Evaluation of Cofiring Bioferment Residue with Coal at Different

Sep 16, 2013 - Hazardous waste treatment facilities in China are in shortage. Co-firing hazardous waste with coal in industrial and utility boilers wo...
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Evaluation of Cofiring Bioferment Residue with Coal at Different Proportions: Combustion Characteristics and Kinetics Yuying Du,† Xuguang Jiang,†,* Xiaojun Ma,‡ Xudong Liu,§ Guojun Lv,† Yuqi Jin,† Fei Wang,† Yong Chi,† and Jianhua Yan† †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38 Zheda Road, Hangzhou 310027, People’s Republic of China ‡ Industrial Technology Research Institute, Zhejiang University, 148 Tianmushan Road, Hangzhou 310028, People’s Republic of China § Zhejiang Jinhua Conba Bio-Pharm.Co., Ltd., 288 Jinqu Road, Jinhua 321016, Zhejiang , People’s Republic of China S Supporting Information *

ABSTRACT: Hazardous waste treatment facilities in China are in shortage. Co-firing hazardous waste with coal in industrial and utility boilers would be a near term, low cost, and low risk option. This work presents a thermogravimetric and kinetic analysis based study on bioferment residue (BR, hazardous waste), coal and their blends to understand their cocombustion characteristics at different proportions. Kinetic data are obtained using the Coats-Redfern method. The co-firing ratio for BR/coal in different combustion types is suggested. It is found that BR has a composition that favors combustion, although its combustion properties are not as good as that of coal. Co-firing BR with coal mainly consists of four stages. Reaction mechanisms vary with combustion stage and blending ratio. BR-coal blends present a sum reactivity of their parent component. With increasing BR cofiring ratio, ignition temperature, and combustion rate basically decrease as well as the total energy output. Provided sufficient reaction time, BR, coal and their blends would be completely burnt below a temperature of 750 °C. Co-firing BR in coal fired boilers preferably below percentage of 10.1−42.2% for grate, 90.7−100% for FB, and 3.1−86.3% for PF. FGD performances, even with a sawdust mass proportion of 25%.5 Biomass cofiring in boilers designed for coal combustion represents an option that promises reduction in emissions of net CO2, SOx, and often NOx.6 Analyses showed that cofiring of woody biomass can lower the costs resulting from flue gas cleaning and byproducts.7 Most of the research in o-firing fields focus on biomass, MSW, solid recovered fuel, sludge, tobacco waste, textile wastes, and other industrial wastes with coal.8−21 There is little literature on cofiring of hazardous waste and coal. To evaluate the potential of cofiring of hazardous waste in industrial and utility boilers, relevant tests have been done in an MSW incineration power plant.22 But the essential process in hazardous waste cofiring is not clear yet, especially the cocombustion behavior when cofired in different proportions. Pharmaceutical plants generate a variety of wastes during manufacturing. Generally, about 200 kg of waste is generated per metric ton of active ingredient produced.23 Pharmaceutical manufacturing wastes contain large amounts of spent solvents and other toxic organics, posing threats to human and environmental health. Because of the dangers, they are identified as hazardous waste (HW02 Toxicity) on the basis of National Hazardous Wastes, as proposed by the Ministry of Environmental Protection of China, on the basis of inclusive lists, as proposed by the European Union, or on the basis of hazardous properties, as followed by the US EPA. Bioferment residue from the pharmaceutical industry in China exists in large amounts and used to be utilized as animal feed. However, this way is forbidden recently for the potential biological toxicity of BR. To find a new disposal way for BR is in urgent need.

