Bio-energy from agricultural residues

Published September 14, 2009

AGRICULTURAL residues are of different types like crop residues, animal manures, animal bedding etc. Crop residues are of two types, field-based and process-based (industrial).

Field-based residues are the parts of different crops left behind in the fields after the grain or seed has been removed during combining and harvesting. Process-based residues are collected in industries after completion of process and final goods production.

There is abundant sources of field-based crop residue in the form of rice-straw, wheat-straw, cotton-stalks, maize-stalks, sugarcane tops etc. According to the United Nation's Food and Agriculture Organisation (FAO) study, Pakistan annually generates around 69 million tons of field-based crop residues. The detail of the quantity of residues produced from each crop is given in the table below

Field-based crop residues are generally considered useless.These are either burned in the fields / homes or buried in the land. Burning of crop residues emits carbon dioxide and smoke which is hazardous for health and a source of ozone with risks to the atmosphere. Although different crop residues contain different nutrients which could be helpful in improving land fertility, the process of disposing of residue to improve land fertility cannot be justified as our farmers mostly do not know what type of nutrients is needed for their land as soil analysis is not much in practice. Conclusively, it can be said that crop residues are not being beneficially utilised and are wasted.

These residues are burnt due to lack of demand. This reduces farmers' income significantly but a major source of energy is lost. About 25-40 per cent of a crop is food while the rest 60-70 per cent is residue, which can be a low cost source of raw material for biomass-energy.

If crops residues are used for production of energy a sufficient amount of it can be produced cutting oil import and improve the profitability of farming sector. According to Dr Anil K. Rajvanshi (a renewable energy resources expert in India), on an average a farmer can get an extra income of $200-300 / acre per year from the residues alone if these are used for producing energy.

Because of easy availability of crop byproduct as a cheaper source and renewing ability, crop residues offer significant potential for increasing the quantity and controlling the rising costs of raw material for generation of energy. The cost of electricity produced from residues though depends upon the type of power plant used (steam turbine or combine cycle), the per kilowatt hour cost is much less (approximate three times) than the thermal (oil-fed) power plants.

Hence crop residues for energy production can be a viable option especially for agricultural countries.

Crop residues can be used for energy production by using different techniques like combustion (direct burning and co-firing) and non-combustion methods (thermo-chemical, biochemical etc.). In direct burning, residues are simply burned and the heat produced is used for heating, cooking or to produce electricity.

In co-firing process crops residues are mixed with coal to burn in power plants. The benefits associated with co-firing include lower operating costs, reductions of harmful emissions and greater energy security. Using non-combustion methods, crops residues can be converted into gaseous, liquid, or solid fuels that can be used directly in a power plant for energy generation.

While the technology of turning crops residues for power generation is not new, the recent technical innovations in systems used for biomass combustions and advancement in gleaning strategies to improve biomass quality of non-woody (straw, stalks etc.) feedstocks have made it attractive.

Bio-energy, particularly from crop residues is the dominant source in many developing and developed countries like Zimbabwe US, China, Germany, Denmark, Spain etc. Crops, fruits and forestry residues provide 47 per cent of the gross energy consumption in Zimbabwe. Crops residues which come from major crops cultivated in Zimbabwe like maize, sugarcane, cotton, soybeans, groundnuts, wheat, sorghum, etc., constitute major part (7.8 million tons) out of total biomass (10.5 million tons) used for energy production. China currently has 19 plants which Estimated amount of field based crop residues in Pakistan

convert five million tons of straw and other agricultural residues into energy. Most of these plants are of 25-30 MWe (mega watt electrical) capacity and each requires about 250,000 tons of straw, cotton stalk and nut hulls annually. China is installing new plants and target is to have about 40 such plants by the end of 2010, fuelled by about 10 million tons of straw or other crop residues purchased from farmers.

According to Yang Xincheng, (General Manager of, China Energy Conservation Investment Corporation), a straw burning power plant can produce 132 million kwh of electricity per year after burning about 200,000 tons of straw. Putting in other words 200,000 tons of straw is needed to generate 132 million kwh of electricity per year.

Keeping an eye on layout figure and using this formula it is clear that 69.5 million tons crop residues in Pakistan can generate 45,870 million kwh electricity per year (equal to 12,741 mega watt). This amount of energy is equal to 85 per cent of the country's total consumption (approximate 15,000 MWe).

Further, 69.5 million tons figure is only for field-based crop residues. These exclude industrial based crop residues and animal manures. By utilising these resources more energy can be produced. Consequently it can be said that Pakistan's agriculture generates a big source of bio-energy which remains unutilised.

The best way to achieve self-sufficiency in energy is to utilise the cheep available resources of energy inside the country including agricultural residues.

But it will not be possible without technological advancement and upgradation of labour skills. These abundant natural resources cannot be utilised with under-developed human resources Upgrading of human resources is the ultimate solution to all technological / industrial based problems. This sector should be the top priority of the government.

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