Experimental investigation and mathematical modelling of wood combustion in a moving grate boiler

Fuel Processing Technology 91 (2010) 1491–1499

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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c

Experimental investigation and mathematical modelling of wood combustion in a moving grate boiler Xiaohui Zhang a,⁎, Qun Chen a, Richard Bradford b, Vida Sharifi a, Jim Swithenbank a a b

SUWIC, Department of Chemical & Process Engineering, Sheffield University, Sheffield, UK Barnsley Metropolitan Borough Council, Barnsley, UK

a r t i c l e

i n f o

Article history: Received 27 November 2009 Received in revised form 24 May 2010 Accepted 25 May 2010 Keywords: Wood combustion Bioenergy Packed bed reactor Mathematical modeling Pollution

a b s t r a c t The use of biomass to generate energy offers significant environmental advantages for the reduction in emissions of greenhouse gases. The main objective of this study was to investigate the performance of a small scale biomass heating plant: i.e. combustion characteristics and emissions. An extensive series of experimental tests was carried out at a small scale residential biomass heating plant i.e. wood chip fired boiler. The concentrations of CO, NOx, particulate matter in the flue gas were measured. In addition, mathematical modelling work using FLIC and FLUENT codes was carried out in order to simulate the overall performance of the wood fired heating system. Results showed that pollutant emissions from the boiler were within the relative emission limits. Mass concentration of CO emission was 550–1600 mg/m3 (10% O2). NOx concentration in the flue gas from the wood chips combustion varied slightly between 28 and 60 ppmv. Mass concentration of PM10 in the flue gas was 205 mg/m3 (10% O2) The modelling results showed that most of the fuel was burnt inside the furnace and little CO was released from the system due to the high flue gas temperature in the furnace. The injection of the secondary air provided adequate mixing and favourable combustion conditions in the over-bed chamber in the wood chips fired boiler. This study has shown that the use of wood heating system result in much lower CO2 emissions than from a fossil fuel e.g. coal fired heating system. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In July 2009, the UK government announced its Transition Plan for becoming a low carbon country. To drive this transition, UK Government has put in place a legally binding target to cut emissions by at least 80% by 2050 and set of five year “carbon budgets” to 2022 to keep the UK on track [1]. As set in the document, the government expects 40% of the power used in 2020 to come from low carbon sources. For homes and communities, around 15% of the annual emission cuts between now and 2020 will be achieved by making homes more efficient and also the usage of small scale renewable energy systems. As the only carbon based renewable energy source, biomass can replace fossil fuels for heating, power generation and transport. Biomass combustion is one of the main technology routes for bioenergy. The most common process of biomass combustion is burning of wood. Small wood burning boilers are frequently used for heating domestics and residential buildings. There are approximately 70,000 small boilers burning firewood, wood chips, or wood pellets in Denmark alone. In general replacing an oil or coal-fired central ⁎ Corresponding author. Department of Chemical & Process Engineering, Sheffield University, Mappin Street, Sheffield, S1 3JD UK. Tel.: +44 114 222 7563; fax: +44 114 222 7501. E-mail address: [email protected] (X. Zhang). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.05.026

heating boiler with a wood burning system can help to reduce the heating bills. However, current biomass combustion applications are by no means environmentally benign. Biomass is difficult to burn as cleanly and efficiently in most appliances as the commonly used fossil fuels. Compared to natural gas and oil-fired systems, wood-fired residential heating systems are subject to significant flue gas emissions, e.g., particulate, NOx, carbon monoxide and other unburned gaseous pollutants [2,3]. In biomass combustion, fuel-bound nitrogen is the main source of NOx emissions [3,4]. The fuel constituents such as K, Na, S, Cl, etc. contribute to the formation of particulate matter during combustion [5,6]. Moreover, incomplete combustion readily results in high emissions of unburnt pollutants such as CO, soot, and PAH [2]. Flue gas emissions from biomass combustion are closely related not only to the fuel properties but also to the combustion operating conditions in the furnace. The factors that could affect the formation of the pollutants during these processes include excess air ratio, combustion temperature, mixing quality and residence time [7–10]. Reducing the flue gas emissions has now become the main focus for the recent development of biomass heating systems [2]. To this end, it is vital to investigate the flue gas emissions from biomass boilers. Biomass combustion consists of complex physical and chemical processes involving heterogeneous and homogeneous reactions [2]. Depending on fuel particle size, biomass drying,

