Thermal Behaviors and Kinetics of Pingshuo Coal/Biomass Blends during Copyrolysis and Cocombustion

Thermal Behaviors and Kinetics of Pingshuo Coal/Biomass Blendsduring Copyrolysis and CocombustionJian Wang,† Shou-yu Zhang,*,† Xi Guo,† Ai-xia Dong,† Chuan Chen,† Shao-wu Xiong,† Yi-tian Fang,‡

and Wei-di Yin§

†School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, People's Republicof China‡Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 03001, People's Republic of China§Department of Thermal Engineering, Tsinghua University, Beijing, 100084, People's Republic of China

ABSTRACT: The copyrolysis and cocombustion behaviors of Pingshuo coal and the biomasses (sawdust and rice straw) havebeen investigated using a thermogravimetric analyzer. The experimental results indicate that there exist synergetic effects betweenthe biomasses and Pingshuo coal during their coconversion process. The initial temperature of volatile emission from Pingshuocoal and the temperature corresponding to the maximum conversion rate during the copyrolysis change with the biomass mixtureratio. Moreover, it can be deduced from the comparison between the experimental and the calculated DTG curves that thecopyrolysis process is not the sum of Pingshuo coal and the biomass conversion. During their cocombustion process, the largerthe mixture ratio of the biomass is, the lower the ignition temperature and the burnout temperature are, and the larger thecombustion characteristic index is. In addition, the maximum combustion rate and the combustion performance are the bestwhen the mixture ratio of the biomass is 70 wt % in the research. Moreover, the activation energy and the frequency factor of thecopyrolysis and the cocombustion were calculated by the Coats−Redfern method and the first-order reaction model. The resultsshow that the activation energy and the frequency factor change with the mixture ratio of the biomass, and the regularity wasconsistent with the above-mentioned conclusions. Therefore, it can be deduced that the addition of the biomass can facilitate thepyrolysis and the combustion of Pingshuo coal, and improve the utilization field of Pingshuo coal.

■ INTRODUCTION

Because of the global environmental concerns over excessivefossil fuel usage, a new and clean energy has been sought tosubstitute for fossil resources. Compared with petroleum andnatural gas, which are in deeper shortages, the amount of coaldeposit is abundant. Coal, as a traditional fossil fuel, will still bea major energy source in the foreseeable future, especially forChina. Pingshuo coal is a typical thermal coal in China, whichcharacteristically has a high content of sulfur and chlorine.Because of the great amount of CO2, SOx, and NOx emissionduring the direct combustion of coal, it is highly important todevelop the most effective technology to utilize coal as anefficient and clean source of energy. As we know, biomass isone of the most important renewable energy resources, and itsapplication becomes more and more significant for climateprotection.1 Compared with other renewable energy resources,biomass is abundant in annual production, with a geo-graphically widespread distribution in the world. Theapplication of biomass for energy can lead to a zero net CO2emission in a very short life cycle period, since carbon in theform of CO2 and energy are fixed by photosynthesis duringbiomass growth.2 Consequently, the co-utilization of biomass inthe existing coal-fired plants is very appealing nowadays.Besides, its high thermochemical reactivity and high volatilematter yield facilitate the conversion and upgrading of the fuel.To date, many technologies have been developed for the

possible utilization of coal with biomass, such as cocombustion,copyrolysis, cogasification, and coliquefaction. Cocombustion isa promising short-term option for the application with the

renewable fuels.3 Several reasons can be listed for blendingbiomass with coal prior to burning. The cocombustion of coal/biomass blends will help to reduce the consumption of fossilfuels. Sometimes, biofuel is mixed with coal to achieve bettercontrol of the burning process.4 In cocombustion processes, avolatile matter yield greater than 35% is sought in order toprovide a stable flame, which could be attained by usingbiomass.5 What’s more, the ash deposition and foulingproblems on heat surfaces, which are commonly encounteredduring biomass combustion, can be reduced or probablyeliminated by burning coal/biomass blends.6 As we all know,pyrolysis is the first reaction step during coal and biomasscombustion and gasification processes. It plays a key role inclean coal utilization to study their pyrolysis characteristics andprocess properties. Therefore, the copyrolysis and cocombus-tion of Pingshuo coal/biomass are the focus of this research.However, as a typical thermal coal of China, little attention

has been focused on the clean utilization of Pingshuo coal. Theaim of this work was to investigate the thermal properties andkinetic behavior of Pingshuo coal, biomass, and their blendsusing a thermogravimetric analyzer, and the attempts weremade to observe whether there are synergetic effects betweenthe copyrolysis and the cocombustion of Pingshuo coal andbiomass.

