Thermogravimetric studies of the behavior of wheat straw with added coal during combustion

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33 (2009) 50 – 56

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Thermogravimetric studies of the behavior of wheat straw with added coal during combustion Cuiping Wang, Fengyin Wang, Qirong Yang, Ruiguang Liang College of Mechanical & Electronic Engineering, Qingdao University, 266071, China

ar t ic l e i n f o

abs tra ct

Article history:

The combustion behavior of biomass and biomass–coal blends under typical heating

Received 24 October 2006

conditions was investigated. Thermogravimetric analyses were performed on bituminite

Received in revised form

coal, aspen strawdust and wheat straw used alone and blended with different coal weight

10 April 2008

ratios. The behavior of biomass fuels in the burning process (different rates of

Accepted 11 April 2008

volatilization, char burning and heat production) was analyzed, and the effects of a cold

Available online 4 June 2008

molding procedure for wheat straw on the burning properties were investigated. In

Keywords: Wheat straw Aspen sawdust Biomass–coal blends Combustion behavior

addition, the kinetic parameters for the thermal conversion of each fuel were determined. Cold molding led to easier firing, and 5% coal was identified as the ideal ratio to achieve similar heat release characteristics to strawdust. Such a mixed pellet fuel with burning characteristics similar to aspen wood can be produced to take advantage of the wide design basis for wood-fired boilers.

Thermogravimetry

1.

Introduction

Biomass, such as straw, grasses, wood shavings, sawdust, roots, branches, leaves and bark, are used in different forms for energy production. Many technologies have been studied during the last two decades for biomass utilization, such as combustion, pyrolysis, gasification, liquefaction and supercritical fluid extraction, and biochemistry technology [1–6]. Sometimes biofuel products are mixed with semi-fossil peat and fossil coal to achieve better control of the burning process. Methods for direct burning and the production of gaseous or liquid derivatives, e.g. methane or ethanol, are very desirable. Another growing market for biofuels is the production of briquettes and pellets for the consumer market. Biomass pellets can be used in grate furnaces and fluidized bed combustion with many advantages, such as easy storage and transfer of the pellets, higher burning efficiency, lower

Corresponding author. Tel.: +86 532 85953720.

E-mail address: [email protected] (C. Wang). 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.04.013

& 2008 Elsevier Ltd. All rights reserved.

pollution, lower dust and higher heat values. However, the burning efficiency is biomass-specific and varies with the boiler combustion efficiency and output. Few studies [7–11] have been carried out on biomass pellets, and the thermal properties and influence of pelleting processes on pellet burning need to be studied in greater detail. Pellet methods include hot molding (compressed at temperatures 4200 1C), which can affect the structure of cellulose and even lignin [7], and cold molding with the highest compression temperature of 70–80 1C [9–11] has little effect on lignin, which is conductive to an increase in cellulose cementation. The present work was undertaken to determine whether the cold molding method influences the combustion performance and whether the addition of coal can modify the burning velocity and unify the thermal properties of different biomass fuels. Thermogravimetric studies of aspen sawdust, wheat straw and wheat straw blends were carried out.

ARTICLE IN PRESS BIOMASS AND BIOENERGY

2.

chemical and elemental analyses and to TGA/DTA combustion experiments.

Materials and experiments

Wood sawdust pellet is an excellent biomass fuel and is more and more widely used in biomass boilers. The burning characteristics of biomass may very considerably depend on the composition of the raw material used [8]. While, wheat and corn straw products are plentiful in rural China, they are studied in this paper to be a right supplement to sawdust resources.

2.1.

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33 (2009) 50 – 56

2.2.

Chemical and elemental analyses

Elemental analysis was carried out on an Elementar VarioEL III instrument manufactured in Germany. Fuel analysis is reported in Table 1. Biomass typically has a volatile/fixed carbon (VM/FC) ratio 44.0, while the VM/FC ratio for coal is virtually always o1.0. Thus, for biomass fuels, the predominant form of combustion is gas-phase oxidation of the volatile species. Using the addition of coal with a lower VM content, it is possible to adjust the gas-phase combustion of biomass fuels to a similar stage.

