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Nox Control Technology for Large Combustion Installations
T. Schneider et al. (Editors),Atmospheric Ozone Research and its Policy Zmplicatwna 0 1989 Elsevier Science Publishers B.V., Ameterdam - Printed in The Netherlands
NO,
681
CONTROL TECHNOLOGY FOR LARGE COMBUSTION INSTALLATIONS
J. VAN DER KOOIJ Environmental Research Department, N.V. KEMA. P.O. Box 9035, 6800 E T Arnhem (The Netherlands)
ABSTRACT Advances to the state of the art of low NO, combustion technology f o r gas and coal fired boilers offer the possibility for considerable NO, emission reduction. In the Netherlands the utilities have developed on a voluntary basis a concerted NO, Abatement Programme in order to reduce the NOx emissions of both existing and new installations. The programme also comprises a number of demonstration projects in order to assess the applicability of new techniques and to prepare for decisions on cost-effective NO, controls. INTRODUCTION Most of the anthropogenic nitrogen oxides are produced during combustion processes. These processes may occur in stationary or mobile sources. An inventory of the NO, emissions in the Netherlands is presented in Table 1 for the years 1980 and 1985.
TABLE 1 NO, emissions in The Netherlands in 10’ t/a (ref. 1). I980
1985
80 20 44 26
45
83 17 43 20 46
road tcaff ic
264
261
other t canspor tat ion
57 536
57 527
power stations re€ineries industry, combustion industty, processes Other stationary S O U C C B S
Powerstations, refineries and part of the industrial combustion belong to the category of the large combustion installations. Under the idea that 5 0 % of the industrial combustion
602 processes occur in installations w i t h a capacity larger than 50 MWth, approximately 110 ton/a NO, is produced in large combustion installations, equalling 20% of the national emissions. As t h e power stations produce 75% of these emissons, there is a good reason to confine t h e discussion to power station emissions. Another reason is that the emission standards for combustion installations are thus far not applicable to process furnaces whereas government and industry a r e cooperatively investigating the N O x emissions of these installations and the technology to reduce it (ref. 2). Reduction of t h e N O x emissions of the power stations in t h e Netherlands w i l l have a small effect o n ground level concentrations, acid deposition and ozone formation. If however these measures a r e taken in collaboration with neighbouring countries the environmental relevance increases considerably. T h i s can be seen from Table 2 in w h i c h the power plant NO, emissions in a number of countries are given. T h e N O x emissions vary between 12 and 44% of the national emi6sions. w i t h 25% as a n average value. If w e furthermore consider a NO, emission reduction by 408 in agreement vith t h e Proposal for a Council Directive on the limitation of emission of pollutants into the air from large combustion plants w e c a n expect a reduction in t h e average ambient concentrations of NOx typically i n the order of 10% (ref. 3).
TABLE 2 N O X emissions in neighbouring countries. nat iona 1
power: stations
103 t/a
103 t/a
%
Belgium
415
85
20
Denmark
265
115
43
France
2567
297
12
Germany
3 100
850
27
Italy
1595
530
44
00
15
Netherlands
536
683 NOX A B A T E m N T STRATEGY In the United States the New Source Performance Standards came into effect in 1971. This led to the construction of larger furnaces in new power stations and the incorporation of combustion modification. In this respect it is important to note that NOx formation depends strongly on furnace conditions. Even small modifications in the combustion process can greatly influence emission levels. Combustion modifications to reduce NOx are generally based on promoting a more gradual mixing of fuel and air to reduce flame temperature and the use of a richer fuel-air mixture to reduce oxidation of nitrogen in the fuel. In Japan the regulation of NO, emission started in 1973 and was tightened in a number of stages. In early the eighties the standards became so stringent that flue gas treatment was needed in addition to combustion modification. In the Federal Republic of Germany in 1984 a council of environmental ministers promulgated targets for NOx emission standards for both new and existing installations, that were so stringent that flue gas treatment was needed. Moreover the time schedule for the incorporation of the controls was so short that most efforts were directed towards flue gas treatment. In the Netherlands the first applications of combustion modifications occurred in the early seventies. For many power stations the NOx emissions were limited by the provincial authorities in licenses according to the Air Pollution Act. Only in 1987 a General Administrative Order on the emissions of large COmbUStiOn installations came into force (ref. 2). The standards are strict especially for new installations (coal 400 mg/m 3 , oil 300 mg/m3 and gas 200 mg/m 3 ) . However it is expected that the development of low NO, combustion technology has developed so far that with advanced combustion modifications these standards can be met. The differences between these national approaches are so large that it is considered useful to evaluate the state of the art of NOx control technology for large combustion installation in some detail and to describe the efforts of the 1
electric power companies in the Netherlands in their concerted NOx Abatement Programme (ref. 4). This programme is implemented on a volumentary basis. It consists primarily of the application of low N O technology in new and existing inX
684
stallations. In the second part of the programme new technologies are demonstrated to assess their applicability and to prepare for decisions on cost-effective NO, controls.
