A large quantity of biomass is utilized in the rural area as fuel. Briquetting of biomass is a process which acquired considerable significance in the recent past, due to its efficiency increase, reduction in bulk, and having high calorific value of the briquettes. The process also helps to prepare smokeless briquettes, which are more hygienic to rural population, especially the women folk who are always hard of finding enough fuel supplies.

Forest or agricultural biomass is generally difficult to handle because of its bulky and scattered nature, legal and administrative problems, low thermal efficiency and copious liberation of smoke. In order to achieve maximum and efficient exploitation of resources locally available, it is essential to compress them into manageable and compact pieces which have a high thermal value. The process is called briquetting or pelleting. Pellets are compressed biomass briquettes and are usually cylindrical-shaped with a diameter between 25-100 mm. and length varies between 100-400 mm. Briquetting of biomass to increase its bulk density has acquired considerable significance in the recent past. Briquetting consists of applying pressure to a mass of particles with or without a binder and converting it into a compact aggregate. Since biomass is compressed into a dense material, it is also called as densification.

 

Techniques for Making Briquettes

At a time when the supply of fossil fuel and fire wood is not keeping pace with the increasing energy demand in the country, the use of organic residues through briquetting is one of the promising alternatives to meet local needs in various sectors, households, small scale industries, such as bakeries, brick kilns, drying tea, tobacco, spices, power and producer gas generation. It has been observed that density, durability and combustion efficiency of briquettes increases with de crease in the size and increase in pressure during preparation. Three versions of compaction/briquetting machines based on the use of different forms of energy, manually operated, bullock operated, and power-operated, have been designed, fab ricated and tested by different agencies in the country. Mainly two techniques are available for making briquetted fuel from agro wastes.

 

A. Pyrolysed briquetted fuel 

This process involves conversion of wastes/residues into charcoal in the absence of oxygen (pyrolysis) at 275-300°C and the briquetting is done. The principal users of this kind of fuel at present are households, hoteliers, factories, canteens, tea stalls etc: The crop residues viz. rice and wheat straw and husk makes a good material for briquetting. In Table 1 economics of agro-waste as a fuel is presented.

Table 1:  Economics of coal and crop residues for power

Sl.No

Characters

Coal

Feed stocks

 

 

 

Wheat straw

Rice straw

Rice husk

1

Availability

Fixed reserves

Regular with crop

Regular with crop

Regular with crop

2

Maintaining cost

Handling & milling

1/3 cost of coal

1/3 cost of coal

1/3 cost of coal

3

Ash content %

36

8.4

19.2

16.5

4

Volatile matter %

30

73.6

69.7

71.0

5

Technology

Coal powdered boiler

FBC boiler

FBC

FBC

Courtesy P.D. Tyagi, pp 27 FBC : Fluidixed Bed combustion

Pyrolysis:

Pyrolysis is dry distillation of woody biomass. The organic matter is heated above 400°C in the absence of air. This results in the release of a variety of energy rich products, both gaseous and liquid. Solid residue of pyrolysis is charcoal which in itself is a good energy material. The quick heating of organic material yields more gaseous, while slow heating, increases the percentage of the liquid factions. Presence of tars & pyroliginic acids in the products of pyrolysis makes them unsuitaple as engie fuels. The oils commonly formed via pyrolysis, are complex, mixture of hydrocarbons, ketones, aldehydes and other organic compounds, while the gases contain carbon monoxide, carban dioxide hydrogen, ethylene, methane, ethane, and other hydrocarbons. Pyrolysis of woody biomass is a versatile process having potential to provide a wide range of fuels from different types of biomass.

 

B. Direct compaction briquetted fuel 

Two technologies are available for the production of direct compaction briquetted fuel.

1. Binderless technology 
No binding medium is used to compact the biomass. This process involves two steps

(a) Semifluidixing of the biomass through the application of high pressure (at the range of 1200-1400 kg/cm2) under which condition the residue gets heated to about 182°C, and the lignin (material of which cell walls: are made) begins  flow and act as a binder.

(b) Extracting the diversified material around the room temperature. It is seen that when densification is done in the above manner, there is no need to add external agent like glue or binder. The existing units are using imported briquetting machines of 400 kg/cm2 capacity. The manufacture of indigenous briquetting machines has been taken up. These machines are available in the capacity range of 100-3000 kg/h operating on electric power.