Received: September 11, 2012Revised: November 5, 2012Published: November 6, 2012

Article

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© 2012 American Chemical Society 7120 dx.doi.org/10.1021/ef301473k | Energy Fuels 2012, 26, 7120−7126

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■ EXPERIMENTAL PROCEDURESRaw Materials. Three raw materials, sawdust, rice straw, and

Pingshuo coal, were chosen for the investigation. The air-driedsamples were milled, sieved, and classified to obtain the fractions ofuniform particle sizes of 100−200 mesh for both biomass andPingshuo coal. To eliminate the effect of moisture content, thesamples were oven-dried at 105 °C for 2h and then stored in adesiccator for the following test. Table 1 shows the proximate andultimate analyses of the three samples.Sawdust and rice straw were blended with Pingshuo coal to prepare

different binary blends with varying proportions of Pingshuo coal/biomass. Pingshuo coal/biomass ratios in different blend compositionshave been selected as 80:20, 50:50, and 30:70, respectively.Pyrolysis Experiment. Mixed samples (10 mg) were put into an

Al2O3 crucible, and the crucible was placed into a pyrolysis reactor.During the startup, an inert gas was used to purge air out of thepyrolysis reactor, which had already been filled with mixed sampleparticles. The final pyrolysis temperature was designed as 1273 K, andthe heating rate was chosen as 10 K/min. The carrier gas used was N2with a flow rate of 70 mL/min.Combustion Experiment. The method that investigated the

cocombustion of Pingshuo coal char and biomass char was used in theresearch. Each of the samples was pyrolyzed at 1273 K to obtaindifferent char samples; the heating rate was chosen as 10 K/min. Charsamples were pulverized to 100 mesh size. Char samples can then beused to do combustion experiments. The experimental procedureswere the same as those in the pyrolysis experiment; the only differencewas that the carrier gas was the N2/air (10% air) mixture atmosphere.

■ EXPERIMENTAL RESULTS AND DISCUSSIONCopyrolysis. Figures 1 and 2 describe the DTG curves of

the pyrolysis behaviors of biomass/Pingshuo coal blends, andthe pyrolysis characteristics of biomass/Pingshuo coal blendsare shown in Table 2. As shown in the figures, the pyrolysisbehaviors of the biomass/Pingshuo coal blends have threeweight loss areas that present three peaks. The first peak resultsfrom the dewatering phase, the second one is the biomass devolatilization phase, and the third one is the Pingshuo coal

devolatilization phase. The larger the mixture ratio of thebiomass is, the bigger the pyrolysis rate of the biomass is, andthe smaller the pyrolysis rate of Pingshuo coal is.During the copyrolysis of biomass and coal, the initial

temperature of the volatile emission from the coal cannot bedirectly defined by the characteristics of copyrolysis curves.According to the relevant literature,7 it can be assumed that theinitial temperature of the volatile emission from the coal isequal to the terminal temperature of the volatile emission fromthe biomasses. In this study, the terminal temperature of thevolatile emission from sawdust/rice straw was considered as theinitial temperature of the volatile emission from Pingshuo coal.Table 2 demonstrates the pyrolysis characteristics of

biomass/Pingshuo coal blends. Among these data, T1 is theinitial temperature of the volatile emission from the biomass,that is, the temperature corresponding to dx/dt = 0.1 mg/minon the DTG curve.15 T2 is the initial temperature of the volatileemission from Pingshuo coal in the mixture samples. Tmax1 andTmax2 are the temperatures corresponding to the first and thesecond maximum pyrolysis rate, respectively.

Table 1. Proximate and Ultimate Analyses of the Pure Samples

proximate analysis (wt %) ultimate analysis (wt %)

sample Mad Aad Vad FCad Cad Had Oad Nad St,ad Qnet,v,ad (MJ/kg)

Pingshuo coal 2.16 31.26 29.21 37.37 52.05 3.07 8.31 0.79 2.36 19.83sawdust 3.13 1.64 79.67 15.56 46.92 5.63 41.78 0.86 0.04 17.79rice straw 3.63 11.83 64.82 19.72 45.26 4.15 34.71 0.30 0.12 14.94

Figure 1. DTG curves of pyrolysis behaviors of sawdust/Pingshuo coalblends.

