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The effect of air staged, co-combustion of pulverised coal and biomass blends on NOx emissions and combustion efficiency
Fuel 90 (2011) 126–135
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The effect of air staged, co-combustion of pulverised coal and biomass blends on NOx emissions and combustion efficiency S. Munir 1, W. Nimmo ⇑, B.M. Gibbs Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e
i n f o
Article history: Received 5 March 2010 Received in revised form 16 June 2010 Accepted 28 July 2010 Available online 11 August 2010 Keywords: Co-firing Air-staged Un-staged Biomass Over-fire air
a b s t r a c t Co-firing of biomass residues with coal is continuously increasing in it’s application in coal-fired boilers for electricity production. In this study, co-firing experiments were performed using a Russian coal with a range of biomasses, shea meal (SM), cotton stalk (CS), sugarcane bagasse (SBT), sugarcane bagasse (SBR) and wood chips (WC) as biomasses in 5%, 10% and 15% thermal fractions to evaluate their potential as substitute fuel and an agent for NOx control. It was found that the addition of biomass increased NO reduction under both un-staged and air-staged conditions. However, NO reductions obtained under optimum conditions of primary zone stoichiometry (SR1 = 0.9) and over-fire air (OFA) injection port location 3, were found to be significantly higher than un-staged co-firing for the same biomass thermal share in the fuel blend. It was found that the addition of biomass has a positive effect on carbon burnout under the optimum conditions that were determined in the study. A 10% biomass blending ratio (BBR) was found to be optimum for air-staging conditions. When co-fired under optimum air-staged conditions, a 10% BBR of sugarcane bagasse (SBR), shea meal (SM), wood chips (WC), cotton stalk (CS) and sugarcane bagasse (SBT) in coal gave NO reduction of 49%, 51%, 53%, 60% and 72%, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The role of renewable fuels is continuously increasing due to climate change and energy security threats. By April 2009, 78 countries had signed the statute of the International Renewable Energy Agency (IRENA). Members include most countries of the European Union and many developing countries, from Africa to Asia-Pacific to Latin America, including Argentina, Chile, Ghana, India, Pakistan, Morocco, Philippines, Senegal, South Korea, and Tunisi. By early 2009, 73 countries have renewable energy policy targets [1]. EU-25/EU-27 has a binding target of a 20% share of renewables in the energy consumption by 2020 [2]. Despite the increasing share of renewables in energy generation schemes, new technologies are not yet competitive to combat climate change [3]. In this scenario, co-firing biomass residues with coal in traditional coal-fired boilers for electricity production represents the most cost effective and efficient renewable energy and climate change technology [4–6]. During the last 10 years, much progress has been made in the utilization of biomass in coal-fired power stations. Existing biomass power generation (and cogeneration) capacity is about 52 GW [1]. Currently, over 234 units have ⇑ Corresponding author. Tel.: +44 1133432513. E-mail address: [email protected] (W. Nimmo). On leave from the Institute of Chemical Engineering and Technology, University of the Punjab, Lahore, Pakistan. 1
0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.07.052
the experience of co-firing biomass. A country-wise distribution of these power plants is presented in Table 1. Agricultural residues are a form of biomass that is renewable. Despite their abundant availability, the current level of their utilization as a fuel is low [1,4,6,7]. Cotton stalk (Gossypium) is the stem of cotton plant which is residual biomass material from cotton crop and often burned in the field as rotting vegetation may result in damage to future crops due to disease, infestation, etc. [8–11]. The cotton stalk sample (CS) was obtained from agricultural field of Lodhran, Punjab, Pakistan; cultivated during May–June season and handpicked in November–December season. Sugarcane bagasse samples SBT and SBR were collected from known sugar cane fields near Faisalabad and Rahim yar Khan, normally supplied to Tandlianwala sugar mills and JDW sugar mills. Bagasse is the fibrous waste that remains after the recovery of sugar juice via crushing and extraction. Shea meal (SM) is the residue from the nut of the shea tree (Vitellaria paradoxa), after the removal of fatty ‘butter’ and contains the fleshy mesocarp, shell and husk. This biomass material is currently used as fuel in the UK power generating industry [8,9,12]. UK is importing 5420 tons of shea meal annually from Africa for co-firing for electricity production [12]. Shea meal, wood chips (WC) and coal (Russian coal) RC fuels for this study were provided by (RWE npower) UK. In this context, the co-firing potential of cotton stalk, sugar cane bagasse from Pakistan, shea meal from Africa and wood chips has been evaluated for NOx emissions reduction under air-staging configuration.
127
S. Munir et al. / Fuel 90 (2011) 126–135 Table 1 Power plants with experience in co-firing combinations of biomass and fossil fuels. Country Australia Austria Belgium Canada Denmark Finland Germany Indonesia Italy Netherland Norway Spain Sweden Taiwan Thailand UK USA Total
BFB
CFB
CFB,BFB
3
42
1 13
Grate 1
6
4 4 1
PF 8 1 1 7 7 10 4
Unknown
6 22
2 6 6
3
1 48
1 1 7 1 1 2 5 35
1
1
6
2
3
5 17
16 29 98
30
Total 8 5 1 7 12 81 27 2 7 6 1 2 15 1 1 18 40 234
Data source: [4].
2. Experimental 2.1. Experimental set up The experimental furnace is a 20 kW down fired combustor (shown in Fig. 1). It is additionally equipped with three different fuel feeders, air and gas supply systems, calibration set-up, gas measuring analytical equipment, gas cylinder manifolds, water cooled sample probes, char sample collection quenching system, thermocouples, data logger and PC. The schematic diagram of the combustor is given in Fig. 1. For the biomass and coal co-firing tests, an arrangement of two feeders could be utilised. While pre-blending coal with biomass, the main feeder (Rospen) was used in conjunction with a smaller (Dowson DB1-3/4) which permitted mixing of coal and biomass on the spreader tray. Feeders were pre-calibrated before each test run that were performed under un-staged and air-staged conditions. The coal and biomass were transported to the burner by the primary combustion air flow. A part of the secondary air was injected at a distance away from the burner to create a fuel lean zone for the completion of combustion. The overall stoichiometry was kept at 1.16 during the tests. Gas samples were drawn through stainless steel probes from any of the available ports along the length of the furnace. Axial distance measurements of the ports from the burner are given in Table 3. All the gas samples were dried and filtered before entering individual on-line analysers. Instruments were calibrated before each run with certificated BOC special gases mixtures. Oxygen was measured using a Servomex Paramagnetic Analyser 570A; CO, CO2 by NDIR analysers (Analytical Development Company; ABB Easyline IR CO2 analyser); NO and NOx by a chemiluminescence analyser (Signal Ltd. series 440). Each gas analyser and Rtype sheathed thermocouples were connected through a data logging interface (Pdaq 56) and recorded directly to an Excel spreadsheet with a data sampling rate of 10 s using daq-view software. In order that data for NO emission can be obtained at a particular plant condition, stable operation at that condition is achieved. Stability is deemed to occur when the measured variables (mainly O2 and NO) do not drift from a constant average level for a period of up to 10 min. The data is logged every 10 s and post-run analysis for the period of the test within the run involves extracting data which has been averaged over 30–40 sampled data points. Statistical analysis of example data is presented in Table 2 for the two
Fig. 1. Schematic diagram of 20 kW furnace.
Table 2 Example statistics of data analysis method.
O2 mean Standard deviation NO mean Standard deviation
5% SM + RC
15% SM + RC
3.2 0.05 541 4.02
3.3 0.03 479 3.2
Table 3 Axial distance of ports from burner. Port no.
