Ind. Eng. Chem. Res. 1996,34, 1419-1427
1419
Abatement of N20 Emissions from Circulating Fluidized Bed Combustion through Afterburning Lennart Gustavssont and Bo Lecher* Chalmers University of Technology, Department of Energy Conversion, S-412 96 Goteborg, Sweden
A method for the abatement of N20 emission from fluidized bed combustion has been investigated. The method consists of burning a secondary fuel after the normal circulating fluidized bed combustor. Liquefied petroleum gas (LPG),fuel oil, pulverized coal, and wood, as well as sawdust, were used as the secondary fuel. Experiments showed that the N2O emission can be reduced by 90% or more by this technique. The resulting N20 emission was principally a function of the gas temperature achieved in the afterburner and independent of afterburning fuel, but the amount of air in the combustion gases from the primary combustion also influences the results. No negative effects on sulfur capture or on NO or CO emissions were recorded. In the experiments, the primary cyclone of the fluidized bed boiler was used for afterburning. If afterburning is implemented in a plant optimized for this purpose, a n amount of secondary fuel corresponding to 10% of the total energy input should remove practically all N2O. During the present experiments the secondary fuel consumption was greater than 10% of the total energy input due to various losses.
1. Introduction The rate of increase of the concentration of nitrous oxide (N2O) in the atmosphere is 0.2-0.3% annually. It is estimated that this continuous increase is caused by human activities, principally from agriculture and some industries. The impact of N2O is similar to that of C02 in enhancing the greenhouse effect but less important, and N2O is assumed to contribute together with chlorofluorocarbons to depletion of the stratospheric ozone layer. The subject of N2O emissions, including those originating from combustion, has been treated in a series of international workshops, the latest of which was held in Turku (1994). A few years ago, combustion of fossil fuels in stationary plants was believed to be a major source of anthropogenic N2O. After an error of measurement had been discovered, this assessment was reevaluated, and it is now concluded that combustion in stationary plants contributes insignificantly t o the anthropogenic production of N2O. Atmospheric fluidized bed combustion (FBC) differs from other combustion methods by giving a higher N2O emission, that is, an emission on the order of 50 to 150 ppm. The reason why the contribution from FBC to the global N20 production is still small lies in the small number of plants. Due to other environmental advantages of fluidized bed combustion, such as low NO, emissions and the possibility of sulfur capture through lime addition to the bed, it is probable that the number of FBC plants will increase significantly in the future. Should this be the case, then the N2O emission from FBC will become of significance for the global balance of N20, unless something is done to decrease the emission. Due to the small number of plants there is no immediate need for regulations concerning a maximum level of N2O emission, so there is no direct commercial incentive to develop methods for reduction. On the other hand, as long as FBC is successfully introduced commercially and as long as the impact of N2O on global + Swedish National Testing and Research Institute, Box 857, S-50115 BorAs, Sweden.
warming and ozone layer depletion is not re-evaluated, the properties of formation and destruction of N20 under FBC conditions should be further investigated in order t o gain knowledge of methods to decrease the emission. The usefulness of such methods can then be evaluated. The purpose of this work is to investigate a method of decreasing the N20 emission from fluidized bed combustion. The method involves burning of a secondary fuel in order t o raise the temperature of combustion gases, preferably in a separate combustion chamber after the normal combustor. Preliminary investigations have shown that this method, using liquefied petroleum gas (LPG) as an afterburning fuel, can reduce the N20 emission by 60-95%, depending on boiler operating conditions (Leckner and Gustavsson, 1991; Gustavsson and Leckner, 1992). In the following,results from a fullscale investigation also concerning other afterburning fuels are presented. A general characterization of afterburning as an N2O abatement technique is also made. 2. NzO Emissions from FBC
Recently the subject of N2O emissions has been treated in a number of reviews: Mann et al. (1992), Hayhurst and Lawrence (19921, and W6jtowicz et al. (1993). On the basis of these reviews a short summary will be given here about N20 emissions with emphasis on atmospheric circulating fluidized bed (CFB) combustors. The fuel nitrogen, which is the source of NO, and N2O emissions under FBC conditions, is found in the volatiles released and in the fuel char. N2O is formed by homogeneous reactions mostly from hydrogen cyanide (HCN), whereas the other principal volatile nitrogen compound, ammonia (NHs), to a large extent is converted into NO. N20 is also produced during combustion of char. Destruction of the N20 formed occurs by thermal decomposition, by homogeneous reactions with hydrogen or hydroxyl radicals, or on surfaces of various kinds which are present in the fluidized bed. Calcium oxide and char are known to be the most reactive agents of
0888-5885/95l2634-1419$09.00l0 0 1995 American Chemical Society
1420 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