Figure 2. DTG curves of pyrolysis behaviors of rice straw/Pingshuocoal blends.

Table 2. Pyrolysis Characteristics of Biomass/Pingshuo CoalBlends

sample T1 (K) Tmax1 (K) Tmax2 (K) T2 (K)

Pingshuo coal 657.5 722.3 789.2mixtures 1:4 (20% sawdust) 565.9 626.6 710.5 652.3mixtures 1:1 (50% sawdust) 537.9 620.3 708.8 649.6mixtures 7:3 (70% sawdust) 539.8 624.2 713.6 661.2sawdust 527.2 620.9 660.1mixtures 1:4 (20% rice straw) 546.8 593.4 721.5 647.8mixtures 1:1 (50% rice straw) 522.9 595.8 719.2 642.8mixtures 7:3 (70% rice straw) 499.6 597.2 723.1 658.4rice straw 508.3 594.9 643.3

Energy & Fuels Article

dx.doi.org/10.1021/ef301473k | Energy Fuels 2012, 26, 7120−71267121

As shown in Table 2, considering the copyrolysis of thebiomass and Pingshuo coal, the initial temperature of thevolatile emission from Pingshuo coal and the temperaturecorresponding to the maximum pyrolysis rate change with themixture ratio of the biomass. It can be found that thetemperature range of the first DTG peak, which represents thedevolatilization of biomass, shifts to a higher temperature rangeand the temperature range of the second DTG peak shifts to alower temperature range. When the mixture ratio of thebiomass is below 50 wt %, with the increase of the mixtureratio, the temperature corresponding to the maximum pyrolysisrate of Pingshuo coal is lower than that of Pingshuo coalpyrolysis, and the temperature decreases gradually. When themixture ratio increases to 50 wt %, the correspondingtemperature begins to increase, even higher than that ofPingshuo coal pyrolysis. Considering the copyrolysis of thebiomass and Pingshuo coal, it can be deduced that the initialtemperature of the volatile emission from Pingshuo coal islower than that of Pingshuo coal pyrolysis. The experimentalresults indicate that the addition of the biomass leads to theshifts of the temperature range of Pingshuo coal pyrolysis to alower temperature range. For example, the initial temperatureof the volatile emission from Pingshuo coal becomes lower, andso is the temperature corresponding to the maximum pyrolysisrate. Therefore, the appearance of the biomass results in somesynergetic effects. However, the synergetic effects disappeargradually with the increase of the mixture ratio of the biomass,and this may be owing to that the appearance of the biomassrestrains Pingshuo coal pyrolysis.The appearance of the biomass leads to different effects on

Pingshuo coal pyrolysis. On the one hand, biomass character-istically has a higher alkaline content, especially K- and Na-containing matters. The alkali metal elements in biomass canhave some catalysis effects on Pingshuo coal pyrolysis.Moreover, the ratio of hydrogen to carbon of the biomass ismuch higher than that of Pingshuo coal. The ratio of hydrogento carbon is 0.12 for the sawdust and 0.092 for the rice straw,but only 0.059 for Pingshuo coal. Furthermore, during thecopyrolysis process of the biomass and Pingshuo coal, thebiomass pyrolysis produces much hydrogen-containing radicals.Therefore, the hydrogen may transfer from the biomass for thepyrolysis reactions of Pingshuo coal during their copyrolysis.7,8

As we all know, hydrogen has remarkable effects on Pingshuocoal pyrolysis, so the abundant hydrogen in the biomass couldbe considered as the fine supplement for the pyrolysis ofPingshuo coal. Consequently, the hydrogen within the biomassmay transfer to Pingshuo coal and benefit Pingshuo pyrolysis.On the other hand, the biomass will soften during the pyrolysisprocess, and the large amount of biomasses will adhere to thesurface of Pingshuo coal particles before the devolatilization ofPingshuo coal. Consequently, the pores of Pingshuo coal maybe blocked and the devolatilization of Pingshuo coal will berestrained.9 Besides, it needs more heat during the pyrolysisprocess of the biomass; therefore, the initial temperature of thevolatile emission from Pingshuo coal will increase. Maybe it isthe reason why the initial temperature of the volatile emissionfrom Pingshuo coal during the copyrolysis rises. On the basis ofthe above discussion, as the mixture ratio of the biomass isbelow 50%, there are some positive effects on the pyrolysis ofPingshuo coal. However, the synergetic effects disappeargradually with the increase of the mixture ratio of the biomass,and this may be owing to that the appearance of the biomassrestrains the pyrolysis of Pingshuo coal. On the whole, it can be