COMBUSTION MODIFICATIONS FOR GAS/OIL FIRED INSTALLATIONS Since 1980 the amount of fuel oil fired in Dutch power stations has decreased considerably and in the present situation fuel oil is mainly used as a substitute for natural gas. on days when the gas supply is interrupted. Therefore w e have no recent experience with oil fired installations in the Netherlands. However, it is our view that the extremely low N O X concentrations that have been reported in the literature, especially from Japan do not hold for the Dutch situation, because of the differences in fuel oil composition (e.9. sulfur content, residual oil v s . crude oil). Natural gas is a very important fuel for the Dutch power stations. An ambitious programme consists of the repowering of a steam turbine in existing installations with a gas turbine. In these installations the new gas turbine replaces the existing air blowers and the regenerative air heater. The exhaust gases of the gas turbine with an oxygen content of approximately 15% are used instead of fresh air for the combustion of gas or oil in the existing boiler. Increasing the thermal efficiency from a typical value of 40% to 45% and reducing the NOx emission are the main goals of the programme that is applied t o 10 power stations with an equivalent capacity of 3650 MWe (Table 3.1). T h e NO, emission is reduced primarily because of the low combustion temperature in the boiler; as a matter of fact the adiabatic flame temperature is lowered by the inert material in the gas turbine exhaust and the boiler load is reduced to approximately 70% as the other part of the combustion takes place in the gas turbine. A very important point is the selection of the proper gas turbine as there is a close relation between rating of the existing steam turbine and the gas turbine. Therefore there is not a free choice for a gas turbine with a high efficiency an a low NOx emission. The NOx emission of the gas turbine and control measures in the boiler are also important for the NOx emission of the combined cycle. It was expected that the N O X
emission of the power stations could be reduced by 30% and would amount to 100 g/GJ as an average, although some of the installations had high NO, emissions because of the.high thermal load in the furnace. At present it is already known that reductions higher than 5 0 % have been achieved at full load in a number of installations. The usual combustion modifications are applied in a demonstration project of low NO, combustion technology for gas and oil firing in Flevo Station 1. Flevo station 1 has been retrofitted with advanced low NO, burners, after air ports and gas recirculation into the combustion air. The expected emission 3 values are 200 mg/m3 for gas firing and 300 mg/m for oil firing. However the main objective of the project is the application of reburning technology, also known as In Furnace NO, Reduction (Table 3.2). Reburning is a distributed combustionsystem in which part of the fuel is injected into the furnace through burners placed above the top row of main burners. By correct adjustment of the fuel and air distribution. the reburn zone is operated fuel rich, thereby converting NO from primary zone combustion into N2. The aim of the project is to demonstrate the technology for gas and oil firing with rapid load fluctuations and to investigate the optimum NO, emission level, the quality of the combustion process (CO, and unburned carbon) and the mixing processes of upper fuel and over fire air the combustion gases. The NOx goal is 100 mg/m3 for natural gas and 200 mq/m3 for fuel oil.
686 TABLE 3 Concerted NO,
Abatement Programme.
