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Mathematical modeling of combustion in a grate-fired boiler burning straw and effect of operating conditions under air- and oxygen-enriched atmospheres
Renewable Energy 35 (2010) 895–903
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Mathematical modeling of combustion in a grate-fired boiler burning straw and effect of operating conditions under air- and oxygen-enriched atmospheres Zhaosheng Yu*, Xiaoqian Ma, Yanfen Liao School of Electric Power, South China University of Technology, 510640 Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2007 Accepted 10 October 2009 Available online 18 November 2009
A three-dimensional mathematical model has been developed as a tool for furnace structure design and operation conditions optimization when the straw combustion is in oxygen-enriched or conventional air atmospheres. Mathematical methods have been used based on a combination of FLIC (A fluid Dynamic Incinerator Code) code for the in-bed incineration and commercial software FLUENT for the over-bed combustion. Oxygen-enriched atmospheres promote the destruction of most pollutants due to the high oxygen partial pressures and temperatures, which is reflected by very low residual amounts of organic combustion by-products in the bottom ash and flue gas of the straw-fired boiler unit. The predictions indicated that the maximum combustion temperature is around 1500 K, CO emission is 201 vppm and O2 concentration is about 6.9 vol% at furnace exit, and it is shown that mathematical models can serve as a reliable tool for detailed analysis of straw combustion processes in the packed-bed furnace when compared with literature measurement data. In comparison to traditional straw combustion, the deviation of flue gas CO and NO is 27.5% and 62.1%, respectively. The numerical simulation results showed that combustion under the oxygen-enriched atmosphere excelled combustion under conventional air. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Numerical simulation Furnace Oxygen-enriched Grate-fired boilers burning straw
1. Introduction In China, about 7 Gt of straw is available each year. As a renewable and environmental friendly energy source, straw has a natural advantage for the generation of electricity and heat, especially, in remote small villages and islands which lack electricity [1]. As air contains a large amount of nitrogen (volume percentage is about 76%), and oxygen volume percentage is only 21%, the combustion improver is air in traditional straw combustion, it will reduce the burning temperature notably while chemical energy is translated into thermal energy. However, nitrogen is the production source of NOx. In the process of transferring energy, a large amount of nitrogen in the flue gas has small radiation contribution, but absorbs heat, which causes energy losses. Combustion in an oxygen-enriched air atmosphere has the following features [2,3]: (a) it reduces the available energy loss in the process of translating chemical energy into thermal energy; (b) it decreases the concentration of organic pollutants in the exhaust gas; (c) it reduces the thermal energy of exhaust gas to the minimum. Reactions kinetic vary linearly with oxygen concentration because the activation
* Corresponding author. Tel.: þ86 20 87110232; fax: þ86 20 87110613. E-mail address: [email protected] (Z. Yu). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.10.006
energy of combustion in cellulosic materials varies linearly with oxygen concentration [4–6]. Grate-fired boilers burning straw are often associated with high emission levels and relatively poor fuel burnout. Some researchers have developed numerical model of grate combustion, in order to improve the boiler combustion performance [7]. Compared to experimentation, numerical simulation can save time, human resources and money. Modelling and simulation represent one of the main efforts devoted to biomass combustion in grate-fired boilers [8]. Zhou [9,10] modeled the combustion of straw in a bench-top stationary fixed bed with focus on NO formation and reduction. Kaer [7,11] carried out mathematical modeling of a 22 MW grate-fired boilers burning straw incorporating a standalone bed model and a commercial CFD (Computational Fluid Dynamics) code for gas-space computation. They concluded that poor mixing in the furnace is a key issue leading to high emission levels and relatively high amounts of un-burnt carbon in the fly ash. The stand-alone bed model is based on a one-dimensional ‘‘walking-column’’ approach and includes the energy equations for both the fuel and the gas accounting for heat transfer between the two phases. Van [12] employed a two-dimensional model for straw combustion in a moving bed where the bed was assumed to be at a steady state and the time elapsed since ignition was related to a horizontal position on the grate by a simple linear function. But the model was only validated based on fixed bed experiments. Yang
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[13] simulated the combustion of 38 MW grate-fired boilers burning straw and the running condition of power plant by using share software FLIC and commercial software, mainly aiming at combustion time, temperature in boiler, gas outlet (including NOx), carbon content in flying ash and total burning rate. Yu [14,15] modeled the combustion of grate-fired boilers burning straw by varying the ratio of primary air, secondary air and species of straw. Yin [16] established a reliable baseline CFD model for a thermal 108 MW (108 MWth) grate-fired boilers burning biomass, and a sensitivity analysis was carried out on the basis of the design conditions of the boiler to evaluate the effects of different factors in CFD modeling of grate boilers. This paper aims at working out some of the important parameters of a grate-fired boilers burning straw running in conventional operational conditions and oxygen-enriched atmospheres, by taking a 200 t/d grate-fired boiler burning straw as the study object by FLIC (developed by Sheffield University Waste Incineration Centre) and a commercial software FLUENT. The parameters include gas temperature, velocity and concentration distributing of the top of bed, and solid temperature profile of in-bed, burning character parameters of over-bed, CO and NOx emissions.
Continuity:
vrsb vr vðrsb Vs Þ þ Ub sb þ ¼ Ss vt vx vy
(1)
Energy:
vðrsb Hs Þ vðr Hs Þ vðrsb Vs Hs Þ þ Ub sb þ vt vx vy X v vTs v vTs ls ls Dhk þ Sa h0s Ts Tg þ ¼ vx vy vx vy
(2)
Species:
vðrsb Ysi Þ vðr Y Þ vðrsb Vs Ysi Þ þ Ub sb si þ vt vx vy v vðrsb Ysi Þ v vðrsb Ysi Þ Ds þ Ds Ssi ¼ vx vx vy vy
(3)
2. Boiler and fuel For gas phase in-bed, the conservation equations are as follows: The designed combustion capacity of the plant is 8.3 t/h rice straw (density is about 110 kg/m3). The straw proximate and ultimate analyses used in the predictions are given in Table 1. Because of the low caloric nature of the straw volatiles, the combustion takes place at a lower temperature, but with rapid ignition and fast devolatilisation [17]. The grate-fired boiler burning straw employs a vibrating-grate furnace system as is shown in Fig. 1. Both the primary air (PA) and the secondary air are preheated to about 450 K, and the former enters at the bottom of grate, while the latter is supplied from the front wall and rear wall of the furnace. Once entering the furnace at normal temperature, the straw feed rests on the grate, forming a packed-bed, which is moved forward by periodic vibration of the grate. The compositions of volatile gases released from the solids are assumed to be 20.07% CH4, 43.74% CO, 36.12% H2O and 0.07% CO2 by volume based on mass and energy balances. The air to fuel stoichiometric ratio is around 1.85.
