Energy Sources, Part A: Recovery, Utilization, and

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The Characteristics and Power Generation Energetics of Coal, Cattle Dung, Rice Husk, and Their Blends a

b

M. Kumar & S. K. Patel a

Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, Odisha, India s b

Department of Mechanical Engineering, National Institute of Technology, Rourkela, Odisha, India Published online: 21 Feb 2014.

To cite this article: M. Kumar & S. K. Patel (2014) The Characteristics and Power Generation Energetics of Coal, Cattle Dung, Rice Husk, and Their Blends, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 36:7, 700-708, DOI: 10.1080/15567036.2010.545798 To link to this article: http://dx.doi.org/10.1080/15567036.2010.545798

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Energy Sources, Part A, 36:700–708, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/15567036.2010.545798

The Characteristics and Power Generation Energetics of Coal, Cattle Dung, Rice Husk, and Their Blends M. Kumar1 and S. K. Patel2 1

Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, Odisha, India 2 Department of Mechanical Engineering, National Institute of Technology, Rourkela, Odisha, India

In the present investigation, studies on proximate analysis, energy value, and ash fusion temperatures of E grade coal, cattle dung, rice husk, and their various blends have been carried out to assess their power generation potentials. The studied coal was found to have the highest fixed carbon content and energy value, followed by cattle dung and rice husk. In comparison to cattle dung–rice husk mixtures, the similarly prepared coal–cattle dung blends exhibited significantly higher energy values and proved themselves to be more superior fuels for power generation. Coal, cattle dung, and rice husk ashes were tested for their fusion temperatures and the results obtained established higher values, indicating no chance of bed agglomeration during combustion of these fuels and their blends up to about 900–950ıC. Both the energy value and power generation potential of coal–cattle dung blend got reduced with an increase of its dung content, suggesting the power generation process to be more efficient with smaller fraction of dung in the blend. The calculation results showed that an increase in dung content from zero to 90% in the coal blend enhanced the blend requirement from 4,565 to 6,309 tons/year in order to ensure a perpetual supply of electricity for a group of 10–15 villages. Keywords: analysis, ash fusion temperatures, cattle dung, coal, energy value, power generation, rice husk

INTRODUCTION Ever increasing demand and the adoption of centralized power generation from fossil fuels have resulted in inequities, external debt, and environmental degradation. Decentralized power generation from renewable energy sources can be a feasible long-term solution for these problems and to meet the rural and small scale energy needs in a reliable, affordable, and sustainable way. Biomass energy plays a pivotal role in the field of renewable energy due to its availability and sustainability for distributed power generation in remote areas. Agro-residues and animal wastes are the important biomass feedstocks in India due to their vast agricultural base. Estimated annual availability of agricultural residues and cattle dung for energy applications in India is presented in Table 1 (Kumar and Vijay, 2009). As shown in this table, around 150 MT of agricultural residues (rice husk 22 MT) and 430 MT of animal dung could be made available for power generation. Address correspondence to Dr. Saroj Kumar Patel, Department of Mechanical Engineering, National Institute of Technology, Rourkela, Odisha 769 008, India. E-mail: [email protected]

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COAL, CATTLE DUNG, RICE HUSK, AND THEIR BLENDS

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TABLE 1 Estimated Biomass Availability for Energy Conservation in India


Source Animal dung Crop residue Fruit and vegetable residue Fire wood Community waste

Total Availability, MT

Availability for Energy, MT

860 450 35 600 150

430 150 10 100 50

Raw agricultural residues have many disadvantages as an energy feedstock (Purohit et al., 2006). These include (i) variability in quality and relatively lower energy value, (ii) rapid burning and difficult control of its rate, (iii) requirement of frequent refueling, (iv) difficult mechanization for continuous feeding, (v) requirement of a large area for storage, (vi) possibility of spontaneous combustion in storage and more particulate emissions from boilers, and (vii) problems in their transportation and distribution. The majority of these disadvantages may be attributed to the low bulk densities of agricultural residues, which could be improved by briquetting them with or without a binder (cattle dung and others). As indicated by Purohit et al. (2006), a substantial amount of energy is required for briquetting agricultural residues; however, it may be energetically more viable compared to the energy embodied in mining and transportation of coal to the thermal power plants. In view of energy, cost, and environmental problems associated with the use of coals, the possibility of using cattle dung and agro-residues as supplementary fuels in coal-based power plants in India is being explored. For exploitation of biomass residues in electricity generation, characterization of their various properties, such as calorific value, chemical composition, combustibility, ash fusion temperatures, bulk density, etc., is essential. The present article deals with the studies on proximate analysis, energy value, ash fusion temperatures, and assessment of power generation potentials of coal (E Grade), cattle dung, rice husk, and their blends. The main aim has been to compare the results of coal–cattle dung blends with those of cattle dung–rice husk mixtures.

