N2O emission control in coal combustion

N2O emission control in coal combustion

N20 emission Marek A. W6jtowicz*, control in coal combustion Jan R. Pels and Jacob A. Moulijn Department of Chemical Engineering, 2628 BL Delft...

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N20

emission

Marek

A. W6jtowicz*,

control

in coal combustion

Jan R. Pels and Jacob A. Moulijn

Department of Chemical Engineering, 2628 BL Delft, The Netherlands (Received 25 June 1993)

Delft

University

of Technology,

Julianalaan

736,

Interactions among SO,, NO, and N,O, as well as among methods for their abatement, are discussed and guidelines for the development of N,O control in fluidized-bed combustion (FBC) are proposed. An integrated approach to pollution control is advocated rather than treatment of each pollutant separately. Improvements in operating conditions, start-up procedures and process control are examples of low-cost measures. However, innovative combustor design is required to produce low-emission systems that would meet increasingly stringent standards. Temperature optimization itself is insufficient; gas afterburning and solid-catalysed N,O decomposition are identified as the most promising measures. FBC flue-gas treatment is deemed unattractive, unless it can be made inexpensive and easy to install or retrofit. (Keywords: nitrous oxide; emission control; fluidized-bed combustion)

Nitrous oxide (N,O) has emerged as a pollutant of concern because of its strong absorption of infrared radiation (the greenhouse effect) as well as its role in the destruction of stratospheric ozone. The radiative forcing of N,O is more than 200 times that of CO,, on both mass and molar bases’. In addition, N,O is extremely long-lived in the atmosphere (N 160 years) and its atmospheric concentration is increasing at an annual rate of 0.3%‘. It is estimated that N,O is responsible for -6% of the current anthropogenic contribution to the greenhouse effect’. Owing to its longevity, nitrous oxide is stable enough to rise through the troposphere and reach the ozone layer, where it decomposes to form NO: N,O + O-2N0

(1)

Nitric oxide then destroys catalytic cycle: NO+0

,-NO,

NO, + O-

ozone

through

the following

+ 0,

(2)

NO+O,

(3)

The stability of N,O results in a larger impact on the destruction of the ozone layer than that of high NO emissions at ground level. This is because the high reactivity of NO ensures that it is consumed before it reaches the stratosphere. It is estimated that the nitric oxide, formed to a large extent from N,O, causes up to 50-70% of global ozone depletionzm5. Estimates of the amount of N,O formed during combustion of coal show that this form of power generation is currently not a major global source’.2*697. However, concern has been expressed about future release of N,O from fluidized-bed combustors (FBCs) when they

Presented at ‘NO,: Basic Mechanisms of Formation and Destruction, and their Application to Emission Control Technologies’, 20-22 April 1993, London, UK *Currently with Advanced Fuel Research, 87 Church Street, East Hartford, CT 06108, USA 0016-2361/94/09/141&07 c 1994 Butterworth-Heinemann

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become a major techno10gy7. N,O emission from an FBC is usually between 100 and 250 ppmv, which is high in comparison with the moderate 5520ppmv emitted by conventional power stations7-9. High N,O levels are a penalty paid for the relatively low-NO, and SO, operation of fluidized systems. The question thus arises as to how to incorporate N,O control within existing fluidized-bed combustion practice. What N,O control measures can be taken without adversely affecting NO, and SO, emissions? Should FBC design be changed or modified to facilitate N,O removal? These and related issues are examined in this paper. Strong emphasis is placed on an integrated approach to N,O-NO,-SO, abatement.

9

POLLUTANT DESTRUCTION

FORMATION

AND

Both NO, and N,O formed during fluidized-bed combustion originate mainly from the nitrogen present in the carbonaceous fuel (fuel-N). This is the result of the moderate combustion temperature characteristic of FBC operation (1073-1223 K). Under such conditions, the contributions from thermal NO, and prompt NO, are known to be small. SO, is likewise formed from coal-bound sulfur. There is an important difference between the fates of coal-N and coal-S, namely the existence of a benign nitrogenous product of combustion, i.e. molecular nitrogen. No corresponding harmless sulfur-containing species is formed; hence nearly all the coal-bound sulfur released as SO, needs to be captured. This is usually done by limestone or dolomite injection. There appears to be some merit in trying to devise methods of affecting the selectivity of nitrogenous combustion products in such a way that N, is formed rather than NO, and N,O. Although it is relatively easy to promote N,O formation at the expense of NO,, or vice versa, by changing the combustion temperature69’0*1 ‘,