Continuity:
v frg vt
þ
v frg Ug vx
þ
v frg Vg vy
¼ Ss
Momentum: x-momentum:
3. Mathematical modeling 3.1. Transport equations Due to different numerical solving schemes employed and corresponding computer programs, combustion of straw in the furnace is divided into two sections: in-bed combustion and over-bed combustion. In-bed combustion occurs inside the packedbed formed by the straw feed resting on the grate and involves intensive heat and mass exchange between solid and gas phases. Over-bed combustion occurs in the gas phase, i.e., in the over-bed combustion chamber and radiation shaft. The resultant conservation equations for solid phase in-bed are as follows: Table 1 The ultimate and proximate analyses used in the predictions. Ultimate analysis
wt (%)
Proximate analysis
wt (%)
Carbon Hydrogen Oxygen Nitrogen Sulphur
44 5.9 49.25 0.7 0.15
Moisture Ash Fixed carbon Volatile LHV as received
14 4.5 12 69.5 14.9 MJ/kg
Fig. 1. Schematic of the straw-fired furnace.
(4)
Z. Yu et al. / Renewable Energy 35 (2010) 895–903
v frg Vg vt
þ
897
v frg Ug Vg vx
þ
v frg Vg Vg vy
vpg ¼ þ F Vg vy
(6)
Energy:
v frg Hg
v frg Ug Hg
v frg Vg Hg
þ þ vt vx vy X vT vT v v lg g þ lg g þ Sa h0s Ts Tg þ Dhk ¼ vx vy vx vy
(7)
Species:
v frg Ygj vt ¼
þ
v frg Ug Ygj
vx
v frg Ygj v Dg vx vx
þ
! þ
v frg Vg Ygj
v Dg vy
vy ! v frg Ygj vy
þ Ssj þ Sgj
(8)
In the above equations, function F(Ug) and F(Vg) account for bed resistance to the gas flow and can be calculated by using Ergun [18] equation. The resultant conservation equations for gas phase over-bed see FLUENT manual [19].
3.2. NO formation and destruction
Fig. 2. Schematic of mesh scheme employed in the 3D numerical calculations in the furnace.
v frg Ug vt
þ
v frg Ug Ug vx
þ
v frg Vg Ug vy
Fuel-nitrogen is assumed to be the leading source of NO formation in the furnace. Fuel-nitrogen is produced by oxidation of nitrogen contained in the fuel, such as HCN or HN3. In the different oxygen concentration, HCN or HN3 is further oxidized to produce NO and it reduces the total NOx formation by the reaction with NO at the same time. In this paper, NH3 is assumed to be the only intermediate production and the De-Soete [18] model is used to calculate the rates:
134; 400 1 s RT
RNO ¼ 4:0 106 XNH3 XOb 2 exp vpg ¼ þ F Ug vx
(5)
y-momentum:
Fig. 3. Temperature profile of straw inside the burning bed.
Fig. 4. Process rate profile of straw along the bed length.
(9)
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temperature is always less than 1800 K when straw is burning, therefore, in order to simplify the model, this paper is solely concerned with the fuel-Nitrogen. 3.3. Solving the equations
Fig. 5. Gas compositions in flow exiting the bed-top.
113; 400 1 RN2 ¼ 1:8 108 XNH3 XNO exp s RT
(10)
where RNO and RN2 are the NO formation and destruction rates, respectively. It is assumed that the nitrogen remaining in the char after devolatilisation can be heterogeneously oxidized to NO at a rate proportional to the rate of char burnout by a factor related to the relative distribution of carbon and nitrogen in the char [17]. The surface oxidation of the char carbon by NO to form N2 can be calculated from the paper [20]
145:05kJ AE PNO moles=s RT
ðdNO=dtÞ ¼ 4:18 104 exp
(11) 2
Where AE is the external surface area of the char in m /g, and PNO is in atmospheres. When the temperature is above 1800 K, the main contribution to NO production was due to the thermal mechanism situated near the maximum temperature region, while the prompt mechanism significantly contributes to NO destruction situated in fuel-rich regions. The prompt mechanism also contributes to NO production when combustion takes place in oxygen-enriched atmospheres [21]. According to the literature and experimental experience, the
For the simulation of in-bed combustion, the conservation equations (1)–(8) are discretised according to the scheme first proposed by Patankar [22], The FLIC [23–27] code is used for the in-bed simulation and a mesh array of 180 300 was employed. FLUENT version 6.2.1 is used and a total of 825,404 volume meshes are employed for the whole 3D computation space as shown in Fig. 2. The over-bed steady state governing equations are solved using the SIMPLE algorithm and the effect of turbulence on the mean flow field is accounted for using the RNG k-3 model [19]. Radiative heat transfer was modeled using the discrete transfer model [19]. Super-heaters are treated as solid-surfaces with fixed temperature. In-bed simulation provides the profiles of gas temperature, velocity and chemical species concentrations at the bed-top, which are used as boundary conditions for the over-bed combustion simulation. In conversation equation, the over-bed simulation provides the profile of radiation flux incident on the surface of the straw bed. The two simulations iterate again, until convergence criteria are reached. 4. Results and discussion 4.1. Combustion processes of straw on the grate and in the furnace When oxygen concentration is 21 vol% in conventional operational condition, air to fuel stoichiometric ratio is 1.85, the ratio of the primary and secondary air is 6.5:3.5, the straw combustion processes on the grate and in the furnace. Temperature profile inside the bed is illustrated in Fig. 3. As the fresh straw feed enters into the combustion chamber, it forms a bed on the grate. The fresh straw is heated up at two fronts: at the bedtop, strong radiation from the over-bed flame and furnace arch, quickly heats the top-layer of the bed to the ignition point and combustion processes ensue; at the bottom of the bed, preheated primary air heats up the bed at a slower rate, to drive out the moisture in the fuel but not enough to initiate the combustion processes. As a result, a single flame front is formed at the bed-top. As the straw feed moves along by the action of the periodic vibration of the grate, the combustion intensifies and temperature reaches its maximum in the bed, around 1400 K, at around 2.3 m. Bed height decreases steadily for the mass loss and the flame front travels down the bed at a similar rate. Further, along the grate length, flame temperature decreases due to reduced air supply. Flame front falls down to the bottom of the bed, resulting in a short spike in temperature just before the flame dies away and the
Table 2 The balance of the mass flux and heat flux between straw & PA and bed-top exit gases.
