Energy & Fuels 2006, 20, 1879-1885
1879
Enhanced NOx Reduction with SO2 Capture under Air-Staged Conditions by Calcium Magnesium Acetate in an Oil-Fired Tunnel Furnace W. Nimmo,* A. A. Patsias, and P. T. Williams Energy and Resources Research Institute, Houldsworth Building, The UniVersity of Leeds, Leeds LS2 9JT, U.K. ReceiVed January 9, 2006. ReVised Manuscript ReceiVed May 22, 2006
The technique of combustion modification by air staging (over-fire air) for the control of NOx emissions is currently implemented in many coal-fired power stations. This paper presents results from a new process involving the injection of calcium magnesium acetate (CMA), which can reduce SO2 and at the same time enhance NOx reductions above those achievable by air staging alone. The experiments were performed in a 3.5m long, horizontal tunnel furnace with an internal diameter of 500 mm operated at 80 kWth by firing gas-oil. The organic content of CMA behaves like a fuel, and the Ca content calcines principally to CaO for acid gas capture in the furnace at temperatures greater than 1000 °C. The solubility of CMA in water means that concentrated solutions can be sprayed into the furnace as a fine mist, giving the possibility of intimate mixing with combustion gases. The concentration of fuel nitrogen in the fuel could be easily modified by varying the amount of dopant (quinoline) injected into the oil feed to simulate typical levels of NOx emission. SO2 concentrations were set by injecting SO2 gas into the combustion air. NOx reduction studies were performed at staging levels which created near-burner zone stoichiometries (λnbz) of 1.18 (3% O2 dry) at 0% staging to 0.64 at 46% staging. Over this range of staging levels, the injection of CMA improved the reduction of NOx by a further 25-35% for an initial NOx level of 450 ppm at an overall stoichiometry of λ1 ) 1.18. The effect of CMA on NOx reduction was more apparent at lower levels of staging because of higher initial NOx levels. The near-burner zone (nbz) stoichiometry was 0.64 at this condition. SO2 reductions were studied up to a Ca/S ratio of 2.25, where reductions in the region of 80% were achieved for initial levels of 940 ppm.
Introduction The inherent nitrogen and sulfur contents of most fossil fuels pose serious problems for combustion plant operators trying to comply with increasingly stringent NOx and SOx emission control legislation. This has been the driving force behind the development and commercialization of NOx control methodologies involving, principally, modifications to the combustion process itself with the introduction of low-NOx burner technology that may be combined with other additional methods (overfire air, flue gas recirculation, reburning, and/or the application of postcombustion flue gas cleanup such as SNCR/SCR). Where more stringent NOx control is required to meet the targets of the Large Combustion Plant Directive (LCPD, 2001/80/EC), then a layered approach using a combination of the aforementioned technologies can be implemented. In staged-air combustion,1 the overall combustion air is split into separate streams creating locally fuel-rich zones in the pulverized fuel flame, where reduced O2 conditions lower the rates of conversion of fuel-nitrogen into NOx. Advanced forms of this technique can give reductions of up to 60%. Recently, the use of carboxylic salts of calcium has been shown to have the effect of combining NOx and SO2 reduction by the application of one sorbent/ agent.2-4 When injected into a furnace, the organic fraction of * Corresponding author e-mail:
[email protected]. (1) Staiger, B.; Unterberger, S.; Berger, R.; Hein, K. R. G. Development of an air staging technology to reduce NOx emissions in grate fired boilers. Energy 2005, 30 (8), 1429-1438.