IV20 emission

it is not clear how to decrease the combined NO,-N,O emission in favour of N,. One way of tackling the problem would be to enhance NO, and N,O reduction to N, (in the gas phase and/or heterogeneously). Another way might involve prevention of NO,-N,O formation, for instance by acting upon their gas-phase precursors: NH, and HCN. NH3 is known to react mainly to NO, whereas HCN may form either NO or N,O. In what follows, several topics are discussed which are related to N,O-NO,-SO, abatement as we11 as to interactions among these species themselves or among methods for their control.

8 3 ? .% e A 3 k

Figure 2 conversion limestone; ref. 14, for

10-

50 I 1 I I 1 1 1 I 1 800 840 880 920 960 1000 Mean furnace temperature (“C) Effect of temperature and limestone addition on fuel-N to NO and N,O: 0, N,O without limestone; n , N,O with 0, NO without limestone; 0, NO with limestone. From a 0.8 MW(th) FBC burning bituminous coal

Temperature eflects

The effect of temperature on N,O and NO emissions from an FBC is perhaps the most obvious of all, and one about which there is no controversy in the literature. NO levels rise, whereas those of N,O decrease, as temperature increases10~‘2-‘5. This trend is illustrated in Figure 2. Et should be pointed out that the temperature distribution along the entire combustor is important, not just the bed temperature. It can be shown that high temperature favours gas-phase reactions leading to NO, whereas preferential formation of N,O is expected at lower temperatures16*1 ‘. The combined amount of NO and N,O formed from coal-bound nitrogen has been shown to be remarkably constant over a wide range of temperatures6’11*18. Another reason for low N,O levels at high temperatures is the existence of powerful gas-phase mechanisms for N,O removal, either by radicals or through collisions with gas molecules: N,O+H-

74.5

loo0

loo0

1500 Temperature

[Kl

Figure 1 Equilibrium concentrations of selected products ofbituminous coal combustion. Initial composition (vol.%) of the gas mixture: 0,. 5.6; N,, 73.3; CO,, 12; CO, 2.0; H,O, 6.0; NH,, 0.14; HCN, 0.016; CH,, 0.24; H,, 0.67; SO,, 0.052; SO,, 0.00056. Species with zero initial concentration: N,O, NO, NO,, OH, H, 0, N, NH, NH,, HNCO, NCO, CN, HNO, S, SH, NS, SO, COS and H,S. Equilibrium concentrations of species not shown in the figure but present in the initial gas mixture are <3 ppmv over the whole temperature range. From ref. 6

n/l. A. Wdjtowicz et al.

g 2or Y 8 ‘E 15$

Gas-phase equilibrium concentrations of NO, and NJ0

The multireaction equilibrium composition of a gas mixture resulting from devolatilization of a typical bituminous coal has been determined using a classic non-stoichiometric approach based on minimization of the Gibbs energy of the system6. For these computations, a situation was considered in which devolatilization products were brought into contact with oxygen and carbon dioxide at concentrations typical of fluidized-bed combustion. Equilibrium gas concentrations as a function of temperature are presented in Figure 1. The results show that NO levels rise sharply with temperature, which reflects a growing contribution from thermal NO. At a temperature characteristic of fluidized-bed combustion, i.e. N 1123 K, the equilibrium NO concentration is only 50 ppmv. This is of the same order of magnitude as typical NO emission levels recorded in FBC flue gas. The situation is very different for N,O: the equilibrium N,O concentration at the same temperature is virtually zero, so N,O destruction is thermodynamically favoured. The high N,O emissions observed in practice imply limitations associated with reaction kinetics. Unlike the case of NO, the residence time in an FBC is apparently insufficient for N,O to approach its equilibrium concentration. Raising the temperature could improve N,O destruction kinetics, but the effect on NO would be adverse. Therefore, catalytic enhancement of N,O decomposition should be considered an attractive option.

control:

N,+OH

(4)

N,O + OH-

N, + HO,

(5)