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2. Experimental

using K type thermocouples. The analytical data were recorded by a data logger every 30 s throughout the tests. The flue gas was cooled in a water condenser and a desiccator to remove moisture and dust. The concentrations of CO, CO2 and O2 in the flue gas were then measured using the MGA 3000 Multi-gas Analyser [17]. For NOx analysis, the flue gas from the stack passed through a heated line followed by a heated filter (both 150 °C) to prevent condensing and to remove dust from the gas sample. NOx concentration in the flue gas was measured using a Signal 4000VM NOx Analyser [18]. All the analysers were calibrated before measurement. The mass concentration of particulate matter in the flue gas was measured by an eight stage Andersen Impactor (Series 20-800) [19]. After sampling, the impactor was dismantled and the collection plates with particle samples were kept in a desiccator to dry for 24 h. By weighing the mass of particles on each plate, the mass size distribution and total PM10 concentration were obtained. At the end of the experimental tests, samples of bottom ash and fly ash were collected from the boiler.

2.1. Wood chip boiler

2.3. Fuel and ash analysis

An extensive series of tests were carried out at a biomass-fired heating plant. Two residential wood chip boilers (320 kW and 150 kW) were installed at this site to provide space heating and hot water supplies to approximately 166 residential flats. The fuel store measures 9.5 m × 4 m × 3 m giving approximately 100 m3 of useable storage, which provides approximately 1 week supply in winter and 3 to 4 weeks in summer. During the heating season, approximately 14 tonnes of wood chips per week are delivered to the site. The wood chips are transferred to the boilers by a fully automatic dust-free industrial articulated arm feed unit. Measurements of flue gas emissions were carried out at the exit of the 320 kW wood chip boiler. During the measurements, the wood chip boiler was operating at approximately 65% of its maximum continuous rating (MCR). The wood chip boiler operates automatically from fuel feeding, combustion control, to cleaning and ash removal. Wood chips are fed into the boiler by an auger feeder. The fuel is ignited by an automatic hot-air gun ignition system and burned along the moving grate. Primary air is supplied from beneath the grate. The four-shelled combustion chamber helps to maintain high temperature in the furnace. Along both side walls, secondary and tertiary air is introduced to burn fuel completely. The ashes that fall under the grate are automatically transported to the ash container by a rake. High-temperature flue gas is conducted into the heat exchanger from the top of the furnace. To maintain highly efficient heat transfer, the surface of the upright designed heat exchanger can be automatically cleaned. The heat exchanger also has a built-in patented turbulator multi-cyclone dust separator. This dust separator ensures low dust emissions from the boiler. Ash from the cyclone is removed by the worm screws straight to an ash container outside the boiler.

The size of the wood chips ranged from approximately 8 to 22 mm. The proximate analysis of the wood chips was carried out in accordance with British Standard BS 1016-104:1998 [20], as shown in Table 1. The gross calorific value of the fuel sample was determined using the Parr 1261 bomb calorimeter [21]. The moisture content of the wood chips was approximately 30%. Due to this high moisture content, the calorific value was fairly low. The ash content (2.24%) was also rather high compared to that in wood pellets, which is generally around 0.5%. The ultimate analyses of both fuels were performed using the Carlo Erba EA1108 Elemental Analyser. Full elemental analysis including alkali and heavy metals content of the wood chips, bottom/fly ash and PM10 samples were carried out using the Spectro Ciros ICP (inductively coupled plasma) AES (atomic emission spectrometer). As can be seen in Table 2, the wood chips had quite high concentrations of calcium and potassium.