ASSESSMENT OF CATTLE DUNG AS A SUPPLEMENTARY FUEL IN POWER GENERATION With rapid developments in the living standard of people, there has been a mushroom growth in milk production centers (dairies, goshalas, etc.) and thus the number of cattle heads and dung production in the country are rising. Presently, animal dung cakes are being used as a cheap cooking fuel in remote areas in the most inefficient manner and this should be dispensed with a view to conserve it for economical use. As reported in the literature (Bhargava et al., 2004; Roy et al., 2010), the burning of animal dung cake produces higher concentration of carcinogenic polycyclic aromatic hydrocarbons (PAHs) in the breathing zone than that with liquefied petroleum gas (LPG) and firewood, and results in greater levels of DNA damage to rural Indian women. Therefore, alternative technologies for utilizing animal dung should be highly encouraged. There are two possible routes of producing energy from animal waste—biological and thermochemical. The anaerobic digestion of the waste has the disadvantages of higher installation cost, longer reaction time, and the requirements of huge water and large area (Cantrell et al., 2008).

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TABLE 2 Proximate Analysis and Calorific Values of Cattle Dung, Rice Husk, and Their Blends Proximate Analysis, wt%, Dried Basis

Proportion, wt%


Sl. No. 1 2 3. 4 5 6 7 8

Rice Husk

Cattle Dung

Volatile Matter

0 15 30 45 60 75 90 100

100 85 70 55 40 25 10 0

53.40 56.99 59.14 60.20 61.29 62.84 62.90 64.50

Ash

Fixed Carbon

Calorific Value, kcal/kg, Dried Basis

33.16 29.01 29.03 28.00 26.88 26.11 26.00 25.80

13.44 14.00 11.83 11.80 11.83 11.05 11.10 9.70

3,724 3,670 3,603 3,546 3,491 3,426 3,366 3,327

On the other hand, gasification/combustion process has been proved to be attractive and several biomass-based units in a wide range of capacity have been developed. The gasification/combustion of dung cake results in low temperature in the gasifier/combustor due to its high ash content and low heating value, and this reduces the rates of reactions (Roy et al., 2010). As a result, a major portion of the dung char remains unreacted and leaves with the ash. However, animal dung (blended with agro-residues, coal fines, etc.) can be used as a supplementary fuel in the gasifier/boiler. There are certain benefits in mixing cattle dung with biomass/coal. First, the dung has a good binding property for briquetting agricultural residues/coal fines, which may otherwise be difficult to use in a boiler/gasifier. Second, its use will reduce the load on the biomass/coal to make the resource sustainable for power generation. Third, the emission of hazardous gases into the atmosphere from natural decomposition of manure can be avoided if the alternative use of dung becomes viable. For better results, the properties and power generation potentials of different combinations of coal, cattle dung, and biomass need to be understood well before introducing such blended fuel into the gasifiers/boilers.

EXPERIMENTAL Materials Collection In the present investigation, rice husk and cattle dung, procured from the local area, were sundried and crushed mechanically using a mortar and pestle to ensure homogeneity. Thereafter, the rice husk and cattle dung powders ( 72 mesh size) were mixed in different proportions, as shown in Table 2. Blends of E grade non-coking coal (obtained from Belpahar coal mine) and dung powders were also prepared by mixing them in different ratios (Table 3) for comparative study. The air-dried samples of rice husk, dung, coal, and their mixtures were then processed for the determination of their chemical properties and ash fusion temperatures. Evaluation of Chemical Properties Analyses for moisture, volatile matter, ash, and fixed carbon contents were carried out on samples ground to 72 mesh size by standard methods (BIS, 1969), and the energy values were measured by using an oxygen bomb calorimeter (BIS, 1970).


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TABLE 3 Proximate Analysis and Calorific Values of Coal–Cattle Dung Blends Proximate Analysis, wt%, Dried Basis

Proportion, wt%


Sl. No. 1 2 3 4 5 6 7

Cattle Dung 0 15 30 45 60 75 90

Coal

Volatile Matter

100 85 70 55 40 25 10

26.80 28.42 34.38 38.54 45.26 48.36 52.22

Ash

Fixed Carbon

Calorific Value, kcal/kg, Dried Basis

36.08 35.79 34.38 32.29 31.63 31.91 31.78

37.12 35.79 31.24 29.17 23.11 19.73 16.00

5,397 5,280 5,022 4,731 4,452 4,190 3,905

Determination of Ash Fusion Temperatures The four characteristic fusion temperatures (IDT, ST, HT, and FT) of coal, rice husk, and cattle dung ashes were determined as per the German standard test method (German Standard, 1984).