considered that there are some synergetic effects during thecopyrolysis of the biomass and Pingshuo coal.To further understand the interactions between the biomass

and the Pingshuo coal during the copyrolysis, the theoreticalTG curves of the blends were calculated as the sum of thedecomposition curves of each individual component. In thisresearch, from the angles of the pyrolysis weight loss andweight loss rate, the different temperature spots of thebeginning, middle, and ending during the pyrolysis process ofindividual samples were chosen. The weight loss of mixedsamples during the pyrolysis process is obtained as follows10

= ∗ − + ∗C C w C w(1 )ps b (1)

where C is the weight loss of the different temperature spots bycalculating, w is the biomass percentage of the mixed samples, bis for biomass, and ps is for Pingshuo coal.The curves of weight loss rate were obtained by taking the

derivative of the experimental curves and the calculated curves.Figures 3 and 4 are the comparison diagrams between the

experimental curves and the calculated curves of the weight lossrate during the copyrolysis. It can be found that theexperimental curves show some deviations from the calculatedcurves. Whether in the low-temperature or high-temperatureregion, the experimental value is greater than the calculated

Figure 3. Comparison between experimental and calculated DTGcurves of copyrolysis of sawdust/Pingshuo coal blends.

Figure 4. Comparison between experimental and calculated DTGcurves of copyrolysis of rice straw/Pingshuo coal blends.



value during the copyrolysis of sawdust and Pingshuo coal. Thisresult maybe suggests that the high contents of alkali metal andhydrogen contribute to the interactions of sawdust andPingshuo coal.11−13 During the copyrolysis of rice straw andPingshuo coal, in the high-temperature region, the experimentalvalue is greater than the calculated value, whereas it is just theopposite in the low-temperature region. In the main pyrolysistemperature range of rice straw, Pingshuo coal has not evenstarted pyrolysis. Owing to that, Pingshuo coal has little effecton the pyrolysis of rice straw. These results also indicate thatthere are some synergetic effects during the copyrolysis of thebiomass and Pingshuo coal.Cocombustion. Table 3 shows the ash fusion temperature

of the mixture samples. The deformation temperature (DT),

softening temperature (ST), and flowing temperature (FT) ofPingshuo coal are much higher than those of sawdust and ricestraw. What’s more, DT, ST, and FT of the mixture samples arealso much higher than those of the biomasses. SiO2, which isthe main component of Pingshuo coal ash, can restrain thefouling and slagging caused by the emission of K- and Na-containing matters to some extent. Thus, it can be deduced thatthe ash melting performance of the biomass can be improved inthe presence of Pingshuo coal during their cocombustion.Consequently, the slagging and fouling problem may beovercome to some degree during the cocombustion ofPingshuo coal and biomass.Figures 5 and 6 show the DTG curves of combustion

behaviors of Pingshuo coal/biomass chars with differentmixture ratios, and the combustion characteristic parameters

of Pingshuo coal/biomass char blends are shown in Table 4.Pure sample chars were compared to the char blends withrespect to their performance during combustion. As shown inthe figures, the combustion behaviors of Pingshuo coal/biomasschar blends only have one weight loss area caused by the charburning. According to the combustion process, some character-istic parameters can be defined. Ti is the ignition temperature;Th is the burnout temperature. Both of them can be determinedby the method of TG-DTG.14 Wmean is the average burningrate, Wmax and Tmax are the maximum combustion rate and thecorresponding temperature, Rw is the ignition characteristicindex, and S is the combustion characteristic index.15

As seen in Table 4, the ignition temperature and the burnouttemperature of the mixed char are lower than those of Pingshuocoal char. With the increase of the biomass mixture ratio, theignition temperature and the burnout temperature of the mixedchars decrease. Thus, the ignition performance and the burnoutperformance of the char blends increase compared with thoseof Pingshuo coal char. This can be attributed to the highreactivity of the biomass char due to its loose, porous, andhighly disordered carbon structure, high specific area, and highalkaline content.16,17 Furthermore, the ignition temperature andthe burnout temperature of Pingshuo coal/rice straw chars areboth lower than those of the Pingshuo coal/sawdust chars. Thismay be due to the fact that the mineral content of the ricestraw, which is higher than that of the sawdust, results in aslight improvement in the ignition performance and theburnout performance.18