the salt can act as a NOx reductant under fuel-staging or reburn conditions and the Ca component undergoes calcination to form CaO, which can react with SO2. Previous studies5 using calcium magnesium acetate (CMA) as a dual NOx/SO2 control medium concentrated on performance under reburning and advanced reburning conditions. The study presented in this article concentrates on the combination of CMA injection on air-staged flames to investigate the potential for further NOx reductions over and above those achieved with air staging alone while reducing SO2 emission with in-furnace capture on the solid calcareous residues. The formation of NO via fuel-N is strongly dependent on flame stoichiometry. Under fuel-lean conditions, [H] ≈ [OH] ≈ [O] and the reactions of HCN with O radicals are regarded as the rate controlling steps,6 (2) Steciak. J. W.; Levendis, Y. A.; Wise, D. L.; Simons, G. A. Dual SO2-NOx Concentration Reductions by Calcium Salts of Carboxylic Acids. J. EnViron. Eng. 1995, 121 (8), 596-604. (3) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Williams, P. T. Simultaneous Reduction of NOx and SO2 Emissions from Coal Combustion by Calcium Magnesium Acetate. Fuel 2004, 83 (2), 149155. (4) Nimmo, W.; Patsias, A. A.; Hall, W.; Williams, P. T. Characterization of a Process for the In-furnace Reduction of NOx, SO2, and HCl by Carboxylic Salts of Calcium. Ind. Eng. Chem. Res. 2005, 44, 4484-4494. (5) Nimmo, W.; Patsias, A. A.; Hampartsoumian, E.; Gibbs, B. M.; Fairweather, M.; Williams, P. T. Calcium Magnesium Acetate and Urea Advanced Reburning for NO Control with Simultaneous SO2 Reduction. Fuel 2004, 83 (9), 1143-1150. (6) Miller, J. A.; Bowman, C. T. Mechanism And Modeling Of Nitrogen Chemistry In Combustion. Prog. Energy Combust. Sci. 1989, 15 (4), 287338.
10.1021/ef060011z CCC: $33.50 © 2006 American Chemical Society Published on Web 07/04/2006
1880 Energy & Fuels, Vol. 20, No. 5, 2006
Nimmo et al.
HCN + O ) NCO + H
(R1)
HCN + O ) NH + CO
(R2)
HCN + O ) CN + OH
(R3)
with
possible that these intermediates form the bulk of the CHi radicals required for NO reduction and may participate in reaction (R11). An overall schematic diagram of the CMA principal CMA chemistry is shown below.
being significantly slower in rate. Followed by
NCO + O ) NO + CO
(R4)
CN + O ) N + CO
(R5)
NH + O ) NO + H
(R6)
N + O2 ) NO + O
(R7)
HCN + OH f CN + H2O
(R8)
HCN + OH f NH2 + CO
(R9)
Two other principal routes to the formation of NOx in flames are by the oxidation of N2 in the combustion air,10 which is highly temperature dependent, and the reaction of CH radicals with N2,11 which can be significant under fuel-rich conditions. These NOx formation routes are described elsewhere. In summary, the oxidation of hydrogen cyanide, as the key nitrogen containing intermediate formed during combustion of nitrogen bearing fossil fuels, is generally accepted as being the main precursor to the formation of NOx.12,13 The fuel-nitrogen is assumed to convert to HCN at an infinite rate, then to NH3 (rate limiting), and finally to NO or N2 depending on the availability of oxygen.
NH2 + NO f N2 + H2O
(R10)
Experimental Section
By arranging fuel and air distribution to give fuel-rich zones in the flame near the burner so that [H] > [OH] . [O], hydrogenated nitrogenous species may be formed with the following reactions becoming more important under these conditions.
In addition, the reaction of fuel fragments (CHi) with NO that is already formed can lead to NO destruction, especially under more fuel-rich conditions, by a “reburn” mechanism as follows:7,8
CHi + NO f HCN + O
(R11)
NO + NHi f N2 + H2O
(R12)
Hydrogen cyanide produced in reaction (R11) is converted to N2 in the flame via reactions9
O + HCN f NCO + H
(R13)
NCO + H f NH + CO
(R14)
NH + H f N + H2
(R15)
N + NO f N2 + O
(R16)
Also, the probability of carbon particles in the fuel-rich flame depleting NO is increased by the following reactions,
2C + 2NO f N2 + 2CO
(R17)
The CMA [Ca(C2H3O2)2‚Mg(C2H3O2)2] behaves like a secondary fuel upon injection into the furnace, decomposing to acetone, (C3H6O);4 with trace amounts of allene, (C3H4), acetone oxidizes to form six further intermediates above 1000 K, namely, formaldehyde (CH2O), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), and ketene (C2H2O), and it is very (7) Myerson A. L. The Reduction of Nitric Oxide in Simulated Combustion Effluents by Hydrocarbon-Oxygen Mixtures. Proc. Combust. Inst. 1975, 15, 1085. (8) Wendt, J. O. L. Mechanisms Governing the Formation and Destruction of NOx and other Nitrogenous Species in Low NOx Coal Combustion Systems. Combust. Sci. Technol. 1995, 108 (4-6), 323-344. (9) Morley, C. The formation and destruction of hydrogen cyanide from atmospheric and fuel nitrogen in rich atmospheric-pressure flames. Combust. Flame 1976, 27, 189-204.