decomposition, but ashes from the fuel and even quartz sand, sometimes used as a bed material, contribute slightly. The reviews quoted show that a considerable knowledge has been gained concerning various reaction mechanisms occurring in an FBC. However, it is difficult to translate this knowledge to explain with certainty what happens in an FBC. Instead, empirical information has t o be used: The N20 concentration increases rapidly in the bottom part of the combustor and attains a maximum at high levels in the combustor. This means that the N2O emission is rather sensitive to the conditions of the upper part of a combustor such as bed temperature and air supply. A listing of the most important parameters that influence the N2O emission from atmospheric CFB boilers (Leckner and Amand, 1992a,b)is as follows. (1) Fuel: Fuels with a high content of fixed carbon (coke, bituminous coals) yield relatively high emissions of N2O (100-150 ppm), whereas the emission is lower for lignites and peat and negligible for wood. (2) Bed temperature: The emission decreases with increasing bed temperature, and at temperatures above 950 "Cit is almost eliminated. (3) Excess air: The level of excess air is important. The Nz0 emission decreases with a reduction of the excess air. This is seen also in the results presented below. (4) Air supply: The emission is reduced if combustion is partly carried out under substoichiometric conditions, such as in staged combustion. (5) Bed material: Normal addition of limestone for sulfur capture has been shown to give about a 20% reduction in the N20 emission compared to a similar case with a bed of silica sand and ashes. In some tests on boilers, however, no influence of limestone addition has been noted. These dependencies offer possibilities to decrease the NzO emission. The choice of fuel has a significant effect but is in most cases determined by other considerations than N2O. The bed material is also not freely chosen; it is often a result of the fuel used and of limestone addition for sulfur capture. The remaining factors, bed temperature and air supply, can be altered, but there are also drawbacks with such an approach. Staging of the air supply and a temperature rise may seriously affect the NO emission and the sulfur capture, as shown by Lyngfelt and Leckner (1993). There may be possibilities to avoid this inconvenience (Lyngfelt et al., 19951, but in general, any method to reduce the N2O emission involving the combustion chamber seems to affect either the NO and SO2 emissions or the combustion efficiency. A straightforward way to avoid a negative influence on the processes in the fluidized bed combustor would be to maintain normal conditions in the combustor but to raise the temperature of the combustion gases after the fluidized bed combustor in order to take advantage of the temperature dependence of the N2O destruction reactions. This could be done in a separate combustion chamber after the normal CFB combustor. The purpose of the present investigation is to study the performance of such an additional combustion chamber for N20 reduction. 3. Experimental Section
T h e Boiler. The experiments were carried out in the 12 Mw circulating fluidized bed (CFB) boiler at Chalmers University of Technology. The boiler is extensively equipped with measuring systems, including two sets
Figure 1. The cyclone part of the 12 MW CFB boiler at Chalmers University of Technology. Additional fuel was injected through nozzles at A. Additional air for burn-up was supplied in the cyclone outlet.
of gas analyzers ( 0 2 , CO, NO, N20, and Sod. One set of analyzers was connected to a probe in the stack (downstream of the baghouse filter). The other set was supplied with flue gas from a water-cooled probe placed just upstream of the fuel injection points. There are temperature sensors in the bottom bed region of the combustor, in the top region, in the cyclone inlet, and at three points along the duct after the cyclone outlet. The boiler and the measuring equipment have been described in more detail elsewhere (Gustavsson and Leckner, 1992). The boiler has no separate combustion chamber for afterburning of a fuel to raise the temperature of combustion gases. In the absence of such an arrangement, fuel was injected in the entrance of the primary cyclone and the cyclone was used as a combustion chamber for afterburning. The Spectran 647 N20 analyzer, which was used, is known to have a certain cross-sensitivity to methane (Hulgaard et al., 1989). Since the presence of methane could not be excluded in the cyclone inlet and in the stack gases at low excess-air ratios, gas samples were taken in Tedlar bags for subsequent analysis with gas chromatography. When significant methane contents were found, the analyzer readings were corrected according to Hulgaard et al. (1989). In the same way, the CO analyzer used for stack measurements was known to be cross-sensitive to N2O. The readings were corrected according to a calibration carried out during earlier investigations. All measured gas concentrations given in the diagrams and in the text are converted to those of flue gases containing 6%0 2 (dry gas) by means of the local 0 2 concentration. The Fuel Injection Systems. The experiments were run with five injection fuels: liquefied petroleum gas (LPG), pulverized coal, pulverized wood, sawdust, and fuel oil. The fuel was introduced at three points into the entrance duct between the combustor and the cyclone (A in Figure 1). At this position the duct is 1.6 m high and 0.8 m wide. In the case of LPG injection, three burner heads were used, with the tips placed about 100 mm from the duct
Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1421 Table 1. Characteristics of the Iqjection Fuelsa
moisture, % ash, % (dry basis) carbon, % (am0 hydrogen, % (am0 oxygen, % (am0 nitrogen, % (am0 sulfur, % (am0 net calorific value, M J k g (am0
pulverized icoal
pulverized wood
2.2 9.3 83.1 5.4 8.8 1.4 1.3 33.0
6.2 0.3 50.7 6.3 42.3 0.3 0.03 19.0
Table 2. Properties of Primary Fuels
sawdust fuel oil 13.1 0.4 50.7 6.4 42.8 0.1 0.02 19.1
0 0 87 13 0 KO.1
co.01 42.8
a amf = ash and moisture free basis. Liquefied petroleum gas (LPG): ethane < 2.0 vol %, propane > 95.0 vol %, butane < 5.5 vol %, net calorific value 46 MJkg, density 2.02 kg/m3,.