The maximum combustion rate of the mixed chars increases,and the corresponding temperature decreases with the increaseof the biomass mixture ratio. Moreover, compared with theother chars (except biomass char) employed in the research,the combustion rate of the mixed char with the biomassblending ratio of 70 wt % is the highest. In addition, theignition performance, the burnout performance, Rw, and S arealso better than those of the other chars. However, because ofthe high reactivity of the biomass char, more oxygen will reactwith the biomass char first, and this may slow down theoxidation reaction of Pingshuo coal char. When the blendingratio of the biomass is 20 wt %, the fast burning of a smallamount of the biomass char will improve the combustionperformance of the mixed chars. However, when the mixtureratio of the biomass is 50 wt %, less oxygen reacts with

Table 3. Ash Fusion Temperature of the Mixture Samples

sample DT (K) ST (K) FT (K)

Pingshuo coal 1773 >1773 >1773sawdust 1490 1504 1515rice straw 1390 1461 147680% Pingshuo coal + 20% sawdust 1773 >1773 >177350% Pingshuo coal + 50% sawdust 1773 >1773 >177330% Pingshuo coal + 70% sawdust 1733 >1773 >177380% Pingshuo coal + 20% rice straw 1733 >1773 >177350% Pingshuo coal + 50% rice straw 1605 1636 166130% Pingshuo coal + 70% rice straw 1548 1573 1636

Figure 5. DTG curves of combustion behaviors of Pingshuo coal/sawdust chars with different blending ratios.

Figure 6. DTG curves of combustion behaviors of Pingshuo coal/ricestraw chars with different blending ratios.



Pingshuo coal char due to the high reactivity of the biomasschar and the combustion rate of the mixed chars decreases.When the mixture ratio of the biomass is 70 wt %, the positiveeffect caused by the presence of biomass prevailed over thenegative effect, and the combustion rate was the highest in theresearch. This result suggests that the addition of the biomasscan facilitate the combustion process, and there is an optimalblending proportion during the whole combustion process.These conclusions have been confirmed in previous studies.19,20

Kinetic Analysis. The Coats−Redfern method21 and thefirst-order reaction model, which were often used by otherresearchers,10,22−26 were used in the present work for both thecopyrolysis and the cocombustion processes in order to obtainthe parameters of thermal events. On the basis of the Arrheniusequation and the law of mass conservation, the kinetics of thereaction is described as

β− − = − −x T AR E RT E E RTln[ ln(1 )/ ] ln[ / (1 2 / )] /2

(2)

where A is the frequency factor (min−1), E is the activationenergy (kJ/mol), R is the gas constant (8.314 J K−1 mol−1), T isthe absolute temperature (K), x is the loss in mass fraction ormass conversion ratio, and β (K min−1) is a constant heatingrate during the copyrolysis and the cocombustion.Because it can be demonstrated that, for the value of E and

for the temperature range, the expression ln[AR/βE(1 − 2RT/E)] in eq 2 is essentially constant; if ln[−ln(1 − x)/T2] isplotted versus 1/T, a straight line should be obtained. Theactivation energy, E, can be calculated from the slope of theline, −E/R; and the frequency factor A can be calculated from

the intercept term of eq 2, ln(AR/βE). The pyrolysis kineticparameters and the combustion reaction kinetic parameters arelisted in Tables 5 and 6, respectively. The high coefficientvalues indicate that the corresponding reaction modelsatisfactorily fitted the experimental data.With regards to the kinetic parameters, the frequency factor,

A, is more closely related to the material structure, whereas thereactivity of samples is determined by the activation energy, E.As seen in Table 5, during the copyrolysis of Pingshuo coal andsawdust, the first pyrolysis activation energy of the mixture islower than that of the sawdust, and decreases with the increaseof the Pingshuo coal mixture ratio. Besides, it can be seen fromthe DTG curves that Pingshuo coal has begun to pyrolyzeduring the main pyrolysis temperature region of the sawdust.Therefore, this phenomenon can be attributed to theinteractions between Pingshuo coal pyrolysis and sawdustpyrolysis. However, the first pyrolysis activation energy duringthe copyrolysis of Pingshuo coal and sawdust shows somedifferences from that of Pingshuo coal and rice straw blends.The first pyrolysis activation energy during the copyrolysis ofPingshuo coal and rice straw is higher than the value of ricestraw pyrolysis. The pyrolysis temperature of rice straw isrelatively low, and the pyrolysis of rice straw has already endedwhen Pingshuo coal pyrolysis starts. In this sense, Pingshuocoal has little effect on the rice straw pyrolysis in the firstpyrolysis stage. The second pyrolysis is Pingshuo coal pyrolysis.the larger the mixture ratio of the biomass is, the lower thesecond pyrolysis activation energy is. Moreover, the secondpyrolysis activation energy of the blends is lower than the valueof Pingshuo coal pyrolysis. It could be deduced that the high