devolitalisation and char combustion may take place consecutively or in some degree of overlap [3]. It is thus practically very difficult to conduct detailed measurements of flue gas flow, temperature and gas species during biomass combustion. Mathematical modelling provides a powerful tool for simulating biomass combustion and pollutant formation in various furnace geometries [11–13]. In particular, Sheffield University (SUWIC) [14] has developed a mathematical modelling code called FLIC for packed bed combustion which can be de-coupled to FLUENT modeling code. The FLIC code can simulate the heterogeneous reactions inside the burning solid bed during the biomass drying, devolatisation and char combustion. The integration of FLIC and FLUENT modeling codes enables the simulation of whole furnace operation and has been widely used in the recent years in simulating large scale moving bed incinerators [15,16].

3. Mathematical modelling In order to investigate the overall performance of the boiler and its effects on the flue gas emissions, mathematical modelling work was carried out to simulate the wood chip combustion in the furnace. The mathematical model consisted of two sub-models: one model for the burning bed of wood chip on the moving gate, and another model for the gas flow in the freeboard region above the bed. The two models interacted through their respective boundary conditions. The in-bed combustion model (FLIC) of wood chip bed calculated the velocity, temperature and chemical composition of the gas flow exiting the top of the fuel bed, while the out-bed combustion model (FLUENT) calculated the incident radiation flux onto the bed surface [16].

2.2. Flue gas analysis 3.1. In-bed combustion modelling Measurements of the flue gas emissions were carried out at the exit of the 320 kW wood chip boiler. A stainless steel sampling probe was used to sample the flue gas from the centre of the stack duct. At the same time, flue gas temperatures were monitored and recorded

The in-bed wood chip combustion was simulated using FLIC code [16]. The set of governing equations for the mathematical model consists of equations for the conservation of mass, momentum,

Table 1 Properties of the wood chips for the 320 kW residential boiler. Proximate analysis (as received)

Ultimate analysis (daf)

Moisture (wt.%)

Ash (wt.%)

Volatile (wt.%)

Fixed carbon (wt.%)

LCV (MJ/kg)

C (wt.%)

H (wt.%)

O (wt.%)

N (wt.%)

S (wt.%)

29.41

2.24

56.70

11.65

12.84

49.76

5.6

44.39

0.23

0.02

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Table 2 Full elemental analysis of the wood chips (mg/kg as received). Al

B

Ba

Ca

Cd

Cu

Fe

K

Mg

Mn

Na

Ni

P

S

Si

Zn

144

9.6

14.1

4000

0.4

4.6

257

1890

700

64

77

5.3

315

380

102

40

energy and chemical species for both the gas phase and the solid phase in the bed, together with equations for the processes of moisture evaporation, devolatilisation volatile combustion and char gasification. The interaction between the gas and solid phases occurs through the relevant source terms in the conservation equations. For mathematical simulation of the in-bed combustion, the following assumptions were made: (1) Processes investigated were considered to be in quasi-steady state; (2) Heat flux through the boiler shell was assumed to be uniform; In the FLIC modelling, the initial bulk density of the fuel bed was assumed to be 444 kg/m3, given the bed voidage of 0.68. The particle size was assumed to be 15 mm. The computation domain of the fuel bed was discretised into 60 cells along the bed height. Wood chips were assumed to be ignited by over-board radiation at the temperature of 1273 K. Generally, the distribution of primary, secondary and tertiary air varies from boiler to boiler. The estimated ratio of the primary air to the total air quantity is usually from 0.5 to 0.8 in most boilers [22–24]. Using FLIC modelling work, the profiles of gas temperature, velocity and gas composition at the top of the bed were obtained. These results were then input to the FLUENT code as the boundary conditions for the over-bed combustion simulation. 3.1.1. Biomass drying and devolatilisation Wood chips are heated up by over-bed radiation and the convection of counter-current flue gas as they enter the boiler. The wood chips are dried as the moisture is released. The rate of moisture evaporation (Revp, kg/s) can be expressed as [25], Revp