Gas/oil firing Conversion of existing steam boilers to combined cycles power station
capacity (MWe)
commissioning
336 336 328 273 159 645 622
1986 1987 1987 1987 1988 1988 1988 1988 1989 1989
~~
Bergum station 1 Bergum station 2 Harculo station 5 r,age Weide station 5 Merwedehaven station 6 Eems station 2 Hemweg station 7 Waalhaven station 5 Flevo station 3 Waalhavsn station 4
313
451
3 13
Demonstration project o € €or gas and oil firing Flevo station 1
low NO,
combustion technology
capacity 185 MW
commissioninq 1988
- advanced low NO, bucnecs - gas mixing into the combustion air - two stage combustion - infurnace NO, reduction COMBUSTION MODIFICATIONS FOR COAL FIRED INSTALLATIONS I t is known that in a number of coal fired power stations in Japan NOx concentrations have been obtained in the range between 500 and 600 mg/m3 have been obtained. In order to obtain N O concentrations below the desired value of 400 ~ n g / m ~ ~is i tnecessary to equip the boiler with advanced low-NO, burners and two stage combustion. The results obtained in Japan with the PM burner developed by Mitsubishi Heavy Industries for tangentially fired boilers and HT-NR burner developed by Babcock Hitachi for frontwall and horizontally opposed fired boilers prove that the technology has advanced and conforms to the requirements (ref. 5). In the Netherlands the Power Stations Maasvlakte and Borssele have been converted from gas/oil to coal firing. These boilers have tangential firing systems. As the decision about conversion was taken at a time that the P M burner was not yet available for
687 coal firing, the SGR burner was used. Because of enlarged furnace volume, increased over fire air and the application of the Low NOx concentric firing system the boiler manufacturer "de Schelde" considered that NOx Concentrations below 600 mg/m3 can be reached. In the framework of the concerted NOx Abatement Programme a measurement programme is executed (Table 4.1). The preliminary results of Borssele and Maasvlakte are encouraging. For horizontally opposed and frontwall fired installation the experience in the Netherlands with modern combustion modifications is considered to be insufficient. Therefore the decision has been taken to perform a demonstration project in the Maas Station 5 (Table 4.3). The unit will be retrofitted with HTNR burners and after air ports by the boiler manufacturer "Stork Boilers" with a license of Babcock Hitachi. The aim of the project is to show that a NO, concentration of 400 m g / m 3 in new power stations can be obtained by the combination of advanced low NOX burners and two stage combustion in frontwall and horizontally opposed fired boilers. In addition to the primary measures mentioned before, the National Government of the Netherlands is of the opinion that flue gas denitrification has to be applied in future when the results with low NO, combustion technology are insufficient. A demonstration project, fully paid by the government, is in progress in Power Station Gelderland 12. This project is based on MHI high dust SCR technology. Originally the discussion of flue gas treatment technology was dominated by Japanese suppliers and users of SCR systems. However in recent years in Western Europe, with emphasis on Germany, a stormy development has taken place resulting in a large number of installations (ref. 6): 70 pilot SNCR/SCR plants with a total flue gas volume of about 250.000 m3/h 50 demonstration and commercial plants of 12.000 MWel.
688 TABLE 4 Concerted NO,
Abatement Programme.
Coal firing 1 Conversion of gas/oil fired power station to coal firing
Maas station 6 Maasvlakte station 2 Borssele station 12 Maasvlakte station 1
commi ss i o n i np 1986 1987
capacity 223 MW 517 MW 405 MW 517 MW
1987 1988
2 Study in existing power stations Gelderland station 13 AmeK station 8 3 Demonstration project combustion technology Maas station 5
603 MW 645 MW of
low
180 MW
NO,
pulverized
coal
1988
- advanced low NO, burners - two stage combustion 4 Flue gas denitcification
Gelderland station 12
- SCR is
123 MW
1987
50% of flue gas stream
CONCLUDING REMARKS NO, abatement for stationary sources can be realized primarily by combustion modifications. The low-NOx technology that is applied depends on a number of aspects e.g. retrofit or new installations, fuel and boiler type. On the basis of information obtained thus far estimated NO, emission levels for new installations are presented in Table 5. The use of combustion modifications can be limited by side-effects such as - increased fouling - corrosion by reducing atmospheres - increased CO emission - incomplete burn out. In the Netherlands the quality of the fly ash is considered to be of great importance. There are Bome indications that the physical properties of the ash may change because of the lower
688
furnace temperatures, and that this may have consequences for the applicability of fly ash in construction materials. The potential of combustion modifications is even greater if reburning technology is taken into account, with potential emission reduction in the order of 5 0 % . The technology has been proven for natural gas and fuel oil. There are indications that a highly volatile and low nitrogen containing fuel should be used as a reburn fuel. There is a difference of opinion about the feasibility of the technique with coal as upper fuel. If however combustion modifications are insufficient or are considered to form a great risk. the NO, emissions can be reduced by flue gas denitrification. Selective catalytic reduction is the most promising technology. But other techniques like SNCR or the injection of urea can be cost-effective alternatives especially when the necessary emission reduction is limited. From the first results in Western Germany it seems that SCR can be applied without problems. However it is necessary to point out that there is no long term experience. This is especially important in determining catalyst life, when these materials are subjected to frequent temperature excursions, due to changes in boiler load. Major other problems are the use of different coals and different types of coal fired boilers whose ash may contain constituents that prematurely erode or deactivate the catalyst. Also ammonia slip is considered t o be a potential problem, because of the deposition of ammonium bisulphate in the air preheater and possible contamination of fly ash by these ammonium components.