Mass flux [kg/s]
Heat fluxa [MWt] a
Fig. 6. Velocity and temperature at the top.
C H O N S
From straw & PA
From gases on bed-top
Relative error [%]
0.829 0.152 4.173 9.579 0.003 34.283
0.825 0.146 4.131 9.631 0.003 34.555
0.547 3.679 1.010 0.546 0.000 0.786
The heat flux from straw & PA including remaining chemical heat in ash; the heat flux from gases on bed-top including heat transferred to over-bed by radiation.
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rest of the combustion processes, which indicates that locally airlean (or fuel-rich) conditions dominate the bed combustion processes. Not surprisingly, high levels of unburned species (CO and CH4) are observed in the out-of-bed flue gases. The increase in the concentration of these combustible gases in the later part of the combustion processes (after 2.4 m) corresponds to the dry out period in Fig. 4 where all the fuel moisture has been driven out and there is a subsequent surge in the volatile release rate. Fig. 6 shows the calculated velocity and temperature at the fuel bed-top. The gas velocity mainly was determined by the amount and distribution of primary air along the grate length, besides the influence of volatile released from the bed. The gas temperature was affected by the combustion of volatile and fixed carbon. According to calculation results, bed-top gas phase averages (vol% for composition) are 1.76% CH4, 3.60% CO, 11.09% H2O, 7.93% CO2 and 65.81% N2 based on mass and energy balances. The mass and heat fluxes are conserved basically between straw & PA and bed-top exit gases (as shown in Table 2). Fig. 7 shows the velocity vectors in furnace as Z ¼ 1.95 m, and it shows that the bulk of the flow out of the burning bed keeps close to the rear wall of the radiation shaft, with the other half of the
Fig. 7. Velocity vectors in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m).
combustion process completes. The last section of the bed is for gradual cooling of the left-over material (ash). Residence time of straw feed on grate is estimated at 17 min from the furnace entrance to the ash exit. Fig. 4 shows the individual process combustion rates of straw along the grate length. Moisture evaporation occurs immediately after the admission of straw feeds into the combustion chamber. It maintains a roughly constant rate for a significant period of the combustion process and falls gradually to zero in the last stage as total moisture in the bed reduces to nil. Volatile release begins at 0.9 m from the feed entrance (the ignition point). The devolatilisation rate remains roughly constant until the bed begins to dry out at a distance of 2.4 m, after that the rate rises to an enhanced level due to more heat available to increase the bed temperature (instead of driving moisture out). Ignition of char occurs at about 1.6 m and a later stage than the end of the volatile release because char combustion only takes place at a higher temperature level after local volatile matter in the fuel is mostly released. The char burning rate maintains at a more or less constant level throughout the combustion processes. The bed converts solids into gases and Fig. 5 shows the compositions of the resultant gas flow exiting the bed-top as a function of distance along the grate length. Oxygen quickly drops from 21% to below 1% just 0.9 m from the feed entrance and remains zero for the
Fig. 8. Temperature profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in Kelvin.
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CO profile is shown in Fig. 9. High concentration CO comes from the bed-top and is consumed in both the over-bed combustion chamber and the radiation shaft. The high CO region coincides with the main pass of the gas flow and is close to the rear wall. In the volume space near the front wall, there is only low concentration CO. The reaction of CO with oxygen is controlled not only by their physical mixing but also by the temperature level [13]. In the second flow passage, temperature reduces to a lower level and any further CO reaction is very slow. In-bed NO production, the contribution of fuel-NOx and promote NOx to NO formation is influenced by high temperature and rich oxygen concentration, and is probably significant in grate boilers bed. Fig. 10 shows NO profile. The Predicted NO concentration is thick over-bed; thermal NOx is less because NOx production increases with augment of combustion temperature once temperature is higher than 1800 K [28]. The formation of NO is a slow kinetic process and extends further up to the radiation shaft and the maximum local NO concentration reaches 260 vppm. On the other hand, the NOx reduction process occurs at the same time as NO is formed. The balance shifts to the former as gas flows to the
Fig. 9. CO profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in vppm.
shaft space occupied by weaker flows and flow into recirculation zones. The main flow passes the furnace exit and enters the second flow passage where the first-stage steam super-heater is located. There is a distinct difference in the second combustion region, because the fresh secondary air insufflates into the furnace and reacts intensively with the gas which is not burnout. Fig. 8 illustrates predicted temperature profile on the same cross-section plane. When the combustion flame emerges from the bed, it has the highest temperature, around 1500 K. As it flows upward, the temperature drops as heat is lost to the cooler walls. As the flame passes the ‘neck’ of the furnace, temperature raises again due to continued combustion reactions of the left-over combustible species with the fresh secondary air, and maximum flame temperature reaches around 1400 K in the radiation shaft. Temperature in the other half of the shaft space, at the front wall side, is lower as it is not the major combusting region and heat is lost to the water tubes on the walls. In the upper part of the radiation shaft, gas temperature further drops as additional heat is transferred to the second-stage steam super-heater. The average temperature of outlet is 1215 K.
Fig. 10. NO profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in vppm.
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Fig. 13. Temperature profile of straw inside the oxygen-enriched atmospheres burning bed.
Fig. 11. Predicted gas temperature compared to measurement.
upper part of the radiation shaft where temperature is continuously falling. In the second flow passage after the furnace exit, there is little NO formation and reduction due to the low temperature level. Furthermore, partial temperature increasing seems to have no beneficial effect on NO reduction, but induce the larger production of NO, therefore, keeping the furnace temperature wellproportioned is extremely important. 4.2. Comparison of predictions to earlier works The reported trends are compared to earlier modeling study of Kaer [11] on straw combustion where the measurements by Van [12] were quoted. Approximate locations of the measurement ports are illustrated in Fig. 1. The data from the infrared temperature sensor of this work is used as the sensor to roughly measure the temperature at that location. To illustrate the implications of not knowing the exact probe locations, the variation in predicted values within a distance of 0.5 m from the estimated probe location is indicated by error bars in Fig. 11. Fig. 11 compares measured temperatures with calculated values. It shows that predictions
Fig. 12. Predicted CO2, and O2 concentration compared to measurement.
agree quite well with measurements. All calculated values are lower than the measurement value beside of locations 1 and 4. As is shown in Fig. 12, in comparison to measurement, all calculated values of the concentrations of CO2 and O2 at the approximate location, it shows that predictions agree quite well with measurements, and the trends of change are coincident. When the highly-efficient fully burnout of the same type straw takes place in a full-scale grate-fired boilers burning straw, the furnace temperature, species concentrations and a variation tendency should be coincident. However, when compared to the literature measurements, the temperature, CO2 and O2 exhibit certain deviations between the measurements and CFD predictions. This is mainly due to fuel properties, the design of the boiler and its key elements (e.g., grate assembly, air supply systems), and the boiler operation, even the CFD model. This option has not been investigated in further detail.