 = Sp hm Cm;s −Cm;g ; when Ts <100°C; or

ð1Þ

Revp = Q cr = Hevp ; when Ts = 100°C

ð2Þ

where Sp is the surface area of wood chips (m2), hm, the mass transfer coefficient between the solid surface and gas (m/s), Cm,s and Cm,g, the concentrations of moisture at the solid surface and in the gas stream (kg/m3), Ts, the solid temperature (K), Hevp, the evaporation heat of the moisture from wood chips (J/kg), and Qcr, the heat transferred to wood chips by convection and radiation (W), i.e.,


Qcr = Sp hc Tg −Ts




 4 4 + εs σb Sp Tenv −Ts

ð3Þ 2

where hc is the convective heat transfer coefficient (W/m K), Tg, the gas temperature (K), Tenv, the furnace temperature (K). For wood chips devolatilisation, a one-step global model [26] is used to describe the rate of volatile release (dV/dt, s− 1),   dV E = Av exp − v ðV∞ −V Þ dt RTs

ð4Þ

where Av is the pre-exponential factor of the devolatilisation rate (s− 1), Ev, the activation energy of the devolatilisation (J/mol), V∞, the ultimate yield of volatile, and V, the remaining volatile in wood chips. 3.1.2. Combustion of volatile matter The products of wood chip devolatilisation mainly consist of CO, CO2, H2 and other hydrocarbons. Tars are usually another main product during pyrolysis. The composition of tar is complex because

there are more than 100 different hydrocarbons present in the material [27,28]. For simplicity, the composition of volatile gases released from the wood chips was assumed to be 22.81% C2H4, 38.2% CO2 and 38.99% H2O by volume based on the mass and energy balances. In this combustion model, precise estimation of the devolatilisation products is not crucial for the final modelling result. It should be noted that FG-Biomass-Functional-Group Pyrolysis Model could be used to predict the devolatilisation product distribution [16]. A two-step global reaction model is adopted for the combustion of C2H4 and CO in the intermediate product. C2 H4 þ 2O2 →2CO þ 2H2 O

ð5Þ

CO þ 1=2O2 →CO2

ð6Þ

Volatile matters are mixed with air and burned in/over the bed once they are released from wood chips. The mixing rate (Rmix, kg/m3 s) inside the bed is assumed to be proportional to the energy loss through the bed, which can be expressed as [25,29], ( Rmix = Cmix ρgas 150 ×

Dg ð1−ψÞ2 = 3 d2p ψ

+ 1:75 ×

Vg ð1−ψÞ1 = 3 dp ψ

)

( ) Cfuel CO2 ; Sfuel SO2

× min

ð7Þ

where Cmix is an empirical constant, ρgas, the density of the volatiles (kg/m3), Dg, the diffusivity of air (m2/s), Vg, the air velocity (m/s), dp, the fuel size (m), ψ, the local void fraction of the fuel bed, C, the mass fractions of the gaseous reactants, and S, the stoichiometric coefficients [25]. In the modelling work, the reaction rates of the volatiles are assumed to be controlled by the mixing process [16]. 3.1.3. Char combustion The main products of char combustion are CO and CO2, C þ αO2 →2ð1−αÞCO þ ð2α−1ÞCO2

ð8Þ

where the ratio of CO/CO2 can be given by [30] CO=CO2 ¼ 2500expð−6420=TÞ

ð9Þ

for temperatures ranging from 730 to 1170 K. The overall char combustion rate is then obtained from [25] Rchar =