1. INTRODUCTION In 2011, the generation of industrial hazardous wastes in China was 34.31 million tons, up to 24.0% of which is in interim storage waiting for disposal, according to the 2011 Environmental Statistics Annual Report by the Chinese Environmental Protection Department. In China, considerable attention has been focused on the growing amounts of stored hazardous waste, but its disposal capacity still cannot meet the demand. Literally hundreds of methods to dispose of hazardous wastes exist.1 Of all of the methods, incineration is capable of the highest degree of destruction and control of hazardous waste. Besides dedicated hazardous waste combustion incinerators and boilers, industrial and utility boilers can also provide the high temperature that hazardous waste incineration demands. These boilers are generally designed for coal with flue gas cleaning systems available. By replacing coal with certain fractions of hazardous waste in the coal fired boilers, energy of waste materials would be recovered, toxicity destroyed, and volume reduced drastically. Dedicated hazardous waste disposal facilities are in short supply in China, but the resources of industrial and utility boilers are rich. According to projections based on related data in boiler units in China, the total thermal power of industrial boilers have reached 2 843 700 MW by April 2013, and that of coal-fired power plants was 709 672 MW in 2010 and is expected to reach 960 000 MW in 2015. The coal consumption of boiler in China is 1 750 000 000 t in 2010, accounting for 54% of coal production. Hence, cofiring hazardous waste with coal in industrial and utility boilers is a near term, low cost, and low risk option for disposal of hazardous waste in China. Co-firing has been practiced many different ways.2 More and more new FBC installations are designed to burn more than one fuel, especially various biofuels.3 Biomass cofiring has been demonstrated for most of boiler types, more than a hundred of which are in Europe.4 In the pine sawdust cofiring test carried out in coal fired utility boilers, cofiring is found to have no negative effect on the boiler, ESP and © 2013 American Chemical Society

Received: August 2, 2013 Revised: September 13, 2013 Published: September 16, 2013 6295

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Table 1. Compositions of BR and Coal (on Air Dry Basis) sample coal BR

M

A

V

Fc

Qgr,d

Qnet,d

Qnet,ad

C

H

N

St

O

%

%

%

%

MJ/kg

MJ/kg

MJ/kg

%

%

%

%

%

20.61 14.21

29.25 67.29

44.13 8.79

23.39 18.64

22.61 16.44

20.52 14.60

55.6 40.12

3.16 8.84

1.08 1.19

0.43 0.41

10.81 25.52

8.31 9.71

In this paper, the thermogravimetric (TG) and kinetic analysis is carried out on the blends of bioferment residue (BR), a pharmaceutical manufacturing waste, and coal at different proportions, to obtain cocombustion characteristics such as ignition temperature, maximum burning rate, combustible matter contents, and activation energy.

BR and coal at different proportions, TG experiments under heating rate of 30 °C/min are performed.

3. THEORETICAL BASIS Kinetic parameters are the basic parameters to describe the combustion performance of fuel and oxygen. On the basis of the results measured by TG experiments, kinetic parameters for cocombustion of BR and coal can be calculated. Since combustion process can be considered as a series of volatile release and combustion behaviors, the activation energy fits the following kinetic equation:

2. EXPERIMENTAL SECTION The BR used in experiments is generated during spectinomycin manufacturing at Biological Pharmaceutical Co., Zhejiang, Jinhua Conba. The corresponding biological fermentation processes are as follows. Glucose, starch, corn steep liquor, fish meal, yeast extract, soybean oil, potassium dihydrogen phosphate, and other materials are put into the fermenter. After sterilization by steam, strains are accessed in the fermenter and continuously mixed for 30 h. After fermentation, fermentation broth is acidified and pumped into the frame to get filtered and washed. The filtrate has its pH adjusted to 6.5−7.0 with the diluted NaOH solution and then is filtered again, if necessary, 3−4 kg/m3 perlite will be added to aid filter. Spectinomycin is extracted with resins from the solution, purified using solvent, and then crystallized to get the product. The filtration process generates large amounts of spent filter cake (wet BR) including cell debris, filter aid, as well as some residual spectinomycin.22 The BR on an as-received basis (the wet BR) is detected to have a moisture content of 68.68%. It is still argued that MSW in China with a moisture content up to 65% is too moist for incineration.24 This is because wet wastes must be first dried before rising in temperature, generating volatiles, and eventually catching fire. In contrast, dry BR decomposes swiftly and hence burns rapidly. The samples used in TG experiments are on an as-received basis for coal and air-dry basis for BR. Coal and BR are ground to fine particles without sieving prior to experiments and then detected with 80% in a particle size of 0.01−0.8 mm by Malvern particle size analyzer. Proximate analysis, calorific value determination and ultimate analysis are performed on samples of coal and BR using the Coal Industry Analysis Method (GB/ T 212-2008), method of Determination of Calorific Value of Coal (GB/T 213-2008), and elemental analyzer 1ECOCMNS932, respectively. The proximate and ultimate analysis of BR and coal on an air-dry basis as well as a calorific value on a different basis are shown in Table 1. Thermogravimetric analysis is carried out using a Mettler Toledo TGA/SDTA851e thermo analyzer, with a temperature range of 25−1100 °C, accuracy of ±0.25 °C, and repeatability of ±0.15 °C. It has a measurement range of 0−1000 mg, with a sensitivity of 0.1 μg. The temperature control and data collection are done by the computer automatically. Since coke starts to burn at about 800 °C, and volatile matter evolution is generally completed below 950 °C,25 the experiments are set at a temperature range of 25−1000 °C. About 10 mg of sample is placed into 150 μL alumina crucible suspended on a balance under an oxidizing atmosphere. A flow rate of 50 mL/min is applied for 21%O2/79%N2 constantly. The combustion behavior of pure coal and pure BR is studied at three heating rates of 10, 30, and 50 °C/min. For blends of