Distance from burner (cm)
1 2 3 4 5 6 7 8 9 Flue
56.5 86.5 116.5 146.5 161.5 191.5 221.5 236.5 259.5 299
128
S. Munir et al. / Fuel 90 (2011) 126–135
thermal fractions of 5% and 15% SM-coal blend under air-staged conditions. In order to compare the effects of air-staging and biomass addition, the test of RC firing without co-firing and air-staging while keeping SR = 1.16 was taken as base line and NOx reduction (%) was defined by equation:
NOred% ¼
ðNOÞbaseline @ 6% O2 ðNOÞBBR @ 6% O2 ðNOÞbaseline @ 6% O2
100
ð1Þ
The base line value of NO was taken at the exit without the addition of any biomass/staging. This was then corrected at 6% O2. Similarly, all the NO values for coal–biomass blends (staged and un-staged) were also corrected at 6% O2 to avoid any dilution effect. The proximate and ultimate analyses along with bulk densities and HHV of the samples are given in Table 3. The calorific values were determined by using a Parr 6200 oxygen bomb calorimeter. For the determination of surface area, the char samples were analyzed using surface area and pore size analyzer Model (Quanta Chrome Nova 2200e). Ash analysis was done using a PANalytical Axios Advanced XRF spectrometer aided with PANalytical IQ + Semiquantitative software. The VM/FC of all the biomass samples was found to be 3.5–10.5 times higher than VM/FC of coal (Table 5). This is an indication of significant difference in volatility between the coal and the biomasses. Cellulose and lignin are generally recognized as main components in agricultural residues. The weight fraction, except for the cellulose and lignin fraction, corresponds to the fraction of acidsoluble hydrocarbons in the biomass. Fig. 2 shows the differences in the biomass samples structure. The cellulosic content in the biomasses may enhance the ignition characteristics since cellulose compounds have the structure of branching chain of polysaccharides and no aromatic compounds, which are easily volatilized [13].
stoichiometric air) is kept less than 1. A portion of the secondary air (over-fire air) is injected later in the furnace a creating fuel lean zone to complete the combustion. The region downstream of the over-fire air ports is the burnout zone. The combustion in the fuel rich, sub-stoichiometric primary region reduces the formation of thermal–NOx and fuel–NOx [14]. Mixing of combustion air with fuel in two stages by creating a fuel rich zone and fuel lean zone delays the combustion process and is an effective means of NO control in coal-fired boilers [14,15]. The success of air the staged combustion technique primarily depends on the location of OFA injection and the stoichiometric ratio in the primary combustion zone (SR1) [14]. To find out the optimum OFA location along with optimum SR1, the furnace was optimized with RC and RC-SM blend with 10% BBR. The overall stoichiometric ratio at the furnace exit was kept at 1.16. Three stoichiometric ratios in the primary combustion zone, SR1 = 1, 0.9 and 0.8 were used along with each OFA port location from port 2 to port 5 (Fig. 1). Fig. 3 shows the relationship among the OFA injection location, SR1 and NO reduction efficiency. This was found that a decrease in SR1 has a positive effect on the NO reduction % for every OFA nozzle location. This was expected as lower is the SR1, stronger is the reducing environment in the primary zone which is conducive for NOx reduction. The OFA nozzle location determines the lengths of the primary zone and burn out zone. A downward shift of the OFA nozzle location from port 2 increases the primary zone length and decreases the burn out zone length and vice versa (Fig. 1). Figs. 3 and 4 revealed that maximum NO reduction levels were found at SR1 = 0.8 for all the OFA nozzle locations with minimum carbon burnout. This was found that for each step increase in the primary zone length (by moving
3. Results and discussion 3.1. Determination of optimum OFA injection location and SR1 The general principle of air staging is to create a distinct fuel rich (oxygen deficient) zone and a subsequent fuel lean zone inside the furnace. The primary zone stoichiometric ratio (SR1, actual air/
Fig. 2. Structural composition of biomass samples.
Fig. 3. (a) Effect of SR1 and the location of OFA nozzle on NO with RC. (b) Effect of SR1 and the location of OFA nozzle on NO with 10% SM-coal blend.
S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 4. (a) Effect of OFA nozzle location and SR1 on the carbon burn out for RC. (b) Effect of OFA nozzle location and SR1 on the carbon burn out for 10% SM-RC blend.
OFA nozzle from port 2 to 3 and 3 to 4 and so on) has a positive effect on the NO reduction (Fig. 3) and negative effect on carbon
129
burn out (Fig. 4). This is due to the fact that each step increase in the primary zone length increases the residence time in the oxygen deficient, fuel rich primary zone (Fig. 5a). This was in agreement with the findings of Ribeirete and Costa, 2009 [16]. A strong reducing environment in primary combustion zone restrain coal combustion, and large amount of unburned char enters burn out zone [14]. If the length of the burn out zone is short and temperature is not sufficiently high, carbon burnout efficiency can drop. Figs. 3 and 4 showed that a decrease in the primary zone stoichiometry (SR1) has a positive effect on NO reduction and negative effect on carbon burnout. This could be due to the fact that the lower the fuel rich zone stoichiometry the stronger is the reducing environment and longer residence time in the fuel rich zone (Fig. 5b). Increasing the residence time in the fuel rich zone favours the decomposition of N species (NO, NH3, HCN) resulting a significant reduction in NOx emissions. Decreasing SR1 can lead to an increase of reducing species formed in the primary combustion zone, which is conducive to NOx destruction [17]. But at the same time when SR1 is too low, pulverised coal combustion remains incomplete in the primary zone and large amounts of unburnt char enter the burnout zone which may affect the combustion efficiency. There is a trade off between NO reduction and carbon burnout while selecting an optimum OFA nozzle location and SR1. Therefore, too low or too high SR1 will result either low NO reduction with high carbon burn out or high NO reduction with poor carbon burnout at all OFA nozzle locations. Similarly, too short or too long a residence time in the primary zone will result in either, higher carbon burnout with low NO, or high NO reduction with low carbon burnout. In Fig. 6, NO reduction and carbon burnout have been plotted. From Fig. 6, an optimal position is estimated where the carbon burnout begins to level off as SR1 increases. It is also clear from this comparison that the addition of biomass benefits both carbon burnout and NO reduction at this condition. Keeping in view the above discussion and results exhibited in Figs. 3, 4 and 6, an OFA nozzle location at port 3 and SR1 = 0.9 were found to be optimum.
Fig. 5. (a) Effect OFA injection location on residence time. (b) Effect of primary zone stoichiometry on residence time. Air-staged conditions.
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 6. Effect of SR1 on carbon burnout of 10% BBR blends and NO reduction showing optimal condition at SR1 = 0.9 (OFA = port 3) (RC + SBT – , RC + SM – d, RC + CS – j, RC + WC – N, RC + SBR – ., RC – H).
Fig. 7. Effect of BBR on NO reduction under un-staged co-firing conditions.
3.2. Co-combustion of biomass with coal 3.2.1. Co-firing Three blends of each biomass and coal with 5%, 10% and 15% BBR (thermal) were used in this study for un-staged and air-staged tests. The furnace was operated at an overall stoichiometry (SR) of 1.16. It was found that an increase in BBR has positive effects on NO reduction as shown in Fig. 7. This could be linked to an increase in volatility of the fuel by the addition of biomass. Tillman (2000) reported that an increase in volatility (VM/FC) of fuel has positive effect on NOx reduction until the volatility reaches unity. An increase in VM/FC beyond 1 has no positive effect on the NOx reduction during co-firing [18]. A similar effect observed while increasing the BBR for SBT from 10% to 15%. As shown in Fig. 8 that an increase in BBR from 10% to 15% raised VM/FC level from 0.98 to 1.15 resulting a decrease in NO reduction from 21% to 18%.
It is evident from Table 4 that SBT, having significantly higher fuel nitrogen content than WC, SBR and CS, gave considerably higher NO reduction than WC, SBR and CS when co-fired. The results showed in Fig. 7 are in agreement with the findings of Spliethoff and Hein, 1998 that a correlation of the NO emissions and the nitrogen content of the biomass are not likely [7,19]. 3.2.2. Air-staged co-combustion of biomasses with coal The NO levels and corresponding NO reductions have been plotted in Fig. 9 for various primary zone stoichiometry (SR1) levels. It was found that while co-firing coal–biomass blends, a decrease in primary zone stoichiometry (SR1) showed a more positive effect on the NO reduction compared to coal alone for the same level of SR1. As SR1 = 0.9 and OFA location at port 3 were found to be optimum operating parameters, the results plotted in Fig. 9 corresponds to OFA location port 3. The NO reduction was found to be 42% in the case of RC staged firing with SR1 = 0.9. All the coal–biomass
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 8. (a) Effect of BBR on volatility under un-staged co-firing conditions. (b) Effect of volatility on NO reduction under un-staged co-firing conditions.
Table 4 Proximate and ultimate analysis and HHV of the fuel samples. Fuels as received basis
SM CS RC WC SBT SBr a
Ultimate analysis
Proximate analysis
C (%)
H (%)
Oa (%)
N (%)
S (%)
Ash (%)
FC (%)
VM (%)
H2O (%)
41.70 45.2 60.36 42.2 33.6 42.34
5.0 4.40 4.5 4.94 5.3 5.62
32.32 40.5 8.35 35.48 36.27 37.13
2.47 1.0 1.84 0.28 1.5 0.24
0.09 0.0 0.30 0.10 0.0 0.001
4.29 4.9 14 1.70 11.05 9.56
24.58 18 45.48 11.90 13.86 17.11
57 73.1 29.87 71.1 62.81 68.23
14.13 4.0 10.65 15.3 12.28 5.1
Bulk density (kg/m3)
HHV (MJ/kg)
490 310 620 270 160 180
17.70 17.70 27.29 16.39 11.80 17.37
Calculated by difference.