630 TABLE 5 Estimated NOx emission on levels for new installations in mg/mi (ref. 5 ) .
coa 1
oil
gas
1000-1400
500-700
300- 600
combustion modifications
600-900
200-300
150-270
advanced low NO, burners) combustion modifications)
300- 600
130-250
5 0 - 110
advanced low NO, burners) combustion modifications) 1 SCR
65-130
(6% 02)
without control
advanced low NO, burners) combustion modifications) r e bu r ni ng 1
(120-300)
(3%0 2 )
50
65-130
( 3 % 02)
25
25-50
REFERENCES Milieuprogramma 1 9 8 8 - 1 9 9 1 , Tweede Kamer, vergaderjaar 1 9 8 7 - 1 9 8 8 , 2 0 2 0 2 nrs. 1 - 2 . General Administrative Order "Emission standards for combustion installationsaa,Staatsblad 1987. nr. 1 6 4 , 2 8 april 1987.
E C COM ( 8 3 ) 7 0 4 final. Proposal for a Council Directive on the limitation of emissions of pollutants into the air from large combustion plants. W E N . Voorstel Totaal Programma NOx-uitworpbeperkende Maatregelen (maart 1 9 8 6 ) . J.G. Witkamp, J . van der Kooij, M.E.A. Hermans. status of low NO, combustion technology in Japan; a report of a technical visit in KEMA report 02562-MOL 8 6 - 3 0 4 0 . UNIPEDE Sorrento Congress. Actual status of nitrogen oxide reduction technologies in the THERNOX member countries (1988).
NO,
681
CONTROL TECHNOLOGY FOR LARGE COMBUSTION INSTALLATIONS
J. VAN DER KOOIJ Environmental Research Department, N.V. KEMA. P.O. Box 9035, 6800 E T Arnhem (The Netherlands)
ABSTRACT Advances to the state of the art of low NO, combustion technology f o r gas and coal fired boilers offer the possibility for considerable NO, emission reduction. In the Netherlands the utilities have developed on a voluntary basis a concerted NO, Abatement Programme in order to reduce the NOx emissions of both existing and new installations. The programme also comprises a number of demonstration projects in order to assess the applicability of new techniques and to prepare for decisions on cost-effective NO, controls. INTRODUCTION Most of the anthropogenic nitrogen oxides are produced during combustion processes. These processes may occur in stationary or mobile sources. An inventory of the NO, emissions in the Netherlands is presented in Table 1 for the years 1980 and 1985.
TABLE 1 NO, emissions in The Netherlands in 10’ t/a (ref. 1). I980
1985
80 20 44 26
45
83 17 43 20 46
road tcaff ic
264
261
other t canspor tat ion
57 536
57 527
power stations re€ineries industry, combustion industty, processes Other stationary S O U C C B S
Powerstations, refineries and part of the industrial combustion belong to the category of the large combustion installations. Under the idea that 5 0 % of the industrial combustion
602 processes occur in installations w i t h a capacity larger than 50 MWth, approximately 110 ton/a NO, is produced in large combustion installations, equalling 20% of the national emissions. As t h e power stations produce 75% of these emissons, there is a good reason to confine t h e discussion to power station emissions. Another reason is that the emission standards for combustion installations are thus far not applicable to process furnaces whereas government and industry a r e cooperatively investigating the N O x emissions of these installations and the technology to reduce it (ref. 2). Reduction of t h e N O x emissions of the power stations in t h e Netherlands w i l l have a small effect o n ground level concentrations, acid deposition and ozone formation. If however these measures a r e taken in collaboration with neighbouring countries the environmental relevance increases considerably. T h i s can be seen from Table 2 in w h i c h the power plant NO, emissions in a number of countries are given. T h e N O x emissions vary between 12 and 44% of the national emi6sions. w i t h 25% as a n average value. If w e furthermore consider a NO, emission reduction by 408 in agreement vith t h e Proposal for a Council Directive on the limitation of emission of pollutants into the air from large combustion plants w e c a n expect a reduction in t h e average ambient concentrations of NOx typically i n the order of 10% (ref. 3).