⎛ E ⎞ dα A ⎟f ( α ) = exp⎜ − ⎝ RT ⎠ β dt

where α is the degree of conversion for individual combustion stage; A is the Arrhenius pre-exponential factor; β is the heating rate; E is the activation energy for the individual combustion stage; R is the gas constant; T is the reaction temperature; and f(α) is the combustion function. Thermal dynamic computation has many methods. Those commonly used are the differential method and integral method. And both of them include a large number of solutions. The Coats-Redfern method commonly employed in biomass and coal combustion dynamics analysis26−28 is used in this work. This integral method uses a single TG curve to obtain average activation energy according to temperature range. The equation29 is as follows: If n ≠ 1, then, ⎡ 1 − (1 − α)1 − n ⎤ ⎡ AR ⎛ E 2RT ⎞⎟⎤ ⎜1 − ln⎢ ⎥ = ln⎢ ⎥− 2 ⎝ ⎠ ⎣ βE E ⎦ RT ⎣ T (1 − n) ⎦

If n = 1, then, ⎡ AR ⎛ ⎡ −ln(1 − α) ⎤ E 2RT ⎞⎟⎤ ⎜1 − ln⎢ ⎥− ⎥ = ln⎢ 2 ⎝ ⎠ ⎣ ⎦ ⎣ βE E ⎦ RT T

where, n is the order of reaction for the individual decomposition stage.

4. RESULTS AND DISCUSSION 4.1. Compositions of BR and Coal. From Table 1, it can be observed that BR has less ash, higher volatiles, less fixed carbon, and much the same moisture content when compared to coal, because BR is primarily composed of solid cell debris, the combustible organic matters. The low ash content in BR would lower the chance of slagging and fouling. However, Na content in the BR would be increased due to the NaOH solution added to adjust pH during manufacturing, which would probably add to the risk of slagging and fouling. Hence, the effect of cofiring coal with BR on slagging and fouling is complicated and needs further research. The oxidation of volatiles will dominate the combustion process of BR due to its very high volatile content, which also favors burning in boilers. The net heating value of BR is 16.44 MJ/kg on dry basis, similar to most of the biomass, according to a heating value 6296