N,O+M-

N,+O+M

(6)

where M stands for a gas molecule. Heterogeneous destruction of N,O can also be significant, although the contribution of this effect certainly depends on the concentration of particles. In principle, given enough residence time in a combustor, N,O would eventually reach its equilibrium concentration, which happens to be nearly zero. Since the residence time is usually fixed, temperature is clearly the most important variable that controls the rate, and thus also the extent, of N,O decomposition in the reactor. NO emission, however, seems to be thermodynamically limited by the equilibrium concentration, which increases with temperature. It is well documented that sulfur retention in fluidizedbed combustion reaches a maximum at 1073-1223 K19v20. Several explanations have been postulated, ranging from structure impairment (calcination, sintering, pore plugging, sulfation, annealing) to the competition between sulfation and thermal decomposition of CaSO,.

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In summary, the temperature range 1073-1223 K seems to be favourable for FBC operation, owing to a relatively low NO equilibrium concentration and optimum sulfur retention. Within this interval, temperature can be used to alter the selectivity with respect to NO and N,O, without substantial change in the combined NO-N,0 level.

NO from HCN: NO+CO

NCO+ONC0 + OH-

SO,-catalysed radical recombination to the following mechanism29~30:

interaction

among N20, NO and SO2

De Soete” suggested that NO, may interact with SO, in an FBC in a way similar to that responsible for the N,O sampling artefact. So far, very little has been done to explore this possibility. Some preliminary data have been collected22*23 which show that introducing SO, into a laboratory-scale fluidized bed (fired with methylamineor pyridine-doped propane) causes redistribution of nitrogenous combustion products. Less NO and more N,O are formed when SO, is present in the system. A similar result was reported by Amand et aLZ4 for SO2 injection into a circulating fluidized-bed combustor (CFBC). The effect of SO2 on NO is not surprising, in view of the results of premixed flame experiments carried out in the 1970s and early 1980s. For example, Wendt and Ekmann2’ used a methane-air flat flame doped with SO, or H,S to show that sulfur inhibits the formation of NO. They attributed this result to catalysis of oxygen atom recombination reactions by SO, and concluded that fuel desulfurization might lead to increased NO, emissions. Tseregounis and Smith2’j studied fuel-rich, premixed hydrogen and acetylene flames and concluded that the effect of sulfur compounds on NO occurred via direct or indirect interaction with NH, radicals. Corley and Wendt27 used premixed CH,-He-O, flames and found that SO2 injection resulted in reduced NO, N, and NH, levels, whereas the concentration of HCN increased in the post-flame region compared with the reference case. This effect was explained by the interaction of SO, with the cyanide and amine subsystem. In a different study, in which CO-Ar-0, flames were used, Wendt et aL2’ found that the effect of sulfur was to decrease post-flame NO levels and to increase the N, concentration. Modelling, which involved over 60 reactions, showed that the most plausible effect of SO, was to increase the steady-state concentration of nitrogen atoms, and consequently to enhance N, formation. This occurred via direct interactions between N, NO, S and SO. The presence of SO, may significantly reduce the pool of H, OH and/or 0 radicals that are necessary to form NO from NH,- and HCN-related species. Some of these formation routes are outlined below:

(14) proceeds according

HSO, + M

H+S02+M-

Gas-phase

(13)

NO+CO+H

(15) where M represents a gas molecule. Reaction (15) is followed by the rapid bimolecular steps H+HSO 2-H2

+SO,

(16)

OH + HSO ,-H,0+S02

(17)

In the above scheme, HS02 clearly acts as a scavenger of H and OH radicals, which may be at least partly responsible for a decrease in NO levels upon an increase in SO, concentration. Another way in which SO, may reduce the NO concentration is through the effect that radicals have on the oxidation of carbon monoxide. CO is well known to be instrumental in NO reduction to N, on the surface of char31-33: NO + CO-? char IN, + CO,

(18)

Certain radicals, such as 0, OH, or H02, participate in CO oxidation to CO, according to the following reactions: CO+O+MCO + OHCO+HO

,-CO,

CO,+M

(19)

CO,+H

(20)

+ OH

(21)

A decrease in the radical inventory caused by SO, results in the inhibition of CO oxidation and more efficient NO reduction. The effect of SO, on N,O emission is most probably indirect. Increased N,O levels during SO2 injection may result from a reduced pool of radicals which are instrumental in N,O destruction by reactions (4) and (5). In summary, the lowering of SO, levels in a combustor, which may be a result of improved sulfur removal, usually results in higher NO and lower N,O emissions. This effect seems to be much stronger in the case of NO. A schematic representation of the influence of SO, on NO and N,O levels is shown in Figure 3.