TABLE 2 N O X emissions in neighbouring countries. nat iona 1
power: stations
103 t/a
103 t/a
%
Belgium
415
85
20
Denmark
265
115
43
France
2567
297
12
Germany
3 100
850
27
Italy
1595
530
44
00
15
Netherlands
536
683 NOX A B A T E m N T STRATEGY In the United States the New Source Performance Standards came into effect in 1971. This led to the construction of larger furnaces in new power stations and the incorporation of combustion modification. In this respect it is important to note that NOx formation depends strongly on furnace conditions. Even small modifications in the combustion process can greatly influence emission levels. Combustion modifications to reduce NOx are generally based on promoting a more gradual mixing of fuel and air to reduce flame temperature and the use of a richer fuel-air mixture to reduce oxidation of nitrogen in the fuel. In Japan the regulation of NO, emission started in 1973 and was tightened in a number of stages. In early the eighties the standards became so stringent that flue gas treatment was needed in addition to combustion modification. In the Federal Republic of Germany in 1984 a council of environmental ministers promulgated targets for NOx emission standards for both new and existing installations, that were so stringent that flue gas treatment was needed. Moreover the time schedule for the incorporation of the controls was so short that most efforts were directed towards flue gas treatment. In the Netherlands the first applications of combustion modifications occurred in the early seventies. For many power stations the NOx emissions were limited by the provincial authorities in licenses according to the Air Pollution Act. Only in 1987 a General Administrative Order on the emissions of large COmbUStiOn installations came into force (ref. 2). The standards are strict especially for new installations (coal 400 mg/m 3 , oil 300 mg/m3 and gas 200 mg/m 3 ) . However it is expected that the development of low NO, combustion technology has developed so far that with advanced combustion modifications these standards can be met. The differences between these national approaches are so large that it is considered useful to evaluate the state of the art of NOx control technology for large combustion installation in some detail and to describe the efforts of the 1
electric power companies in the Netherlands in their concerted NOx Abatement Programme (ref. 4). This programme is implemented on a volumentary basis. It consists primarily of the application of low N O technology in new and existing inX
684
stallations. In the second part of the programme new technologies are demonstrated to assess their applicability and to prepare for decisions on cost-effective NO, controls.
COMBUSTION MODIFICATIONS FOR GAS/OIL FIRED INSTALLATIONS Since 1980 the amount of fuel oil fired in Dutch power stations has decreased considerably and in the present situation fuel oil is mainly used as a substitute for natural gas. on days when the gas supply is interrupted. Therefore w e have no recent experience with oil fired installations in the Netherlands. However, it is our view that the extremely low N O X concentrations that have been reported in the literature, especially from Japan do not hold for the Dutch situation, because of the differences in fuel oil composition (e.9. sulfur content, residual oil v s . crude oil). Natural gas is a very important fuel for the Dutch power stations. An ambitious programme consists of the repowering of a steam turbine in existing installations with a gas turbine. In these installations the new gas turbine replaces the existing air blowers and the regenerative air heater. The exhaust gases of the gas turbine with an oxygen content of approximately 15% are used instead of fresh air for the combustion of gas or oil in the existing boiler. Increasing the thermal efficiency from a typical value of 40% to 45% and reducing the NOx emission are the main goals of the programme that is applied t o 10 power stations with an equivalent capacity of 3650 MWe (Table 3.1). T h e NO, emission is reduced primarily because of the low combustion temperature in the boiler; as a matter of fact the adiabatic flame temperature is lowered by the inert material in the gas turbine exhaust and the boiler load is reduced to approximately 70% as the other part of the combustion takes place in the gas turbine. A very important point is the selection of the proper gas turbine as there is a close relation between rating of the existing steam turbine and the gas turbine. Therefore there is not a free choice for a gas turbine with a high efficiency an a low NOx emission. The NOx emission of the gas turbine and control measures in the boiler are also important for the NOx emission of the combined cycle. It was expected that the N O X