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multiple peaks are observed for BR. The first two occur at low temperature from 135 to 412 °C, followed by several spikes and then a peak at high temperature from 412 to 665 °C. For BR, the weight loss relative to the first stage from 135 to 412 °C is about 44%, and around 30% for the second stage from 412 to 665 °C. The weight loss is 76% for coal in the single combustion stage. This is because BR produced from biofermentation process is mainly formed by two volatile fractions (67.29 wt % in total) with different reactivity, while coal, a compact aged form of biomass,33 consists primarily of the less reactive volatiles (29.25%) and fixed carbon (44.13%). The combustion of fixed carbon plays a major role in coal combustion; whereas the release and combustion of volatiles dominates the combustion of BR. Coal generates volatiles at temperatures slightly lower than char combustion,33 without an obvious volatile evolution stage, while BR releases volatile in two distinct stages, followed by char combustion. In general, BR has more reactive volatiles and more thermally stable char compared to coal. The weight loss mainly occurs in a temperature range of 260−710 °C for coal and 135−728 °C for BR. Hence, coal has a more concentrated combustion process, and is therefore conducive to stable combustion. At temperatures above 798.34 °C for BR and above 735.21 °C for coal, the weight of sample starts to increase rather than decrease. This is due to polymerization of the nonflammable part, mainly inorganic minerals. From Figures 1 and 2, it can be observed that the combustion process of pure coal and pure BR is quantitatively affected by heating rate. Seen from DTG curves of coal, the lower the heating rate is, the more obvious the coal weight loss rate becomes. This is because the low heating rate provides sufficient reaction time, beneficial to coal combustion with respect to reaction degree and conducive to the ignition and burnout of coal. From TG curves of coal, it can be seen that the total weight loss under a heating rate of 50 °C/min is 83.1%, whereas those under 30 and 10 °C/min are similar to each other and both higher than that under 50 °C/min. This indicates that the heating rate has an effect on the composition of the solid residue in coal combustion. From the DTG curves of BR, it can be seen that the maximum weight loss rate at the low temperature range under heating rate of 10 °C/min is 0.116%/°C, lower than those under 30 and 50 °C/min, whereas at the high temperature range, it is 0.100%/°C under 10 °C/min, higher than those under 30 and 50 °C/min. Furthermore, at low temperatures, the heating rate has different effects on the two subpeaks. The decomposition of hemicellulose (220−350 °C) and cellulose (315−400 °C) in the plant cell debris is responsible for the first two subpeaks.34,35 For the peaks at high temperatures and the second subpeak at low temperature, the weight loss rate of BR increases with the decrease in heating rate, which is similar to that of coal. In contrast, the first subpeak at low temperatures has a maximum value at the middle heating rate of 30 °C/min. This is because large amounts of volatiles are released and burned rapidly at the low temperatures resulting in an oxygendeficient atmosphere around the BR sample. It is also possible that the heating rate has different effects on hemicellulose and cellulose. From the TG curve of BR, it can be seen that the total weight loss is 81.4% under 50 °C/min and decreases with the decrease in heating rate, suggesting that heating rate has an effect on the composition of solid residue. As seen from Figures 1 and 2, combustion has been finished before 750 °C for BR and coal under 10 °C/min. Hence, if the reaction time is

statistic on biomass fuels by the U.S. Department of Energy.30 The above characteristics enhance the opportunity of cofiring BR as a substitute fuel in coal fired boilers. Seen from Table 1, BR has much the same nitrogen (N) and total sulfur (St) content as coal and hence will not modify the composition of flue gases when cofired in coal fired boilers. BR is rich in oxygen (O) content compared to coal, indicating higher thermal reactivity31 and easier ignition.32 BR and coal are quite different in chemical composition, which would result in very different combustion properties. 4.2. Thermogravimetric Analysis of BR and Coal. Figures 1 and 2 give the TG and DTG (differential

Figure 1. TG and DTG curves for coal combustion under heating rates of 10, 30, and 50 °C/min.

Figure 2. TG and DTG curves for BR combustion under heating rates of 10, 30, and 50 °C/min.

thermogravimetric) profiles for pure coal and BR obtained from combustion experiments under different heating rates of 10, 30, and 50 °C/min. The weight loss is expressed as a percentage of the original sample weight. Since the curves under different heating rates present similar overall qualitative results, the TG and DTG profiles under a middle heating rate of 30 °C/min are studied for comparison of combustion behavior between BR and coal. It can be observed that both BR and coal have most of their moisture evaporated before burning. But the subsequent combustion processes of BR and coal are significantly different from each other. Coal has one single significant weight loss stage at a high temperature range of 260−710 °C. In contrast, 6297

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Table 2. Combustion Characteristic Parameters of BR and Coal Tv