I

I co\ I

generation of N-radicals: NH, + OH -NH,+H,O

(7)

NH, + OH-

(8)

NH + H,O

NH+H-N+H,

(9)

NO from NH,: N+OH-

NO+H

(10)

NHtO-

NO+H

(11)

generation of NCO: HCN + O-

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(12)

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Figure 3 Effect oTlowering SO2 level (e.g. by improved sulfur capture) on NO and N,O emission: - indicates causality, ? an increase and 1 a decrease in concentration of a given species

Al20 emission Air staging and excess air

Air staging is a technique used to reduce NO, levels, but its effect on N,O emissions is unclear. In some studies, no significant change in N,O levels was observed upon staging14. In others, a decrease in N,O was reported, together with NO, reduction34. Air staging was also found to increase N,O emissions slightly in a pressurized fluidized-bed combustor (PFBC)35. It was recently shown for a CFBC that N,O emission could be reduced if the secondary air inlet was positioned sufficiently high in the combustor36. Reduction of excess air has usually been reported to result in decreased N,O and NO, levels14,34.35.37. It should be borne in mind that changes in air staging or excess air are difficult to implement without affecting other important combustor operating characteristics, notably char concentration and temperature distribution along the reactor. This was demonstrated by Amand and Leckner3*, who related N,O-NO emission levels to changes in boiler operating conditions induced by varying the excess air. For example, depending on whether changes in the excess air ratio were implemented by changing the fuel feed rate or by changing the air flows and fuel feed rate, the NO levels could exhibit opposite trends in the same CFBC operating under otherwise identical conditions. Evidence was also provided of the existence of an air:fuel ratio effect independent of the influence of temperature. The authors concluded that the beneficial effect of low oxygen concentration on N20 emission was even stronger than that for NO. Excess air has only a minor effect, if any, on SO, formation as long as enough oxygen is present to oxidize coal-bound sulfur. Under extreme oxygen-lean conditions, a transition to coal gasification occurs and H,S becomes the main sulfur-containing product. The effect of excess air on sulfur capture by limestone has been studied in a bench-scale fluidized bed under both super- and substoichiometric conditions”. It was found that sulfur retention decreased from 75 to 67% as the oxygen concentration decreased from 5.6 to 0.7 vol.%. The same trend was observed when the air:fuel ratio was reduced to 0.75. For air:fuel ratios ~0.75, sulfur retention unexpectedly increased, which was explained by a transition from SO, to H,S as a main combustion/ gasification product, with subsequent H,S capture by CaO to form CaS. CaS is an unstable solid product of sulfur capture: it readily reacts with oxygen and decomposes to form SO, and CaO. In general, sulfur retention by limestone is more favourable under oxygenrich conditions and may be adversely affected by air staging. Limestone injection

Limestone addition to an FBC system has been shown to reduce N,O emission and to increase that of NO’4*“9,40. Figure 2 shows conversions of fuel-bound nitrogen to NO and N,O with and without limestone. The effect of limestone on N,O levels seems to be complex and is governed mainly by heterogeneous catalysis, gas-phase N,O-NO,-SO, interactions, and reactions involving NH, and HCN. CaO-catalysed N,O decomposition is certainly a decisive factor in reducing the N,O concentration. Further lowering of N,O may result from reduced SO, levels, as discussed above. Yet it has been observed that limestone addition usually results in only a moderate decrease in N,O levels,

control:

