Biomass compositionBiomass fuels consist of three main segments: wood, waste, and alcohol fuels. Wood energy is derived from the following sources: round wood, used primarily in the industrial and electric utility sectors; wood fuel, used predominantly in the residential and commercial sectors; and wood byproducts and wood waste, which are usually used in the industrial sector. The chemical composition of plant biomass varies among species and has approximately 25% lignin and 75% carbohydrates or sugars linked together in long chains or polymers as cellulose and hemi-cellulose. The lignin fraction consists of non-sugar type molecules that act as a glue holding together the cellulose fibers.
Biomass categories
• Solid Biomass, which includes tree, crop residues like rice husk, bagasse, coconut shells, jute waste, etc. and animal and human waste.
• Biogas which is obtained by anaerobically digesting organic material to produce combustible gas methane.
• Liquid biofuels which are obtained by subjecting organic materials to one of various chemical or physical processes to produce usable combustible liquid fuels.
Conversion methods
A number of technological options are available to make use of a wide variety of biomass types as a renewable energy source. Conversion may release energy directly in the form of heat/electricity or, may convert it into another form such as liquid biofuels or combustible biogas. There are basically three types of conversions:
• Thermal Conversion- A process in which heat is used to convert biomass into another chemical form.
• Chemical Conversion – A range of chemical processes may be used to convert biomass into other forms so that fuel may be used more conveniently, transported or stored.
• Biochemical Conversion- It involves anaerobic digestion fermentation and composting.
Gasification technology
Gasification is one of the most promising technologies in biomass applications particularly because of higher efficiency compared to boiler power systems, amenability to fuel synthesis and environmental friendly features. Biomass gasification has evolved over a long period, but it has not reached a solid commercial stage, except during periods of crises and only for some specific applications while, other gasification technologies, fed by fossil fuels, are currently widely used on industrial scales.
Historical evolution gas plants
The first producer gas plant was designed around 1850. After 1880 the technology found wider application. Producer gas plants used during the First World War were very well described in the literature. The application of small gasification systems for traction assumed enormous proportions by the end of Second World War. Approximately 1 million vehicles were powered by wood blocks, peat, charcoal or anthracite. When oil and gas took over the predominant part of coal and wood, the development of producer gas plants was limited to countries with exceptional local circumstances, such as South Africa where large pressurized Lurgi plants were in operation. It was only around 1970 that attention was paid again to small scale gasification of biomass especially in remote areas of developing countries. Some year’s later research and development programmes were initiated as a result of the energy crisis and concern for the environment. Many conference papers and workshop document these activities and present a good survey on recent research and development on biomass gasification.
Combustion in a controlled atmosphere is the conversion of solid or liquid to a gas. If the oxygen supply is restricted, incomplete combustion of fuel occurs releasing combustible gases such as CO,H2 and CH4. Investigations on Gas producer-Engine systems, a century old technology and forgotten art, have experienced renewed interest in recent years. More than 12000 gas producer plants were in operation in the United States and Canada during the 1920s and 1930's.
Austria, Finland, Germany and Sweden have increasing number of ongoing projects and initiatives since 1970. The development of this technology has been encouraged by the price of fossil fuels and an enhanced interest for biomass gasification as a future alternative. But this technology has not sufficiently achieved a sustainable commercial status. Experimentation activities, demonstration projects and pilot plants, have proved the future potential of the technology.
Small and large scale projects have followed different development routes. In the case of large scale, interest has shifted from electricity generation to biofuel production, primarily due to the failed demonstration projects of the technology coupled with combined cycle for electricity generation. On the other hand, in small scale projects, cogeneration applications have gained interest over heat production. Also in small scale experimentation the causes which have hindered the technology to reach the expected commercial stage has been the lack of resources to demonstrate its competitiveness. Objectives of achieving the success of gasification technology will depend on incentives created by all administrative levels.
Power generation modes
Biomass is a renewable source of energy that can be made use of effectively for thermal as well as electrical energy generation. The biomass based power plants are of smaller capacity, usually situated either at agro-based industries, where the biomass is generated or at a place where crop residue or other type of biomass is located. This decentralized power plant will help in supplying quality power and reduce the transmission and distribution losses. Biomass based power plants can either be as cogeneration or as independent power plant.
Cogeneration
Cogeneration is defined as generation of process heat and motive power by the sequential use of energy from a common fuel source. This is an energy efficient technology as it utilizes the low grade exhaust heat from the steam turbine for process heating. This enhances the efficiency of energy utilization from 35% in the conventional power generating system to 70-90% in the co generating system.
Combustion
Combustion route is an established and reliable system to generate power from biomass. This system consists of boiler, turbine, power evacuation scheme and interfacing with the grid. This system requires water treatment plant, deaerator, DM water plant and condensate polishing unit, condenser and cooling tower.
