Technologies and Pathways in deriving bioenergy - Energy Alternatives India

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Biomass to Energy Pathways:

Biomass is a complex class of feed stocks with significant energy potential to apply different technologies for energy recovery. Typically technologies for biomass energy are broadly classified on the basis of principles of thermo chemistry as combustion, gasification, pyrolysis and biochemistry as anaerobic digestion, fermentation and trans-esterification. Each technology has its uniqueness to produce a major calorific end product and a mixture of by-products. Choice of a processing method often depends on nature and origin of feed stocks, their physio-chemical state and application spectrum of fuel products derived from it.

The flow chart below comprehensively highlights the major biomass conversion technologies, their range of compatible feed stocks and major fuel products for power, heat and transport utilizations.

*Anaerobic digestion (AD) also results in a nutrient-rich digestate as a co-product. This, however, cannot be used as an energy source, and hence is not included here.

Process Description:

A brief description of the technologies for energy generation from biomass is as follows:

Combustion

In this process, biomass is directly burned in presence of excess air (oxygen) at high temperatures (about 800°C), liberating heat energy, inert gases, and ash. Combustion results in transfer of 65%–80% of heat content of the organic matter to hot air, steam, and hot water. The steam generated, in turn, can be used in steam turbines to generate power.

Transesterification

The traditional method to produce biodiesel from biomass is through a chemical reaction called transesterification. Under this method, oil is extracted from the biomass and it is processed using the transesterification reaction to give biodiesel as the end-product.

Alcoholic Fermentation

The process of conversion of biomass to biofuels involves three basic steps:

Converting biomass to sugar or other fermentation feedstock
Fermenting these biomass-derived feedstocks using microorganisms for fermentation.
Processing the fermentation product to produce fuel-grade ethanol and other fuels.

Anaerobic Digestion

In the absence of air, organic matter such as animal manures, organic wastes and green energy crops (e.g. grass) can be converted by bacteria-induced fermentation into biogas (a 40%-75% methane-rich gas with CO₂ and a small amount of hydrogen sulphide and ammonia). The biogas can be used either for cooking/heating applications, or for generating motive power or electricity through dual-fuel or gas engines, low-pressure gas turbines, or steam turbines.

Pyrolysis

Pyrolysis is a process of chemical decomposition of organic matter brought about by heat. In this process, the organic material is heated in absence of air until the molecules thermally break down to become a gas comprising smaller molecules (known collectively as syngas).

The two main methods of pyrolysis are “fast” pyrolysis and “slow” pyrolysis. Fast pyrolysis yields 60% bio-oil, 20% biochar, and 20% syngas, and can be done in seconds. Slow pyrolysis can be optimized to produce substantially more char (~50%) along with organic gases, but takes on the order of hours to complete.

Gasification

In this process, biomass reacts with air under extreme temperatures and results in production of producer gas, to produce power (or) react with pure oxygen to produce synthesis gas for fuel production. The combustible gas, known as producer gas, has a calorific value of 4.5 - 5.0 MJ/cubic meter. A wide range of biomass in the form of wood or agro residue can be used for gasification.

Summary of bioenergy processes, feedstocks and products

Process	Biomass feedstock	Products	Features/ Highlights
Thermal Conversion
Combustion	Diverse biomass	Heat and power	Combustion can be applied for biomass feedstocks with moisture contents up to atleast 60 percent Combustion is ideally suited for power segments which works well beyond 5 MW Combustion is a established technology working on the regular rankine cycle Combustion comprises over 85% of installed capacity for biomass based power production in India (excluding biomass cogeneration) The process works well for most types of biomass
Thermo-chemical Conversion
Gasification	Diverse biomass	Low or medium-Btu producer gas	Gasification systems are well-suited for small-scale applications. The process can work at low scales – as low as 20 kW, and works well up to 2 MW. Currently, less than 125 MW of cumulative installed capacity in India (less than 15% of total biomass power capacity, excluding biomass cogeneration). Gasification can produce a high purity syngas for catalytic conversion processes for the production of liquid biofuels. This process is currently in pilot phase.
Pyrolysis	Wood, Agricultural Waste Municipal Solid Waste	Synthetic Fuel Oil (Biocrude), Charcoal	Pyrolysis is not well established currently in India or elsewhere in the world. Pyrolysis is a simple, low-cost technology capable of processing a wide variety of feedstocks Typically pyrolysis plants work well beyond 2 MW scale.
Biochemical Conversion
Anaerobic Digestion	Agricultural Waste, Municipal Solid and Liquid Wastes, Landfills and Animal Manure	Biogas	Anaerobic digestion is a commercially proven technology and is widely used for recycling and treating wet organic waste and waste waters Anaerobic digesters of various types were widely distributed throughout India and China. Anaerobic digestion is increasingly used in small size, rural and off-grid applications at the domestic and farm-scale. Small scale biogas for household use is a simple, low-cost, low-maintenance technology, which has been used for decades.
Alcohol fermentation	Agricultural Waste, Sugar Or Starch Crops, Wood Waste, Pulp Sludge and Grass Straw etc	Ethanol	Sugar molasses is extensively used as a feedstock for alcoholic fermentation Recent advances in the use of lignocellulosic biomass as a feedstock may allow bioethanol to be made competitively from woody agricultural residues and trees.
Chemical Conversion
Pressing/extraction Transesterification	Oils from plant seeds and nuts etc, Fats from animal tissues	Biodiesel	Transesterification is a fairly simple and well-understood route to produce biodiesel from biomass. Glycerol, a by-product obtained from the process is difficult to be removed. Meanwhile it can be used as fuel in stationary applications, or can be converted into other high-value products Jatropha is used as a source for biodiesel production in India. Food crops such as soybean are also used as sources in some countries.

India Biomass Energy