The NOx and SO2 reduction studies were carried out using a horizontal tunnel furnace, refractory-lined to an internal diameter of 500 mm. The furnace overall was 3 m in length and equipped with ports for the insertion of probes and injectors (Figure 1). The furnace was fired using a pressure jet, gas-oil burner (Dunphy TLO2.12) operating at ∼80 kW. The burner produced a turbulent diffusion flame ∼750 mm in length. Combustion air to the burner was separated into primary and secondary streams by a swirl plate on the burner head with a centrally positioned oil spray nozzle (Figure 2). Typical in-flame temperatures for this type of burner, measured using a suction pyrometer, peaked at ∼1500-1590 °C depending on the degree of staging.14 Air staging of the combustion system was accomplished by the introduction of tertiary air through four equidistant injection ports on a 150 mm pitch center diameter from the center axis of the burner, with all flows directed parallel to the main burner flow. Air staging creates a near-burner zone (nbz) stoichiometry which can be fuel-rich. Staging levels of up to 46% were achieved with this method, giving the desired range of burner stoichiometries, λnbz of 0.6-1.18 (at λ1 ) 1.18) and λnbz of 0.76-1.38 (at λ1 ) 1.38). Levels of SO2 were set at ∼750 ppmv by injecting pure SO2 from cylinders (BOC) into the primary burner air supply, and the levels of NOx in the flue gas were set by doping the gas-oil fuel with quinoline (VWR, GPR, 97%). Quinoline was chosen (10) Zeldovich, Ya. B.; Sadovnikov, P. Ya.; Frank-Kamanetski, D. A. Oxidation of Nitrogen in Combustion; Translated by M. Shelef; Academy of Sciences of the USSR, Institute of Chemical Physics: Moscow-Leningrad, 1947. (11) Fenimore, C. P. Formation of Nitric Oxide in premixed Flames. Proc. Combust. Inst. 1971, 13, 373. (12) Hampartsoumian, E.; Nimmo, W.; Clarke, A. G.; Williams, A. The Formation of NH3, HCN and N2O in an Air-Staged Fuel-Oil Flame. Combust. Flame 1991, 85, 499-504. (13) Hill, S. C.; Smoot, L. D. Modeling of nitrogen oxides formation in combustion systems. Prog. Energy Combust. Sci. 2000, 26, 417-458. (14) Hampartsoumian, E.; Nimmo, W. An Experimental Investigation of Sulphur-Nitrogen Interactions in Turbulent, Spray Flames. Combust. Sci. Technol. 1995, 110-111, 487-504.
NOx Reduction with SO2 Capture under Air-Staged Conditions
Energy & Fuels, Vol. 20, No. 5, 2006 1881
Figure 1. Schematic diagram of the tunnel furnace combustion facility.
Figure 2. Schematic diagram of flame zone, showing burner air distribution: λ1 ) 1.18.