Ti

DTGmax

Tmax

Th

TGend

Tend

DTGmean

sample

°C

°C

%/°C

°C

°C

%

°C

%/°C

BR coal

103.0 293.9

168.4 401.7

0.126 0.179

264.9 548.4

713.3 666.1

20.13 14.14

798.3 735.2

0.103 0.121

Cb

S

2.86 × 10−04 2.65 × 10−04

1.05 × 10−04 1.29 × 10−04

5.49 × 10−11 1.29 × 10−10

amount for BR and coal obtained from TG analysis is different from the ash content by proximate analysis. This is because the analyses are performed on a different basis of sample and the heating rate has a different effect on different samples. Seen from Table 2, both the DTGmax and DTGmean are higher for coal than for BR, suggesting that BR burns less intensely compared to coal. Combustible index Ci is introduced to reflect the overall combustibility of sample.

sufficient, then both BR and coal may be burn out below a temperature of 750 °C. 4.3. Combustion Characteristics of BR and Coal. A series of parameters calculated from the TG data are used to characterize the combustion of BR and coal. Table 2 gives the characteristic parameters for pure BR and pure coal based on the TG results under a heating rate of 30 °C/min. The initial release temperature of volatile, Tv, is the temperature when the weight of sample starts to descrease after the moisture evaporation.32 Since coal sample used in experiments is on an as-received basis with a moisture content between 12.0 and 14.8 wt %, its DTG curves show a clear moisture loss peak. BR sample is on air-dry basis with moisture evaporation stage partly overlapped by volatile evolution stage. The initial release temperature of volatiles is observed at 293.9 °C for coal and 103.0 °C for BR. Hence, the volatile matter in BR starts to evolve at a relatively low temperature. The ignition temperature, Ti, reflects the difficulty of sample in ignition. It can be determined by TG and DTG curves. Make the vertical over the main peak point on DTG curves and the vertical intersect with the TG curve at one point. Make the tangent on the TG curve over the intersection and the tangent intersect with the beginning horizontal line at another point, which is considered as the ignition point. For BR, multiple peaks are observed on its DTG curve interrupting the determination of ignition temperature. In this situation, the first, also the maximum, peak point on DTG curve is chosen as an intersection to make vertical. Seen from Table 2, the ignition temperature is 168 °C for BR and 402 °C for coal, indicating that BR starts to burn at a lower temperature compared to coal. The burnout temperature, Th, is determined by the turning point at a weight loss rate of 0.01%/°C when the intense weight loss of sample slows down. As seen in Table 2, the burnout temperature is 713 °C for BR and 666 °C for coal. BR has a higher burnout temperature due to its more thermal stable char content compared to coal. The maximum weight loss rate DTGmax indicates the maximum reaction rate. TGmax and Tmax are the corresponding weight loss and temperature. Seen from Table 2, the maximum weight loss rate is 0.126%/°C at a temperature of 264.89 °C for BR and 0.179%/°C at 548.37 °C for coal. This indicates that the evolution and combustion of volatiles in BR is more intense at low temperatures and coal combustion concentrates at high temperatures. The mean weight loss rate DTGmean is the weight loss rate averaged over the temperature range investigated. DTGmean =

Ci

DTGmax Ti + 273.15

Ci =

The burnout index, Cb, is introduced to reflect the overall burnout performance of sample. Cb =

DTGmean Th + 273.15

The combustion index, S, is introduced to compare the overall combustion characteristic of samples. S=

C iC b Th − Ti

As seen from Table 2, BR has higher Ci, lower Cb, and lower S than coal. The higher Ci is attributed to the high volatile content in BR, while the Cb is probably due to the more thermally stable char content in BR compared with coal. Although BR has better combustibility than coal, adding BR is likely to lower the overall combustion character of coal since the burnout performance of BR is low. 4.4. Thermogravimetric Analysis of BR and Coal Blends. To study cocombustion properties of BR and coal, the TG, DTG, and DTA (differential thermal analysis) curves obtained from cocombustion of BR and coal are shown in Figures 3, 4, and 5 with BR blending ratios of 0, 20, 40, 60, 80, and 100 wt %. It can be observed that the TG and DTG curves for BR-coal blends lie between those for pure BR and pure coal. As the DTG curves shown in Figure 4, the cocombustion of BR and coal mainly consists of four stages. The first stage at temperatures below 135 °C is mainly caused by the evaporation of moisture in BR-coal blends; the second stage, from 135 to 405 °C, is primarily due to the rapid evolution and combustion of thermally less stable volatiles in BR; the third stage, from 405 to 670 °C, is attributed to the release of thermally more stable volatiles from both BR and coal and the combustion of char in coal; and the fourth stage, from 670 to 730 °C, is mainly due to the combustion of char in BR. With the increase in BR blending ratio, the peak below 135 °C corresponding to the moisture evaporation decreases, the peak between 103 and 405 °C corresponding to the evolution of reactive volatiles increases, the peak between 405 and 670 °C corresponding to the combustion of less reactive volatile in blends and char in coal decreases, and the peak above 670 °C corresponding to char combustion of BR increases. The heat released or absorbed from sample reaction causes the temperature difference ΔT between sample and reference,

