Fuel Processing Technology 91 (2010) 1491–1499

Contents lists available at ScienceDirect

Fuel Processing Technology

j ourna l homepage: www.e lsev ie r.com/ locate / fuproc

Experimental investigation and mathematical modelling of wood combustion in amoving grate boiler

Xiaohui Zhang a,⁎, Qun Chen a, Richard Bradford b, Vida Sharifi a, Jim Swithenbank a

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

⁎ Corresponding author. Department of Chemical &University, Mappin Street, Sheffield, S1 3JD UK. Tel.: +4222 7501.

E-mail address: Zhangxh2003@gmail.com (X. Zhang

0378-3820/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.fuproc.2010.05.026

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 November 2009Received in revised form 24 May 2010Accepted 25 May 2010

Keywords:Wood combustionBioenergyPacked bed reactorMathematical modelingPollution

The use of biomass to generate energy offers significant environmental advantages for the reduction inemissions of greenhouse gases. The main objective of this study was to investigate the performance of asmall scale biomass heating plant: i.e. combustion characteristics and emissions. An extensive series ofexperimental tests was carried out at a small scale residential biomass heating plant i.e. wood chip firedboiler. 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 overallperformance of the wood fired heating system. Results showed that pollutant emissions from the boiler werewithin 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. Massconcentration of PM10 in the flue gas was 205 mg/m3 (10% O2) The modelling results showed that most ofthe fuel was burnt inside the furnace and little CO was released from the system due to the high flue gastemperature in the furnace. The injection of the secondary air provided adequate mixing and favourablecombustion conditions in the over-bed chamber in the wood chips fired boiler. This study has shown that theuse of wood heating system result in much lower CO2 emissions than from a fossil fuel e.g. coal fired heatingsystem.

Process Engineering, Sheffield4 114 222 7563; fax: +44 114

).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In July 2009, the UK government announced its Transition Plan forbecoming a low carbon country. To drive this transition, UKGovernment has put in place a legally binding target to cut emissionsby at least 80% by 2050 and set of five year “carbon budgets” to 2022to keep the UK on track [1]. As set in the document, the governmentexpects 40% of the power used in 2020 to come from low carbonsources. For homes and communities, around 15% of the annualemission cuts between now and 2020 will be achieved by makinghomes more efficient and also the usage of small scale renewableenergy systems. As the only carbon based renewable energy source,biomass can replace fossil fuels for heating, power generation andtransport. Biomass combustion is one of the main technology routesfor bioenergy. The most common process of biomass combustion isburning of wood. Small wood burning boilers are frequently used forheating domestics and residential buildings. There are approximately70,000 small boilers burning firewood, wood chips, or wood pellets inDenmark alone. In general replacing an oil or coal-fired central

heating boiler with a wood burning system can help to reduce theheating bills.

However, current biomass combustion applications are by nomeans environmentally benign. Biomass is difficult to burn as cleanlyand efficiently in most appliances as the commonly used fossil fuels.Compared to natural gas and oil-fired systems, wood-fired residentialheating systems are subject to significant flue gas emissions, e.g.,particulate, NOx, carbon monoxide and other unburned gaseouspollutants [2,3]. In biomass combustion, fuel-bound nitrogen is themain source of NOx emissions [3,4]. The fuel constituents such as K,Na, S, Cl, etc. contribute to the formation of particulate matter duringcombustion [5,6]. Moreover, incomplete combustion readily results inhigh emissions of unburnt pollutants such as CO, soot, and PAH [2].

Flue gas emissions from biomass combustion are closely relatednot only to the fuel properties but also to the combustion operatingconditions in the furnace. The factors that could affect the formation ofthe 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 forthe recent development of biomass heating systems [2].

To this end, it is vital to investigate the flue gas emissions frombiomass boilers. Biomass combustion consists of complex physicaland chemical processes involving heterogeneous and homogeneousreactions [2]. Depending on fuel particle size, biomass drying,

1492 X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

devolitalisation and char combustion may take place consecutively orin some degree of overlap [3]. It is thus practically very difficult toconduct detailed measurements of flue gas flow, temperature and gasspecies during biomass combustion. Mathematical modelling pro-vides a powerful tool for simulating biomass combustion andpollutant formation in various furnace geometries [11–13]. Inparticular, Sheffield University (SUWIC) [14] has developed amathematical modelling code called FLIC for packed bed combustionwhich can be de-coupled to FLUENTmodeling code. The FLIC code cansimulate the heterogeneous reactions inside the burning solid bedduring the biomass drying, devolatisation and char combustion. Theintegration of FLIC and FLUENT modeling codes enables thesimulation of whole furnace operation and has been widely used inthe recent years in simulating large scale moving bed incinerators[15,16].