wall. The total capacity was about 1.8 MW. The capacity could be varied continuously down to zero. The gas pressure before the burner heads was about 1 bar gauge. Gas was supplied from pressure cylinders via an evaporator and a gas train with measuring and regulating devices. The solid fuels (pulverized coal, pulverized wood, and sawdust) were fed from a container with a variable speed screw feeder through hoses to three tubings (Q, = 65 mm) mounted in the duct wall. No special injection nozzles were used. The fuel transport from the feeder into the cyclone entrance duct was accomplished by the negative pressure in the duct. Different fuel flows were set by varying the speed of the screw feeder. The fuel container was mounted on load cells, so that the fuel consumption could be followed. The fuel oil was injected through nozzles which were mounted in the three 65 mm tubings. The oil supply was a two-pipe system, and by varying the pressure on the return side of the nozzles, different amounts of fuel oil could be introduced into the cyclone entrance duct. The fuel oil consumption was determined by flow meters, mounted in the feed and return lines of the system. No air was added a t the location of the fuel injection; instead, the oxygen content of the gases leaving the combustion chamber was used for the combustion of the fuel. Only in the cases of solid injection fuel, was a small leakage of air in the screw feeder employed to facilitate the fuel transport into the duct. In certain cases, when the boiler was operating at low excess-air ratios, additional air was introduced downstream of the cyclone outlet (see Figure 1)in order to achieve burnout. The characteristics of the injection fuels are given in Table 1. The particle size distribution of the pulverized coal was such that 31%by weight had a diameter below 0.18 mm and 77% by weight had a diameter below 0.7 mm. For the pulverized wood, 90% by weight had a diameter below 1 mm. The Experimental Program. The experiments comprised a study of the influence of bed temperature, excess-air ratio, and injection-fuel ratio (IFR, defined as the ratio of injected fuel power and primary fuel power) on N2O reduction by afterburning. In addition, the effects on NO, CO, and SO2 emissions were measured. Normally the tests were run on a bed consisting of silica sand and fuel ashes with no limestone added for desulfurization. The primary fuel was bituminous coal with a low sulfur content. High-sulfur coal (HS) was burned, and limestone was added in a few runs. Due to the extended time schedule of the experiments,
moisture, % ash, % (dry basis) carbon, % (amf) hydrogen, % oxygen, % (am0 nitrogen, % (am0 sulfur, % (am0 net calorific value, M J k g (am0
(am
LS1
LS2
HS
8.0 11.8 81.1 5.1 11.7 1.4 0.7 31.8
14.2 7.4 80.3 5.4 12.0 1.5 0.8 31.6
9.8 7.6 84.7 5.9 5.1 1.6 2.7 31.8
low-sulfur coal from two lots (LS1 and LS2) had to be used, but the differences in main characteristics were small, as shown in Table 2. Runs were carried out at bed temperatures of 850 and 900 "C and a t 0 2 contents in the flue gases from the combustor of approximately 5%, 3.5%, and 2%. The boiler load was 8 M W , and the operation was normal for commercial CFB boilers. In each case, three tests were run at different injection fuel ratios in addition to the reference test without fuel injection. Since injection of LPG at, for example, IFR = 10% corresponds to a decrease of 0 2 in the flue gases of about 1.3% units, additional air was introduced after the cyclone outlet in almost all of the cases of 0 2 = 3.5% and 2% to avoid excessive CO levels in the flue gases. The maximum injection fuel ratios for LPG, fuel oil, and pulverized wood were about 18%. For pulverized coal the maximum IFR was about 30%. Samples of the material separated in the secondary cyclone and in the baghouse filter were taken at the highest IFR ratio in each series for subsequent analysis of combustibles.