PO2 1 kr

+

1 kd

ð10Þ

where, kr and kd (kg/atm m2 s) are the combustion kinetic rate and diffusion rate respectively. 3.2. Out-of-bed combustion modelling Gas phase reactions are simulated in the full geometry of the furnace using the FLUENT code (Version 6.3.26). The standard k − ε two-equation turbulence model and P1 radiation model were employed to solve the conservation equations for momentum, heat and mass transfer together with various gas reactions. The radiation absorption coefficient was calculated as function of characteristic cellsize and gas concentrations [16]. In biomass fired appliances, most of the NO is derived from fuelnitrogen. Fuel-nitrogen is released during the pyrolysis process as HCN or NH3. The kind of intermediates and ratio between HCN and NH3 are dependent on the fuel type and heating rates [16]. In terms of combustion process in packed bed with biomass fuel, HCN is the main

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product from volatile nitrogen [31,32]. For this study, it was assumed that the ratio of volatile-N to char-N was equal to that of volatile to fixed carbon content in the fuel samples. In the FLIC modelling, char N is assumed to convert into the NO and all volatile-N into HCN. In the FLUENT modelling, NO is assumed to be formed from the HCN intermediate by the De Soete mechanism [33]. In the FLUENT modeling work, a total of 217,249 column meshes were employed for the 3D computation of the furnace geometry as shown in Fig. 1. To optimize the convergence and computing time, the grids were set finer at the inlet of fuel and air and coarser towards the exit. 4. Results and discussion 4.1. CO, CO2 and O2 concentrations Fig. 2 shows the concentrations of CO, CO2 and O2 in the flue gas from the wood chip boiler. The gas concentrations fluctuated slightly in accordance with the intermittent fuel feeding operation during the sampling period. The oxygen concentration in the flue gas was approximately 17.4 ± 0.3%, slightly varying in the range of 16.8–17.9%. This concentration corresponded to an equivalence ratio of approximately 0.2 or an excess air ratio of 5. The CO2 concentration varied between 3.1 and 4.6%, with an average of 3.9 vol.%. In order to mix the combustion air with the flue gas sufficiently, the excess air ratio in the small-scale wood chips fired boilers has to be above 1.5 [2]. For this boiler, the moisture content in the wood chips is 30%. High excess air ratio is thus necessary to dry the fuel and to maintain stable combustion in the bed. However, too much excessive air also leads to low combustion temperature in the chamber and a reduction in thermal efficiency. Because of this high excess air ratio, the heat loss due to flue gas (stack loss) for this boiler was approximately 23%. This reduced the overall thermal efficiency to approximately 76%. CO concentration in the flue gas from the wood chip boiler fluctuated significantly from 200 to 500 ppmv, with an average of 319 ppmv during the sampling period. Hence, the emission factor of CO based on the nominal boiler heat output was approximately 524 mg/MJ. CO is the major intermediate product of char and hydrocarbon combustion. The emission of CO implies incomplete combustion in the chamber. The emission levels depend on the availability of oxygen, the combustion temperature, and the residence time in the furnace. The CO emissions from this wood chip boiler

Fig. 1. Grid profile on the surface of the wood chips boiler.

Fig. 2. Concentrations of CO, CO2 and O2 in flue gas from the wood chip boiler.

could be attributed to the high excess air ratio, which reduces the combustion and temperature and the residence time in the furnace. Fig. 3 presents the corrected CO mass concentrations at the reference O2 concentration of 10 vol.%, together with the monitored temperature of the flue gas at the boiler exit. As can be seen, average CO emission from the wood pellet boiler is much lower than the emission limit value (2000 mg/m3 Class2) specified in BS EN 3035:1999 [34]. In the Process Guidance Note published by Defra [35], the CO emission limit value was set at 250 mg/m3 (at reference O2 concentration of 11%) for combustion of solid waste fuel in appliances between 400 kW and 1 MW. For a wood fuel fired boiler with heat output of 150–500 kW, German Standard DIN 4702 and Swiss Ordinance on Air Pollution Control both define the CO emission limit value (ELV) to be 1000 mg/m3 at the reference O2 concentration of 13 vol.%. CO emissions from this measured wood chip boiler were below the specified limit and in compliant with this ELV. 4.2. NOx emission Fig. 4 presents the NOx concentration in the flue gases from the wood chips fired boiler. During the sampling period, NOx concentration in the flue gas from the wood chips combustion varied between 28 and 60 ppmv, with an average of 42.4 ppmv. Consequently, the emission factor of NOx (as NO2) based on the nominal boiler heat output was 113 mg/MJ. This emission factor is lower than that in the AEA report to Defra [36], which provides technical guidance in order to support local authorities in assessing the impact of biomass boilers on air quality. Moreover, this emission factor is very close to those measured in some commercial residential boilers fired with wood pellets in Sweden [37]. When corrected to emissions at the reference