2. Experimental

2.1. Wood chip boiler

An extensive series of tests were carried out at a biomass-firedheating plant. Two residential wood chip boilers (320 kW and150 kW) were installed at this site to provide space heating and hotwater supplies to approximately 166 residential flats. The fuel storemeasures 9.5 m×4 m×3 m giving approximately 100 m3 of useablestorage, which provides approximately 1 week supply in winter and 3to 4 weeks in summer. During the heating season, approximately14 tonnes of wood chips per week are delivered to the site. The woodchips are transferred to the boilers by a fully automatic dust-freeindustrial articulated arm feed unit. Measurements of flue gasemissions were carried out at the exit of the 320 kW wood chipboiler. During the measurements, the wood chip boiler was operatingat 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 fedinto the boiler by an auger feeder. The fuel is ignited by an automatichot-air gun ignition system and burned along the moving grate.Primary air is supplied from beneath the grate. The four-shelledcombustion chamber helps to maintain high temperature in thefurnace. Along both sidewalls, secondary and tertiary air is introducedto burn fuel completely. The ashes that fall under the grate areautomatically transported to the ash container by a rake.

High-temperature flue gas is conducted into the heat exchangerfrom the top of the furnace. To maintain highly efficient heat transfer,the surface of the upright designed heat exchanger can be automat-ically cleaned. The heat exchanger also has a built-in patentedturbulator multi-cyclone dust separator. This dust separator ensureslow dust emissions from the boiler. Ash from the cyclone is removedby the worm screws straight to an ash container outside the boiler.

2.2. Flue gas analysis

Measurements of the flue gas emissions were carried out at theexit of the 320 kW wood chip boiler. A stainless steel sampling probewas used to sample the flue gas from the centre of the stack duct. Atthe same time, flue gas temperatures were monitored and recorded

Table 1Properties of the wood chips for the 320 kW residential boiler.

Proximate analysis (as received)

Moisture Ash Volatile Fixed carbon LCV(wt.%) (wt.%) (wt.%) (wt.%) (MJ/kg)

29.41 2.24 56.70 11.65 12.84

using K type thermocouples. The analytical data were recorded by adata logger every 30 s throughout the tests.

The flue gas was cooled in a water condenser and a desiccator toremove moisture and dust. The concentrations of CO, CO2 and O2 inthe flue gas were then measured using the MGA 3000 Multi-gasAnalyser [17]. For NOx analysis, the flue gas from the stack passedthrough a heated line followed by a heated filter (both 150 °C) toprevent condensing and to remove dust from the gas sample. NOx

concentration in the flue gas was measured using a Signal 4000VMNOx Analyser [18]. All the analysers were calibrated before measure-ment. Themass concentration of particulatematter in the flue gas wasmeasured by an eight stage Andersen Impactor (Series 20-800) [19].After sampling, the impactor was dismantled and the collection plateswith particle samples were kept in a desiccator to dry for 24 h. Byweighing the mass of particles on each plate, the mass sizedistribution and total PM10 concentration were obtained. At the endof the experimental tests, samples of bottom ash and fly ash werecollected from the boiler.

2.3. Fuel and ash analysis

The size of the wood chips ranged from approximately 8 to 22 mm.The proximate analysis of the wood chips was carried out inaccordance with British Standard BS 1016-104:1998 [20], as shownin Table 1. The gross calorific value of the fuel sample was determinedusing 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 inwood pellets, which is generally around 0.5%. The ultimate analyses ofboth fuels were performed using the Carlo Erba EA1108 ElementalAnalyser.

Full elemental analysis including alkali and heavy metals contentof the wood chips, bottom/fly ash and PM10 samples were carried outusing the Spectro Ciros ICP (inductively coupled plasma) AES (atomicemission spectrometer). As can be seen in Table 2, the wood chips hadquite high concentrations of calcium and potassium.