M. A. Wdjtowicz et al.

typically by G30%40. This modest effect of limestone on N,O is surprising, in view of the highly favourable kinetics of CaO-catalysed N,O decomposition41-43. One reason may be the fact that part of the limestone present in a combustor is sulfated and thus relatively inactive4’. In addition, sulfated limestone is a potential catalyst for NO reduction, with N,O formed as a product. Under oxygen-lean conditions, this source of N,O may become important, relative to N,O formation from char-N2’. Furthermore, N,O can be formed as a by-product of CaO-catalysed NH, oxidation to N043. Steady-state formation of N,O has also been observed over a CaO surface in the simultaneous presence of HCN and NO as well as HCN and 0243. The net result of the entire scheme depends on the fuel and combustion conditions, but an overall reduction in N,O levels is the usual outcome of limestone addition. Increased emissions of NO due to limestone injection result from several effects. First, catalytically enhanced oxidation of NH, to NO has been well documented43. Second, limestone addition leads to a decrease in SO, levels, which in turn has been shown to bring about an increase in NO emission. Amand et ~1.~~ and DamJohansen er ~1.~~indicate that CO may also be implicated in the reaction scheme, perhaps in more than one way (Ca-catalysed CO oxidation and a gas-phase interaction with SO,). Finally, increased NO levels resulting from limestone addition may be partly due to the CaOcatalysed conversion of HCN to NH345. Such an HCN-NH3 redistribution would also contribute to a reduction in N,O levels. Recent work by Hulgaard* shows that NO reduction may also occur on limestone, especially when SO, is present. This is supported by the experiments by Lyngfelt and Leckner46, who reported NO reduction on sulfated limestone (CaSO,/CaS) under oxygen-lean conditions46. In summary, N,O levels usually decrease, whereas those of NO increase, when limestone is added to the combustion system. The effect seems to be stronger for NO than for N,O. NO, control by injection of additives (NHS, urea, etc.)

Injection of ammonia, urea or cyanuric acid is one of the measures for reducing NO, emissions; this technique is often termed selective non-catalytic reduction (SNCR). In general, SNCR involves NO, reduction to N, and H,O in the temperature region 1173-l 373 K. Such a high temperature is necessary to ensure that NH, can be formed from NH,. A similar method of NO, abatement combines ammonia injection into the flue gas with the use of a solid catalyst (typically V,O, and TiO,). The process is known as selective catalytic reduction (SCR) and is normally carried out in the temperature range 573-673 K. A general problem associated with both SNCR and SCR is the emission of excess ammonia into the atmosphere, so-called ammonia slip. Ammonia storage, handling and transport may pose additional difficulties, as most countries require special conditions to be met. Therefore the use of urea is often preferred, especially in small-scale applications. A detrimental effect of SNCR on N,O levels has been reported’4,47,48, as discussed in detail by Hjalmarsson49. In general, the increase in N,O depends on the temperature, the additive used and its feed rate, the location of the injection point, and the efficiency of NO, reduction. The increase in N,O emissions due to SNCR

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(AN,O) is roughly correlated with the amount of NO, reduced by this process (ANO,). The value of the ratio AN,O/ANO, has been reported at between 5 and 50%49. There has been concern about possible formation of N,O due to SCR, but the available commercial-scale data show that this does not occur4*.

reduced emissions of the others; this should be combined with control measures taken against excessive levels of the selected pollutant. Two examples of such an approach are outlined below. In both cases, limestone injection is used as an SO, control measure.

(4 N,O

EMISSION

CONTROL

In view of the existence of interactions among N,O, NO, and SO,, as well as among some of the known pollution control technologies, an integrated approach to N,O-NO,-SO, abatement is needed. The following measures are suggested:

combustor

design to produce

low-emission

high

control}

(after

high temperature (~1173K)

low N,O, high NO, (most of fuel-N converted to NOJ

l

limestone addition (Ca/S = 3)

low SO,, low N,O,

low excess air (- 1.2 may be possible owing to improved process control)

low N,O, (approach

0

a gas afterburningin

high

NO,

the

low NO, (1) used)

low N,O

cyclone

Post-combustion measures are also a possibility but should perhaps be limited to situations in which they are inexpensive and easy to install or retrofit. It is a great advantage of fluidized-bed combustion that pollution control is performed in situ, with no need for postcombustion flue-gas treatment. The advantages of approach (1) are its relatively low cost and the fact that it requires only minor modifications to existing plant. Although highly recommended, it is unlikely to bring all emissions below acceptable levels. New processes may have to be devised in which knowledge of pollutant formation and destruction is judiciously applied to produce low-emission combustors (approach (2)). It may prove advantageous to adopt the policy of sacrificing low emissions of one pollutant for the sake of

a ammonia

injection

(SNCR)

Q-V{Low

NO,,

limestone/catdyst

oxygen-rich itage tertiary air

&..-J -____. -----.