Table 4. Combustion Characteristic Parameters of Biomass/Pingshuo Coal Chars with Different Blending Ratios

char Ti (K) Th (K) Wmean (mg·min−1) Wmax (mg·min−1) Tmax (K) Rw S × 10−10 (mg2·min−2·°C−3)

Pingshuo coal 842.52 1049.15 0.2995 0.5226 883.52 2.2264 6.320780% Pingshuo coal + 20% sawdust 835.97 1072.7 0.2947 0.5676 885.83 1.9652 6.615650% Pingshuo coal + 50% sawdust 828.66 1024.45 0.3848 0.6054 879.78 2.0377 10.650030% Pingshuo coal + 70% sawdust 823.64 972.53 0.5255 0.8274 870.22 2.1888 20.5520sawdust 771.06 909.49 0.7211 0.5634 830.30 2.9960 25.803080% Pingshuo coal + 20% rice straw 812.62 1034.48 0.2991 0.5760 872.41 2.2796 7.789250% Pingshuo coal + 50% rice straw 773.99 1014.82 0.2001 0.3462 852.65 2.3339 3.744030% Pingshuo coal + 70% rice straw 761.87 960.81 0.4136 0.7196 797.47 2.5810 18.1557rice straw 655.23 798.73 0.3365 0.6001 699.84 3.1536 26.3845

Table 5. Pyrolysis Kinetic Parameters of Biomass/Pingshuo Coal Blends

reaction 1 reaction 2

char

range oftemperature

T (K)activation energyE (kJ·mol−1)

frequencyfactor

A (min−1)

relatedcoefficient

R2

range oftemperature

T (K)activation energyE (kJ·mol−1)

frequencyfactor

A (min−1)

relatedcoefficient

R2

Pingshuo coal 629−758 44.94 66.55 0.9970sawdust 501−658 57.13 9211.42 0.9947rice straw 476−627 53.86 6102.52 0.998280% Pingshuo coal +20% sawdust

506−663 54.64 1522.78 0.9969 665−770 30.78 9.73 0.9916

50% Pingshuo coal +50% sawdust

506−679 55.40 4054.57 0.9955 667−743 7.81 0.09 0.9968

30% Pingshuo coal +70% sawdust

480−685 56.29 5015.59 0.9937 686−772 4.02 0.03 0.9729

80% Pingshuo coal +20% rice straw

554−627 46.61 360.99 0.9935 696−791 26.52 3.38 0.9910


529−628 56.01 5071.59 0.9970 668−795 16.01 0.55 0.9964


480−685 61.48 32862.3 0.9999 686−772 11.18 0.36 0.9947



ash yield and hydrogen content of the biomass is a significantfactor affecting the pyrolysis behavior of Pingshuo coal.Therefore, there may be some synergetic effects occurringduring the copyrolysis of the biomass and Pingshuo coal.From Table 6, obviously, it can be seen that the activation

energy of both the sawdust char and the rice straw char is lowerthan that of Pingshuo coal char and the activation energy ofsawdust char is higher than the value of rice straw char. Alongwith the change of the biomass mixture ratio, the activationenergy and the frequency factor of the mixed char varyregularly. The activation energy of the mixed char lies betweenthat of the biomass char and that of the Pingshuo coal char.The larger the mixture ratio of the biomass is, the lower theactivation energy of the mixed char is. What’s more, theseresults are consistent with the conclusions drawn by the changein the maximum combustion rate of the mixed char. Thesynergistic effect existing between Pingshuo coal char and thebiomass chars can be confirmed, and the addition of thebiomass can facilitate the burning process of the mixed char.