3. Mathematical modelling

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

3.1. In-bed combustion modelling

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

Ultimate analysis (daf)

C H O N S(wt.%) (wt.%) (wt.%) (wt.%) (wt.%)

49.76 5.6 44.39 0.23 0.02

Table 2Full 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

1493X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

energy and chemical species for both the gas phase and the solidphase in the bed, together with equations for the processes ofmoisture evaporation, devolatilisation volatile combustion and chargasification. The interaction between the gas and solid phases occursthrough the relevant source terms in the conservation equations. Formathematical simulation of the in-bed combustion, the followingassumptions were made:

(1) Processes investigated were considered to be in quasi-steadystate;

(2) Heat flux through the boiler shell was assumed to be uniform;

In the FLIC modelling, the initial bulk density of the fuel bed wasassumed tobe444 kg/m3, given thebedvoidage of 0.68. Theparticle sizewas assumed to be 15 mm. The computation domain of the fuel bedwasdiscretised into 60 cells along thebedheight.Wood chipswere assumedto be ignited by over-board radiation at the temperature of 1273 K.Generally, the distribution of primary, secondary and tertiary air variesfrom boiler to boiler. The estimated ratio of the primary air to the totalair 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 boundaryconditions for the over-bed combustion simulation.

3.1.1. Biomass drying and devolatilisationWood chips are heated up by over-bed radiation and the

convection of counter-current flue gas as they enter the boiler. Thewood chips are dried as the moisture is released. The rate of moistureevaporation (Revp, kg/s) can be expressed as [25],

Revp = Sphm 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 transfercoefficient between the solid surface and gas (m/s), Cm,s and Cm,g, theconcentrations of moisture at the solid surface and in the gas stream(kg/m3), Ts, the solid temperature (K), Hevp, the evaporation heat ofthe moisture from wood chips (J/kg), and Qcr, the heat transferred towood chips by convection and radiation (W), i.e.,

Qcr = Sphc Tg−Ts� �

+ εsσbSp T4env−T4

s

� �ð3Þ

where hc is the convective heat transfer coefficient (W/m2 K), Tg, thegas temperature (K), Tenv, the furnace temperature (K).

For wood chips devolatilisation, a one-step global model [26] isused to describe the rate of volatile release (dV/dt, s−1),

dVdt

= Av exp − EvRTs

� �V∞−Vð Þ 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 ultimateyield of volatile, and V, the remaining volatile in wood chips.

3.1.2. Combustion of volatile matterThe products of wood chip devolatilisation mainly consist of CO,

CO2, H2 and other hydrocarbons. Tars are usually another mainproduct during pyrolysis. The composition of tar is complex because

there are more than 100 different hydrocarbons present in thematerial [27,28]. For simplicity, the composition of volatile gasesreleased 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 energybalances. In this combustion model, precise estimation of thedevolatilisation products is not crucial for the final modelling result.It should be noted that FG-Biomass-Functional-Group Pyrolysis Modelcould be used to predict the devolatilisation product distribution [16].

A two-step global reaction model is adopted for the combustion ofC2H4 and CO in the intermediate product.

C2H4 þ 2O2→2COþ 2H2O ð5Þ

COþ 1=2O2→CO2 ð6ÞVolatile matters are mixedwith 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 throughthe bed, which can be expressed as [25,29],

Rmix = Cmixρgas 150 ×Dgð1−ψÞ2 =3

d2pψ+ 1:75 ×

Vgð1−ψÞ1 =3dpψ

( )× min

Cfuel

Sfuel;CO2

SO2

( )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 massfractions of the gaseous reactants, and S, the stoichiometric coeffi-cients [25]. In the modelling work, the reaction rates of the volatilesare assumed to be controlled by the mixing process [16].

3.1.3. Char combustionThe main products of char combustion are CO and CO2,

C þ αO2→2ð1−αÞCO þ ð2α−1ÞCO2 ð8Þwhere the ratio of CO/CO2 can be given by [30]

CO=CO2 ¼ 2500expð−6420=TÞ ð9Þfor temperatures ranging from 730 to 1170 K. The overall charcombustion rate is then obtained from [25]

Rchar =PO2

1kr

+ 1kd

ð10Þ

where, kr and kd (kg/atm m2 s) are the combustion kinetic rate anddiffusion rate respectively.

3.2. Out-of-bed combustion modelling

Gas phase reactions are simulated in the full geometry of thefurnace using the FLUENT code (Version 6.3.26). The standard k−εtwo-equation turbulence model and P1 radiation model wereemployed to solve the conservation equations for momentum, heatand mass transfer together with various gas reactions. The radiationabsorption coefficient was calculated as function of characteristic cell-size and gas concentrations [16].