Figure 4 Schematic operating according description)

representation to pollution

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of a low-SO,-NO,N,O combustor control scheme (B) (see text for

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9

NO, control (ammonia slip? increased N,O?)

high NzO} + {N20

control}:

l

low temperature (- 1073 K)

low NO,, high N,O (most of fuel-N converted to N,O)

l

two-stage operation (O,-lean and O,-rich)

low NO,

0

limestone addition only in the O,-rich section

low SO,, low NO, (no catalytic enhancement of NH, oxidation during devolatilization, no catalytic conversion of HCN to NH,, HCN reacts to form N,O rather than NO,, high SO, in O,-lean section), high N,O in O,-lean section, lower N,O in O,-rich section

catalytic N,O decomposition

N,O

control

afterburning

N,O

control

fluegas

1420

NO,} +(NO,

0

(1) minimization of emissions through improvements in operating conditions and process control of boilers (improved combustion efficiency); (2) innovative systems.

{Low N,O, Leckner5’):

schematic representation of a combustor operating according to scheme (B) is shown in Figure 4. The first, fuel-rich, stage of combustion takes place in a bubbling fluidized bed with an exceptionally short freeboard. Coal is continuously supplied to this part of the system and no limestone is added at this stage. The second combustion stage is divided from the first by a weir, over which char particles are entrained into the oxygen-rich environment. The sand particles used as the bed material are too heavy to spill over the weir, however, and stay in the bubbling bed. The whole combustion arrangement operates according to the principle of staged combustion, with secondary air introduced at the bottom of the second stage. This part of the system functions as a CFBC, with particles of the bed material, limestone and a suitable N,O decomposition catalyst recycled through a cyclone. In the embodiment shown in Figure 4, limestone and the catalyst are introduced with tertiary air through tangential ports situated along the height of the CFBC. The highly turbulent swirl generated by such an arrangement is an optional feature of the system; the

N20

tertiary air is likewise optional. In fact, solids can be introduced at any convenient place in the second stage of the combustor. The fuel for reburning can be supplied either to the cyclone or to the upper part of the CFBC (not shown in Figure 4). The main features of this combustion system are that (i) the circulation of limestone is restricted only to the second, oxygen-rich, stage, and (ii) catalytic N,O decomposition and gas reburning (a high-temperature, radical-rich zone) are used as N,O control measures. There are a number of advantages associated with the above arrangement. Because limestone is excluded from the fuel-rich zone in which most of the coal devolatilization occurs, ammonia oxidation to NO is not catalytically enhanced by CaO; neither is HCN conversion to NH, catalysed. The bubbling FBC operates at the lower end of the usual temperature range (i.e. m 1073-l 123 K), which favours formation of N,O rather than NO. High concentrations of sulfur as well as an oxygen-lean environment also help to keep NO, levels low. Finally, it may prove advantageous to add to the bubbling bed a catalyst promoting NO-to-N, and/or NO-to-N,0 conversion (e.g. completely sulfated limestone could be used, provided that the combustion conditions are such that the conversion of CaSO,/CaS to CaO is negligible). By the time the gas reaches the second, oxygen-rich, zone of the system, most of the volatile nitrogen has already reacted to N,O or N,. In this way, the prevailing conditions in the bubbling bed are low-NO,, high-N,0 (and high-SO,), in accordance with strategy (B) outlined above. The function of the second stage of the system is (i) to complete the combustion of char and volatiles and (ii) to provide adequate SO, and N,O control, without sacrificing the already low NO, levels. The latter should pose no problems, as it is well known that N,O is not readily oxidized to NO, even under strongly oxidizing conditions5 ‘. Gustavsson and that gas afterburning in a Leckner52,53 demonstrated CFBC cyclone successfully reduced N,O levels by -40%. No detrimental effect on NO emission was observed, and a significant decrease in CO levels was also reported. Catalytic decomposition ofN,O has also shown much promise for N,O abatement41*42. Because of the high temperature in the second combustion stage, it is expected that relatively inexpensive catalysts can be used. The strongly oxidizing environment of the second stage also creates favourable conditions for efficient SO, capture. The above examples illustrate how N,O abatement can be incorporated into the framework of existing SO, and NO, control measures. More work in this area is certainly needed.