RESULTS AND DISCUSSION Chemical Characteristics of Coal, Cattle Dung, Rice Husk, and Their Blends The quality of solid fuel affects the economy, output, and efficiency of the boiler operation. Proximate analysis gives indications about the availability of carbon and ash in the boilers. Data listed in Table 2 indicate that the distinction between the studied biomass residues with respect to proximate analysis and energy value is more pronounced and the cattle dung has higher ash and fixed carbon contents, and calorific value than those of rice husk. Furthermore, the results (Tables 2 and 3) established significantly lower energy values and fixed carbon contents in all the studied cattle dung–rice husk blends than those of the similarly prepared coal–dung blends, indicating reduced power generation potentials in the former and it may not be economically viable to run the boiler only on these biomass residues. The higher calorific value in the fuel can be attributed to its higher C and H contents. As shown in Table 2, both the calorific value and fixed carbon content get reduced with an increase in rice husk content in the biomass fuel blend. Hence, the increased use of rice husk in power generation through co-firing with cattle dung will render the combustion process much less efficient. It is further evident from Table 3 that the quality (heating value) of the coal blend becomes inferior at higher contents of cattle dung. As a result, a low temperature is expected to be achieved in the gasifier/boiler with a high fraction of cattle dung/rice husk in the coal blend. After considering all of these together, the authors suggest the use of a small fraction of cattle dung/rice husk in power generation through co-firing with the presently used coals (E and F grades) in boilers/gasifiers. However, a relatively higher ash content in coal (E to G grades)–cattle dung mixture (Table 3) may slightly increase suspended particulate matter (SPM) emission from the boilers. In any engineering design, economy is a major factor to reckon with. Although the cattle dung– rice husk is a technically inferior fuel for the purpose of power generation, their costs are low compared to those of coals and woody biomasses. The on-site cost of biomass fuel is calculated by considering different components like collection, preparation, drying, and transportation costs.

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TABLE 4 Ash Fusion Temperatures of Studied Biomass and Coal Materials


Ash Fusion Temperatures, ı C Ash Material

IDT a

ST a

HT a

FT a

Coal Cattle dung Rice husk

1,212 1,028 1,107

1,319 1,198 1,286

1,508 1,388 1,460

1,552 1,430 1,510

a IDT: Initial deformation temperature; ST: Softening temperature; HT: Hemispherical temperature; FT: Flow temperature.

Based on these components, the on-site cost of cattle dung in India may be taken as Rs. 250/per ton (Singh and Sooch, 2004). Assessment of Ash Fusion Temperatures The ash fusion temperature (AFT) results are widely used to give an indication to the designers and operators of boilers/reactors about the likely characteristics (slagging and fouling) of deposited ash. The major ash-related problem encountered in combustion/gasification is the bed agglomeration, which may result in total defluidization and unscheduled shutdown. In order to ensure no agglomeration and ample availability of oxygen inside the boiler/gasifier, the softening temperature (ST) of ash should be at least 150–200ıC more than the operation temperature of the reactor. The AFT results (Table 4) indicate that the rice husk ash has somewhat higher fusion temperature values (1,107–1,510ıC) than those of cattle dung ash (1,028–1,430ıC). As suggested by Mansaray and Ghaly (1997) and Baxter et al. (1998), this difference is believed to be due to relatively higher content of silica in rice husk. Rice husk ash contains more than 95% silica, which gives a rigid cage-like structure to the ash sample (Natarajan et al., 1998), and this structure may not easily permit the deformation of cube in the ash fusion test. Both the ashes, in general, exhibited a softening temperature 1,200ıC, indicating safe combustion operation in boilers up to about 1,000ı C with these biomass residues. From the data presented in Table 4, it appears that the coal–cattle dung blends will have somewhat lower AFTs than those of coal–rice husk mixtures. This indicates that the use of the latter is technically more viable for combustion at higher temperatures. However, the use of coal–cattle dung blends is not expected to be problematic during combustion in the boilers up to about 1,000ıC. Materials Required for a Planned Decentralized Power Generation In the present study, a group of 10–15 villages (consisting of around 3,000 families) has been considered for the estimation of coal, cattle dung, and rice husk requirements to feed a proposed co-fired power plant. The electricity requirement for irrigation, domestic work, lighting, and smallscale industries in these villages may be around 20,000 kWh/day (73 105 kWh/year) for which a power plant of this capacity could be planned. The requirements of coal, cattle dung, rice husk, and their blends for a power plant of the above capacity have been estimated in Tables 5 and 6. As shown in these tables, about 4,565, 6,618, and 7,404 tons of coal, cattle dung, and rice husk would be required to meet the yearly electricity requirement of 73 105 kWh from each of them individually. The evaluated amounts of fuels for co-firing of a power plant with different blends of coal and cattle dung (Table 5) indicate that the