Fig. 3. Corrected CO concentration (mg/m3 at 10% O2) and temperature of flue gas in the wood chip boiler.

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Fig. 6. Elemental compositions in the bottom ash, fly ash and PM10. Fig. 4. Measured NOx concentration and temperature of the flue gases from the wood chip boiler.

O2 concentration of 10 vol.%, the NOx mass concentrations in the flue gas fluctuated between 90 and 150 mg/m3. In biomass fired appliances, air staging and fuel staging are two primary measures for NOx abatement [3]. Though this boiler was operated using air staging, the wet fuel required high primary air flow to maintain stable combustion in the bed. Highly excessive primary air undermined the formation of the reduction zone in the combustion chamber.

The PM emission factor from this wood chip boiler was 126 mg/MJ based on the input fuel net calorific value. This value was lower than that (240 mg/MJ) reported in Defra Technical Guidance [36]. Fig. 6 compares the elemental compositions in the bottom ash, fly ash and PM10 samples. Obviously, fly ash and PM10 produced from the combustion of wood chips contain high contents of K, Ca and S. The content of potassium was highest in PM10, and lowest in the bottom ash. This is expected as the full elemental analysis of the fuel shows high content of calcium and potassium. It has been demonstrated that the bottom ash mainly consists of calcium and the fly ash particles in the flue gas contain high concentration of potassium [37].

4.3. Particulate matter and bottom ash Fig. 5 presents the mass size distribution and accumulated concentrations of particulate matter in the flue gas from the wood chip boiler. Most of the particulate matter emitted from wood chips combustion was in the range from 2.1–10 μm. This may be due to the accumulation of fine particles within the moist environment, considering that the flue gas temperature at the boiler exit was close to the dew point. The measured PM10 emission from the wood chip boiler was 66.3 mg/m3, fairly close to the measurement made by Ehrlich et al. [38]. They measured PM emissions from small-scale firing units in Germany and reported that the PM10 concentrations from a 450 kW log wood firing plant with multi-cyclone were 54.0–56.7 mg/m3. The corrected PM10 concentration from the wood chip boiler at 10 vol.% of O2 reference concentration was approximately 205 mg/m3. At 11% of O2 reference concentration, the corrected PM10 concentration was 186 mg/m3, which was within the emission limit (200 mg/m3) specified in the Defra PG Note for solid waste combustion in appliances between 0.4 and 3 MW [35]. Moreover, this concentration was corrected to be 149 mg/m3 at 13% O2 reference concentration. This value was close to that measured by Obernberger et al. [39] who measured PM emissions from a 440 kW moving grate boiler to be 160 mg/Nm3.

Fig. 5. PM concentration in flue gases from the coal and wood chip boiler.

4.4. Mathematical modelling results Fig. 7 presents the temperature profile for the fuel bed calculated from the FLIC modelling. Fig. 8 shows the individual process rates for moisture evaporation, devolatilisation and char combustion in the burning fuel bed. As shown, once wood chips are fed into the furnace, evaporation of considerable amount of moisture gives rise to immediate weight loss and a low temperature level on the top surface of the fuel bed. As wood chips absorb the radiation heat flux from the high temperature flue gas in the furnace, the bed temperature increases and volatiles are released, resulting in the fuel ignition and combustion. During the volatiles release and combustion periods, the temperature of the top surface remains at about 1000 K. The combustion layer is thin because of the given high flow rate of the primary air. Char combustion starts at the middle of the moving grate where most volatiles are being released. Due to the relatively large size of wood chips, the char combustion and the volatile release processes occur simultaneously towards the end of the grate. The char burning rate increases to approximately 50 kg/m2 h, as shown in Fig. 8.