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Improvements in operating conditions, start-up procedures and process control are examples of low-cost measures. These are unlikely, however, to satisfy the growing need to meet increasingly stringent emission standards. Innovative combustor design is required to produce new low-emission combustion systems. It may prove convenient to use temperature to improve the selectivity of coal-N conversion to NO or N,O. In this way, emissions of only one of these pollutants would have to be controlled. It should be kept in mind, however, that for practical purposes, temperature variation in an FBC is limited. with fuel The lower limit of - 1073 K is associated ignition, incomplete combustion and poor sulfur capture, the upper one (- 1223 K) with high NO, levels, melting of ash and again inefficient sulfur removal. Yet this range of temperatures is wide enough to provide a variation in NO/N,0 levels of at least an order of magnitude. In addition to temperature, the most promising N,O control measures seem to be gas afterburning and solid-catalysed N,O decomposition. More research is needed on ways of increasing overall coal-N conversion to N, rather than NO,/N,O.

ACKNOWLEDGEMENT Financial support by the Commission Communities through grant JOUF gratefully acknowledged.

of the European 0047-C(SMA) is

REFERENCES 1

2

3 4 5

6 1

8

9

10

CONCLUSIONS No N,O control technologies are available at the moment, and the following guidelines for their development are proposed. In view of the existing interactions among NO,, SO, and N,O, as well as among the techniques for their abatement, an integrated approach to pollution control is advocated rather than treatment of each pollutant separately. Since there is considerable experience with successful NO, and SO, control in fluidized-bed combustion (staged combustion, ammonia injection, limestone addition), it seems reasonable to make N,O control compatible with the existing technologies.

emission

I1

12 13

14

15

Houghton, J. T., Jenkins, G. J. and Ephraums, J. J. (Eds). ‘Climatic Change: the IPCC Scientific Assessment’, Cambridge University Press, 1990 Levine, J. S. In Proceedings of the Fifth International Workshop on Nitrous Oxide Emissions (NIRE/IFP/EPA/SCEJ), Tsukuba, 1992, p. KL-1-l Sloss, L. L. ‘NO, emissions from coal combustion’, Report IEACR/36, IEA Coal Research, London, 1991 Wayne, R. P. ‘Chemistry of Atmospheres’, Oxford University Press, 1985, pp. 113-173 Levine, J. In ‘EPA/IFP European Workshop on the Emission of Nitrous Oxide from Fossil Fuel Combustion’, EPA-600/989-089, US Environmental Protection Agency, 1989, p. 11 Wojtowicz, M. A., Pels, J. R. and Moulijn, J. A. Fuel Process. Z&to/. 1993, 34, I De Soete, G. G. In Proceedings of the LNETI/EPA/IFP European Workshop on the Emission ofNitrous Oxide, LNETI, Lisbon, 1990, pp. 4145 Hulgaard, T. ‘Nitrous Oxide from Combustion’, Ph.D. thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby, 1991 Andersson, C., Brannstrom-Norberg, B.-M. and Hanell, B. ‘Nitrous Oxide Emissions from Different Combustion Sources, Report no. U(V) 1989/31, Vattenfall (Sweden), 1989 Gavin, D. G. and Dorrington, M. A. In Proceedings, 1991 International Conference on Coal Science, ButterworthHeinemann, Oxford, 1991, pp. 347-350 Wojtowicz, M. A., Pels, J. R. and Moulijn, J. A. ‘The trade-off between NO, and N,O formation in fluidised-bed combustion of coal’, Paper to Conference ‘NO,r: Basic Mechanisms of Formation and Destruction, and their Application to Control _. Technologies’, London, 1993 Aho, M. J. and Rantanen, .I. T. Fuel 1989. 68. 586 Amand, L. E. and Andersson, S. In Proceedings, 10th International Conference on Fluidized Bed Combustion, Vol. 1, ASME, New York, 1989, pp. 49-56 Hiltunen, M., Kilpinen, P., Hupa, M. and Lee, Y. Y. In Proceedings, 1 Ith International Conference on Fluidized Bed Combustion, ASME, New York, 1991, pp. 687-694 Wojtowicz, M. A., Oude Lohuis, J. A., Tromp, P. J. J. and

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20 21 22

23

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