Preparation

The agriculture residues of wheat straws were from rural North-China during the summer. After the seeds were removed, the wheat straws were air dried for about 2 months. Then the wheat straws were pulverized to millimeter-size in the feed grinder. Another biomass material was aspen wood sawdust, which was collected from the furniture factory in Qingdao city; the aspen wood sawdust was grown in north China about 5–10 years old. The sawdusts were dried at room conditions (20 1C) for 1 month and screen sizing. A small (75 kg h1) laboratory-scale pelleter did the cold molding at the same palletizing condition, separately to press the wheat straw powder and aspen sawdusts into 6-mm-diameter pellets, with of 5–15-mm-length, necessarily adding 5% wt. water into the powder and uniformly mixed in advance. Dry density of the pellets was 1.45 kg m3, approximately calculated by weighing the pellet samples after room conditions for half a month. Bituminite coal (made in Datong, Shanxi Province, China) powder was chosen as an additive for mixing with wheat straw powder; the mean size of the additive coal was approximately 80 mm, and the mixing ratios of 5%, 8% and 11% were selected. The chosen materials are all very common and the pellets are of low cost; accurately weighed samples were subjected to

2.3.

Combustion in TGA/DTA

Seven samples were analyzed, as shown in Table 2. Their combustions were carried out in a TGA/SDTA model 851 instrument (Mettler Toledo, Switzerland). The linear temperature gradient in TGA experiments was 10 K min1 for coal and 15 K min1 for wheat straw and the blends. From the weight loss (TG) process, derivative thermogravimetric (DTG) and difference in thermal analysis (DTA) curves can be plotted. The released heat can be calculated from the DTA curve as described previously [12]: Z 1 ½T  Tignition  dt ¼ bS ¼ b Dw h, (1) DQ ¼ b ignition

where b is the heat transfer constant from the sample to the metal wall; S is the area under the DTA curve; Dw is the average width of the heat release peak; and h is the thermopositive peak height. Neglecting the difference of b constant between these biomass species, the area under the DTA curve reflects the heat releasing state.

Table 1 – Fuel analysis (applied basis data) Chemical analysis (%)

Sawdust Wheat straw (1) Wheat straw (2) Coal

Elemental analysis (%)

VM

FC

Ash

Water

N

C

H

O

S

65.06 66.8 68.28 17.45

14.27 15.0 15.43 52.80

3.61 9.28 8.92 28.55

17.05 9.0 7.35 1.19

0.37 1.01 0.71 0.66

38.14 36.9 38.2 59.19

5.22 5.52 5.56 3.71

35.96 37.9 39 5.51

0.23 0.4 0.31 1.50

LHV (kJ kg1)

VM/FC ratio

13992 13882 14275 23425

4.56 4.45 4.42 0.33

Table 2 – The samples and their marks used in the paper Samples

Marks

Bituminite coal

Aspen wood sawdust

Unmolded wheat straw

Wheat straw pellet

89% wheat straw+11% coal pellet

92% wheat straw+8% coal pellet

95% wheat straw+5% coal pellet

Coal

Sawdust

Wheat straw (1)

Wheat straw (2)

WS89%+C11%

WS92%+C8%

WS95%+C5%

ARTICLE IN PRESS BIOMASS AND BIOENERGY

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33 (2009) 50 – 56

12

100

Results and discussion

10

12

B

60

1.5

M

A

6 1.0

40

4 C

20

D

8

DTG DTA

1.5 DTA

60 A

1.0

40

2

20

2 C 400

500

600 T [K]

700

400

500

600

Fig. 1 – Thermal properties of the bituminite coal additive.

12

2.5

TG DTG 10 DTA

2.0

1.5 DTA

m/m0 [%]

8 60 A

6 1.0 4

B

20

2

500

600 T [K]

700

800

DTG DTA

B

6 1.0 4

0 500

600 T [k]

WS89%+C11% WS92%+C8% WS95%+C5%

700

800

D

2

0.5

0 0.0 900

Fig. 3 – Thermal properties of wheat straw (1).

dm/dT [mgK-1]

1.5

1.0

0.5

1.5

C

900

2.0

8

60

400

2.5

DTG [mgK-1]

TG

DTA

m/m0 [%]

10

20

800

Fig. 5 – TG curves for biomass–coal blends.

2.0

12

A

500 600 700 Temperature [K]

2.5

100

40

400

0.5

Fig. 2 – Thermal properties of aspen strawdust.