In biomass fired appliances, most of the NO is derived from fuel-nitrogen. Fuel-nitrogen is released during the pyrolysis process asHCN or NH3. The kind of intermediates and ratio between HCN andNH3 are dependent on the fuel type and heating rates [16]. In terms ofcombustion process in packed bed with biomass fuel, HCN is the main

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

1494 X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

product from volatile nitrogen [31,32]. For this study, it was assumedthat the ratio of volatile-N to char-N was equal to that of volatile tofixed carbon content in the fuel samples. In the FLIC modelling, char Nis assumed to convert into the NO and all volatile-N into HCN. In theFLUENT modelling, NO is assumed to be formed from the HCNintermediate by the De Soete mechanism [33].

In the FLUENT modeling work, a total of 217,249 column mesheswere employed for the 3D computation of the furnace geometry asshown in Fig. 1. To optimize the convergence and computing time, thegrids were set finer at the inlet of fuel and air and coarser towards theexit.

4. Results and discussion

4.1. CO, CO2 and O2 concentrations

Fig. 2 shows the concentrations of CO, CO2 and O2 in the flue gasfrom the wood chip boiler. The gas concentrations fluctuated slightlyin accordance with the intermittent fuel feeding operation during thesampling period. The oxygen concentration in the flue gas wasapproximately 17.4±0.3%, slightly varying in the range of 16.8–17.9%.This concentration corresponded to an equivalence ratio of approx-imately 0.2 or an excess air ratio of 5. The CO2 concentration variedbetween 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 tobe above 1.5 [2]. For this boiler, the moisture content in the woodchips is 30%. High excess air ratio is thus necessary to dry the fuel andto maintain stable combustion in the bed. However, too muchexcessive air also leads to low combustion temperature in thechamber and a reduction in thermal efficiency. Because of this highexcess air ratio, the heat loss due to flue gas (stack loss) for this boilerwas approximately 23%. This reduced the overall thermal efficiency toapproximately 76%.

CO concentration in the flue gas from the wood chip boilerfluctuated significantly from 200 to 500 ppmv, with an average of319 ppmv during the sampling period. Hence, the emission factor ofCO based on the nominal boiler heat output was approximately524 mg/MJ. CO is the major intermediate product of char andhydrocarbon combustion. The emission of CO implies incompletecombustion in the chamber. The emission levels depend on theavailability of oxygen, the combustion temperature, and the residencetime in the furnace. The CO emissions from this wood chip boiler

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

could be attributed to the high excess air ratio, which reduces thecombustion and temperature and the residence time in the furnace.

Fig. 3 presents the corrected CO mass concentrations at thereference O2 concentration of 10 vol.%, together with the monitoredtemperature of the flue gas at the boiler exit. As can be seen, averageCO emission from the wood pellet boiler is much lower than theemission limit value (2000 mg/m3 Class2) specified in BS EN 303-5:1999 [34]. In the Process Guidance Note published by Defra [35], theCO emission limit value was set at 250 mg/m3 (at reference O2

concentration of 11%) for combustion of solid waste fuel in appliancesbetween 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 PollutionControl both define theCOemission limit value (ELV) to be 1000 mg/m3

at the reference O2 concentration of 13 vol.%. CO emissions from thismeasured wood chip boiler were below the specified limit and incompliant with this ELV.

4.2. NOx emission

Fig. 4 presents the NOx concentration in the flue gases from thewood chips fired boiler. During the sampling period, NOx concentra-tion in the flue gas from the wood chips combustion varied between28 and 60 ppmv, with an average of 42.4 ppmv. Consequently, theemission factor of NOx (as NO2) based on the nominal boiler heatoutput was 113 mg/MJ. This emission factor is lower than that in theAEA report to Defra [36], which provides technical guidance in orderto support local authorities in assessing the impact of biomass boilerson air quality. Moreover, this emission factor is very close to thosemeasured in some commercial residential boilers fired with woodpellets 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 thewood chip boiler.

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 woodchip boiler.

1495X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

O2 concentration of 10 vol.%, the NOx mass concentrations in the fluegas fluctuated between 90 and 150 mg/m3.