Table 5 Volatility and particle size of the samples. Fuels
VM/FC
Particle volume mean diameter [4,3] (lm)
SM CS WC SBT SBR RC
2.32 4.06 5.97 4.53 3.99 0.656
150.29 209.89 586.25 743.11 586 85.292
blends exhibited higher NO reduction compared to coal alone for SR1 = 0.9 (Fig. 9). A 10% BBR was found to be optimum for coal–biomass blends (Figs. 9 and 10) under the conditions studied. A comparison of the air staged experiments for maximum NO reduction with remarkable air-staging investigations by other researchers is given in Table 6. For optimum 10% BBR (thermal), NO reductions were found to be 72%, 60%, 53%, 51% and 49% for SBT, CS, WC, SM and SBR, respectively. The difference in the % NO reduction for bagasse samples (SBT and SBR) could be linked to the difference in their proximate and ultimate constituents (Table 4) and volatility (Fig. 8a and Table 5). During combustion, fuel bound nitrogen reacts with the released volatiles from fuel and generally tends to form HCN and NHi. These intermediate components subsequently react to form NOx in an oxidizing environment and N2 in a reducing environment [15,20–22]. The formation of HCN and NHi,(1,2,3), depends on the hydrocarbon release during fragmentation. The volatile matter in
the biomasses is higher than coal (Tables 4 and 5) and the volatiles from biomass devolatilization are mainly the combustibles –CO, H2, CxHy [6]. The addition of biomass to replace coal increases the volatility of the fuel. The dominant volatile-nitrogen compound in coal combustion is HCN and in biomass combustion is NH3. The ratio of HCN to NH3 is 0.9 in coal combustion and 0.1 in wood combustion [23]. The formation of NH3 and HCN increases with increasing fuel volatility [24] and gas-phase combustion becomes predominant [18]. Furthermore, most of the fuel nitrogen in biomass is initially converted ammonia and subsequently by reaction to NHi radicals during combustion. The N species reduces NO to molecular nitrogen (essentially providing an in situ thermal DeNOx source) [6,19,25,26]. The mechanistic pathways for formation and reduction of nitrogen oxides during air-staged coal combustion were described by Beer (2000). Bai (2000) described mechanism of wood chips combustion. The reactions involved in two staged biomass combustion has been described by Nussbaumer (2003) [20,21,27]. In the light of above elucidated referred work, possible routes for the NO reduction in near burner reducing zone during two staged biomass coal co-combustion are shown in Fig. 10. Therefore, the synergistic effect of the addition of biomass to control NOx under staged conditions becomes apparent. As biomasses contain less carbon and are highly oxygenated compared to coal (Table 4), the amount of theoretical air required for combustion is less than coal combustion, alone. It is anticipated that the addition of biomass without increasing SR1 level can cause locally stronger reducing environments within the flame in the primary zone resulting in a negative effect on carbon burnout and
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 9. Effect of SR1 and BBR on NO, OFA port 3, air-staged conditions.
NO emissions. The wider range of biomass particle combustion rates related to size and higher volatility may modify the combustion pattern compared with a normal coal flame so that volatile-N from coal may enter the flame after rapid consumption of O2 by more reactive biomass combustion [7]. The CO concentrations measured in primary zone for different biomass-coal blends are shown in Fig. 12. This could explain why, that in the case of 15% BBR, some of the biomass-coal blends showed lower NO reduction and carbon burnouts than 10% BBR blends of the same biomasses (Figs. 10 and 12). In order to compare the effect of biomass addition on burn out, un-staged coal firing burnout was taken as base line (Fig. 13a) for un-staged co-firing tests for same SR = 1.16. To compare the effect of biomass addition under air-staged conditions, burnout of staged coal firing for SR1 = 0.9 was taken as base line (Fig. 13b). In both the cases, burnout improved with an increased biomass share. This was in agreement with the findings of Spliethoff and Hein (1998) [19]. It is clear that the addition of biomass has a positive effect on carbon burnout (except WC Fig. 13). This could be linked to the higher volatility of biomasses (Table 5), higher reactivity and porosity of the biomass chars as compared to coal [19,23,28–33]. The char surface area of the biomasses were found to be 21.712, 5.44, 4.832, 4.549 and 3.958 m2/g for SBT, SBR, WC, SM and CS compared to 0.128 m2/g of RC. In the case of WC, the bulk density to particle size relation could be a limiting factor. WC and SBR have the same particle size and the density of WC is considerably higher than SBR (Tables 4 and 5). Furthermore, the cellulose to lignin ratio of WC was found to be 0.8 compared to SBR of 5.86 (Fig. 2). The axial temperature profiles of the biomass-coal blends co-combustion for SR1 = 0.9 and OFA injection location port 3 are shown in Fig. 13. A temperature difference of 75–120 °C was observed in the primary zone for RC1-WC blend (Fig. 14). This could be due to higher particle size, density and moisture content (Tables 4 and 5) causing a delay on the ignition of WC permitting the coal to ignite ahead of the WC preferentially consuming O2 in the near burner zone resulting lower burnout (Fig. 13). A maximum of 21% NO reduction was found in the case of 10% SBT-coal un-staged co-firing (Fig. 7). When co-fired under optimum
Fig. 10. Possible routes for NOx reduction during co-combustion of biomass with coal in the primary zone.
133
S. Munir et al. / Fuel 90 (2011) 126–135 Table 6 A comparison of the air staged experiments conducted for this study and other researchers. Primary fuel
Secondary fuel
Reactor type
Un-staged NO (ppm) for 6% O2 in the primary zone
Initial primary zone stoichiometric ratio (SR1)
Primary zone stoichiometric ratio range (SR1)
Primary zone residence time (s)
Maximum NO reduction efficiency at primary zone stoichiometric ratio (SR1)
References
Bituminous Russian coal 90% RC 90% RC 90% RC 90% RC 85% RC Hard coal
N/A
20 kW pulverised fuel combustor -do-do-do-do-do0.5 MW pulverised fuel combustor -do-do-do-do-do-do1.5 MW under stoker furnace 100 kW laboratory furnace
854
1.16
0.8–1.2
1.17
67% (0.80)
This study
811 828 794 673 769 1384
1.16 1.16 1.16 1.16 1.16 12
0.8–1.2 0.8–1.2 0.8–1.2 0.8–1.2 0.8–1.2 0.6–1.2
1.25 1.29 1.20 1.21 1.16 0.9
67% (0.8) 73% (0.8) 54% (0.8) 76% (0.8) 71% (0.8) 38% (0.6)
This This This This This [19]
1160 1575 935 1450 1225 1775 631 458
12 12 12 12 12 12 12 1.15
0.77–1.2 0.55–1.2 0.65–120 0.55–1.2 0.52–1.2 0.6–12 0.4–12 0.8–1.15
0.9 2.5 0.9 2.5 2.5 2.5 0.5 0.3
30% (0.88) 77% (0.7) 39% (0.82) 81% (0.70) 76% (0.60) 79% (0.60) 50% (0.70) 80% (0.80)
[19] [19] [19] [19] [19] [19] [21] [16]
Hard coal Hard coal Hard coal Hard coal Hard coal Hard coal UF chipboard Bituminous Russian coal
10% 10% 10% 10% 15% N/A
SM CS WC SBT SBR
10% 10% 40% 25% 40% NA N/A N/A
straw straw straw straw straw
study study study study study
Fig. 12. Effect of BBR on CO concentration in the primary zone, SR1 = 0.9, air-staged conditions. (RC1 + WC – N, RC1 + SBT – , RC1 + SM – d, RC1 + CS – j, RC1 + SBR – .). Fig. 11. Effect of BBR on NO, OFA port 3, SR1 = 0.9, air-staged conditions.