■ CONCLUSIONS

(1) There are three weight loss areas during the pyrolysisprocess of the biomass and Pingshuo coal blend. The firstis the dewatering phase, the second is the biomassdevolatilization phase, and the third is the Pingshuo coaldevolatilization phase. During the copyrolysis process,the temperature range of the second DTG peak causedby biomass devolatilization shifts to a higher temperaturerange and the temperature range of the third DTG peakshifts to a lower temperature range. The initialtemperature of the volatile emission from Pingshuocoal and the temperature corresponding to the maximumpyrolysis rate change with the mixture ratio of thebiomass. The comparison between the experimental andthe calculated curves of the weight loss rate indicates thatthere exists some synergetic effect during the copyrolysisprocess.

(2) The chars from the biomass and Pingshuo coal blendsare subjected to only one main combustion step. Theignition temperature and the burnout temperature of themixed char decrease with the increase of the biomassmixture ratio, and the combustion performance of themixtures is promoted.

(3) When the biomass mixture ratio is 70 wt %, themaximum combustion rate of the mixed char is thebiggest in the research. Moreover, the ignition perform-ance, the combustion performance, and the burnoutperformance all increase, and the addition of the biomasscan improve the combustion performance of Pingshuocoal.

(4) The first-order reaction model may be the mainmechanism responsible for the copyrolysis and thecocombustion process. The model satisfactorily andsimultaneously represents the results obtained from theconversion of the different samples. The variation in theactivation energy calculated during the copyrolysis andthe cocombustion is consistent with the experimentaldata. The addition of the biomass facilitates thecopyrolysis and the cocombustion processes.

(5) Because the experiments have been performed using athermobalance apparatus, there exist some differencesbetween the experimental conditions and the realpractice. However, these problems could be solvedusing a drop-tube reactor with a high heating rate, andthe experiment will be performed in our future studies.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was financially supported by the National Programon Key Basic Research Project (973 Program) of China (No.2012CB214900).

■ REFERENCES(1) Thomas, N. V. Combustion and co-combustion of biomass:Fundamentals, technologies, and primary measures for emissionreduction. Energy Fuels 2003, 17, 1510−1521.(2) Mangut, V.; Sabio, E.; Gan an, J.; Gonzalez, J. F.; Ramiro, A.;Gonzalez, C. M.; Roman, S.; Al-Kassir, A. Thermo-gravimetric study ofthe pyrolysis of biomass residues from tomato processing industry.Fuel Process. Technol. 2006, 87, 109−115.(3) Robinson, A. L.; Junker, H.; Baxter, L. L. Pilot-scale investigationof the influence of coal-biomass cofiring on ash deposition. EnergyFuels 2002, 16, 343−355.(4) Wang, C.; Wang, F.; Yang, Q.; Liang, R. Thermo-gravimetricstudies of the behavior of wheat straw with added coal duringcombustion. Biomass Bioenergy 2009, 33, 50−56.(5) Biagini, E.; Lippi, F.; Petarca, L.; Tognotti, L. Devolatilization rateof biomasses and coal-biomass blends: An experimental investigation.Fuel 2002, 81, 1041−1050.(6) Haykiri, A.; Yaman, H. Effect of co-combustion on the burnout oflignite/biomass blends: A Turkish case study.Waste Manage. 2008, 28,2077−2084.(7) Yan, W. P.; Chen, Y. Y. Interaction performance of co-pyrolysisof biomass mixture and coal of different rank. Proc. Chin. Soc. Electr.Eng. 2007, 27, 80−86.

Table 6. Combustion Reaction Kinetic Parameters of Biomass/Pingshuo Coal Chars with Different Blending Ratios

char range of temperature T (K) activation energy E (kJ·mol−1) frequency factor A (min−1) related coefficient R2

Pingshuo coal 843−950 199.92 1.1888 × 1011 0.9965sawdust 771−878 137.50 9.5479 × 107 0.9994rice straw 655−763 110.61 3.389 × 107 0.996580% Pingshuo coal + 20% sawdust 836−922 161.98 6.4831 × 108 0.990350% Pingshuo coal + 50% sawdust 829−942 158.51 4.7022 × 108 0.994730% Pingshuo coal + 70% sawdust 824−920 80.85 11504.75 0.998380% Pingshuo coal + 20% rice straw 813−915 179.50 6.7426 × 109 0.995050% Pingshuo coal + 50% rice straw 774−893 236.15 5.2943 × 1014 0.995330% Pingshuo coal + 70% rice straw 762−816 134.38 1.1765 × 108 0.9580



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Thermal Behaviors and Kinetics of Pingshuo Coal/Biomass Blends during Copyrolysis and Cocombustion