emission of the power stations could be reduced by 30% and would amount to 100 g/GJ as an average, although some of the installations had high NO, emissions because of the.high thermal load in the furnace. At present it is already known that reductions higher than 5 0 % have been achieved at full load in a number of installations. The usual combustion modifications are applied in a demonstration project of low NO, combustion technology for gas and oil firing in Flevo Station 1. Flevo station 1 has been retrofitted with advanced low NO, burners, after air ports and gas recirculation into the combustion air. The expected emission 3 values are 200 mg/m3 for gas firing and 300 mg/m for oil firing. However the main objective of the project is the application of reburning technology, also known as In Furnace NO, Reduction (Table 3.2). Reburning is a distributed combustionsystem in which part of the fuel is injected into the furnace through burners placed above the top row of main burners. By correct adjustment of the fuel and air distribution. the reburn zone is operated fuel rich, thereby converting NO from primary zone combustion into N2. The aim of the project is to demonstrate the technology for gas and oil firing with rapid load fluctuations and to investigate the optimum NO, emission level, the quality of the combustion process (CO, and unburned carbon) and the mixing processes of upper fuel and over fire air the combustion gases. The NOx goal is 100 mg/m3 for natural gas and 200 mq/m3 for fuel oil.
686 TABLE 3 Concerted NO,
Abatement Programme.
Gas/oil firing Conversion of existing steam boilers to combined cycles power station
capacity (MWe)
commissioning
336 336 328 273 159 645 622
1986 1987 1987 1987 1988 1988 1988 1988 1989 1989
~~
Bergum station 1 Bergum station 2 Harculo station 5 r,age Weide station 5 Merwedehaven station 6 Eems station 2 Hemweg station 7 Waalhaven station 5 Flevo station 3 Waalhavsn station 4
313
451
3 13
Demonstration project o € €or gas and oil firing Flevo station 1
low NO,
combustion technology
capacity 185 MW
commissioninq 1988
- advanced low NO, bucnecs - gas mixing into the combustion air - two stage combustion - infurnace NO, reduction COMBUSTION MODIFICATIONS FOR COAL FIRED INSTALLATIONS I t is known that in a number of coal fired power stations in Japan NOx concentrations have been obtained in the range between 500 and 600 mg/m3 have been obtained. In order to obtain N O concentrations below the desired value of 400 ~ n g / m ~ ~is i tnecessary to equip the boiler with advanced low-NO, burners and two stage combustion. The results obtained in Japan with the PM burner developed by Mitsubishi Heavy Industries for tangentially fired boilers and HT-NR burner developed by Babcock Hitachi for frontwall and horizontally opposed fired boilers prove that the technology has advanced and conforms to the requirements (ref. 5). In the Netherlands the Power Stations Maasvlakte and Borssele have been converted from gas/oil to coal firing. These boilers have tangential firing systems. As the decision about conversion was taken at a time that the P M burner was not yet available for
687 coal firing, the SGR burner was used. Because of enlarged furnace volume, increased over fire air and the application of the Low NOx concentric firing system the boiler manufacturer "de Schelde" considered that NOx Concentrations below 600 mg/m3 can be reached. In the framework of the concerted NOx Abatement Programme a measurement programme is executed (Table 4.1). The preliminary results of Borssele and Maasvlakte are encouraging. For horizontally opposed and frontwall fired installation the experience in the Netherlands with modern combustion modifications is considered to be insufficient. Therefore the decision has been taken to perform a demonstration project in the Maas Station 5 (Table 4.3). The unit will be retrofitted with HTNR burners and after air ports by the boiler manufacturer "Stork Boilers" with a license of Babcock Hitachi. The aim of the project is to show that a NO, concentration of 400 m g / m 3 in new power stations can be obtained by the combination of advanced low NOX burners and two stage combustion in frontwall and horizontally opposed fired boilers. In addition to the primary measures mentioned before, the National Government of the Netherlands is of the opinion that flue gas denitrification has to be applied in future when the results with low NO, combustion technology are insufficient. A demonstration project, fully paid by the government, is in progress in Power Station Gelderland 12. This project is based on MHI high dust SCR technology. Originally the discussion of flue gas treatment technology was dominated by Japanese suppliers and users of SCR systems. However in recent years in Western Europe, with emphasis on Germany, a stormy development has taken place resulting in a large number of installations (ref. 6): 70 pilot SNCR/SCR plants with a total flue gas volume of about 250.000 m3/h 50 demonstration and commercial plants of 12.000 MWel.