air-staged conditions, a 10% BBR of sugarcane bagasse (SBR), shea meal (SM), wood chips (WC), cotton stalk (CS) and sugarcane bagasse (SBT) in coal gave NO reduction of 49%, 51%, 53%, 60% and 72%, respectively. It seems that in the case of un-staged experiments, fuel volatility played positive role in NO reduction. In the case of air-staged co-firing experiments, operating parameters like primary zone stoichiometry (SR1), OFA injection port location and temperature exploited the NOx formation kinetics significantly. It was found that the air-staging technique has a synergistic effect on biomass-coal co-firing for NOx reductions (Figs. 9 and 11) and carbon burnout (Fig. 13). It was found that NO emissions decrease as the distance between staged air injector and burner increase because of an increase in the residence time of the particles in the primary zone. However, if the distance exceeds from 116.5 cm (OFA injection location port 3) (Table 3), carbon burnout decreases significantly (Figs. 4 and 5). A reduction in primary zone stoichiom-
etry was found to cause a decrease in both NO emissions and carbon burnout (Figs. 4, 6 and 9). This was in agreement with the findings of Ribeirete and Costa (2009), Li et al. (2009) and Spliethoff and Hein (1998) [14,16,19]. 4. Slagging and fouling Inorganic constituents in the fuel are the main contributor to slagging and fouling [25,34]. The major elements including alkali metals (K, Na), alkaline earth metals (Ca, Mg), silicon, chlorine and sulphur are involved in reactions leading to ash slagging and fouling [34,35]. Biomasses have lower ash content compared to coal (Table 4). At the same time, biomass ash materials are rich in alkali and alkaline earth metals [7] and these are effective fluxes for alumina-silicates and lower the ash fusion temperature resulting an increase in slagging and fouling propensity [31]. Despite the lower ash fraction in biomass, co-firing of biomass with coal can
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 14. Temperature profiles of coal and coal–biomass blends, SR1 = 0.9, air-staged conditions.
Fig. 13. (a) Effect of BBR on burnout with SR = 1.16 (un-staged co-combustion). (b) Effect of BBR on burnout with SR1 = 0.9, OFA port 3, air-staged conditions.
lead to an enhanced slagging and fouling propensity due to its lower fusion temperature depending on chemical and mineralogical composition of fly ash as well as on conditions (temperature and velocity distribution, reducing or oxidizing atmosphere and many others) in the furnace [35]. The ash chemical composition (from air staged co-combustion experiments) for 10% optimum BBR is shown in Table 7. Ash chemical composition of the coal–biomass blends with 10% BBR does not differ significantly from the pure coal ash (Table 7). This was in agreement with the findings of Pronobis (2005) and Grammelis et al. (2006) [35,36]. Pronobis (2005) concluded that
the chemical composition of the biomass-coal blends ash does not differ significantly from the pure coal ash provided BBR (thermal) remains less than 20% [35]. Gramelis et al. (2006) are of the view point that biomass exploitation as secondary fuel in co-combustion processes is technically and economically feasible up to 20% w/w and the produced ash could be further utilised without any major treatment [36]. Therefore, the traditional correlations for the coal ash can be used as predictors for slagging and fouling tendency of ashes from fuel mixtures with 10% BBR. Traditional slagging and fouling indices are given in Table 8.
Table 8 Traditional slagging and fouling indices.a. Slagging (basic 10 acidic compounds ratio) Index
Table 7 Air staged (SR1 = 0.9) co-fired ash composition for 10% BBR of different blends.
a
Components (%)
RC
RC + SM
RC + CS
RC + WC
RC + SBT
RC + SBR
Na2O MgO A12O3 SiO2 p2o5 K2O CaO TiO2 Fe2O3 SO3 Othersa
0.35 1.386 23.07 63.35 0.444 2.18 1.5 0.995 5.75 0.058
0.32 1.68 21.57 60.98 0.7 2.5 2.69 0.9 6.78 0.223
0.36 1.7 22.77 59.9 0.58 2.83 2.27 0.965 5.6 0.06
0.36 1.36 22.98 63.27 0.49 2.28 1.55 0.998 5.74 0.06
0.44 1.83 22.66 61.73 0.5 2.38 2.25 0.95 5.66 0.079
0.33 1.36 21.1 62.36 0.51 2.86 2.39 0.92 5.9 0.089
Include V2O5, Cr2O3, SrO, ZrO2, BaO, Mn3O4, NiO, CuO, ZnO, PbO, HfO2.
Simplified B/A Slagging (Babcock)index
Fouling index
Ratio – slag viscosity index
a
Source: [35,37,38].
B/A < 0.5, low slagging inclination 05 < B/A < 1.0, medium B/A = 1.00, high B/A P 1.7S, severe O3 þCaOþMgO 0.75 < R(B/A) low RðB=AÞ ¼ Fe2SiO 2 þAl2 O3 slagging Rs < 0.6. low slagging Rs ¼ AB Sd inclination Sd ¼ % of S on dry basis Rs = 0.6–2.0, medium Rs = 2.0–2.6, high Rs > 2.6. extremely high B Fu 6 .6, low fouling F u ¼ A ðNa2 O þ K2 OÞ inclination Fu = 0.6–40, high Fu P 40, extremely high SiO2 SR > 72, low slagging SR ¼ SiO2 þMgOþCaOþFe 100 2 O3 inclination 72 P SR > 65, medium SR 6 65, high B A
¼
Fe2 O3 þCaOþNa2 OþK2 OþMgO SiO2 þAl2 O3 þTiO2
S. Munir et al. / Fuel 90 (2011) 126–135 Table 9 Calculated values of the traditional ash deposition indices. Index
SBt
SBR
CS
SM
WC
RC
Rb B/A B/A(+P) R(B/A) RS Fu SR
12.56 0.1472 0.153 0.115 0.039 0.415 86.37
12.84 0.1522 0.158 0.116 0.043 0.485 86.60
12.76 0.15.26 0.1595 0.116 0.043 0.487 86.22
13.97 0.1674 0.176 0.135 0.051 0.472 88
11.29 0.1294 0.135 0.1 0.039 0.342 87.97
10.56 0.1202 0.1255 0.0931 0.0409 0.2977 88.74
Rb represents the percentage of basic constituents in the ash. The minimum ash softening temperature occurs with 35% < Rb < 55%. Although the influence of P2O5 on ash fusibility depends on the form in which it occurs in the fly ash, it seems appropriate to add P2O5 content to the B group in B/A index. The new ratio is called B/A(+P) [35,37]. The calculated values of the traditional ash deposition indices for optimum air staged (SR1 = 0.9), 10% BBR compared to un-staged RC (SR = 1.16) are given in Table 9. The values of the slagging and fouling (S&F) indices for coal– biomass blends with 10% BBR were found to be in the ranges attributed for low slagging and fouling inclination (Tables 8 and 9). It was also noticed that S&F indices for coal–biomass blends with 10% BBR are in proximity with pure coal indices (Table 9). 5. Conclusions Five world biomass samples were co-fired under un-staged and air-staged configurations in a 20 kW down-fired coal combustor to investigate their (biomasses) potential as substitute fuel and an agent to control NOx. Biomasses were pre-blended with coal in 5%, 10% and 15% thermal fractions. An increase in biomass blending ratio (BBR) showed an increase in fuel volatility and increase in NO reduction. A maximum of 21% NO reduction was obtained with coal-SBT co-firing blend. In air-staging experiments, primary zone stoichiometry (SR1) was varied from 0.8 to 1.16 for each of the OFA injection port location from port 2 to 5. It was found that a decrease in primary zone stoichiometry (SR1) and an increase in residence time, by varying OFA port location in the primary zone, both have a positive effect on NO reduction for coal and coal–biomass blends co-firing. A 10% BBR was found to be optimum with respect to NO reduction and corresponding carbon burnouts under air-staged conditions. The investigated biomass-coal blends exhibited significantly higher NO reductions under air-staged conditions compared to un-staged co-firing for the same BBR. NO reductions between 49% and 72% were obtained under optimum air-staged conditions of SR1 = 0.9, OFA injection location = port 3 and BBR = 10% (thermal) compared to 1.4–21% NO reductions under un-staged co-firing. It was found that the air-staging technique has a synergistic effect on biomass-coal co-firing for NOx reductions and carbon burnout. The addition of biomass as secondary fuel up to 10% BBR (thermal) in co-combustion was found to have no adverse effect on slagging and fouling propensity. Acknowledgement The authors wish to express their gratitude to Dr John Smart (RWE npower) for providing the shea meal, wood chips and Russian coal samples for this study. References [1] REN21. Renewables global status report: 2009 update. Paris; 2009. 17p.