Fig. 7. Predicted solid temperature distribution in the moving grate (K).

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Fig. 8. Predicted individual processes of the wood chips combustion on the moving grate.

Fig. 9. Predicted composition of gases released from wood chips combustion.

Fig. 9 shows the predicted composition of gases released from fixed bed combustion of wood chips. As the volatiles are released, CO2 and CO concentrations increase. High excess primary air ratio ensures sufficient mixing between the volatiles and the oxygen. Thus, C2H4 keeps at a low level of around 1%. When char begins to burn, CO concentration grows rapidly whereas O2 concentration decreases. However, the oxygen concentration in the burning fuel bed is

significantly high. It should be noted that the excess primary air ratio is generally around 0.7 because of the way the combustion air is staged/injected into the furnace [3]. Due to the short residence time in the burning fuel bed, the hot combustible gases continue their combustion processes in the overbed chamber. Secondary air is injected into the combustion chamber from the sidewalls to ensure complete combustion of unburned gases. Tertiary air is added to prevent the combustion chamber from overheating. The velocity vector profile of the flue gas on the central plane of the furnace is shown in Fig. 10(a). Fig. 10(b) presents the velocity profile on the cross-section plane through a secondary air jet. As shown, the secondary air enhances the mixing of the flue gas with oxygen which helps in burn out of combustible gases. The injection of secondary air gives rise to a reverse flow above the secondary jets. This reverse flow facilitates and stabilises the fuel combustion. However, due to their low momentum compared to the primary air, the secondary air jets can only penetrate into one third of the furnace width. Thus, an induced reverse flow is observed close to the sidewalls. This could lead to a near wall flame in the chamber. Moreover, the secondary air jets hardly exert any mixing in the gas flow towards the back end of the bed (z = 1.8 m, corresponding to the origin of the x-coordinate in Fig. 7 where the fuel is fed into the boiler). With the introduction of the tertiary air, the flue gas velocity increases to 14 m/s. This may greatly reduce the residence time of the flue gas in the chamber. The temperature profiles in the furnace are shown in Fig. 11. Obviously, adequate mixing between the secondary air and combustible gases released from the moving fuel bed improves the gas-phase combustion in the furnace. In the area near the secondary air jets (Fig. 11a), the flue gas temperature rises to above 1200 K. However, as shown in Fig. 11b, the secondary air injection results in intensive combustion and thus forms the high temperature zones near the jets due to the reverse flow near the sidewalls. Inevitably, furnace temperature then decreases by the excessive tertiary air with low temperature. As the furnace is well insulated, heat loss hardly occurs through the shell. Consequently, the average temperature at the outlet remains approximately 964 K. CO is one of the major indicators of the incomplete combustion of wood chips. Fig. 12 presents the profile of CO concentrations in the combustion chamber. High concentration of CO in the furnace are formed and released from wood chips devolatilisation as well as the combustion of char in the fuel bed and C2H4 in the gas phase. The oxidation of CO to CO2 is controlled by the flue gas mixing and the

Fig. 10. Velocity profiles in the combustion chamber of the wood chip fired boiler (a) on the central plane (x = 0.2 m) and (b) on a cross-section through a secondary air jet (z = 0.725 m).