688 TABLE 4 Concerted NO,
Abatement Programme.
Coal firing 1 Conversion of gas/oil fired power station to coal firing
Maas station 6 Maasvlakte station 2 Borssele station 12 Maasvlakte station 1
commi ss i o n i np 1986 1987
capacity 223 MW 517 MW 405 MW 517 MW
1987 1988
2 Study in existing power stations Gelderland station 13 AmeK station 8 3 Demonstration project combustion technology Maas station 5
603 MW 645 MW of
low
180 MW
NO,
pulverized
coal
1988
- advanced low NO, burners - two stage combustion 4 Flue gas denitcification
Gelderland station 12
- SCR is
123 MW
1987
50% of flue gas stream
CONCLUDING REMARKS NO, abatement for stationary sources can be realized primarily by combustion modifications. The low-NOx technology that is applied depends on a number of aspects e.g. retrofit or new installations, fuel and boiler type. On the basis of information obtained thus far estimated NO, emission levels for new installations are presented in Table 5. The use of combustion modifications can be limited by side-effects such as - increased fouling - corrosion by reducing atmospheres - increased CO emission - incomplete burn out. In the Netherlands the quality of the fly ash is considered to be of great importance. There are Bome indications that the physical properties of the ash may change because of the lower
688
furnace temperatures, and that this may have consequences for the applicability of fly ash in construction materials. The potential of combustion modifications is even greater if reburning technology is taken into account, with potential emission reduction in the order of 5 0 % . The technology has been proven for natural gas and fuel oil. There are indications that a highly volatile and low nitrogen containing fuel should be used as a reburn fuel. There is a difference of opinion about the feasibility of the technique with coal as upper fuel. If however combustion modifications are insufficient or are considered to form a great risk. the NO, emissions can be reduced by flue gas denitrification. Selective catalytic reduction is the most promising technology. But other techniques like SNCR or the injection of urea can be cost-effective alternatives especially when the necessary emission reduction is limited. From the first results in Western Germany it seems that SCR can be applied without problems. However it is necessary to point out that there is no long term experience. This is especially important in determining catalyst life, when these materials are subjected to frequent temperature excursions, due to changes in boiler load. Major other problems are the use of different coals and different types of coal fired boilers whose ash may contain constituents that prematurely erode or deactivate the catalyst. Also ammonia slip is considered t o be a potential problem, because of the deposition of ammonium bisulphate in the air preheater and possible contamination of fly ash by these ammonium components.
630 TABLE 5 Estimated NOx emission on levels for new installations in mg/mi (ref. 5 ) .
coa 1
oil
gas
1000-1400
500-700
300- 600
combustion modifications
600-900
200-300
150-270
advanced low NO, burners) combustion modifications)
300- 600
130-250
5 0 - 110
advanced low NO, burners) combustion modifications) 1 SCR
65-130
(6% 02)
without control
advanced low NO, burners) combustion modifications) r e bu r ni ng 1
(120-300)
(3%0 2 )
50
65-130
( 3 % 02)
25
25-50
REFERENCES Milieuprogramma 1 9 8 8 - 1 9 9 1 , Tweede Kamer, vergaderjaar 1 9 8 7 - 1 9 8 8 , 2 0 2 0 2 nrs. 1 - 2 . General Administrative Order "Emission standards for combustion installationsaa,Staatsblad 1987. nr. 1 6 4 , 2 8 april 1987.
E C COM ( 8 3 ) 7 0 4 final. Proposal for a Council Directive on the limitation of emissions of pollutants into the air from large combustion plants. W E N . Voorstel Totaal Programma NOx-uitworpbeperkende Maatregelen (maart 1 9 8 6 ) . J.G. Witkamp, J . van der Kooij, M.E.A. Hermans. status of low NO, combustion technology in Japan; a report of a technical visit in KEMA report 02562-MOL 8 6 - 3 0 4 0 . UNIPEDE Sorrento Congress. Actual status of nitrogen oxide reduction technologies in the THERNOX member countries (1988).