4.3. Straw burning in oxygen-enriched atmospheres When Oxygen concentration is 29 vol%, which is 8 vol% higher than conventional operational condition, air to fuel stoichiometric ratio is 1.85, the ratio of primary air and the second air is 6.5:3.5, the total primary and secondary air rate is about 27.6 vol% less than conventional operational condition, straw combustion processes on the grate and in the furnace.
Fig. 14. Process rate profile of straw burning in oxygen-enriched atmospheres along the bed length.
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Z. Yu et al. / Renewable Energy 35 (2010) 895–903 Table 3 The predicted results and literature [11] measurement. Air Model 6.9 vol% O2 CO 201 vppm 10.3 vol% H2O NO 135.4 vppm Furnace temperature 1215 K Combustible in bottom ash 6.9 wt% Combustion efficiency 99.1%
Measurement 7.1 vol% 214 vppm 11.6 vol% 165 vppm 1187 K 6.2 wt% 99.4%
Oxygen enrichment air model 5.8 vol% 157.6 vppm 13.3 vol% 357.5 vppm 1270 K 5.2 wt% 99.7%
completion of combustion. The CO concentration is 157.6 vppm, about 28% less than conventional operational condition. 4.4. Comparison of straw combustions in oxygen-enriched and conventional air operational conditions Table 3 summarizes comparison of the predicted results to literature [13] available measurement data of the plant for flue gas composition, furnace temperature, bottom ash carbon content and overall combustion efficiency. The furnace exit NO, temperature, combustion efficiency and H2O in oxygen-enriched atmospheres is higher than in conventional air, the deviation is 62.1%, 4.3%, 6.0% and 22.6% respectively; furnace exit CO concentration, oxygen content and combustible in bottom ash in oxygen-enriched atmosphere is less than conventional operational condition, the deviation is 27.5%, 19.0% and 16.1% respectively. Agreement between simulation results and literature plant operation data is satisfactory in conventional operational conditions. There is high NO production because of rich oxygen and high combustion temperature, but it can be reduced by flue gas recycling and management. 5. Conclusions The following conclusions have been drawn as a result of this study:
Fig. 15. Burning in oxygen-enriched atmospheres, CO profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in vppm.
It can be seen from Figs. 13 and 14, in comparison to conventional operational condition, straw volatiles release and burns more intensively in oxygen-enriched atmospheres because higher oxygen concentrations increase straw combustion rate and temperature. The highest temperature is about 1500 K, about 100 K higher than conventional combustion condition. The maximum value of volatile release rate is about 1950 kg/m2 h, compared to conventional operational conditions, the deviation is about 38%, the char combustion lasts a longer time. When the oxygen amount is invariable and the primary air flow rate is less than conventional operational conditions, not only does the gases mix poorly leading to high CO levels but also the straw entrained in the flue gas travels either in a low temperature region with fast char oxidation rates or in a relatively high temperature region with rich oxygen, so that over-bed CO concentration is higher than conventional operational conditions, It can be seen from Fig. 15, at the location where the CO profile begins to even out in the second pass the temperature is too high for CO and the residual carbon in the fly ash to be oxidized at a significant rate. It indicates that rich oxygen secondary air ensures the adequate
a) Mathematical methods based on a combination of an FLIC code for the in-bed incineration and a commercial software FLUENT for the over-bed combustion were validated for detailed analysis of straw combustion processes in the packed-bed furnace; b) The predicted results of in-bed shows that the combustion of straw is initially intensively on the vibration grate under an conventional air atmosphere; the temperature reaches its maximum in the bed, around 1400 K at around 2.3 m along the bed length; volatile release begins at 0.9 m and dries out at a distance of 2.4 m from the feed entrance; c) The simulation results of the over-bed combustion under conventional air atmosphere indicate that flame, from the bed, has the highest temperature around 1500 K, the maximum combustion flame temperature is around 1400 K in the second flow passage, and the furnace exit average temperature and CO concentration are about 1215 K and 201 vppm respectively. Compared with the earlier work, it is proved that the simulation results agree well with the relative literature [11] data; d) The burning of straw in the bed is sub-stoichiometric (fuelrich). Coupled with short residence time in the bed, most of the fuel-N is released from the bed as NH3 and very little NO is formed inside the bed; NO formation mainly occurs in the overbed combustion chamber and in the radiation shaft of the furnace, the furnace exit NO concentration is 135.4 vppm, the prediction is a good agreement with literature [13] measurement data;
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e) To Model the combustion of straw under an oxygen-enriched atmosphere in the same boiler. In comparison with the conventional operational condition, the oxygen-enriched combustion predictions indicates that NO production is increasing, but furnace temperature and CO emission can be improved, the deviation of furnace exit temperature, CO concentration, O2 and NO is 4.3%, 27.5%, 19.0% and 62.1%, respectively. Acknowledgments This work was supported by Natural Science Foundation of Guangdong Province (China) Research Team (No. 003045) and the Doctorate Foundation of South China University of Technology. Nomenclature
D H hs h0s
Dhk R S Sa t T U V x X y Y
r l
mass diffusion coefficients, m2/s enthalpy, J/kg convective mass transfer coefficient between solid and gas, m/s convective heat transfer coefficient between solid and gas, W m2/K heat effect for kth process or reaction, W m3 reaction rate (kg/m3 s), gas universal constant mass source term, kg m3/s particle surface area, m2 time, s temperature, K horizontal velocity, m/s vertical velocity, m/s co-ordinate in the direction of grate length, m solid mass loss fraction, mass fraction of pollutants co-ordinate in the direction of bed height, m mass fraction of species density, kg/m3 thermal conductivity, W/m K
Subscripts b bed g gas i species in solid, i.e., moisture, volatile matter, fixed carbon and ash j species in gas, i.e., O2, CO, CO2, CH4, N2, NO, NH3 s solid sb bulk density References [1] Singal SK, Varun, Singh RP. Rural electrification of a remote island by renewable energy sources. Renewable Energy 2007;32:2491–501. [2] Gohlke O, Busch M. Reduction of combustion by-products in WTE plants: O2 enrichment of underfire air in the M S process. Chemosphere 2001;42: 545–50.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Mathematical modeling of combustion in a grate-fired boiler burning straw and effect of operating conditions under air- and oxygen-enriched atmospheres Zhaosheng Yu*, Xiaoqian Ma, Yanfen Liao School of Electric Power, South China University of Technology, 510640 Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2007 Accepted 10 October 2009 Available online 18 November 2009