In biomass fired appliances, air staging and fuel staging are twoprimary measures for NOx abatement [3]. Though this boiler wasoperated using air staging, the wet fuel required high primary air flowtomaintain stable combustion in the bed. Highly excessive primary airundermined the formation of the reduction zone in the combustionchamber.

4.3. Particulate matter and bottom ash

Fig. 5 presents the mass size distribution and accumulatedconcentrations of particulate matter in the flue gas from the wood chipboiler. Most of the particulate matter emitted from wood chipscombustion was in the range from 2.1–10 μm. This may be due to theaccumulation offineparticleswithin themoist environment, consideringthat thefluegas temperature at theboiler exitwas close to thedewpoint.

The measured PM10 emission from the wood chip boiler was66.3 mg/m3, fairly close to the measurement made by Ehrlich et al.[38]. They measured PM emissions from small-scale firing units inGermany and reported that the PM10 concentrations from a 450 kWlog wood firing plant with multi-cyclone were 54.0–56.7 mg/m3. Thecorrected PM10 concentration from the wood chip boiler at 10 vol.% ofO2 reference concentration was approximately 205 mg/m3. At 11% ofO2 reference concentration, the corrected PM10 concentration was186 mg/m3, which was within the emission limit (200 mg/m3)specified in the Defra PG Note for solid waste combustion inappliances between 0.4 and 3 MW [35]. Moreover, this concentrationwas corrected to be 149 mg/m3 at 13% O2 reference concentration.This value was close to that measured by Obernberger et al. [39] whomeasured PM emissions from a 440 kW moving grate boiler to be160 mg/Nm3.

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

The PM emission factor from this wood chip boiler was 126 mg/MJbased on the input fuel net calorific value. This value was lower thanthat (240 mg/MJ) reported in Defra Technical Guidance [36].

Fig. 6 compares the elemental compositions in the bottom ash, flyash and PM10 samples. Obviously, fly ash and PM10 produced from thecombustion of wood chips contain high contents of K, Ca and S. Thecontent of potassium was highest in PM10, and lowest in the bottomash. This is expected as the full elemental analysis of the fuel showshigh content of calcium and potassium. It has been demonstrated thatthe bottom ash mainly consists of calcium and the fly ash particles inthe flue gas contain high concentration of potassium [37].

4.4. Mathematical modelling results

Fig. 7 presents the temperature profile for the fuel bed calculated fromthe FLIC modelling. Fig. 8 shows the individual process rates for moistureevaporation, devolatilisation and char combustion in the burning fuel bed.As shown, once wood chips are fed into the furnace, evaporation ofconsiderable amount of moisture gives rise to immediate weight loss anda low temperature level on the top surface of the fuel bed. As wood chipsabsorb the radiation heat flux from the high temperature flue gas in thefurnace, the bed temperature increases and volatiles are released,resulting in the fuel ignition and combustion. During the volatiles releaseand combustion periods, the temperature of the top surface remains atabout 1000 K. The combustion layer is thin because of the given highflowrate of theprimary air. Char combustion starts at themiddle of themovinggrate where most volatiles are being released. Due to the relatively largesize ofwood chips, the char combustion and the volatile release processesoccur simultaneously towards the end of the grate. The char burning rateincreases to approximately 50 kg/m2 h, as shown in Fig. 8.

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

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

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

1496 X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

Fig. 9 shows the predicted composition of gases released fromfixed bed combustion of wood chips. As the volatiles are released, CO2

and CO concentrations increase. High excess primary air ratio ensuressufficient mixing between the volatiles and the oxygen. Thus, C2H4

keeps at a low level of around 1%. When char begins to burn, COconcentration grows rapidly whereas O2 concentration decreases.However, the oxygen concentration in the burning fuel bed is

Fig. 10. Velocity profiles in the combustion chamber of the wood chip fired boiler (a) on(z=0.725 m).

significantly high. It should be noted that the excess primary airratio is generally around 0.7 because of the way the combustion air isstaged/injected into the furnace [3].

Due to the short residence time in the burning fuel bed, the hotcombustible gases continue their combustion processes in the over-bed chamber. Secondary air is injected into the combustion chamberfrom the sidewalls to ensure complete combustion of unburned gases.Tertiary air is added to prevent the combustion chamber fromoverheating. The velocity vector profile of the flue gas on the centralplane of the furnace is shown in Fig. 10(a). Fig. 10(b) presents thevelocity profile on the cross-section plane through a secondary air jet.