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[2] REN21. Renewables 2007 global status report. Paris; 2008. 40p. [3] Kavouridis K, Koukouzas N. Coal and sustainable energy supply challenges and barriers. Energy Policy 2008;36:693–703. [4] IEA bioenergy task 32. Technical status of biomass co-firing. Netherland; 2009. 10p. [5] Zhang L, Xu C, Champagne P. Overview of recent advances in thermo-chemical conversion of biomass. Energy Convers Manage 2010;51:969–82. [6] Werther J, Saenger M, Hartge E-U, Ogada T, Siagi Z. Combustion of agricultural residues. Prog Energy Combust 2000;26:1–27. [7] Munir S, Nimmo W, Gibbs BM. Co-combustion of agricultural residues with coal: turning waste into energy. Energy Fuel 2010;24:2146–53. [8] Munir S, Daood SS, Nimmo W, Cunliffe AM, Gibbs BM. Thermal analysis and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal under nitrogen and air atmospheres. Bioresour Technol 2009;100:1413–8. [9] Munir S, Nimmo W, Gibbs BM. 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Fuel journal homepage: www.elsevier.com/locate/fuel
The effect of air staged, co-combustion of pulverised coal and biomass blends on NOx emissions and combustion efficiency S. Munir 1, W. Nimmo ⇑, B.M. Gibbs Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e
i n f o
Article history: Received 5 March 2010 Received in revised form 16 June 2010 Accepted 28 July 2010 Available online 11 August 2010 Keywords: Co-firing Air-staged Un-staged Biomass Over-fire air
a b s t r a c t Co-firing of biomass residues with coal is continuously increasing in it’s application in coal-fired boilers for electricity production. In this study, co-firing experiments were performed using a Russian coal with a range of biomasses, shea meal (SM), cotton stalk (CS), sugarcane bagasse (SBT), sugarcane bagasse (SBR) and wood chips (WC) as biomasses in 5%, 10% and 15% thermal fractions to evaluate their potential as substitute fuel and an agent for NOx control. It was found that the addition of biomass increased NO reduction under both un-staged and air-staged conditions. However, NO reductions obtained under optimum conditions of primary zone stoichiometry (SR1 = 0.9) and over-fire air (OFA) injection port location 3, were found to be significantly higher than un-staged co-firing for the same biomass thermal share in the fuel blend. It was found that the addition of biomass has a positive effect on carbon burnout under the optimum conditions that were determined in the study. A 10% biomass blending ratio (BBR) was found to be optimum for air-staging conditions. When co-fired under optimum air-staged conditions, a 10% BBR of sugarcane bagasse (SBR), shea meal (SM), wood chips (WC), cotton stalk (CS) and sugarcane bagasse (SBT) in coal gave NO reduction of 49%, 51%, 53%, 60% and 72%, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The role of renewable fuels is continuously increasing due to climate change and energy security threats. By April 2009, 78 countries had signed the statute of the International Renewable Energy Agency (IRENA). Members include most countries of the European Union and many developing countries, from Africa to Asia-Pacific to Latin America, including Argentina, Chile, Ghana, India, Pakistan, Morocco, Philippines, Senegal, South Korea, and Tunisi. By early 2009, 73 countries have renewable energy policy targets [1]. EU-25/EU-27 has a binding target of a 20% share of renewables in the energy consumption by 2020 [2]. Despite the increasing share of renewables in energy generation schemes, new technologies are not yet competitive to combat climate change [3]. In this scenario, co-firing biomass residues with coal in traditional coal-fired boilers for electricity production represents the most cost effective and efficient renewable energy and climate change technology [4–6]. During the last 10 years, much progress has been made in the utilization of biomass in coal-fired power stations. Existing biomass power generation (and cogeneration) capacity is about 52 GW [1]. Currently, over 234 units have ⇑ Corresponding author. Tel.: +44 1133432513. E-mail address: [email protected] (W. Nimmo). On leave from the Institute of Chemical Engineering and Technology, University of the Punjab, Lahore, Pakistan. 1
0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.07.052
the experience of co-firing biomass. A country-wise distribution of these power plants is presented in Table 1. Agricultural residues are a form of biomass that is renewable. Despite their abundant availability, the current level of their utilization as a fuel is low [1,4,6,7]. Cotton stalk (Gossypium) is the stem of cotton plant which is residual biomass material from cotton crop and often burned in the field as rotting vegetation may result in damage to future crops due to disease, infestation, etc. [8–11]. The cotton stalk sample (CS) was obtained from agricultural field of Lodhran, Punjab, Pakistan; cultivated during May–June season and handpicked in November–December season. Sugarcane bagasse samples SBT and SBR were collected from known sugar cane fields near Faisalabad and Rahim yar Khan, normally supplied to Tandlianwala sugar mills and JDW sugar mills. Bagasse is the fibrous waste that remains after the recovery of sugar juice via crushing and extraction. Shea meal (SM) is the residue from the nut of the shea tree (Vitellaria paradoxa), after the removal of fatty ‘butter’ and contains the fleshy mesocarp, shell and husk. This biomass material is currently used as fuel in the UK power generating industry [8,9,12]. UK is importing 5420 tons of shea meal annually from Africa for co-firing for electricity production [12]. Shea meal, wood chips (WC) and coal (Russian coal) RC fuels for this study were provided by (RWE npower) UK. In this context, the co-firing potential of cotton stalk, sugar cane bagasse from Pakistan, shea meal from Africa and wood chips has been evaluated for NOx emissions reduction under air-staging configuration.
127
S. Munir et al. / Fuel 90 (2011) 126–135 Table 1 Power plants with experience in co-firing combinations of biomass and fossil fuels. Country Australia Austria Belgium Canada Denmark Finland Germany Indonesia Italy Netherland Norway Spain Sweden Taiwan Thailand UK USA Total
BFB
CFB
CFB,BFB
3
42
1 13
Grate 1
6
4 4 1
PF 8 1 1 7 7 10 4
Unknown
6 22
2 6 6
3
1 48
1 1 7 1 1 2 5 35
1
1
6
2
3
5 17
16 29 98
30
Total 8 5 1 7 12 81 27 2 7 6 1 2 15 1 1 18 40 234
Data source: [4].
2. Experimental 2.1. Experimental set up The experimental furnace is a 20 kW down fired combustor (shown in Fig. 1). It is additionally equipped with three different fuel feeders, air and gas supply systems, calibration set-up, gas measuring analytical equipment, gas cylinder manifolds, water cooled sample probes, char sample collection quenching system, thermocouples, data logger and PC. The schematic diagram of the combustor is given in Fig. 1. For the biomass and coal co-firing tests, an arrangement of two feeders could be utilised. While pre-blending coal with biomass, the main feeder (Rospen) was used in conjunction with a smaller (Dowson DB1-3/4) which permitted mixing of coal and biomass on the spreader tray. Feeders were pre-calibrated before each test run that were performed under un-staged and air-staged conditions. The coal and biomass were transported to the burner by the primary combustion air flow. A part of the secondary air was injected at a distance away from the burner to create a fuel lean zone for the completion of combustion. The overall stoichiometry was kept at 1.16 during the tests. Gas samples were drawn through stainless steel probes from any of the available ports along the length of the furnace. Axial distance measurements of the ports from the burner are given in Table 3. All the gas samples were dried and filtered before entering individual on-line analysers. Instruments were calibrated before each run with certificated BOC special gases mixtures. Oxygen was measured using a Servomex Paramagnetic Analyser 570A; CO, CO2 by NDIR analysers (Analytical Development Company; ABB Easyline IR CO2 analyser); NO and NOx by a chemiluminescence analyser (Signal Ltd. series 440). Each gas analyser and Rtype sheathed thermocouples were connected through a data logging interface (Pdaq 56) and recorded directly to an Excel spreadsheet with a data sampling rate of 10 s using daq-view software. In order that data for NO emission can be obtained at a particular plant condition, stable operation at that condition is achieved. Stability is deemed to occur when the measured variables (mainly O2 and NO) do not drift from a constant average level for a period of up to 10 min. The data is logged every 10 s and post-run analysis for the period of the test within the run involves extracting data which has been averaged over 30–40 sampled data points. Statistical analysis of example data is presented in Table 2 for the two
Fig. 1. Schematic diagram of 20 kW furnace.
Table 2 Example statistics of data analysis method.
O2 mean Standard deviation NO mean Standard deviation
5% SM + RC
15% SM + RC
3.2 0.05 541 4.02
3.3 0.03 479 3.2
Table 3 Axial distance of ports from burner. Port no.