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TABLE 5 Energy Values and Calculation of Material Requirements for a Planned Decentralized Power Generation from Coal–Cattle Dung Blends


Proportion of Component, wt% Sl. No.

Coal

Cattle Dung

1 2 3 4 5 6 7 8

100 85 70 55 40 25 10 0

0 15 30 45 60 75 90 100

Calorific Value, kcal/ton, Dried Basis 5,397 5,280 5,022 4,731 4,452 4,190 3,905 3,724

103 103 103 103 103 103 103 103

Assumptions used in the calculations for this study are as follows: i) Conversion efficiency of thermal power plants D 30%; ii) Overall efficiency of power plants D 85%. Total energy from 1 ton of coal–cattle dung blends at 30% efficiency of power plant: i) For 100% coal D 5,397 103 0.3 kcal D 1,881 kWh; ii) For 85% coal and 15% cattle dung D 5,280 103 0.3 kcal D 1,841 kWh; iii) For 70% coal and 30% cattle dung D 5,022 103 0.3 kcal D 1,751 kWh; iv) For 55% coal and 45% cattle dung D 4,731 103 0.3 kcal D 1,649 kWh; v) For 40% coal and 60% cattle dung D 4,452 103 0.3 kcal D 1,552 kWh; vi) For 25% coal and 75% cattle dung D 4,190 103 0.3 kcal D 1,461 kWh; vii) For 10% coal and 90% cattle dung D 3,905 103 0.3 kcal D 1,361 kWh; viii) For 100% cattle dung D 3,724 103 0.3 kcal D 1,298 kWh. Power generation at 85% overall efficiency: i) From 100% coal D 1,881 0.85 D 1,599 kWh/ton; ii) From 85% coal and 15% cattle dung D 1,841 0.85 D 1,565 iii) From 70% coal and 30% cattle dung D 1,751 0.85 D 1,488 iv) From 55% coal and 45% cattle dung D 1,649 0.85 D 1,402 v) From 40% coal and 60% cattle dung D 1,552 0.85 D 1,319 vi) From 25% coal and 75% cattle dung D 1,461 0.85 D 1,242 vii) From 10% coal and 90% cattle dung D 1,361 0.85 D 1,157 viii) From 100% cattle dung D 1,298 0.85 D 1,103 kWh/ton.

kWh/ton; kWh/ton; kWh/ton; kWh/ton; kWh/ton; kWh/ton;

Amount of blends required to supply electricity (73 105 kWh) for the whole year: Component Proportion, wt%

Material Required, Tons

Sl. No.

Coal

Cattle Dung

Coal

Cattle Dung

Total

1 2 3 4 5 6 7 8

100 85 70 55 40 25 10 0

0 15 30 45 60 75 90 100

4,565 3,965 3,434 2,864 2,214 1,470 631 0

0 700 1,472 2,343 3,321 4,409 5,678 6,618

4,565 4,665 4,906 5,207 5,535 5,878 6,309 6,618

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TABLE 6 Energy Values and Calculation of Material Requirements for a Planned Decentralized Power Generation from Cattle Dung–Rice Husk Blends Proportion of Component, wt%


Sl. No.

Cattle Dung

Rice Husk

100 85 70 55 40 25 10 0

0 15 30 45 60 75 90 100

1 2 3 4 5 6 7 8

Calorific Value, kcal/ton, Dried Basis 3,724 3,670 3,603 3,546 3,491 3,426 3,366 3,327

