, the conversion of NH3 to NO, was almost 60%. This was in good agreement with experiments of Liu & Schmidli (1996), as shown in Fig. 10. Liu and Schmidli added small amounts of NH3 to a LCV blast furnace gas to examine the NH3 conversion to NO, in a gas turbine model combustor at atmospheric pressure. The conversion of NH3 to NO, appears to be independent of the concentration of NH3 in the fuel gas, except for an indication that at very high NH3 concentrations in the fuel gas the conversion appears to be smaller. This may be caused by insufficient residence time in the combustor. The conversion of NH3 (and HCN) to NO, can be epressed by creating a so called ‘richlean flame’. In a rich-lean combustor the combustion air is added to the combustor in portions, such that a fuel-rich primary zone is created in which conversion of NH3 (and HCN) to molecular nitrogen is enhanced (Kelsall & Laughlin, 1995; Toof, 1986). Kelsall & Laughlin (1995) measured the NH3 to NO, conversion in such a ‘rich-lean’ gas turbine combustor at a pressure of 4 bar (absolute). They added 500-3000 ppm NH3 to syngas. The reported conversion ratio was about 45%; again almost independent of the NH3 concentration in the fuel gas. Kramlich et al. (1989) proposed the following reaction mechanism for the conversion of NH3 to NO and NzO: [NH3+OH++NH2+HzO]

(R3)

[NH,+OH-NH+H,O]

(R4)

[NH+OeNO+H]

(R5)

114

l? D. J. Hoppesteyn, W de Jong, J. Andries, K. R. G. Hein

NH+OHc*NO+H,

NH+NO++N,O+H

(R6)

(R7)

A part of the NO can oxidize to NOz according to the following reaction: [NO + HO,++NO,+

OH]

(R8)

NO+O+Mc*NOz+M

(R9)

NO+O,+-+NO,+O

(RIO)

NO+OH++N02+H

(RII)

In the present experiments the outflow of nitrogen in the form of nitrogen oxides exceeded the amount released by the conversion of NH3 by a factor 3 of approximately. It was not possible to establish to what extent this discrepancy should have been attributed to imperfections in the gas analysis system or to the other NO, formation pathways not normally associated with the low combustion temperatures measured here.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the contribution of Mr C. Boeren and Mr J. Chedoe, who performed most of the experiments within the framework of their graduate study at Delft University of Technology. They are also indebted to Prof. D. G. H. Latzko, emeritus professor and former head of the Laboratory for Thermal Power Engineering, for his comments and suggestions. This project is partly funded by the European Union as part of the Joule II R&D programme on clean coal technology: contract JOU2-CT92-0154. APPENDIX The main combustible components in the coal derived LCV fuel gas are CO, H2 and CH4. The LCV fuel gas must contain a minimum amount of these gases to be combustible. The minimum amount of the combustible component in an inert mixture is defined by the lower flammability limit (LFL). According to Hustad & Sonju (1988) the LFL of CO, H2 and CH, can be calculated from the following equations: [LFL.=

[LFL,,=5[1-O.O129(T-25)]]

CONCLUSIONS The following conclusions to the AFBG:

15 [l -O.O0095(T-25)]]

are in order with respect

The thermal conversion efficiency increased with the equivalence ratio; limestone addition had a negative effect on the conversion efficiency. As the equivalence ratio increased, the conversion of fuel nitrogen to NH3 decreased without limestone, and increased with limestone. Without limestone addition the conversion of fuel sulphur to SO2 increased with increasing equivalence ratio, while the conversion to H2S and COS decreased. The total sulphur retention decreased with increasing equivalence ratio. With limestone addition the conversion of fuel sulphur to SO* was significantly decreased, while the conversion to H2S showed a maximum at an equivalence ratio of O-67. The formation of COS was independent of the addition of limestone. With respect to the burning fuel gas in the combustor:

of coal-derived

LCV

Stable combustion was found possible down to a calorific value of 157 MJ/mi. The combustion efficiency varied between 98.72 and 99.96%. Nearly 60% of the NH3 in the fuel gas was converted to NO,.