Distance from burner (cm)
1 2 3 4 5 6 7 8 9 Flue
56.5 86.5 116.5 146.5 161.5 191.5 221.5 236.5 259.5 299
128
S. Munir et al. / Fuel 90 (2011) 126–135
thermal fractions of 5% and 15% SM-coal blend under air-staged conditions. In order to compare the effects of air-staging and biomass addition, the test of RC firing without co-firing and air-staging while keeping SR = 1.16 was taken as base line and NOx reduction (%) was defined by equation:
NOred% ¼
ðNOÞbaseline @ 6% O2 ðNOÞBBR @ 6% O2 ðNOÞbaseline @ 6% O2
100
ð1Þ
The base line value of NO was taken at the exit without the addition of any biomass/staging. This was then corrected at 6% O2. Similarly, all the NO values for coal–biomass blends (staged and un-staged) were also corrected at 6% O2 to avoid any dilution effect. The proximate and ultimate analyses along with bulk densities and HHV of the samples are given in Table 3. The calorific values were determined by using a Parr 6200 oxygen bomb calorimeter. For the determination of surface area, the char samples were analyzed using surface area and pore size analyzer Model (Quanta Chrome Nova 2200e). Ash analysis was done using a PANalytical Axios Advanced XRF spectrometer aided with PANalytical IQ + Semiquantitative software. The VM/FC of all the biomass samples was found to be 3.5–10.5 times higher than VM/FC of coal (Table 5). This is an indication of significant difference in volatility between the coal and the biomasses. Cellulose and lignin are generally recognized as main components in agricultural residues. The weight fraction, except for the cellulose and lignin fraction, corresponds to the fraction of acidsoluble hydrocarbons in the biomass. Fig. 2 shows the differences in the biomass samples structure. The cellulosic content in the biomasses may enhance the ignition characteristics since cellulose compounds have the structure of branching chain of polysaccharides and no aromatic compounds, which are easily volatilized [13].
stoichiometric air) is kept less than 1. A portion of the secondary air (over-fire air) is injected later in the furnace a creating fuel lean zone to complete the combustion. The region downstream of the over-fire air ports is the burnout zone. The combustion in the fuel rich, sub-stoichiometric primary region reduces the formation of thermal–NOx and fuel–NOx [14]. Mixing of combustion air with fuel in two stages by creating a fuel rich zone and fuel lean zone delays the combustion process and is an effective means of NO control in coal-fired boilers [14,15]. The success of air the staged combustion technique primarily depends on the location of OFA injection and the stoichiometric ratio in the primary combustion zone (SR1) [14]. To find out the optimum OFA location along with optimum SR1, the furnace was optimized with RC and RC-SM blend with 10% BBR. The overall stoichiometric ratio at the furnace exit was kept at 1.16. Three stoichiometric ratios in the primary combustion zone, SR1 = 1, 0.9 and 0.8 were used along with each OFA port location from port 2 to port 5 (Fig. 1). Fig. 3 shows the relationship among the OFA injection location, SR1 and NO reduction efficiency. This was found that a decrease in SR1 has a positive effect on the NO reduction % for every OFA nozzle location. This was expected as lower is the SR1, stronger is the reducing environment in the primary zone which is conducive for NOx reduction. The OFA nozzle location determines the lengths of the primary zone and burn out zone. A downward shift of the OFA nozzle location from port 2 increases the primary zone length and decreases the burn out zone length and vice versa (Fig. 1). Figs. 3 and 4 revealed that maximum NO reduction levels were found at SR1 = 0.8 for all the OFA nozzle locations with minimum carbon burnout. This was found that for each step increase in the primary zone length (by moving
3. Results and discussion 3.1. Determination of optimum OFA injection location and SR1 The general principle of air staging is to create a distinct fuel rich (oxygen deficient) zone and a subsequent fuel lean zone inside the furnace. The primary zone stoichiometric ratio (SR1, actual air/
Fig. 2. Structural composition of biomass samples.
Fig. 3. (a) Effect of SR1 and the location of OFA nozzle on NO with RC. (b) Effect of SR1 and the location of OFA nozzle on NO with 10% SM-coal blend.
S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 4. (a) Effect of OFA nozzle location and SR1 on the carbon burn out for RC. (b) Effect of OFA nozzle location and SR1 on the carbon burn out for 10% SM-RC blend.
OFA nozzle from port 2 to 3 and 3 to 4 and so on) has a positive effect on the NO reduction (Fig. 3) and negative effect on carbon
129
burn out (Fig. 4). This is due to the fact that each step increase in the primary zone length increases the residence time in the oxygen deficient, fuel rich primary zone (Fig. 5a). This was in agreement with the findings of Ribeirete and Costa, 2009 [16]. A strong reducing environment in primary combustion zone restrain coal combustion, and large amount of unburned char enters burn out zone [14]. If the length of the burn out zone is short and temperature is not sufficiently high, carbon burnout efficiency can drop. Figs. 3 and 4 showed that a decrease in the primary zone stoichiometry (SR1) has a positive effect on NO reduction and negative effect on carbon burnout. This could be due to the fact that the lower the fuel rich zone stoichiometry the stronger is the reducing environment and longer residence time in the fuel rich zone (Fig. 5b). Increasing the residence time in the fuel rich zone favours the decomposition of N species (NO, NH3, HCN) resulting a significant reduction in NOx emissions. Decreasing SR1 can lead to an increase of reducing species formed in the primary combustion zone, which is conducive to NOx destruction [17]. But at the same time when SR1 is too low, pulverised coal combustion remains incomplete in the primary zone and large amounts of unburnt char enter the burnout zone which may affect the combustion efficiency. There is a trade off between NO reduction and carbon burnout while selecting an optimum OFA nozzle location and SR1. Therefore, too low or too high SR1 will result either low NO reduction with high carbon burn out or high NO reduction with poor carbon burnout at all OFA nozzle locations. Similarly, too short or too long a residence time in the primary zone will result in either, higher carbon burnout with low NO, or high NO reduction with low carbon burnout. In Fig. 6, NO reduction and carbon burnout have been plotted. From Fig. 6, an optimal position is estimated where the carbon burnout begins to level off as SR1 increases. It is also clear from this comparison that the addition of biomass benefits both carbon burnout and NO reduction at this condition. Keeping in view the above discussion and results exhibited in Figs. 3, 4 and 6, an OFA nozzle location at port 3 and SR1 = 0.9 were found to be optimum.
Fig. 5. (a) Effect OFA injection location on residence time. (b) Effect of primary zone stoichiometry on residence time. Air-staged conditions.
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 6. Effect of SR1 on carbon burnout of 10% BBR blends and NO reduction showing optimal condition at SR1 = 0.9 (OFA = port 3) (RC + SBT – , RC + SM – d, RC + CS – j, RC + WC – N, RC + SBR – ., RC – H).
Fig. 7. Effect of BBR on NO reduction under un-staged co-firing conditions.
3.2. Co-combustion of biomass with coal 3.2.1. Co-firing Three blends of each biomass and coal with 5%, 10% and 15% BBR (thermal) were used in this study for un-staged and air-staged tests. The furnace was operated at an overall stoichiometry (SR) of 1.16. It was found that an increase in BBR has positive effects on NO reduction as shown in Fig. 7. This could be linked to an increase in volatility of the fuel by the addition of biomass. Tillman (2000) reported that an increase in volatility (VM/FC) of fuel has positive effect on NOx reduction until the volatility reaches unity. An increase in VM/FC beyond 1 has no positive effect on the NOx reduction during co-firing [18]. A similar effect observed while increasing the BBR for SBT from 10% to 15%. As shown in Fig. 8 that an increase in BBR from 10% to 15% raised VM/FC level from 0.98 to 1.15 resulting a decrease in NO reduction from 21% to 18%.
It is evident from Table 4 that SBT, having significantly higher fuel nitrogen content than WC, SBR and CS, gave considerably higher NO reduction than WC, SBR and CS when co-fired. The results showed in Fig. 7 are in agreement with the findings of Spliethoff and Hein, 1998 that a correlation of the NO emissions and the nitrogen content of the biomass are not likely [7,19]. 3.2.2. Air-staged co-combustion of biomasses with coal The NO levels and corresponding NO reductions have been plotted in Fig. 9 for various primary zone stoichiometry (SR1) levels. It was found that while co-firing coal–biomass blends, a decrease in primary zone stoichiometry (SR1) showed a more positive effect on the NO reduction compared to coal alone for the same level of SR1. As SR1 = 0.9 and OFA location at port 3 were found to be optimum operating parameters, the results plotted in Fig. 9 corresponds to OFA location port 3. The NO reduction was found to be 42% in the case of RC staged firing with SR1 = 0.9. All the coal–biomass
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 8. (a) Effect of BBR on volatility under un-staged co-firing conditions. (b) Effect of volatility on NO reduction under un-staged co-firing conditions.