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Fig. 11. Temperature (unit: K) contours in the combustion chamber of the wood chip fired boiler (a) on the central plane (x = 0.2 m) and (b) on the cross-section z = 0.75 m.

temperature in the combustion chamber. Consistent with the velocity and temperature profiles shown in Figs. 10 and 11, CO is burned and the CO concentrations decrease rapidly when mixed with secondary air jets. Although additional tertiary air is introduced into the

Fig. 12. CO profile (unit: mg/m3) in the furnace.

chamber, the relatively low temperature slows down CO oxidation. Thus no further decrease in CO concentrations is observed after the tertiary air jets. Moreover, with the high excess air ratio in the furnace, the residence time of the flue gas is shortened to approximately 1.4 s. Consequently, the concentration of CO in the flue gas at the chamber exit remains above 400 mg/m3. Figs. 13 and 14 show the concentration profiles of O2 and NO in the furnace of the wood chip boiler respectively. In this boiler, staged combustion is obtained by separating the primary and secondary air in two combustion zones. However, high excessive primary air results in high O2 concentrations (a minimum of 12%) in the primary combustion zone, as shown in Fig. 13(a). This leads to the formation of NO from the char-N and a small portion of HCN (from volatile-N) in the primary combustion zone. As the secondary air is introduced into the furnace, the reverse flow above the jets intensifies the mixing and combustion. The O2 concentration near the vortex consequently decreases to a minimum of around 10% (Fig. 13b). In these high temperature zones, HCN reacts with O2 to produce NO which attains a maximum, about 80 ppm, then decreases to 40 ppm at the outlet of the furnace , as shown in Fig. 14. The flue gas temperature and gas composition obtained from the computational modelling work are compared with the measurement results, as shown in Table 3. As can be seen, the modelling results agree well with those from the experimental measurements.

Fig. 13. Profile of O2 concentrations (unit: %) in the combustion chamber of the wood chip fired boiler (a) on the central plane (x = 0.2 m) and (b) on the cross-section z = 0.725 m.

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Acknowledgement The authors would like to thank the Engineering and Physical Science Research Council (EPSRC Supergen Biomass/Biofuels II Consortium) for their financial support for this research work. The authors would also like to take this opportunity to thank Barnsley Metropolitan Borough Council for their technical support and access to the small scale heating plant.

References

Fig. 14. NO profile (unit: ppmv) in the combustion chamber.

5. Conclusions Various experimental tests were carried out on the 320 kW wood chip-fired residential boiler. In addition, FLIC and FLUENT computational codes were used to model and investigate the overall performance of the 320 kW wood chip fired residential boiler. The main findings are as follows: (a) Mass concentration of CO emission was 550–1600 mg/m3 (10% O2), which is below the ELV specified in BS EN 303-5:1999. NOx concentration in the flue gas from the wood chips combustion varied slightly between 28 and 60ppmv. The emission factor of NOx was 113 mg/MJ, i.e. lower than Defra Technical Guidance (150 mg/MJ). Mass concentration of PM10 in the flue gas was 205 mg/m3 (10% O2), matching the ELV (200 mg/m3) in BS EN 303-5:1999, however the Emission Factor of PM10 was 126 mg/ MJ, i.e. significantly lower than the limiting value (240 mg/MJ) specified in Defra Technical Guidance. (b) The wood chip boiler was operating at approximately 65% of its MCR. Due to the high excess air ratio, the stack loss was about 23% of the input energy. This led to the thermal efficiency of the boiler being reduced to 76% (based on net calorific value). (c) In the wood chips fired boiler, due to the high flue gas temperature in the furnace, most of the fuel burns out and little CO is released from the furnace. The injection of the secondary air provides adequate mixing and favourable combustion conditions in the over-bed chamber. (e) The CFD simulation and the experimental results show “high temperature” zones near the walls close to the secondary jets. The number of secondary jets could be reduced and make them larger in diameter in order to get better jet-flow penetration. Further reduction in CO emissions, can be achieved by reducing the total combustion air quantity.

Table 3 Comparison of experimental data and simulation results for wood chips boiler. Volume fraction

Experimental data Simulation

O2

CO

CO2

NO

%

mg/m3

%

ppmv

16.9–18.2 17.38

313–781 403.5

3.06–4.57 3.61

27.8–65.1 40.6

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