As shown, the secondary air enhances the mixing of the flue gaswith oxygen which helps in burn out of combustible gases. Theinjection of secondary air gives rise to a reverse flow above thesecondary jets. This reverse flow facilitates and stabilises the fuelcombustion. However, due to their low momentum compared to theprimary air, the secondary air jets can only penetrate into one third ofthe furnace width. Thus, an induced reverse flow is observed close tothe sidewalls. This could lead to a near wall flame in the chamber.Moreover, the secondary air jets hardly exert any mixing in the gasflow towards the back end of the bed (z=1.8 m, corresponding to theorigin of the x-coordinate in Fig. 7 where the fuel is fed into theboiler). With the introduction of the tertiary air, the flue gas velocityincreases to 14 m/s. This may greatly reduce the residence time of theflue gas in the chamber.

The temperature profiles in the furnace are shown in Fig. 11.Obviously, adequate mixing between the secondary air and combus-tible gases released from the moving fuel bed improves the gas-phasecombustion in the furnace. In the area near the secondary air jets(Fig. 11a), the flue gas temperature rises to above 1200 K. However, asshown in Fig. 11b, the secondary air injection results in intensivecombustion and thus forms the high temperature zones near the jetsdue to the reverse flow near the sidewalls. Inevitably, furnacetemperature then decreases by the excessive tertiary air with lowtemperature. As the furnace is well insulated, heat loss hardly occursthrough the shell. Consequently, the average temperature at theoutlet remains approximately 964 K.

CO is one of the major indicators of the incomplete combustion ofwood chips. Fig. 12 presents the profile of CO concentrations in thecombustion chamber. High concentration of CO in the furnace areformed and released from wood chips devolatilisation as well as thecombustion of char in the fuel bed and C2H4 in the gas phase. Theoxidation of CO to CO2 is controlled by the flue gas mixing and the

the central plane (x=0.2 m) and (b) on a cross-section through a secondary air jet

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.

1497X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

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

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

Fig. 13. Profile of O2 concentrations (unit: %) in the combustion chamber of the wood chip fi

chamber, the relatively low temperature slows down CO oxidation.Thus no further decrease in CO concentrations is observed after thetertiary 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 chamberexit remains above 400 mg/m3.

Figs. 13 and 14 show the concentration profiles of O2 and NO in thefurnace of the wood chip boiler respectively. In this boiler, stagedcombustion is obtained by separating the primary and secondary airin two combustion zones. However, high excessive primary air resultsin high O2 concentrations (a minimum of 12%) in the primarycombustion zone, as shown in Fig. 13(a). This leads to the formation ofNO from the char-N and a small portion of HCN (from volatile-N) inthe primary combustion zone. As the secondary air is introduced intothe furnace, the reverse flow above the jets intensifies the mixing andcombustion. The O2 concentration near the vortex consequentlydecreases to a minimum of around 10% (Fig. 13b). In these hightemperature zones, HCN reacts with O2 to produce NOwhich attains amaximum, about 80 ppm, then decreases to 40 ppm at the outlet ofthe furnace , as shown in Fig. 14.

The flue gas temperature and gas composition obtained from thecomputational modelling work are compared with the measurementresults, as shown in Table 3. As can be seen, the modelling resultsagree well with those from the experimental measurements.

red boiler (a) on the central plane (x=0.2 m) and (b) on the cross-section z=0.725 m.

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

1498 X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

5. Conclusions

Various experimental tests were carried out on the 320 kW woodchip-fired residential boiler. In addition, FLIC and FLUENT computa-tional codes were used to model and investigate the overallperformance of the 320 kW wood chip fired residential boiler. Themain 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 combustionvaried slightly between 28 and 60ppmv. The emission factor ofNOx was 113 mg/MJ, i.e. lower than Defra Technical Guidance(150 mg/MJ). Mass concentration of PM10 in the flue gas was205 mg/m3 (10% O2), matching the ELV (200 mg/m3) in BS EN303-5:1999, 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 itsMCR. Due to the high excess air ratio, the stack loss was about23% of the input energy. This led to the thermal efficiency of theboiler being reduced to 76% (based on net calorific value).

(c) In the wood chips fired boiler, due to the high flue gastemperature in the furnace, most of the fuel burns out and littleCO is released from the furnace. The injection of the secondaryair provides adequate mixing and favourable combustionconditions in the over-bed chamber.