as a model fuel-N compound, on the basis of previous experience,14 as most closely simulating the behavior of inherent fuel-nitrogen compounds. It has a relatively high boiling point (237 °C), which is within the distillation range of the gas-oil used and avoids the problems of flash evaporation. The quinoline was metered by calibrated peristaltic pump (Cole Palmer, Masterflex), and the desired baseline emission levels were set using an on-line NOx analyzer (Analysis Automation Ltd, model 440, chemiluminescence). Calcium magnesium acetate (Cryotech, U.S.A.) solutions were made up to a strength of 250 g/1000 mL with deionized water and fed by calibrated peristaltic pump (Cole Palmer, Masterflex) to a water-cooled injector with a multihole twin-fluid atomizer. Access ports were located along the length of the furnace at 430 mm (port 5), 735 mm (port 6), 910 mm (port 7), 1060 mm (port 8), 1215 mm (port 9), 1535 mm (port 10), 1685 mm (port 11), 1990 mm (port 13), and 2315 mm (port 14). The injection point was varied between 910 mm and 1990 mm from the burner, but normal operation was with the injector in the former position (port 7). Injection
of CMA was always postflame, even at high staging levels where the flame length was extended beyond the normal operating length of ∼400 mm from the burner. Combustion conditions in the oil flame were set using an on-line, zirconia cell O2 analyzer (Servomex 2700) with a hightemperature silicon carbide filter on the probe tip. Other gas concentrations were measured using an ABB Advanced Optima system consisting of a Magnos 17 (thermomagnetic) analyzer for dry O2 and a Uras (NDIR) system for CO, CO2, and SO2. Sample gas was quenched and dried via coalescing filters and integrated permeation drier prior to entering the analyzer systems. All instruments were calibrated using certified gas compositions from pressurized cylinders (BOC Special gases). Temperatures in the furnace were measured by thermocouples (Type K and R) placed at locations on the furnace wall and in the gas flow, including the exhaust. All gas concentrations and furnace temperatures were logged (Iotech Multiscan/1200) and stored on a PC for subsequent analysis. In addition, the furnace pressure was monitored (Furness Controls, FC016) so that consistent operating conditions could be maintained between experiments. Results Experiments were performed to satisfy the following objectives: • To identify the optimum tertiary/primary air ratio for NO reduction by air staging and characterize the effect of flame stoichiometry on emissions and combustion efficiency. • To determine the effect on NO emissions of the injection of calcium magnesium acetate for SO2 reduction. • To test two stoichiometries, λ1 ) 1.18 and λ1 ) 1.38, both comparing NO reductions with and without calcium magnesium acetate; SO2 reductions were investigated at λ1 ) 1.38 (Ca/S ratios of 1, 1.6, and 2.25). Air staging, as described here, is the separation of the combustion air into separate flows relative to the fuel. The air to the burner was reduced while air was introduced via tertiary air ports (Figure 2) to maintain the same overall stoichiometry. This air apportionment can be related to a percentage air staging, calculated by
1882 Energy & Fuels, Vol. 20, No. 5, 2006
AS (%) ) 100 ×
Nimmo et al.
( ) QTert QTotal
(E1)
where AS (%) ) air-staging percentage, QTert ) total tertiary air added for the staged combustion (∼100-600 L/min), and QTotal ) total air available to the system. A derived stoichiometry can be calculated for the near-burner zone as follows
( ( ))
λnbz ) λ1 1 -
QTert QTotal
(E2)
where λnbz ) stoichiometry at the near-burner zone, and
λ1 ) overall stoichiometry ) (Air/Fuel)Actual/(Air/Fuel)Stoichiometric (E3) The relationship between levels of air staging and near-burner zone stoichiometry calculated using eq E2 is shown in Table 1. Up to a λnbz of 0.95 (equivalent to ∼20% air staging), the total NO concentration increases by ∼20% of the initial value of 131 ppm to ∼158 ppm. NO emission increases as the flame stoichiometry approaches λnbz ) 1 because of increases in temperature as diluent (heat sink) N2 is reduced proportionately with the combustion air flow. Gas temperature measurement inside the furnace was performed to characterize the conditions at the point of CMA injection. A type R thermocouple (with ceramic sheath) was inserted at varying port positions to a depth of ∼110 mm inside the furnace (Figure 3). Three main conditions were examined: • Condition 1, no air staging or water injection (triangles) • Condition 2, as for condition 1 but with water injection equivalent to a Ca/S ratio of 1.75 (squares); no air staging. • Condition 3, water injection (as for condition 2) and 