100 − TGend Tend − 25

where, TGend is the lowest weight of sample and Tend is the corresponding temperature. The combustible matter content in sample can be calculated by (10 0− TGend). As seen in Table 2, the value of TGend is 20.13% for BR and 14.14% for coal, suggesting that the amount of solid residue from combustion would be higher for BR on an air-dry basis than coal on an asreceived basis under the same heating rate. The solid residue 6298

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170 °C, after which volatiles evolve and combust simultaneously, igniting the char. Hence the combustion of volatiles and char are partially overlapped. The nature of the shoulder peak below 405 °C on DTA curves corresponding to combustion of thermally less stable volatiles remain almost unaltered. Hence, the effect of blending BR with coal on the heat transfer pattern remains unchanged below 405 °C. However, the energy released in the third stage of blend combustion decreases with increasing BR blending ratio, observed by the relevant decrease in the maximum peak of the DTA curve. This is primarily attributed to the reduced coal char content. The area under the DTA curve that indicates the total energy output is reduced with the increase in BR blending ratio. This is due to the blending of a lower calorific value BR with higher calorific value coal. So for a given energy output, more amounts of BR are to be burned. This may demand an increase in the capacity of the feeding equipment and air handling equipment. 4.5. Combustion Characteristics of BR and Coal Blends. Figure 6 gives the ignition and burnout temperature

Figure 3. TG curves of BR and coal cocombustion under 30 °C/min.

Figure 4. DTG curves of BR and coal cocombustion under 30 °C/ min. Figure 6. ignition and burnout temperature of BR-coal blends.

of BR and coal blends. With the increases in BR proportion, the ignition temperature of blends decreases slowly at a BR blending ratio below 50% and significantly above 50%, while the burnout temperature gradually increased. The decrease in ignition temperature is due to the improvement in the devolatilization characteristics of blends with the addition of BR. However, the energy supplied by volatiles in BR is not sufficient enough to reduce the ignition temperature significantly when the BR blending ratio below 50%. A BR blending ratio above 50% may result in a very low ignition temperature, which probably leads to the blends catching fire too early before expected.32 Co-combustion of BR and coal has little effect on the burnout temperature. A higher combustion rate after ignition is more likely to form a high combustion temperature, more stable combustion, and a higher degree of burnout within the given residence time. In the thermogravimetric analysis, the maximum weight loss rate reflects the reaction rate of sample. Figure 7 gives the maximum weight loss rate DTGmax, the mean weight loss rate DTGmean, and the total weight loss TGend. It can be observed that blending BR with coal leads to significant changes in the maximum weight loss rate. With the increase in BR blending

Figure 5. DTA curves of BR and coal cocombustion under 30 °C/min.

and the caused heat flow is in direct proportion to the temperature difference. Thus the DTA curve shown in Figure 5 can provide the energy information associated with blend’s cocombustion process. As the temperature increases, volatiles evolve first but do not combust until a temperature of 136.5− 6299