(e) The CFD simulation and the experimental results show “hightemperature” zones near the walls close to the secondary jets.The number of secondary jets could be reduced andmake themlarger in diameter in order to get better jet-flow penetration.Further reduction in CO emissions, can be achieved by reducingthe total combustion air quantity.

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

Volume fraction

O2 CO CO2 NO

% mg/m3 % ppmv

Experimental data 16.9–18.2 313–781 3.06–4.57 27.8–65.1Simulation 17.38 403.5 3.61 40.6

Acknowledgement

The authors would like to thank the Engineering and PhysicalScience Research Council (EPSRC Supergen Biomass/Biofuels IIConsortium) for their financial support for this research work. Theauthors would also like to take this opportunity to thank BarnsleyMetropolitan Borough Council for their technical support and accessto the small scale heating plant.

References

[1] HM Government, The UK Low Carbon Transition Plan: National strategy forclimate and energy, 9780108508394, 2009.

[2] S. van Loo, J. Koppejan, The Handbook of Biomass Combustion and Co-firing,Earthscan, London, 2007.

[3] T. Nussbaumer, Combustion and co-combustion of biomass: fundamentals,technologies, and primary measures for emission reduction, Energ Fuel 17(2003) 1510–1521.

[4] I. Obernberger, Decentralized biomass combustion: state of the art and futuredevelopment, Biomass Bioenerg 14 (1998) 33–56.

[5] C. Boman, Particulate and Gaseous Emissions from Residential Biomass Combus-tion, ETPC Report 05-03, 2005.

[6] T. Nussbaumer, C. Czasch, N. Klippel, L. Johansson, C. Tullin, Particulate Emissionsfrom Biomass Combustion in IEA Countries, Bioenergy Task, vol. 32, InternationalEnergy Agency (IEA), ISBN: 3-908705-18-5, 2008.

[7] R.P. Van der Lans, L.T. Pedersen, A. Jensen, P. Glarborg, K. Damjohansen, Modellingand experiments of straw combustion in a grate furnace, Biomass Bioenerg 19(2000) 199–208.

[8] Y.B. Yang, V.N. Sharifi, J. Swithenbank, Effect of air flow rate and fuel moisture onthe burning behaviors of biomass and simulated municipal solid wastes in packedbeds, Fuel 83 (2004) 1553–1562.

[9] W. Zhao, Z.Q. Li, D. Wang, Q.Y. Zhu, R. Sun, B.H. Meng, G. Zhao, Combustioncharacteristics of different parts of corn straw and NO formation in a fixed bed,Bioresour. Technol. 99 (2008) 2956–2963.

[10] C. Boman, Particulate and Gaseous Emissions from Residential Biomass Combus-tion, ETPC Report 05-03, 2005.

[11] C. Yin, L. Rosendahl, S.K. Kar, T.J. Condra, Use of numerical modeling in design forco-firing biomass in wall-fired burners, Chem. Eng. Sci. 59 (2004) 3281–3292.

[12] M. Miltner, A. Miltner, M. Harasek, A. Friedl, Process simulation and CFDcalculations for the development of an innovative baled biomass-fired combus-tion chamber, Appl. Therm. Eng. 27 (2007) 1138–1143.

[13] L. Ma, J.M. Jones, M. Pourkashanian, A. Williams, Modelling the combustion ofpulverised biomass in an industrial combustion test furnace, Fuel 86 (2007)1959–1965.

[14] Y.B. Yang, Y.R. Goh, R. Zakaria, V. Nasserzadeh, J. Swithenbank, Mathematicalmodelling of MSW incineration on a travelling bed, Waste Manage 22 (2002)369–380.

[15] C. Ryu, Y.B. Yang, V. Nasserzadeh, J. Swithenbank, Thermal reaction modeling of alarge municipal solid waste incinerator, Combust. Sci. Technol. 176 (2004)1891–1907.

[16] Y.B. Yang, R. Newman, V. Sharifi, J. Swithenbank, J. Ariss, Mathematical modellingof straw combustion in a 38 MWe power plant furnace and effect of operatingconditions, Fuel 86 (2007) 129–142.

[17] BS EN 15058, Stationary Source Emissions – Determination of the MassConcentration of Carbon Monoxide (Co) – Reference Method: Non-DispersiveInfrared Spectrometry, 2006.

[18] BS EN 14792, Stationary Source Emissions – Determination of Mass Concentrationof Nitrogen Oxides (NOx) – Reference Method: Chemiluminescence, 2005.