40% air staging (circles). The dashed line in Figure 3 represents the injection point (for all reported results) for the water and calcium magnesium acetate at 910 mm, Port 7. The overall stoichiometry was λ1 ) 1.18. At condition 1, temperatures were recorded as 1200 °C at port 9 followed by a steady decline to 1157 °C at port 14. The subsequent injection of water at port 7 for condition 2 changes the temperature profile. The presence of water and the addition of 40 L/min of nitrogen to form the spray plume alters the temperature characteristic downstream of the injector. A 6070 °C reduction was found at each point compared to condition 1. Indeed, the temperature reduction noted at port 8, i.e., the point most likely to be most effected by the spray, does show the greatest reduction. Temperature thereafter remains fairly constant, albeit at a lower value of ∼1125 °C at ports 9, 10, Table 1. Relationship between Level of Air Staging and Near-Burner Zone Stoichiometry λ ) 1.38 (6% O2 in flue) tertiary air flow (L/min)
air staging (%)
λnbz
0 100 120 200 300 400 500 600 700 800
0.0 5.6 6.7 11.1 16.7 22.2 27.8 33.3 38.9 44.4
1.38 1.3 1.29 1.23 1.18 1.07 1.00 0.92 0.84 0.77
λ ) 1.18 (3% O2 in flue) air staging (%)
λnbz
0 6.75 8.07 13.4 20.2 26.9 33.6 40.3 47.1
1.18 1.1 1.06 1.00 0.92 0.84 0.76 0.69 0.61
Figure 3. Axial temperature profile for the tunnel furnace with and without air staging and water injection at injection port P7. Baseline condition (2), baseline and water injection (equivalent to Ca/S ) 1.75) (9), baseline, water injection and staged to 40% (b).
and 11, where a steady decline to 1085 °C is can be observed from port 13 to 14. Condition 3 differed from condition 2 by the addition of staged air to a level of 40% of the total air available while keeping the oil flow rate unchanged. The main difference was the overall increase in temperature immediately before and after the injector. This increase was more significant after the injector, where the presence of the spray did not significantly affect the temperature (5 °C) compared to 20 °C at condition 2. The temperature profile midway along the furnace was unlike the flat trend found in the previous conditions; instead, there was a fairly uniform decrease in temperature from port 8 to port 14, though always maintaining a higher temperature over those measured at condition 2. From these results, it is clear that, by reducing the burner air flow, a lower volume of combustion air is being heated for virtually the same specific heat release by combustion compared with the unstaged condition. This effect on temperature will have a direct effect on the formation of thermal NO in the hottest parts of the flame. Baseline NO Emissions (Undoped Flame) To identify the baseline levels of emission of NOx and SO2 from gas-oil combustion without the addition of dopant SO2 gas or quinoline, measurements were made at an initial nearburner zone stoichiometry of λ1 ) 1.18 (3% oxygen, dry). The initial levels of NOx are a combination of “prompt”, “thermal NOx”, and “fuel NOx” (from a small amount of fuel-N inherent in the gas-oil (0.05 wt %)). A correction for the small contribution of the fuel-NOx component to this emission can be made on the assumption, used in previous work,14 that typically 25-50% of this inherent nitrogen may be converted to NO depending on the level of staging employed. From mass balance computations, maximum fuel-N conversion would give a theoretical emission of 65 ppm for the unstaged condition, resulting in an estimated emission level of 33 ppm (50% conversion) at λ1 ) 1.18. When doping with quinoline, the fuelNO emission data can be estimated from the difference between the measured total NO concentration and the contribution from thermal NO. No attempt has been made to quantify prompt NO in this work, and it is assumed to form only a small proportion of the thermal NO values reported.
NOx Reduction with SO2 Capture under Air-Staged Conditions
Figure 4. Baseline test results showing the effect of staging on thermal NO as a function of near-burner zone stoichiometry (λnbz), for λ1 ) 1.18. NO emission levels also shown (corrected to 3% O2).
Energy & Fuels, Vol. 20, No. 5, 2006 1883
unstaged flames, and for the purposes of discussion, the stoichiometry will be described in terms of the dry O2 levels measured in the flue gas. The overall stoichiometry was allowed to change for this test (λ1 ) λnbz) as the burner air was increased and as a means of characterizing the emissions for a range of burner oxygen levels. At the slightly fuel-lean condition, 80% at Ca/S levels of 2.25. At an overall stoichiometry of λ ) 1.18, improvement in NO reduction was seen to be most effective at low levels of air staging (