4. Results
Emission Characteristics of the Boiler. A background t o the results from afterburning is given by the emission characteristics of the boiler without afterburning. The most important parameter influencing the emission is combustor temperature. CFB boilers, like the present one, are controlled by the bottom bed temperature. Normally, the top temperature of the combustion chamber is about the same as the bottom bed temperature. In the present boiler differences in the excess-air ratio and the flue gas recirculation ratio caused the top temperature to deviate, normally less than f5 "C or at most f 1 0 "C, from the bottom bed temperature. The cyclone inlet temperature, in turn, was 1-5 "C lower than the combustor top temperature. For the purpose of the present work these variations are small and bottom and top temperatures will both be used to characterize bed temperature. Figure 2 shows NzO emissions for the reference cases without fuel injection, as a function of combustor top temperature. It is evident that the temperature of the combustor strongly influences the N20 emission. An increase in (bottom) bed temperature from 850-900 "C reduces the N20 emissions from about 150 ppm (6%0 2 ) to about 90 ppm (6% 02). The dependence on the top temperature is even stronger. Also the dependence of N2O emission on excess-air ratio is confirmed in Figure 2 and explicitly shown in Figure 3. A lower excess-air ratio significantly reduces the N20 emission. Figure 4 shows NO emission as a function of the combustor top temperature. Clearly, an increase in the combustor temperature leads to an NO emission which increases by the same order as N2O is decreasing. A lower excess-air ratio reduces the NO emission.
1422 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
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To complete the picture, the CO emission also has to be considered. A n increasing combustor temperature is beneficial for the reduction of the CO emission. The reduction of the excess-air ratio down to 0 2 = 2% in the combustor did not lead to an increased CO emission. The correlations given above are based on data from experiments without limestone addition. In principle they are valid also for lime-in-bed conditions. In this
Figure 5. NzO emission vs oxygen content in flue gases during ahrburning with LPG.Excess-air ratios (expressed as 0 2 concentration) refer to values before afterburning. In cases with 0 2 = 3.5% and 2 6 , additional air was introduced after the cyclone.
case, however, the negative influence of a rise in bed temperature and a reduction of the excess-air ratio on sulfur capture must also be considered. This, as well as an increasing NO emission with bed temperature, confirms the drawbacks of a simple in-bed NzO reduction strategy (which would involve an increase in the bed temperature and/or a reduction of the air supply). Afterburning. The ceramic lining of the cyclone walls is cooled by water tubes. This cooling, as well as heat losses to the surroundings, causes a significant temperature drop between the cyclone inlet and the cyclone gas outlet. Without additional fuel this drop amounted to around 45 and 55 "C a t bed temperatures of 850 and 900 "C, respectively,both at 5%0 2 . The drop was slightly lower a t lower excess-air ratios. During fuel injection the cyclone outlet temperature rises as a result of the combustion of the fuel injected, the IFR. The rise in temperature is also influenced by various losses to be discussed later on. In the following, results are presented from injection of pulverized coal, pulverized wood, sawdust, and fuel oil and, for comparison, from LPG. The results are generally shown as NzO emission versus cyclone outlet temperature. The combustion of additional fuel in the cyclone and the corresponding rise in temperature may affect the other emissions, NO, SOZ, and CO. The resulting emissions of these species are also shown as a function of the cyclone outlet temperature. NzO Emission. In Figure 5,the Nz0 emission as a function of the cyclone outlet temperature is shown for afterburning with LPG. The bed temperature, which is principally the same as the cyclone inlet temperature, was 850 "C. As fuel was injected into the cyclone inlet duct, the cyclone outlet temperature rose. This was accompanied by a steep decrease in NzO emission, so that at a cyclone temperature of 950 "C only a very small N2O emission remained. At lower excess-air ratios this low N2O value was achieved at lower cyclone outlet temperatures. Typically, an LPG injection giving a cyclone outlet temperature of 950 "C resulted in N2O emissions decreasing from about 150 ppm without injection of fuel to below 30 ppm when the boiler was operated at 5% 0 2 and below 10 ppm when operating at 3.5% or 2% 0 2 . An increase in the cyclone outlet temperature of 100 "C required an IFR of about 12%,
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1423 200
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