A three-dimensional mathematical model has been developed as a tool for furnace structure design and operation conditions optimization when the straw combustion is in oxygen-enriched or conventional air atmospheres. Mathematical methods have been used based on a combination of FLIC (A fluid Dynamic Incinerator Code) code for the in-bed incineration and commercial software FLUENT for the over-bed combustion. Oxygen-enriched atmospheres promote the destruction of most pollutants due to the high oxygen partial pressures and temperatures, which is reflected by very low residual amounts of organic combustion by-products in the bottom ash and flue gas of the straw-fired boiler unit. The predictions indicated that the maximum combustion temperature is around 1500 K, CO emission is 201 vppm and O2 concentration is about 6.9 vol% at furnace exit, and it is shown that mathematical models can serve as a reliable tool for detailed analysis of straw combustion processes in the packed-bed furnace when compared with literature measurement data. In comparison to traditional straw combustion, the deviation of flue gas CO and NO is 27.5% and 62.1%, respectively. The numerical simulation results showed that combustion under the oxygen-enriched atmosphere excelled combustion under conventional air. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Numerical simulation Furnace Oxygen-enriched Grate-fired boilers burning straw
1. Introduction In China, about 7 Gt of straw is available each year. As a renewable and environmental friendly energy source, straw has a natural advantage for the generation of electricity and heat, especially, in remote small villages and islands which lack electricity [1]. As air contains a large amount of nitrogen (volume percentage is about 76%), and oxygen volume percentage is only 21%, the combustion improver is air in traditional straw combustion, it will reduce the burning temperature notably while chemical energy is translated into thermal energy. However, nitrogen is the production source of NOx. In the process of transferring energy, a large amount of nitrogen in the flue gas has small radiation contribution, but absorbs heat, which causes energy losses. Combustion in an oxygen-enriched air atmosphere has the following features [2,3]: (a) it reduces the available energy loss in the process of translating chemical energy into thermal energy; (b) it decreases the concentration of organic pollutants in the exhaust gas; (c) it reduces the thermal energy of exhaust gas to the minimum. Reactions kinetic vary linearly with oxygen concentration because the activation
* Corresponding author. Tel.: þ86 20 87110232; fax: þ86 20 87110613. E-mail address: [email protected] (Z. Yu). 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.10.006
energy of combustion in cellulosic materials varies linearly with oxygen concentration [4–6]. Grate-fired boilers burning straw are often associated with high emission levels and relatively poor fuel burnout. Some researchers have developed numerical model of grate combustion, in order to improve the boiler combustion performance [7]. Compared to experimentation, numerical simulation can save time, human resources and money. Modelling and simulation represent one of the main efforts devoted to biomass combustion in grate-fired boilers [8]. Zhou [9,10] modeled the combustion of straw in a bench-top stationary fixed bed with focus on NO formation and reduction. Kaer [7,11] carried out mathematical modeling of a 22 MW grate-fired boilers burning straw incorporating a standalone bed model and a commercial CFD (Computational Fluid Dynamics) code for gas-space computation. They concluded that poor mixing in the furnace is a key issue leading to high emission levels and relatively high amounts of un-burnt carbon in the fly ash. The stand-alone bed model is based on a one-dimensional ‘‘walking-column’’ approach and includes the energy equations for both the fuel and the gas accounting for heat transfer between the two phases. Van [12] employed a two-dimensional model for straw combustion in a moving bed where the bed was assumed to be at a steady state and the time elapsed since ignition was related to a horizontal position on the grate by a simple linear function. But the model was only validated based on fixed bed experiments. Yang
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[13] simulated the combustion of 38 MW grate-fired boilers burning straw and the running condition of power plant by using share software FLIC and commercial software, mainly aiming at combustion time, temperature in boiler, gas outlet (including NOx), carbon content in flying ash and total burning rate. Yu [14,15] modeled the combustion of grate-fired boilers burning straw by varying the ratio of primary air, secondary air and species of straw. Yin [16] established a reliable baseline CFD model for a thermal 108 MW (108 MWth) grate-fired boilers burning biomass, and a sensitivity analysis was carried out on the basis of the design conditions of the boiler to evaluate the effects of different factors in CFD modeling of grate boilers. This paper aims at working out some of the important parameters of a grate-fired boilers burning straw running in conventional operational conditions and oxygen-enriched atmospheres, by taking a 200 t/d grate-fired boiler burning straw as the study object by FLIC (developed by Sheffield University Waste Incineration Centre) and a commercial software FLUENT. The parameters include gas temperature, velocity and concentration distributing of the top of bed, and solid temperature profile of in-bed, burning character parameters of over-bed, CO and NOx emissions.
Continuity:
vrsb vr vðrsb Vs Þ þ Ub sb þ ¼ Ss vt vx vy
(1)
Energy:
vðrsb Hs Þ vðr Hs Þ vðrsb Vs Hs Þ þ Ub sb þ vt vx vy X v vTs v vTs ls ls Dhk þ Sa h0s Ts Tg þ ¼ vx vy vx vy
(2)
Species:
vðrsb Ysi Þ vðr Y Þ vðrsb Vs Ysi Þ þ Ub sb si þ vt vx vy v vðrsb Ysi Þ v vðrsb Ysi Þ Ds þ Ds Ssi ¼ vx vx vy vy
(3)
2. Boiler and fuel For gas phase in-bed, the conservation equations are as follows: The designed combustion capacity of the plant is 8.3 t/h rice straw (density is about 110 kg/m3). The straw proximate and ultimate analyses used in the predictions are given in Table 1. Because of the low caloric nature of the straw volatiles, the combustion takes place at a lower temperature, but with rapid ignition and fast devolatilisation [17]. The grate-fired boiler burning straw employs a vibrating-grate furnace system as is shown in Fig. 1. Both the primary air (PA) and the secondary air are preheated to about 450 K, and the former enters at the bottom of grate, while the latter is supplied from the front wall and rear wall of the furnace. Once entering the furnace at normal temperature, the straw feed rests on the grate, forming a packed-bed, which is moved forward by periodic vibration of the grate. The compositions of volatile gases released from the solids are assumed to be 20.07% CH4, 43.74% CO, 36.12% H2O and 0.07% CO2 by volume based on mass and energy balances. The air to fuel stoichiometric ratio is around 1.85.