[LFLo-,

= 53[1 -O.O0085(T-25)]]

Where T is the inlet temperature of the fuel gas in “C (in our experiments: 130°C f 1O’C). The resulting flammability limits for CO, H2 and CH4 at 130°C are 13.6, 4.4 and 4*9%, respectively. While intended for ambient temperatures, Hustad & Sonju (1988) have shown that Le Chatelier’s rule for calculating the LFL for mixtures of gases yields fairly good results up to temperatures of 500°C. 100

LF Lmix = i

i= 1

(CiILFLi)

According to this equation the present fuel gas cannot be combusted when the equivalence ratio is 0.82 and barely reaches its LFL when the equivalence ratio is 0.72 without limestone addition. REFERENCES Chomiak, J., Longwell, J. P. & Sarofim, A. F. (1989). Combustion of low calorific value gases; problems and prospects Prog. Energy Comb. Sci., l&109-129. DeCorso, M. Newby, R., Anson, D., Wenglarz, R. & Wright, I. (1996). Coal/biomass fuels and the gas turbine: utilization of solid fuels and their derivatives. Paper presented at International Gas Turbine and Aeroengine Congress and Exhibition, June 10-13, Birmingham.

Coal gasification and combustion of LCVgas

Faaij, A. (1997). Energy from biomass and waste, Ph.D Thesis, University of Utrecht, The Netherlands. Hansen, P. F. B. (1991). Sulphur capture in fluidized bed combustors. PhD Thesis, Technical University of Denmark, Lyngby. Hustad, J. E. & Sonju, 0. K. (1988). Experimental studies of lower flammability limits of gases and mixtures at elevated temperatures. Combustion and Flame, 71. 283-294.

Kelsall, G. J. &. Laughlin, K. (1995). The development of co-gasification for coal/biomass and other coal/waste mixtures. Final Report EC research project, COALCT92-0002, vol III, University of Stuttgart. Kramlich, J. C., Cole, J. A., McCarthy, J. M. & Steven W., Lanier (1989). Mechanisms of nitrous oxide formation in coal flames. Combustion and Flame, 77, 375-384. Kurkela, F. & Stihlberg, P. (1992). Air gasification of peat, wood and brown coal in a pressurized fluidized bed reactor. I. Carbon conversion, gas yields and tar formation Fuel Processing Technology, 31, 1-21. Kurkela, E., Laatikainen, J. and Stahlberg, P. (1995). Cogasification of biomass and coal. APAS final report. Lefebvre, A. H. (1983). Gas Turbine Combustion. Hemisphere Publishing Corp Bristol, UK. Leppalahti, J. (1993). Formation and behaviour of nitrogen compounds in an ICJCC process. Bioresource Technology, 46, 65-70.

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Liu, Y. & Schmidli, J. (1996). Experiments with a gas turbine model combustor firing blast-furnace gas. Paper presented at International Gas Turbine and Aeroengine Congress and Exhibition, June 10-13, Birmingham. Meier, J. G., Hung, W. S. Y. & Sood, V. M. (1986). Development and application of industrial gas turbines for medium-BTU gaseous fuels. J. Eng. Gas Turbines and Power, 108, 182-190.

Moulijn, J. A. & Kapteijn, F. (1995). Towards a unified theory of reactions of carbon with oxygen containing molecules. Carbon, 33, 8 1155-1165. Santos, R. R. (1993). Dry low NO, reduction of aircraft derivative gas turbines. IGTZ, 8 ASME COGENTURBO. Schobert, H. H. (1990). The Chemistry qf Hydrocarbon Fuels. Butterworths, London. Stambler, 1. (1995). Dry low NO, becoming standard for new heavy frame machines. Gas Turbine World, MarchApril. Toof, J. L. (1986). A model for the prediction of thermal, prompt and fuel NO,r emission from turbine systems. J. Eng. Gas Turbines and Power, 108, 340-347.

Van Ree, R., Oudhuis, A. B. J., Faaij. A. and Curvers, A. P. W. M. (1995). Modelling of a biomass integrated gasifiericombined cycle (BIG/CC) system with the flowsheet simulation programme ASPEN PLUS. Report ECN-C-95: Petten.