103 103 103 103 103 103 103 103

Assumptions used in the calculations for this study are as follows: i) Conversion efficiency of thermal power plants D 30%; ii) Overall efficiency of power plants D 85%. Total energy from 1 ton of cattle dung–rice husk blends at 30% efficiency of power plant: i) For 100% cattle dung D 3,724 103 0.3 kcal D 1,298 kWh; ii) For 85% cattle dung and 15% rice husk D 3,670 103 0.3 kcal D 1,279 kWh; iii) For 70% cattle dung and 30% rice husk D 3,603 103 0.3 kcal D 1,256 kWh; iv) For 55% cattle dung and 45% rice husk D 3,546 103 0.3 kcal D 1,236 kWh; v) For 40% cattle dung and 60% rice husk D 3,491 103 0.3 kcal D 1,217 kWh; vi) For 25% cattle dung and 75% rice husk D 3,426 103 0.3 kcal D 1,195 kWh; vii) For 10% cattle dung and 90% rice husk D 3,366 103 0.3 kcal D 1,174 kWh; viii) For 100% rice husk D 3,327 103 0.3 kcal D 1,160 kWh. Power generation at 85% overall efficiency: i) From 100% cattle dung D 1,298 0.85 D 1,103 kWh/ton; ii) From 85% cattle dung and 15% rice husk D 1,279 0.85 D iii) From 70% cattle dung and 30% rice husk D 1,256 0.85 D iv) From 55% cattle dung and 45% rice husk D 1,236 0.85 D v) From 40% cattle dung and 60% rice husk D 1,217 0.85 D vi) From 25% cattle dung and 75% rice husk D 1,195 0.85 D vii) From 10% cattle dung and 90% rice husk D 1,174 0.85 D viii) From 100% rice husk D 1,160 0.85 D 986 kWh/ton.

1,087 kWh/ton; 1,068 kWh/ton; 1,051 kWh/ton; 1,035 kWh/ton; 1,016 kWh/ton; 998 kWh/ton;

Amount of blends required to supply electricity (73 105 kWh) for the whole year: Component Proportion, wt% Sl. No. 1 2 3 4 5 6 7 8

Material Required, Tons

Cattle Dung

Rice Husk

Cattle Dung

Rice Husk

Total

100 85 70 55 40 25 10 0

0 15 30 45 60 75 90 100

6,618 5,709 4,785 3,820 2,821 1,796 732 0

0 1,007 2,050 3,126 4,232 5,389 6,583 7,404

6,618 6,716 6,835 6,946 7,053 7,185 7,315 7,404



707

requirement of the former to generate 73 105 kWh of electricity decreases from 4,565 to 631 tons with an increase in cattle dung content in the blend from zero to 90%. Table 5 also shows the increased requirement of blend at higher cattle dung content and this can be attributed to the lower energy value of dung than coal. Results (Table 6) for the variation in the amount of species required to generate 73 105 kWh of electricity per year from different blends of cattle dung and rice husk clarify that the requirement of dung decreases and that of blend increases with a rise in rice husk content in the blend and this is because of relatively lower calorific value/combustible content in rice husk. Furthermore, comparison of the data presented in Tables 5 and 6 indicate that to produce the above said amount of electricity, the requirement of coal–dung blend is much lower than that for dung–rice husk blend, suggesting the latter to be a more inferior fuel than the former. CONCLUSIONS On the basis of the results of the present investigation, the following conclusions may be drawn: 1. The studied E grade coal showed the highest energy value and fixed carbon content, followed by cattle dung and rice husk. 2. The results established significantly higher calorific values and fixed carbon contents in all of the studied coal–cattle dung blends than those of the similarly prepared cattle dung–rice husk blends, recommending the former to be superior fuel in power generation than the latter. 3. Coal and rice husk ashes exhibited higher fusion temperatures (ST: 1,285–1,320ıC, HT: 1,460–1,510ıC) and ensured for no bed sintering in the boiler up to the operation temperature of about 1,000ıC. 4. The fusion temperature results of cattle dung ash have been reported to be lower (IDT–FT: 1,028–1,430ıC), indicating the safe boiler operation up to about 900–950ıC. 5. The quality (heating value) of coal–cattle dung blend worsened rapidly with increase in its dung content, suggesting the power generation process to be more efficient with a smaller fraction of dung in the blend. 6. Calculation results have shown that the requirement of coal–cattle dung blend to generate 73 105 kWh of electricity per year (for a group of 10–15 villages) increased from 4,565 to 6,309 tons with an increase in dung content from zero to 90%. 7. The heating values and calculation results proved cattle dung–rice husk blends to be much less effective in power generation than coal–cattle dung blends. 8. The sole use of rice husk as a fuel in power generation does not appear to be economically feasible. 9. Altogether, the authors recommend the exploitation of cattle dung and rice husk in power generation after blending with coal.

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Energy Sources, Part A: Recovery, Utilization, and

Energy Sources, Part A: Recovery, Utilization, and

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