2. Using binder
The briquetting machines operate at lower pressures (500- 1000 kg/cm2) and are powered by electricity. Briquettes produced by this technology requires binders like molasses, ligno-sulphonates etc.

 Briquettes produced by this process are suitable neither for household use nor for hotels, canteens etc. because they release a great deal of smoke while burning. These briquettes however, find use as industrial fuel in boilers in isolated pockets of the country. In Table 2 economics of binderless briquetting in India are presented.

Table 2: Economics of binderiess briquetting in India

Sl.No

Particulars

Small machines  Rs

Big machines Rs

1

Average cost

1,00,000

10,00,000

2

Delivered cost (add 15%)

15,000

1,15,000

3

Building, land & development cost

2,00,000

2,00,000

4

Fitting of power, water, etc transformer

3,19,000

4,19,000

5

Fixed assets

25,000

1,00,000

6

Preoperative expenses

50,000

50,000

7

Total financial outlay

7,09,000

19,19,000

A

Production average

150 kg/hr

1500 kg/hr

B

For 20 hr. & 300 days

900 tons

9000 tons

C

Raw material

1,80,000

18,00,000

D

Power, water, labour etc. maintenance, depreciation Rs 186 per tonne

1,67,400

16,74,400

E

Production cost

3,47,400

34,74,000

F

Sale price @ Rs 550 per tonne

4,95,000

49,50,000

G

Net profit/year

1,47,600

14,76,000

The different types of feed stocks and are used for briquetting fuel. These briquettes have different properties as regards calorific values, churning strength etc. in the Table no. 3 properties of different briquettes are presented.

Table 3:  Properties of briquettes

Sl.No

Properties of briquettes moisture (8-10%)

Rice husk

Rice husk & Molasses

G. nut shell

Rice straw  & husk & charcoal

Cotton stalk charcoal

Sugar-cane pith & coir pith char-coal

Rice husk char-coal

Maize/ sorghum char-coal

1

Av.length mm.

96

21

12-30

-

-

-

24.40

24.40

2

Av.diameter mm.

66

50

48

-

-

-

25

25

3

Av.weight gm.

46.4

28

-

-

-

-

-

-

4

Av.volume cc.

35.5

39.25

-

-

-

-

9.8-19.6

9.8-19.6

5

Bulk dens-ity g/cc

0.445

0.71

1.05

0.95-1.2

0.95-1.2

0.95-1.2

0.81

0.81

6

Av.chirning over stren-gth

10,000 kg in total surface

3292

4 N/cm2

-

-

-

-

139

7 N/cm2

530 6 N/cm2

7

Calorific value kcal/kg

4090

4500

4750

4000

5200

5600

3500

2000

All the briquettes fuel prepared from all the feed stock except rice husk charcoal and maize/sorghum stalk charcoal have high calorific value over 4000 kcal/kg.

 

III. Briquetting Presses 

A large number of machines (presses) are designed and developed to suit commercial as well as for rural areas in cottage industries. 

A. Briquetting presses 

Types Briquetting presses may be of screw or piston type. In the first case, the biomass is screwed forward under high pressure through a nozzle (funnel shaped) in which case, the briquettes gets its cylindrical shape. In the second case, the same forward pressure is effected with a piston. There are two methods to retain their cylindrical form. One of them operates with binding agent and the other without binding agent. Briquetting without biding agent requires a raw material with a maximum moisture content of 15 percent and a briquetting press which can generate a pressure of at least 1000 kg/cm2. The minimum pressure can vary depending upon the nature of raw material pressed. Briquetting with binding material does not require the same low moisture content which varies de- pending upon the-raw material. The four chief parameters of the briquetting process are: moisture, particle size, binding agent, and level of pressure.

 

B. Briquetting press for rural area 

A briquetting press suitable for use in villages should be able to meet the following requirements: 

  1. It must be inexpensive.
  2. It must be very easy to operate.

iii. It must be reparable using local tools and common sense.

  1. It must be energy economical.
  2. It must be suitable for all types of agriculture wastes.
  3. It should not require sophisticated storage space for the raw material, or the finished product or drying equipment.