Table 4 Proximate and ultimate analysis and HHV of the fuel samples. Fuels as received basis
SM CS RC WC SBT SBr a
Ultimate analysis
Proximate analysis
C (%)
H (%)
Oa (%)
N (%)
S (%)
Ash (%)
FC (%)
VM (%)
H2O (%)
41.70 45.2 60.36 42.2 33.6 42.34
5.0 4.40 4.5 4.94 5.3 5.62
32.32 40.5 8.35 35.48 36.27 37.13
2.47 1.0 1.84 0.28 1.5 0.24
0.09 0.0 0.30 0.10 0.0 0.001
4.29 4.9 14 1.70 11.05 9.56
24.58 18 45.48 11.90 13.86 17.11
57 73.1 29.87 71.1 62.81 68.23
14.13 4.0 10.65 15.3 12.28 5.1
Bulk density (kg/m3)
HHV (MJ/kg)
490 310 620 270 160 180
17.70 17.70 27.29 16.39 11.80 17.37
Calculated by difference.
Table 5 Volatility and particle size of the samples. Fuels
VM/FC
Particle volume mean diameter [4,3] (lm)
SM CS WC SBT SBR RC
2.32 4.06 5.97 4.53 3.99 0.656
150.29 209.89 586.25 743.11 586 85.292
blends exhibited higher NO reduction compared to coal alone for SR1 = 0.9 (Fig. 9). A 10% BBR was found to be optimum for coal–biomass blends (Figs. 9 and 10) under the conditions studied. A comparison of the air staged experiments for maximum NO reduction with remarkable air-staging investigations by other researchers is given in Table 6. For optimum 10% BBR (thermal), NO reductions were found to be 72%, 60%, 53%, 51% and 49% for SBT, CS, WC, SM and SBR, respectively. The difference in the % NO reduction for bagasse samples (SBT and SBR) could be linked to the difference in their proximate and ultimate constituents (Table 4) and volatility (Fig. 8a and Table 5). During combustion, fuel bound nitrogen reacts with the released volatiles from fuel and generally tends to form HCN and NHi. These intermediate components subsequently react to form NOx in an oxidizing environment and N2 in a reducing environment [15,20–22]. The formation of HCN and NHi,(1,2,3), depends on the hydrocarbon release during fragmentation. The volatile matter in
the biomasses is higher than coal (Tables 4 and 5) and the volatiles from biomass devolatilization are mainly the combustibles –CO, H2, CxHy [6]. The addition of biomass to replace coal increases the volatility of the fuel. The dominant volatile-nitrogen compound in coal combustion is HCN and in biomass combustion is NH3. The ratio of HCN to NH3 is 0.9 in coal combustion and 0.1 in wood combustion [23]. The formation of NH3 and HCN increases with increasing fuel volatility [24] and gas-phase combustion becomes predominant [18]. Furthermore, most of the fuel nitrogen in biomass is initially converted ammonia and subsequently by reaction to NHi radicals during combustion. The N species reduces NO to molecular nitrogen (essentially providing an in situ thermal DeNOx source) [6,19,25,26]. The mechanistic pathways for formation and reduction of nitrogen oxides during air-staged coal combustion were described by Beer (2000). Bai (2000) described mechanism of wood chips combustion. The reactions involved in two staged biomass combustion has been described by Nussbaumer (2003) [20,21,27]. In the light of above elucidated referred work, possible routes for the NO reduction in near burner reducing zone during two staged biomass coal co-combustion are shown in Fig. 10. Therefore, the synergistic effect of the addition of biomass to control NOx under staged conditions becomes apparent. As biomasses contain less carbon and are highly oxygenated compared to coal (Table 4), the amount of theoretical air required for combustion is less than coal combustion, alone. It is anticipated that the addition of biomass without increasing SR1 level can cause locally stronger reducing environments within the flame in the primary zone resulting in a negative effect on carbon burnout and
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 9. Effect of SR1 and BBR on NO, OFA port 3, air-staged conditions.
NO emissions. The wider range of biomass particle combustion rates related to size and higher volatility may modify the combustion pattern compared with a normal coal flame so that volatile-N from coal may enter the flame after rapid consumption of O2 by more reactive biomass combustion [7]. The CO concentrations measured in primary zone for different biomass-coal blends are shown in Fig. 12. This could explain why, that in the case of 15% BBR, some of the biomass-coal blends showed lower NO reduction and carbon burnouts than 10% BBR blends of the same biomasses (Figs. 10 and 12). In order to compare the effect of biomass addition on burn out, un-staged coal firing burnout was taken as base line (Fig. 13a) for un-staged co-firing tests for same SR = 1.16. To compare the effect of biomass addition under air-staged conditions, burnout of staged coal firing for SR1 = 0.9 was taken as base line (Fig. 13b). In both the cases, burnout improved with an increased biomass share. This was in agreement with the findings of Spliethoff and Hein (1998) [19]. It is clear that the addition of biomass has a positive effect on carbon burnout (except WC Fig. 13). This could be linked to the higher volatility of biomasses (Table 5), higher reactivity and porosity of the biomass chars as compared to coal [19,23,28–33]. The char surface area of the biomasses were found to be 21.712, 5.44, 4.832, 4.549 and 3.958 m2/g for SBT, SBR, WC, SM and CS compared to 0.128 m2/g of RC. In the case of WC, the bulk density to particle size relation could be a limiting factor. WC and SBR have the same particle size and the density of WC is considerably higher than SBR (Tables 4 and 5). Furthermore, the cellulose to lignin ratio of WC was found to be 0.8 compared to SBR of 5.86 (Fig. 2). The axial temperature profiles of the biomass-coal blends co-combustion for SR1 = 0.9 and OFA injection location port 3 are shown in Fig. 13. A temperature difference of 75–120 °C was observed in the primary zone for RC1-WC blend (Fig. 14). This could be due to higher particle size, density and moisture content (Tables 4 and 5) causing a delay on the ignition of WC permitting the coal to ignite ahead of the WC preferentially consuming O2 in the near burner zone resulting lower burnout (Fig. 13). A maximum of 21% NO reduction was found in the case of 10% SBT-coal un-staged co-firing (Fig. 7). When co-fired under optimum
Fig. 10. Possible routes for NOx reduction during co-combustion of biomass with coal in the primary zone.
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S. Munir et al. / Fuel 90 (2011) 126–135 Table 6 A comparison of the air staged experiments conducted for this study and other researchers. Primary fuel
Secondary fuel
Reactor type
Un-staged NO (ppm) for 6% O2 in the primary zone
Initial primary zone stoichiometric ratio (SR1)
Primary zone stoichiometric ratio range (SR1)
Primary zone residence time (s)
Maximum NO reduction efficiency at primary zone stoichiometric ratio (SR1)
References
Bituminous Russian coal 90% RC 90% RC 90% RC 90% RC 85% RC Hard coal
N/A
20 kW pulverised fuel combustor -do-do-do-do-do0.5 MW pulverised fuel combustor -do-do-do-do-do-do1.5 MW under stoker furnace 100 kW laboratory furnace
854
1.16
0.8–1.2
1.17
67% (0.80)
This study
811 828 794 673 769 1384
1.16 1.16 1.16 1.16 1.16 12
0.8–1.2 0.8–1.2 0.8–1.2 0.8–1.2 0.8–1.2 0.6–1.2
1.25 1.29 1.20 1.21 1.16 0.9
67% (0.8) 73% (0.8) 54% (0.8) 76% (0.8) 71% (0.8) 38% (0.6)
This This This This This [19]
1160 1575 935 1450 1225 1775 631 458
12 12 12 12 12 12 12 1.15
0.77–1.2 0.55–1.2 0.65–120 0.55–1.2 0.52–1.2 0.6–12 0.4–12 0.8–1.15
0.9 2.5 0.9 2.5 2.5 2.5 0.5 0.3
30% (0.88) 77% (0.7) 39% (0.82) 81% (0.70) 76% (0.60) 79% (0.60) 50% (0.70) 80% (0.80)
[19] [19] [19] [19] [19] [19] [21] [16]
Hard coal Hard coal Hard coal Hard coal Hard coal Hard coal UF chipboard Bituminous Russian coal
10% 10% 10% 10% 15% N/A
SM CS WC SBT SBR
10% 10% 40% 25% 40% NA N/A N/A
straw straw straw straw straw
study study study study study
Fig. 12. Effect of BBR on CO concentration in the primary zone, SR1 = 0.9, air-staged conditions. (RC1 + WC – N, RC1 + SBT – , RC1 + SM – d, RC1 + CS – j, RC1 + SBR – .). Fig. 11. Effect of BBR on NO, OFA port 3, SR1 = 0.9, air-staged conditions.