[19] ISO/DIS 23210-1. Draft EN ISO 23210 Stationary source emissions – Determina-tion of PM 10/2.5 mass concentration in flue gas, Part 1: Measurement at lowconcentrations by use of impactors, 2009.

[20] BS 1016-104, Methods for Analysis and Testing of Coal and Coke – Part 104:Proximate Analysis, 1998.

[21] BS 1016-105, Methods for Analysis and Testing of Coal and Coke – Part 105:Determination of Gross Calorific Value, 1992.

[22] R.L. Bain, R.P. Overend, K.R. Craig, Biomass-fired power generation, Fuel ProcessTechnol 54 (1998) 1–16.

[23] S.K. Kær, Numerical modelling of a straw-fired grate boiler, Fuel 83 (2004)1183–1190.

[24] R. Scharler, I. Obernberger, G. Langle, J. Heinzle, CFD Analysis of Air Staging andFlue Gas Recirculation in Biomass Grate Furnaces, Proceedings of the 1st WorldConference on Biomass for Energy and Industry. Sevilla, Spain. 2, 2000,pp. 1935–1939.

[25] Y.B. Yang, H. Yamauchi, V. Nasserzadeh, J. Swithenbank, Effects of fueldevolatilisation on the combustion of wood chips and incineration of simulatedmunicipal solid wastes in a packed bed, Fuel 82 (2003) 2205–2221.

[26] L.D. Smoot, D.T. Patt, Pulverized-coal Combustion and Gasification, Plenum Press,New York, 1979.

[27] C.D. Blasi, Heat, momentum and mass transport through a shrinking biomassparticle exposed to thermal radiation, Chem Eng Sci 51 (1996) 1121–1132.

[28] K. Papadikis, S. Gu, A.V. Bridgwater, H. Gerhauser, Application of CFD to model fastpyrolysis of biomass, Fuel Process Technol 90 (2009) 504–512.

1499X. Zhang et al. / Fuel Processing Technology 91 (2010) 1491–1499

[29] M.L. Hunt, C.L. Tien, Non-Darcian convection in cylindrical beds, J. Heat Transfer110 (1988) 378–384.

[30] J.A. Arthur, Reactions between carbon and oxygen, Trans. Faraday Soc. 47 (1951)164–178.

[31] C. Yin, L. Rosendahl, S.K. Kar, T.J. Condra, Use of numerical modeling in design forco-firing biomass in wall-fired burners, Chem. Eng. Sci. 59 (2004) 3281–3292.

[32] L. Liang, R. Sun, J. Fei, S. Wu, X. Liu, K. Dai, N. Yao, Experimental study on effects ofmoisture content on combustion characteristics of simulated municpal solidwastes in a fixed bed, Bioresour. Technol. 99 (2008) 7238–7246.

[33] FG-BioMass – Functional-Group Pyrolysis Model, version 10.0.13, Advanced FuelResearch, Inc., 87 Church Street, East Hartford, CT 06108, 2006. Website: http://www.afrinc.com.

[34] BS EN 303-5, Heating boilers – Part 5: Heating boilers for solid fuels, hand andautomatically fired, nominal heat output of up to 300 kW – Terminology,requirements, testing and marking, 1999.

[35] Defra, Secretary of State's Guidance for Combustion of Fuel Manufactured from orComprised of Solid Waste in Appliances between 0.4 and 3 MW Rated ThermalInput, 2004.

[36] AEA, Technical Guidance: Screening assessment for biomass boiler. Report to theDepartment of Environment, Food and Rural Affairs, ED48673005/R2655, 2008.

[37] L.S. Johansson, C. Tullin, B. Leckner, P. Sjovall, Particle emissions from biomasscombustion in small combustors, Biomass Bioenerg. 25 (2003) 435–446.

[38] C. Ehrlich, G. Noll, W.D. Kalkoff, G. Baumbach, A. Dreiseidler, PM10, PM2.5 andPM1.0 – Emissions from industrial plants – Results from measurementprogrammes in Germany, Atmos. Environ. 41 (2007) 6236–6254.

[39] T. Obernberger, T. Brunner, M. Joller, Characterisation and Formation of Aerosolsand Fly-Ashes from Fixed-Bed Biomass Combustion, in: T. Nussbaumer (Ed.),Aerosols from Biomass Combustion, 2001.