C. Briquetting machines availibility 

A number of agencies/institutions/organisations have manufactured machines to popularise biomass based fuel briquetting in the country. Some of them are listed below: 

  1. Amteep Machines Tools Pvt. Ltd., Faridabad.
  2. Biomass Energy System, Madras
  3. Ion Exchange, Bombay
  4. Alternate Energy Sources, Pondicherry
  5. Desai Coal Company, Kolhapur
  6. Indian Institute of Technology, Delhi.
  7. IN COR, Visakhapatnam, A.P.
  8. National Small Scale Industries Institute, Okhla, Delhi
  9. School of Applied Research, Vishrambag, Sangli.
  10. Nimbkar Agri Res.Institute, Phaltan.
  11. Mis Latech Fuels (P) Ltd., Chandigarh
  12. Mis Bharat Industrial Corporation, Calcutta.
  13. Post Harvest Technology Centre, Jabalpur.

 A good beginning has been made and it is hoped that this sector will help to open up a renewable, inexhaustible and non-polluting source of energy supply consistent with due considerations for environmental protection in the near future.

 IV. Agro Waste Products

Briquettes are made out of a large number of waste feed stocks, with a binder or without binder. But the most common waste feed stocks used are rice husk, sugarcane trash, sunflower stalks and cotton stalks.

 

A. Rice husk

Almost 90% of the units use rice husk as the principal raw material. The annual raw material requirement for a 4 tonnes/day briquitted fuel unit is about 2500 tonnes of rice husk. It is estimated that to meet this demand, there must be at least 4 rice shellers in the vicinity of the briquetting unit to ensure continuous operation. A special feature of the briquettes made by this process is that they bum without emitting any smoke and can be used as domestic fuel. The average calorific value of this fuel when made from rice husk is 3400 kcal/kg. 

Rice husk is a bulky material posing significant transportation problems. Therefore efforts need to be made to develop an integrated system, so that the surplus energy could be used for setting up agro-industrial complexes. Maximum and efficient utilization of rice husk cannot only improve environmental conditions but also create many employment opportunities through development of secondary and territory industries. Rice husk is widely used by majority of units due to its greater availability.

B. Sugarcane trash

Briquettes made from sugarcane trash are a better fuel than burning them directly. The cost of briquetting sugarcane trash with power machines come to, Rs. 252 per tonnes. This compares favourably, with the fuel wood costs of Rs. 500-600 per  tonnes. Similar analysis for hand powered briquetting machines however gives the cost of production of Rs. 747 per tonne. This is because of economy of scale, since the production from a hand powered machine is only 25 tonnes per year as compared to 4800 tonnes per year for the powdered one (Table 4). Hand powered machines are. therefore not becoming popular in rural areas.

Table 4 : Production costs of briquettes from sugarcane leaves

Sl. No

Items

Powered machine

1 tonne/hr (4800 tonne/yr)

Hand-powered machines 25 kg/8 hr shift (7.5 tonne per yr)

1

Capital cost

 

 

A

Land, building 1 acre @ 50000/

3000 sq.ft.cost of building

50000

3,00,000

-

-

B

Plant & machinery

1150000

5000

 

Total

1500000

5000

2

Running cost per year

 

 

A

Maintenance cost 35%

402500

500

B

Interest 18% on capital

270000

900

C

Power cost

132000

-

D

Harvesting cost of Sug.leaves Rs 80 tonne

384000

600

E

Labour cost

24000

3600

 

Total

1212500

5600

 

Cost of briquetting

“ “

“”

 

 

4800

7.5

 

 

= Rs 252/tonne

= Rs 747/tonne

C. Sunflower stalks 

In India, Maharashtra accounted for the maximum production of sunflower in 1985-86. The total production in the country exceeded 0.3 million tonnes. The production of sunflower stalk was also around 0.3 million tonnes. The sunflower stalks are used widely as a household fuel. They can be converted into briquettes (higher heating value 3900 kcal/kg at 6.7% moisture). The briquettes burn with an average efficiency of 67% and produces little smoke. They are slow in igniting and contain a high percentage of ash. Sunflower stalk is also available in large quantities in Kamataka, 42.1 %, Tamilnadu 8.0%, AP. 3.4% besides Maharashtra 45.0%. In minor quantities it is also available in V.P., Rajasthan and Haryana.