air-staged conditions, a 10% BBR of sugarcane bagasse (SBR), shea meal (SM), wood chips (WC), cotton stalk (CS) and sugarcane bagasse (SBT) in coal gave NO reduction of 49%, 51%, 53%, 60% and 72%, respectively. It seems that in the case of un-staged experiments, fuel volatility played positive role in NO reduction. In the case of air-staged co-firing experiments, operating parameters like primary zone stoichiometry (SR1), OFA injection port location and temperature exploited the NOx formation kinetics significantly. It was found that the air-staging technique has a synergistic effect on biomass-coal co-firing for NOx reductions (Figs. 9 and 11) and carbon burnout (Fig. 13). It was found that NO emissions decrease as the distance between staged air injector and burner increase because of an increase in the residence time of the particles in the primary zone. However, if the distance exceeds from 116.5 cm (OFA injection location port 3) (Table 3), carbon burnout decreases significantly (Figs. 4 and 5). A reduction in primary zone stoichiom-
etry was found to cause a decrease in both NO emissions and carbon burnout (Figs. 4, 6 and 9). This was in agreement with the findings of Ribeirete and Costa (2009), Li et al. (2009) and Spliethoff and Hein (1998) [14,16,19]. 4. Slagging and fouling Inorganic constituents in the fuel are the main contributor to slagging and fouling [25,34]. The major elements including alkali metals (K, Na), alkaline earth metals (Ca, Mg), silicon, chlorine and sulphur are involved in reactions leading to ash slagging and fouling [34,35]. Biomasses have lower ash content compared to coal (Table 4). At the same time, biomass ash materials are rich in alkali and alkaline earth metals [7] and these are effective fluxes for alumina-silicates and lower the ash fusion temperature resulting an increase in slagging and fouling propensity [31]. Despite the lower ash fraction in biomass, co-firing of biomass with coal can
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S. Munir et al. / Fuel 90 (2011) 126–135
Fig. 14. Temperature profiles of coal and coal–biomass blends, SR1 = 0.9, air-staged conditions.
Fig. 13. (a) Effect of BBR on burnout with SR = 1.16 (un-staged co-combustion). (b) Effect of BBR on burnout with SR1 = 0.9, OFA port 3, air-staged conditions.
lead to an enhanced slagging and fouling propensity due to its lower fusion temperature depending on chemical and mineralogical composition of fly ash as well as on conditions (temperature and velocity distribution, reducing or oxidizing atmosphere and many others) in the furnace [35]. The ash chemical composition (from air staged co-combustion experiments) for 10% optimum BBR is shown in Table 7. Ash chemical composition of the coal–biomass blends with 10% BBR does not differ significantly from the pure coal ash (Table 7). This was in agreement with the findings of Pronobis (2005) and Grammelis et al. (2006) [35,36]. Pronobis (2005) concluded that
the chemical composition of the biomass-coal blends ash does not differ significantly from the pure coal ash provided BBR (thermal) remains less than 20% [35]. Gramelis et al. (2006) are of the view point that biomass exploitation as secondary fuel in co-combustion processes is technically and economically feasible up to 20% w/w and the produced ash could be further utilised without any major treatment [36]. Therefore, the traditional correlations for the coal ash can be used as predictors for slagging and fouling tendency of ashes from fuel mixtures with 10% BBR. Traditional slagging and fouling indices are given in Table 8.
Table 8 Traditional slagging and fouling indices.a. Slagging (basic 10 acidic compounds ratio) Index
Table 7 Air staged (SR1 = 0.9) co-fired ash composition for 10% BBR of different blends.
a
Components (%)
RC
RC + SM
RC + CS
RC + WC
RC + SBT
RC + SBR
Na2O MgO A12O3 SiO2 p2o5 K2O CaO TiO2 Fe2O3 SO3 Othersa
0.35 1.386 23.07 63.35 0.444 2.18 1.5 0.995 5.75 0.058
0.32 1.68 21.57 60.98 0.7 2.5 2.69 0.9 6.78 0.223
0.36 1.7 22.77 59.9 0.58 2.83 2.27 0.965 5.6 0.06
0.36 1.36 22.98 63.27 0.49 2.28 1.55 0.998 5.74 0.06
0.44 1.83 22.66 61.73 0.5 2.38 2.25 0.95 5.66 0.079
0.33 1.36 21.1 62.36 0.51 2.86 2.39 0.92 5.9 0.089
Include V2O5, Cr2O3, SrO, ZrO2, BaO, Mn3O4, NiO, CuO, ZnO, PbO, HfO2.
Simplified B/A Slagging (Babcock)index
Fouling index
Ratio – slag viscosity index
a
Source: [35,37,38].
B/A < 0.5, low slagging inclination 05 < B/A < 1.0, medium B/A = 1.00, high B/A P 1.7S, severe O3 þCaOþMgO 0.75 < R(B/A) low RðB=AÞ ¼ Fe2SiO 2 þAl2 O3 slagging Rs < 0.6. low slagging Rs ¼ AB Sd inclination Sd ¼ % of S on dry basis Rs = 0.6–2.0, medium Rs = 2.0–2.6, high Rs > 2.6. extremely high B Fu 6 .6, low fouling F u ¼ A ðNa2 O þ K2 OÞ inclination Fu = 0.6–40, high Fu P 40, extremely high SiO2 SR > 72, low slagging SR ¼ SiO2 þMgOþCaOþFe 100 2 O3 inclination 72 P SR > 65, medium SR 6 65, high B A
¼
Fe2 O3 þCaOþNa2 OþK2 OþMgO SiO2 þAl2 O3 þTiO2
S. Munir et al. / Fuel 90 (2011) 126–135 Table 9 Calculated values of the traditional ash deposition indices. Index
SBt
SBR
CS
SM
WC
RC
Rb B/A B/A(+P) R(B/A) RS Fu SR
12.56 0.1472 0.153 0.115 0.039 0.415 86.37
12.84 0.1522 0.158 0.116 0.043 0.485 86.60
12.76 0.15.26 0.1595 0.116 0.043 0.487 86.22
13.97 0.1674 0.176 0.135 0.051 0.472 88
11.29 0.1294 0.135 0.1 0.039 0.342 87.97
10.56 0.1202 0.1255 0.0931 0.0409 0.2977 88.74
Rb represents the percentage of basic constituents in the ash. The minimum ash softening temperature occurs with 35% < Rb < 55%. Although the influence of P2O5 on ash fusibility depends on the form in which it occurs in the fly ash, it seems appropriate to add P2O5 content to the B group in B/A index. The new ratio is called B/A(+P) [35,37]. The calculated values of the traditional ash deposition indices for optimum air staged (SR1 = 0.9), 10% BBR compared to un-staged RC (SR = 1.16) are given in Table 9. The values of the slagging and fouling (S&F) indices for coal– biomass blends with 10% BBR were found to be in the ranges attributed for low slagging and fouling inclination (Tables 8 and 9). It was also noticed that S&F indices for coal–biomass blends with 10% BBR are in proximity with pure coal indices (Table 9). 5. Conclusions Five world biomass samples were co-fired under un-staged and air-staged configurations in a 20 kW down-fired coal combustor to investigate their (biomasses) potential as substitute fuel and an agent to control NOx. Biomasses were pre-blended with coal in 5%, 10% and 15% thermal fractions. An increase in biomass blending ratio (BBR) showed an increase in fuel volatility and increase in NO reduction. A maximum of 21% NO reduction was obtained with coal-SBT co-firing blend. In air-staging experiments, primary zone stoichiometry (SR1) was varied from 0.8 to 1.16 for each of the OFA injection port location from port 2 to 5. It was found that a decrease in primary zone stoichiometry (SR1) and an increase in residence time, by varying OFA port location in the primary zone, both have a positive effect on NO reduction for coal and coal–biomass blends co-firing. A 10% BBR was found to be optimum with respect to NO reduction and corresponding carbon burnouts under air-staged conditions. The investigated biomass-coal blends exhibited significantly higher NO reductions under air-staged conditions compared to un-staged co-firing for the same BBR. NO reductions between 49% and 72% were obtained under optimum air-staged conditions of SR1 = 0.9, OFA injection location = port 3 and BBR = 10% (thermal) compared to 1.4–21% NO reductions under un-staged co-firing. It was found that the air-staging technique has a synergistic effect on biomass-coal co-firing for NOx reductions and carbon burnout. The addition of biomass as secondary fuel up to 10% BBR (thermal) in co-combustion was found to have no adverse effect on slagging and fouling propensity. Acknowledgement The authors wish to express their gratitude to Dr John Smart (RWE npower) for providing the shea meal, wood chips and Russian coal samples for this study. References [1] REN21. Renewables global status report: 2009 update. Paris; 2009. 17p.
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