Continuity:
v frg vt
þ
v frg Ug vx
þ
v frg Vg vy
¼ Ss
Momentum: x-momentum:
3. Mathematical modeling 3.1. Transport equations Due to different numerical solving schemes employed and corresponding computer programs, combustion of straw in the furnace is divided into two sections: in-bed combustion and over-bed combustion. In-bed combustion occurs inside the packedbed formed by the straw feed resting on the grate and involves intensive heat and mass exchange between solid and gas phases. Over-bed combustion occurs in the gas phase, i.e., in the over-bed combustion chamber and radiation shaft. The resultant conservation equations for solid phase in-bed are as follows: Table 1 The ultimate and proximate analyses used in the predictions. Ultimate analysis
wt (%)
Proximate analysis
wt (%)
Carbon Hydrogen Oxygen Nitrogen Sulphur
44 5.9 49.25 0.7 0.15
Moisture Ash Fixed carbon Volatile LHV as received
14 4.5 12 69.5 14.9 MJ/kg
Fig. 1. Schematic of the straw-fired furnace.
(4)
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v frg Vg vt
þ
897
v frg Ug Vg vx
þ
v frg Vg Vg vy
vpg ¼ þ F Vg vy
(6)
Energy:
v frg Hg
v frg Ug Hg
v frg Vg Hg
þ þ vt vx vy X vT vT v v lg g þ lg g þ Sa h0s Ts Tg þ Dhk ¼ vx vy vx vy
(7)
Species:
v frg Ygj vt ¼
þ
v frg Ug Ygj
vx
v frg Ygj v Dg vx vx
þ
! þ
v frg Vg Ygj
v Dg vy
vy ! v frg Ygj vy
þ Ssj þ Sgj
(8)
In the above equations, function F(Ug) and F(Vg) account for bed resistance to the gas flow and can be calculated by using Ergun [18] equation. The resultant conservation equations for gas phase over-bed see FLUENT manual [19].
3.2. NO formation and destruction
Fig. 2. Schematic of mesh scheme employed in the 3D numerical calculations in the furnace.
v frg Ug vt
þ
v frg Ug Ug vx
þ
v frg Vg Ug vy
Fuel-nitrogen is assumed to be the leading source of NO formation in the furnace. Fuel-nitrogen is produced by oxidation of nitrogen contained in the fuel, such as HCN or HN3. In the different oxygen concentration, HCN or HN3 is further oxidized to produce NO and it reduces the total NOx formation by the reaction with NO at the same time. In this paper, NH3 is assumed to be the only intermediate production and the De-Soete [18] model is used to calculate the rates:
134; 400 1 s RT
RNO ¼ 4:0 106 XNH3 XOb 2 exp vpg ¼ þ F Ug vx
(5)
y-momentum:
Fig. 3. Temperature profile of straw inside the burning bed.
Fig. 4. Process rate profile of straw along the bed length.
(9)
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temperature is always less than 1800 K when straw is burning, therefore, in order to simplify the model, this paper is solely concerned with the fuel-Nitrogen. 3.3. Solving the equations
Fig. 5. Gas compositions in flow exiting the bed-top.
113; 400 1 RN2 ¼ 1:8 108 XNH3 XNO exp s RT
(10)
where RNO and RN2 are the NO formation and destruction rates, respectively. It is assumed that the nitrogen remaining in the char after devolatilisation can be heterogeneously oxidized to NO at a rate proportional to the rate of char burnout by a factor related to the relative distribution of carbon and nitrogen in the char [17]. The surface oxidation of the char carbon by NO to form N2 can be calculated from the paper [20]
145:05kJ AE PNO moles=s RT
ðdNO=dtÞ ¼ 4:18 104 exp
(11) 2
Where AE is the external surface area of the char in m /g, and PNO is in atmospheres. When the temperature is above 1800 K, the main contribution to NO production was due to the thermal mechanism situated near the maximum temperature region, while the prompt mechanism significantly contributes to NO destruction situated in fuel-rich regions. The prompt mechanism also contributes to NO production when combustion takes place in oxygen-enriched atmospheres [21]. According to the literature and experimental experience, the
For the simulation of in-bed combustion, the conservation equations (1)–(8) are discretised according to the scheme first proposed by Patankar [22], The FLIC [23–27] code is used for the in-bed simulation and a mesh array of 180 300 was employed. FLUENT version 6.2.1 is used and a total of 825,404 volume meshes are employed for the whole 3D computation space as shown in Fig. 2. The over-bed steady state governing equations are solved using the SIMPLE algorithm and the effect of turbulence on the mean flow field is accounted for using the RNG k-3 model [19]. Radiative heat transfer was modeled using the discrete transfer model [19]. Super-heaters are treated as solid-surfaces with fixed temperature. In-bed simulation provides the profiles of gas temperature, velocity and chemical species concentrations at the bed-top, which are used as boundary conditions for the over-bed combustion simulation. In conversation equation, the over-bed simulation provides the profile of radiation flux incident on the surface of the straw bed. The two simulations iterate again, until convergence criteria are reached. 4. Results and discussion 4.1. Combustion processes of straw on the grate and in the furnace When oxygen concentration is 21 vol% in conventional operational condition, air to fuel stoichiometric ratio is 1.85, the ratio of the primary and secondary air is 6.5:3.5, the straw combustion processes on the grate and in the furnace. Temperature profile inside the bed is illustrated in Fig. 3. As the fresh straw feed enters into the combustion chamber, it forms a bed on the grate. The fresh straw is heated up at two fronts: at the bedtop, strong radiation from the over-bed flame and furnace arch, quickly heats the top-layer of the bed to the ignition point and combustion processes ensue; at the bottom of the bed, preheated primary air heats up the bed at a slower rate, to drive out the moisture in the fuel but not enough to initiate the combustion processes. As a result, a single flame front is formed at the bed-top. As the straw feed moves along by the action of the periodic vibration of the grate, the combustion intensifies and temperature reaches its maximum in the bed, around 1400 K, at around 2.3 m. Bed height decreases steadily for the mass loss and the flame front travels down the bed at a similar rate. Further, along the grate length, flame temperature decreases due to reduced air supply. Flame front falls down to the bottom of the bed, resulting in a short spike in temperature just before the flame dies away and the
Table 2 The balance of the mass flux and heat flux between straw & PA and bed-top exit gases.