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blending ratio. It can be observed that the Ci and S basically decrease with increasing BR blending ratio, except for the turning point with a BR blending ratio of 80%. This is also caused by the shift of the maximum peak to the second stage of cocombustion. Although BR is more combustible than coal, the overall combustibility of blends is not improved by adding BR. With the increase in BR blending ratio burnout index, Cb, is gradually reduced, suggesting the degradation in burnout performance of blends. Generally, blending BR would lower the combustible, burnout, and combustion performance of coal. The index of Ci, Cb, and S for coal is only slightly reduced when the BR blending ratio is below 40%, indicating acceptable combustion performances. 4.6. Thermodynamic Analysis of BR and Coal Blends. Kinetic parameters for cocombustion of BR and coal are calculated based on the TG data at a heating rate of 30 °C/min, as shown in Table 3, where T represents the temperature range of the reaction stage, and r represents the linear correlation coefficient. Combustion primarily occurs in one single stage for coal and in two distinct stages for BR and its blends with coal. And the kinetic calculation is performed according to their combustion stages. As shown in Table 3, the high values of r suggest that the results obtained using the Coats-Redfern method match well with the TG experimental data. Coal combustion (300−700 °C) has the reaction order n close to 1 and is considered as a series of consecutive first order reactions.36 BR has the reaction order n of 1.4 in the first stage and 1.3 in the second, indicating different reaction mechanisms from coal. In the case of BR-coal blends, the reaction order n basically decreases in the first stage with the increase in BR blending ratio, but behaves oppositely in the second stage. For BR, coal and their blends, reaction order n is higher in the first stage than in the second one. This suggests that the reaction mechanisms of cocombustion vary with the stage and BR blending ratio. Activation energy is the dominating factor in the reactivity equation.27 Activation energy is 43.46 kJ/mol in the first stage and 104.35 kJ/mol in the second stage for BR combustion, both lower than woody biomass.37 The lower activation energy in the first stage is due to thermally less stable volatiles in BR, which are less resistant to the heat. Coal has one primary combustion interval of 300−700 °C with an activation energy of 82.18 kJ/mol. The activation energy of coal at a low temperature range of 100−300 °C is also calculated (69.13 kJ/ mol) for comparison. The first combustion stage of BR requires lower activation energy than coal, but the second stage requires higher. Because in the second stage, less reactive volatiles in BR and coal start to burn as well as the large amounts of char in coal, releasing lots of heat, but the small amounts of char in BR stay unburned until a temperature of around 670 °C. The energy released by char combustion reduces the activation energy required in the second stage. In general, BR has higher

Figure 7. Weight loss characteristics of BR-coal blends.

ratio, the values of DTGmax and DTGmean of the mixture basically decreases, with the exception of the DTGmax value at the BR blending ratio above 80%. This is because the maximum weight loss rate peak of BR-coal blends shifts from the third stage to the second when the BR blending ratio is above a value of 80%. The value of DTGmax is a little lower than DTGmean, which is not expected to happen. Because the DTGmax value is calculated by the derivation of discrete TG data points and computation, error exists. Basically, with the increasing BR blending ratio, the combustion rate of blends decreases, and so does the combustion stability. It also can be observed that the amount of solid residue formed during BR-coal cocombustion depends on the individual blend components and is approximately proportional to the BR blending ratio. Figure 8 gives the combustible index, Ci, burnout index, Cb, and combustion index, S, for BR-coal blends as a function of BR

Figure 8. Combustion characteristics of BR-coal blends.

Table 3. Kinetic Parameters for BR and Coal Blends sample BR BR 80% BR 60% BR 40% BR 20% coal

T (°C) 135−412 140−400 150−390 170−370 180−350 190−300

E (kJ/mol)

A (min−1)

r (%)

n

T (°C)

E (kJ/mol)

43.46 44.74 48.07 56.91 63.01 69.13

× × × × × ×

98.21 97.54 97.36 96.31 96.45 94.14

1.4 1.4 1.45 1.55 1.5 1.5

412−665 400−665 390−665 370−665 350−665 300−700

104.35 105.05 103.24 91.84 81.81 82.18

5.24 7.26 1.66 1.36 5.56 5.70

03

10 1003 1004 1005 1005 1006

6300

A (min−1)

r (%)

n

× × × × × ×

97.82 98.01 98.34 98.30 98.17 98.69

1.3 1.3 1.2 1.1 1.05 1.1

2.88 3.29 2.27 3.46 7.08 7.29

1006 1006 1006 1005 1004 1004

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reactivity than coal at low temperatures and lower reactivity at high temperatures. Activation energy for BR-coal blends is plotted as a function of blending ratio in Figure 9, to provide a quick comparison of combustion reactivity between blends with different proportion.