80

300

0 0.0 900

0 400

20

D

C

60

40

DTG [mgK-1]

80

40

WS89%+C11% WS92%+C8% WS95% C5%

80

100

0.0 0 900

0.5

0 0.0 900 1000 1100

700 800 T [K]

800

Fig. 4 – Thermal properties of wheat straw (2).

m/m0 [%]

300

0.5

D

0

100

0

6

B

2.0

DTA

m/m0 [%]

80

2.0

TG

4

TG DTG 10 DTA 8

N

DTG [mgK-1]

IG

100

2.5

80 m/m0 [%]

Results of the TGA analysis are shown in Figs. 1–6, in which the TG, DTG and DTA curves are reported for the experimental samples as a function of temperature. According to

2.5

DTG [mgK-1]

52

0.0 300

400

500

600

700

800

900

Temperature [K]

Fig. 6 – DTG curves for biomass–coal blends.

the fluctuation characteristics of the three curves, four typical combustion stages are apparent. A, the dewatering period; B, volatilization and burning; C, char burning; and D, burnout, as shown in Figs. 1–4.

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BIOMASS AND BIOENERGY

1.9 and 0.65 mg K1, 2.9 and 1.4 mg K1, and 3.75 and 1.55 mg K1 for mixing ratios of 11%, 8% and 5%, respectively. The figures show that all residues are higher than the ash contents determined by the chemical analysis. This is possibly related to the cylindrical shape of the burning chamber, which may influence air easily reaching the fuel at the bottom, the combustion needs longer time to complete the burnout stage till the ash residues coincidence to the chemical analysis.

Thermal properties of coal and biomass materials

To determine the ignition temperature, two points on the TG curve should first be identified. One (marked as M) is the point at which a vertical line from the sharp DTG peak (highest dm/dT value) crosses the TG curve. The other (marked as N) is the point at which volatilization begins. A tangent to the TG curve at M and another horizontal tangent to N are drawn. The point at which these lines cross is marked as IG, which corresponds to the ignition temperature. This process is shown in Fig. 1, and the ignition point for coal is approximately 748 K. The DTA curve has two clear thermopositive peaks for volatile burning and char burning. The thermopositive peak for volatile burning is short and lower, so less heat is released. The stage C thermopositive peak is high, representing a large release of heat. The burning characteristics of the biomass and biomass– coal blends are very similar, as shown in Figs. 2–6, with very similar TG and DTG curves. At the start of DTG, there is a higher peak for the biomass fuels compared to coal because of their higher water content. Stage B lasts for a longer time, while stage C is shorter. At the same linear temperature gradient, aspen sawdust burns more slowly than wheat straw (1) in the volatilization period. The DTG curve shows a peak at approximately 1.4 mg K1, which is lower than that for wheat straw (2.25 mg K1). Differences between wheat straw and sawdust in the DTG curves are mostly during the volatilization period. For stage D, the residue is lower than that for coal based on the TG curve. Thus, improvement of wheat straw as a replacement for sawdust fuel can be designed to adjust its volatilization rate. The DTA curves in Figs. 2–4 show that heat release for wheat straw in the volatilization period finishes earlier (narrow thermopositive DTA peak) than that for aspen sawdust. In the char burning stage, aspen sawdust begins to release heat earlier, and the thermopositive peak is wider. For the blends, the differences generally indicate that the lower the mixing ratio, the faster the weight loss. In Fig. 6, the two DTG peaks are very slender. Although the width of the two peaks is similar for the three fuel blends, the peaks are at

3.2.

Effect of cold molding on combustion

Comparison of Figs. 3 and 4 indicates that the molding process has some effects on the combustion of wheat straw. Except the similar dewatering period, the other three periods show differences to some degree. The two DTG peaks of the straw powder are all very slender, while the two peaks of the straw pellet powder are wider (in 470–625 and 705–775 K). The peaks points of the pellet powder are at 1.5 and 1 mg K1, which are lower than those for wheat straw (1) (2.25 and 1.35 mg K1, respectively). After cold molding, the width of the thermopositive peak in stage B is smaller and the heat release area of VM becomes lower, the peak in stage C is higher so the heat release area of FC is bigger. This indicates that the molding procedure relatively reduces the VM content and correspondingly increases the FC content. Overall, the heat released by wheat straw (2) is greater, which implies increased burning efficiency. For stage D of uncomplete burnout, the residue for wheat straw pellet powder is approximately 16.5% (Fig. 4) compared with approximately 20% for wheat straw 1) (Fig. 3). A lower residue correlates to better burning efficiency under the same combustion conditions. All the analysis results are listed for comparison in Table 3.

3.3.

Kinetic parameters

Activation energy and pre-exponential factors are discussed here to evaluate the difficulty and intensity of thermal chemistry reactions for the different fuels, under the same combustion conditions of terminal temperature and heat-up rate.