Mass flux [kg/s]
Heat fluxa [MWt] a
Fig. 6. Velocity and temperature at the top.
C H O N S
From straw & PA
From gases on bed-top
Relative error [%]
0.829 0.152 4.173 9.579 0.003 34.283
0.825 0.146 4.131 9.631 0.003 34.555
0.547 3.679 1.010 0.546 0.000 0.786
The heat flux from straw & PA including remaining chemical heat in ash; the heat flux from gases on bed-top including heat transferred to over-bed by radiation.
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rest of the combustion processes, which indicates that locally airlean (or fuel-rich) conditions dominate the bed combustion processes. Not surprisingly, high levels of unburned species (CO and CH4) are observed in the out-of-bed flue gases. The increase in the concentration of these combustible gases in the later part of the combustion processes (after 2.4 m) corresponds to the dry out period in Fig. 4 where all the fuel moisture has been driven out and there is a subsequent surge in the volatile release rate. Fig. 6 shows the calculated velocity and temperature at the fuel bed-top. The gas velocity mainly was determined by the amount and distribution of primary air along the grate length, besides the influence of volatile released from the bed. The gas temperature was affected by the combustion of volatile and fixed carbon. According to calculation results, bed-top gas phase averages (vol% for composition) are 1.76% CH4, 3.60% CO, 11.09% H2O, 7.93% CO2 and 65.81% N2 based on mass and energy balances. The mass and heat fluxes are conserved basically between straw & PA and bed-top exit gases (as shown in Table 2). Fig. 7 shows the velocity vectors in furnace as Z ¼ 1.95 m, and it shows that the bulk of the flow out of the burning bed keeps close to the rear wall of the radiation shaft, with the other half of the
Fig. 7. Velocity vectors in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m).
combustion process completes. The last section of the bed is for gradual cooling of the left-over material (ash). Residence time of straw feed on grate is estimated at 17 min from the furnace entrance to the ash exit. Fig. 4 shows the individual process combustion rates of straw along the grate length. Moisture evaporation occurs immediately after the admission of straw feeds into the combustion chamber. It maintains a roughly constant rate for a significant period of the combustion process and falls gradually to zero in the last stage as total moisture in the bed reduces to nil. Volatile release begins at 0.9 m from the feed entrance (the ignition point). The devolatilisation rate remains roughly constant until the bed begins to dry out at a distance of 2.4 m, after that the rate rises to an enhanced level due to more heat available to increase the bed temperature (instead of driving moisture out). Ignition of char occurs at about 1.6 m and a later stage than the end of the volatile release because char combustion only takes place at a higher temperature level after local volatile matter in the fuel is mostly released. The char burning rate maintains at a more or less constant level throughout the combustion processes. The bed converts solids into gases and Fig. 5 shows the compositions of the resultant gas flow exiting the bed-top as a function of distance along the grate length. Oxygen quickly drops from 21% to below 1% just 0.9 m from the feed entrance and remains zero for the
Fig. 8. Temperature profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in Kelvin.
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CO profile is shown in Fig. 9. High concentration CO comes from the bed-top and is consumed in both the over-bed combustion chamber and the radiation shaft. The high CO region coincides with the main pass of the gas flow and is close to the rear wall. In the volume space near the front wall, there is only low concentration CO. The reaction of CO with oxygen is controlled not only by their physical mixing but also by the temperature level [13]. In the second flow passage, temperature reduces to a lower level and any further CO reaction is very slow. In-bed NO production, the contribution of fuel-NOx and promote NOx to NO formation is influenced by high temperature and rich oxygen concentration, and is probably significant in grate boilers bed. Fig. 10 shows NO profile. The Predicted NO concentration is thick over-bed; thermal NOx is less because NOx production increases with augment of combustion temperature once temperature is higher than 1800 K [28]. The formation of NO is a slow kinetic process and extends further up to the radiation shaft and the maximum local NO concentration reaches 260 vppm. On the other hand, the NOx reduction process occurs at the same time as NO is formed. The balance shifts to the former as gas flows to the
Fig. 9. CO profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in vppm.
shaft space occupied by weaker flows and flow into recirculation zones. The main flow passes the furnace exit and enters the second flow passage where the first-stage steam super-heater is located. There is a distinct difference in the second combustion region, because the fresh secondary air insufflates into the furnace and reacts intensively with the gas which is not burnout. Fig. 8 illustrates predicted temperature profile on the same cross-section plane. When the combustion flame emerges from the bed, it has the highest temperature, around 1500 K. As it flows upward, the temperature drops as heat is lost to the cooler walls. As the flame passes the ‘neck’ of the furnace, temperature raises again due to continued combustion reactions of the left-over combustible species with the fresh secondary air, and maximum flame temperature reaches around 1400 K in the radiation shaft. Temperature in the other half of the shaft space, at the front wall side, is lower as it is not the major combusting region and heat is lost to the water tubes on the walls. In the upper part of the radiation shaft, gas temperature further drops as additional heat is transferred to the second-stage steam super-heater. The average temperature of outlet is 1215 K.
Fig. 10. NO profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in vppm.
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Fig. 13. Temperature profile of straw inside the oxygen-enriched atmospheres burning bed.
Fig. 11. Predicted gas temperature compared to measurement.
upper part of the radiation shaft where temperature is continuously falling. In the second flow passage after the furnace exit, there is little NO formation and reduction due to the low temperature level. Furthermore, partial temperature increasing seems to have no beneficial effect on NO reduction, but induce the larger production of NO, therefore, keeping the furnace temperature wellproportioned is extremely important. 4.2. Comparison of predictions to earlier works The reported trends are compared to earlier modeling study of Kaer [11] on straw combustion where the measurements by Van [12] were quoted. Approximate locations of the measurement ports are illustrated in Fig. 1. The data from the infrared temperature sensor of this work is used as the sensor to roughly measure the temperature at that location. To illustrate the implications of not knowing the exact probe locations, the variation in predicted values within a distance of 0.5 m from the estimated probe location is indicated by error bars in Fig. 11. Fig. 11 compares measured temperatures with calculated values. It shows that predictions
Fig. 12. Predicted CO2, and O2 concentration compared to measurement.
agree quite well with measurements. All calculated values are lower than the measurement value beside of locations 1 and 4. As is shown in Fig. 12, in comparison to measurement, all calculated values of the concentrations of CO2 and O2 at the approximate location, it shows that predictions agree quite well with measurements, and the trends of change are coincident. When the highly-efficient fully burnout of the same type straw takes place in a full-scale grate-fired boilers burning straw, the furnace temperature, species concentrations and a variation tendency should be coincident. However, when compared to the literature measurements, the temperature, CO2 and O2 exhibit certain deviations between the measurements and CFD predictions. This is mainly due to fuel properties, the design of the boiler and its key elements (e.g., grate assembly, air supply systems), and the boiler operation, even the CFD model. This option has not been investigated in further detail.