Figure 10. Calorific values of wet BR and coal blends.

chain grate combustion, 5.02 MJ/kg for fluidized bed combustion (FB), 20 MJ/kg for pulverized coal combustion (PF), and 17 MJ/kg for a PF used natural gas reburning process. On the basis of the LHV limits for different combustion types, the scope of the proportion for cofiring BR and coal are given in Table 4.

Figure 9. Activation energy for BR and coal blends.

Table 4. Scope of BR Co-Firing Ratio in Coal Fired Boilers

The activation energy at low temperature decreases with the increase in BR blending ratio, similar to those for woody biomass blending with coal.28 This is consistent with the influence of BR blending ratio on ignition temperature. At high temperature, the activation energy increases with blending BR, although it is reduced slightly when the BR blending ratio is below 20%. Activation energy of BR-coal blends basically reflects a sum reactivity of the parent component. With the increase in BR blending ratio, the energy released from char combustion decreases, requiring more heat from external and increasing activation energy of blends. Hence, the effect of BR blending ratio on a blends reactivity varies with the cocombustion stage. For the blends of 60% BR and above, activation energy at high temperatures is higher than 100 kJ/ mol and adverse to combustion stability and burnout, although that at low temperature is far lower than coal. However, coal reactivity is slightly enhanced as a consequence of a low addition of BR (20%). 4.7. Scope of Proportion for Co-Firing BR and Coal. To ensure heating, ignition, and burnout, boilers should reach the adequate temperature. European Union Codes stipulate that auxiliary burners must be installed when the flue gas temperature below 850 °C. Excessive temperature is undesirable as well in the case of ash deposition.2 The heat generation rate of a given boiler is primarily determined by the air and the fuel. And calorific value of fuel is a key factor. Since the moisture content required for biomass in combustion is high (up to 60%38), wet BR may be directly cofired with coal. A lower heating value (LHV) for blends of wet BR and coal is calculated in Figure 10. The LHV is 3.43 MJ/kg for wet BR, even less than the Chinese MSW (5 MJ/kg) that is generally mixed with 15% coal when incinerated.39 The calorific value of fuel would be reduced significantly by blending wet BR. Grate, fluidized bed, and pulverized coal combustion have been successfully used in biomass cocofiring.4 The minimum LHV of fuel in a coal-fired boiler is around 18.83MJ/kg for

furnace type Qnet (MJ/kg) BR (wt %)

grate furnace limit dry air dry wet

18.83 42.2% 29.1% 10.1%

FB

PF

PF-reformed

5.02 100% 100% 90.7%

20.00 12.8% 8.8% 3.1%

17.00 86.3% 59.5% 20.6%

If wet BR is directly cofired, then the mixing ratio should not be more than 10.1 wt % for grate combustion, 90.7% for FB, 3.1% for PF, and 20.6% for reformed PF. The percentage of BR cofiring in coal-fired boilers is preferably below 10.1−42.2% for grate, 90.7−100% for FB, 3.1−12.8% for PF, and 20.6−86.3% for reformed PF. According to the percentage of biomass cofired in different power plants with experience,40 the proportion of biomass ranges from 15% to 100% for grate, 80−100% for BFB, 40−90% for CFB, and 0−100% for PF. There are still many other issues to be considered in practice.

5. CONCLUSIONS This study presents an investigation on the cofiring characteristics of BR with coals, using TG and kinetic analysis. The results are as follows: (1) BR has higher volatile content and lower caloric value than coal. It has two volatile combustion stage, followed by a char combustion stage, while coal burns intensely in one single stage with no obvious volatile release stage observed. BR is easier to ignite but more difficult in the complete combustion compared to coal. However, provided sufficient reaction time, BR, coal, and their blends can be completely burned below 750 °C. The heating rate has different effects on the different stages of BR combustion. (2) Co-combustion of BR and coal mainly consists of four stages: the moisture evaporation stage (