Table 3 – Summary of TGA analysis Fuel

Coal Strawdust Wheat straw (1) Wheat straw (2) WS89%+C11% WS92%+C8% WS95%+C5%

Temperature (K)

Volatilization and burning

Range (K)

E (kJ mol1)

k0

Burnout residue (%)

680–710 640–670 615–710

710–900 670–770 710–755

92.9 52.8 171.7

9.76E+3 16.6 3.87E+8

26 13 19

2.24E+4

625–705

705–775

154.2

1.98E+7

12

9.85E+5 3.77E+6 8.88E+8

620–710 605–710 610–705

710–800 710–770 705–760

152.6 202.9 209.6

7.3E+6 1.45E+11 2.64E+11

21 20 18.5

Dewatering

Ignition

Range (K)

E (kJ mol1)

k0

380 400 390

748 552 550

515–680 480–640 475–615

205.5 91.12 109.3

2.13E+13 2.41E+4 2.11E+6

390

535

470–625

88.2

395 390 390

550 552 550

480–620 500–605 500–610

106.4 111.5 114.7

Transition period (K)

Char burning

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As previously explained [13] for a first-order thermal chemistry reaction, the devolatilization kinetics and the rate of weight loss can be expressed as a global Arrhenius decomposition Eq. (2) and mass action law (3): (2)

dm ¼ ka dt

(3)



-5.5 -6.0 -6.5 -7.0 -7.5

where k is the decomposition rate, k0 is the pre-exponential frequency factor for the rate coefficient (s1); E is the activation energy (kJ mol1); R is the universal gas constant 8.3143 kJ K1 mol1; T is absolute temperature (K); and a is the remaining quota of solid fuel. Let A ¼ k=a ¼ ðdm=dtÞ=a; after logarithmic transformation, the equation becomes lnðAÞ ¼ lnðk0 Þ 

coal strawdust wheat straw(1) wheat straw(2) WS95%+C5%

-5.0

ln (A)

k ¼ k0 eE=RT

-4.5

E RT

-8.0 -8.5 -9.0 -9.5 1.3

(4)

1.4

1.5

1.6 1.7 1000/T

1.8

1.9

2.0

Fig. 8 – Linear relationship for fuels in FC firing.

Let Y ¼ ln(A), a ¼ ln(k0), b ¼ E/R, and x ¼ 1/T; then Eq. (4) becomes: Y ¼ a þ bx.

(5)

Using the known temperature T and the corresponding calculated Y, the coefficients b and a can be obtained by fitting the line in Originpro 7.0 software, so that E and k0 can be calculated. Because volatilization and char burning are two independent periods, and the burning kinetic reactivity can be regarded as the rising section, the data for T and Y corresponding to volatilization and char kinetic burning are filtered for fitting (Figs. 7 and 8, respectively). E and k0 are calculated using the fitted coefficient, as listed in Table 3. E for the volatilization period is greater than that for char burning, which is in agreement with the high ignition temperature and the char inflammability. The char burning stage shows a better degree of correlation. For the biomass fuels, the data for ln(A) and 1000/T show better linear correlation to the fitting lines in the volatilization period (the main burning stage, as for coal) in Fig. 7, but the dependence relationship is poorer in the char burning stage,

-7

coal strawdust wheat straw(1) wheat straw(2) WS95%+C5

ln (A)

-8

-9

-10

-11

-12 1.5

1.6

1.7

1.8 1000/T

1.9

2.0

Fig. 7 – Linear relationship for fuels in VM firing.

as observed in Fig. 8. We deduced that biomass char burning is always too short to obtain enough measured points, so the fitting error is large. The data for wheat straw and WS–coal blends are almost linear, but are far away from the data for sawdust and coal. The values calculated k0 and E for the char burning stage are greater than that for coal (compared in Table 3). Because char firing involves charcoal and biomass char for blends of 8% and 5%, the linear relationship in the char burning stage is poorer and E and k0 are also greater. Perhaps this is why previous papers only dealt with the kinetics of pure fuel combustion. It is evident from Table 3 that E and k0 for sawdust are low, which indicates that this fuel is easy to burn and the burning ranges last longer, so sawdust is a good fuel. The cold molding process reduced the ignition temperature for wheat straw by approximately 15 K. In addition, the temperature range in the volatilization stage was 475–615 K for wheat straw (1) and 470–625 K for wheat straw (2), an increase of approximately 15 K. The increase is approximately 25 K for the char burning stage. The activation energy for the oxidation reaction also decreased from 109.3 to 88.2 kJ mol1 in volatilization and from 171.7 to 154.2 kJ mol1 in char burning after molding. These improvements in combustion highlight the advantages of cold molding of wheat straw biomass.

3.4.