 

D. Cotton stalks 

Cotton is the most important cash crop in India. In respect of production, India occupies the fourth place in the world. The production & availability of cotton stalks (1985- 86) was 4392.3 thousand tonnes. The main states contributing were Gujrat (1013.1 tonnes), Maharashtra (966.3), Punjab (714.9 tonnes), Haryana (380.1 tonnes), A.P. (366.9 tonnes), Tamil Nadu (279.9), Kamataka (256.5) and Rajasthan (141.3tonnes). The availability of cotton stalks has increased significantly during the last few years. 

At present after the pickings are over, the cotton plant is uprooted by the farmers and used for fuel purposes. Cotton stalks are woody residue and are valued as a household fuel. They are light, (density 0.21) and have a heating value of 3300 k cal/kg at 12% moisture content. The Chemical composition of stalks consists of cellulose 41.93%, pentosan 18.99%, lignin 27.18% and benzene/alcohol extractions 9.3%. Cotton stalks can be carbonised to char which is suitable for briquetting and can be used as smaller fuel. For briquetting, char obtained from the kilns is screened, through a 3 mm screen and crushed and mixed with moist clay and molasses as binders, and lime as binder-cum-energy extender. The mixture obtained with 30-35% moisture content is then briquetted either in extraduers or roller briquetting machines, commercially available in India. These briquettes are then dried. In the following Table No.5, specifications and utilities required for briquetting cotton stalks are presented.

Table 5 Specifications lor briquettes from cotton stalks

Sl.no

Briquettes (%)

Composition

Raw material and utilities (1 tonne of briquettes fuel)

1

Moisture

5-7

Cotton stalks

4.25 tonnes

2

Volatiles

7-9

(0.85 tonnes char)

 

3

Carbon

51

Lime

100 kg

4

Ash

35

Molasses

50 kg

5

Higher heating value

4450 Kcal/kg

Bentonite

50 kg

 

 

 

Power

56 Kwh

 

 

 

Water

500 litres

 

 

 

Rice husk or some equivalent fuel

600 kg

V. Other Forms of Biomass Fuel

Besides using the waste feedstock and other biomass to prepare briquetting and pellatisation of these material they are also used for some other purposes, viz. for energy supply as fuel. These processes may be called as wood conversion processes, which gives fuel of different energy value.

 

A. Carbonisation

The process is essentially a removal of volatile materials in the absence of air to give a high carbon product charcoal. This charcoal (Caloric val.6600- 7800 Kcal/kg) contains 20- 25% volatiles and 75-80% fixed carbon on a moisture-free basis. It burns smokelessly and can be preserved for a long periods. This process of the wood conversion above 280oC liberates energy, hence the reaction is exothermic.

B. Gasification 

Partial combustion of wood and air yields, gaseous mixtures comprising of carbon-dioxide, carbon-mono-oxide, hydrogen, methane, nitrogen etc. which may have as much as 85% of the heat value of biomass. It is therefore a highly efficient conversion process. The gas mixtures popularly known as producer gas, freed from solid particles and condensates is a good fuel for internal combustion engines. Thermo chemical gasification can be carried out through various processes, the classification is based on the agent used in the process. Air gasification, oxygen gasification, hydrogen gasification, and pyrolysis are the four main processes. Air gasification is the most common and the one considered to be the most simple; A variety of gasification equipments have been developed and are in practice.

C. Liquefaction of biomass 

In this process, the biomass contains 85% moisture is reacted with carbon monoxide and steam in the presence of an alkaline catalyst at 250 to 400°C and under conditions of high pressure. This process yields an oil which has an energy content of about 40 MJ/Kg. It is feasible to convert about 25% of the cellulose content to the residue in oil through liquefaction. Some variations in this process have been used to control the product characteristics. For example, a high energy gas is produced if the reaction temperatures is doubled and the process reduced to half.

Conclusion

It may be stated that there are a number of options to convert woody biomass and waste feed stocks into different forms of energy for use in various sectors household, agriculture, agro-industries, transport, and public utilities, but more data needs to be generated on the different processes before the conversion of technologies can be considered for extensive application. This call for giving high priority to testing of viable technologies for energy purposes on pilot scale under different sets of situations in the country.

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