4.3. Straw burning in oxygen-enriched atmospheres When Oxygen concentration is 29 vol%, which is 8 vol% higher than conventional operational condition, air to fuel stoichiometric ratio is 1.85, the ratio of primary air and the second air is 6.5:3.5, the total primary and secondary air rate is about 27.6 vol% less than conventional operational condition, straw combustion processes on the grate and in the furnace.
Fig. 14. Process rate profile of straw burning in oxygen-enriched atmospheres along the bed length.
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Z. Yu et al. / Renewable Energy 35 (2010) 895–903 Table 3 The predicted results and literature [11] measurement. Air Model 6.9 vol% O2 CO 201 vppm 10.3 vol% H2O NO 135.4 vppm Furnace temperature 1215 K Combustible in bottom ash 6.9 wt% Combustion efficiency 99.1%
Measurement 7.1 vol% 214 vppm 11.6 vol% 165 vppm 1187 K 6.2 wt% 99.4%
Oxygen enrichment air model 5.8 vol% 157.6 vppm 13.3 vol% 357.5 vppm 1270 K 5.2 wt% 99.7%
completion of combustion. The CO concentration is 157.6 vppm, about 28% less than conventional operational condition. 4.4. Comparison of straw combustions in oxygen-enriched and conventional air operational conditions Table 3 summarizes comparison of the predicted results to literature [13] available measurement data of the plant for flue gas composition, furnace temperature, bottom ash carbon content and overall combustion efficiency. The furnace exit NO, temperature, combustion efficiency and H2O in oxygen-enriched atmospheres is higher than in conventional air, the deviation is 62.1%, 4.3%, 6.0% and 22.6% respectively; furnace exit CO concentration, oxygen content and combustible in bottom ash in oxygen-enriched atmosphere is less than conventional operational condition, the deviation is 27.5%, 19.0% and 16.1% respectively. Agreement between simulation results and literature plant operation data is satisfactory in conventional operational conditions. There is high NO production because of rich oxygen and high combustion temperature, but it can be reduced by flue gas recycling and management. 5. Conclusions The following conclusions have been drawn as a result of this study:
Fig. 15. Burning in oxygen-enriched atmospheres, CO profile in the over-bed combustion chamber and radiation shaft on plane Z ¼ 1.95 m (range 0–3.9 m). Values are shown in vppm.
It can be seen from Figs. 13 and 14, in comparison to conventional operational condition, straw volatiles release and burns more intensively in oxygen-enriched atmospheres because higher oxygen concentrations increase straw combustion rate and temperature. The highest temperature is about 1500 K, about 100 K higher than conventional combustion condition. The maximum value of volatile release rate is about 1950 kg/m2 h, compared to conventional operational conditions, the deviation is about 38%, the char combustion lasts a longer time. When the oxygen amount is invariable and the primary air flow rate is less than conventional operational conditions, not only does the gases mix poorly leading to high CO levels but also the straw entrained in the flue gas travels either in a low temperature region with fast char oxidation rates or in a relatively high temperature region with rich oxygen, so that over-bed CO concentration is higher than conventional operational conditions, It can be seen from Fig. 15, at the location where the CO profile begins to even out in the second pass the temperature is too high for CO and the residual carbon in the fly ash to be oxidized at a significant rate. It indicates that rich oxygen secondary air ensures the adequate
a) Mathematical methods based on a combination of an FLIC code for the in-bed incineration and a commercial software FLUENT for the over-bed combustion were validated for detailed analysis of straw combustion processes in the packed-bed furnace; b) The predicted results of in-bed shows that the combustion of straw is initially intensively on the vibration grate under an conventional air atmosphere; the temperature reaches its maximum in the bed, around 1400 K at around 2.3 m along the bed length; volatile release begins at 0.9 m and dries out at a distance of 2.4 m from the feed entrance; c) The simulation results of the over-bed combustion under conventional air atmosphere indicate that flame, from the bed, has the highest temperature around 1500 K, the maximum combustion flame temperature is around 1400 K in the second flow passage, and the furnace exit average temperature and CO concentration are about 1215 K and 201 vppm respectively. Compared with the earlier work, it is proved that the simulation results agree well with the relative literature [11] data; d) The burning of straw in the bed is sub-stoichiometric (fuelrich). Coupled with short residence time in the bed, most of the fuel-N is released from the bed as NH3 and very little NO is formed inside the bed; NO formation mainly occurs in the overbed combustion chamber and in the radiation shaft of the furnace, the furnace exit NO concentration is 135.4 vppm, the prediction is a good agreement with literature [13] measurement data;
Z. Yu et al. / Renewable Energy 35 (2010) 895–903
e) To Model the combustion of straw under an oxygen-enriched atmosphere in the same boiler. In comparison with the conventional operational condition, the oxygen-enriched combustion predictions indicates that NO production is increasing, but furnace temperature and CO emission can be improved, the deviation of furnace exit temperature, CO concentration, O2 and NO is 4.3%, 27.5%, 19.0% and 62.1%, respectively. Acknowledgments This work was supported by Natural Science Foundation of Guangdong Province (China) Research Team (No. 003045) and the Doctorate Foundation of South China University of Technology. Nomenclature
D H hs h0s
Dhk R S Sa t T U V x X y Y
r l
mass diffusion coefficients, m2/s enthalpy, J/kg convective mass transfer coefficient between solid and gas, m/s convective heat transfer coefficient between solid and gas, W m2/K heat effect for kth process or reaction, W m3 reaction rate (kg/m3 s), gas universal constant mass source term, kg m3/s particle surface area, m2 time, s temperature, K horizontal velocity, m/s vertical velocity, m/s co-ordinate in the direction of grate length, m solid mass loss fraction, mass fraction of pollutants co-ordinate in the direction of bed height, m mass fraction of species density, kg/m3 thermal conductivity, W/m K
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