Comparison of combustion similarity

Fig. 9 shows the heat released for different ratios of coal added to wheat straw (1). Aspen sawdust released heat more continuously from the volatilization stage to char burning, and the transition between the two peaks is a shallow trough. For wheat straw (1), there is clearly a transition to lower heat release, and the heat release in the char burning stage lasts for a shorter time. The lower the mixing ratio, the higher the heat release; the two peaks are at 6.8 and 7.6 for 11 wt% coal, 7.7 and 9.8 for 8 wt% coal, and 8.3 and 9.95 for 5 wt% coal. For weight ratios of 8% and 5%, heat release in the transition stage is higher for the latter, and the two peaks and the transition stage are close to those for sawdust, although the

ARTICLE IN PRESS BIOMASS AND BIOENERGY

two peaks occur at lower and higher temperatures, respectively, compared to sawdust. With decreasing weight ratio, the transition period is increasingly like a third peak, so the sample with 5% coal releases heat in a similar manner to sawdust and shows the most ideal combustion similarity. Similar results for modifying and improving other biomass resources can be used to identify those best suited to supplement sawdust.

3.5.

Comparison of combustion characteristic factor

A parameter called the combustion characteristic factor (CCF) [14] can be used as a criterion for fuel combustion performance, defined as S¼

ðdw=dtÞmax ðdw=dtÞmean , T2i Th

(6)

where (dw/dt)max is the maximum burning velocity (%min1); (dw/dt)mean is the average burning velocity (%min1); Ti is the ignition temperature (K); and Th is the burnout temperature (K). This factor, which encompasses the ease of ignition, the firing velocity and burnout temperature, is a comprehensive parameter, used here to compare the combustion performance of biomass fuels. CCF values were calculated for several biomass fuels and are listed in Table 4. The values are greater than 2 for all the biomass fuels, indicating their good general burning perfor-

10

DTA

8 6 strawdust wheat straw(1) WS89%+C11% WS92%+C8% WS95%+C5%

4

2

0 500

600

700 T [K]

800

900

55

33 (2009) 50 – 56

mance. If mixed with a suitable ratio of coal additive, for example, lower than 8%, the CCF is improved, to a value as high as 4.98 for a ratio of 5%. Thus, the conclusion can be drawn that 5 wt% coal as an additive can improve the combustion performance of wheat straw as a pellet fuel.

4.

Conclusions

In this study, TGA–DTA experiments were performed on a number of biomass species relevant for use as fuels. Some quantitative characteristics during dewatering, volatilization, char burning and burnout stages are listed and compared. All biomass fuels showed volatilization as the predominant combustion process, almost with its short char conflagration stage and longer burn-off stage. Combustion of wheat straw (1) showed a longer transition stage between volatilization and char burning, so mixing with a coal additive represents a method to modify the biofuel and obtain a more continuous heat-release process. It was observed that the cold molding procedure is favorable for firing properties, such as lower ignition temperature, shorter transition stage between volatile burning and char burning, extended heat release, and a reduction in combustion residue. Thus, biomass fuel pellets are superior to the direct burning of biomass powder. A basic kinetic analysis for each species is provided. The kinetic parameters reveal that sawdust is a premium fuel for easy burning. The comprehensive parameter CCF for all biomass fuels in this project was greater than 2, indicating good combustion performance. When 5 wt% coal was added to wheat straw, the blend showed an improved CCF of 4.98. In general, the above TGA–DTA analysis revealed that when coal is added to biomass, the volatilization rate is modified, the heat released is affected and the combustion residue is reduced under the same final combustion temperature; thus, the combustion efficiency is increased. A project involving the addition of coal to improve the combustion of biomass fuel pellets is feasible. By adding a coal to less than satisfactory agricultural residues, a mixed pellet fuel with burning characteristics similar to aspen wood can be produced to take advantage of the wide design basis for wood-fired boilers. For wheat straw as a substitute for aspen sawdust, 5 wt% of bituminite coal is the ideal additive ratio to achieve close heat release to that for the aspen sawdust.

Fig. 9 – Heat release process.

Table 4 – Combustion characteristic factor S for biomass fuels

Strawdust Wheat straw WS89%+C11% WS92%+C8% WS95%+C5%

(dm/dt)max

(dm/dt)mean

Ti (K)

Th (K)

S  10–7

Order

18.4 28.3 22.94 32.47 39.93

3.37 3.15 2.79 2.87 2.87

552 550 550 552 550

770 755 800 770 760

2.643 3.9 2.644 3.97 4.98

5 3 4 2 1

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