Renewable Energy - Biomass (Part 2)

In the last post, I talked about the general idea of biomass energy and it's potential in Malaysia. So for today, I'll continue on the part 2 for Biomass, how to harness the biomass energy from it's sources (wood, agriculture residues, animal waste and sewage (yuck!)) into the more manageable form like pellets, liquid and gases. Biomass is the oldest source of energy as early civilizations learn how to use and control fire. Among all the renewable energy, biomass has the most diversity in term of energy conversion. There are three types of biomass energy conversion; thermal, chemical and biochemical.

1) Thermal Conversion

Thermal conversions are processes in which heat is the dominant mechanism to convert the biomass into another chemical form. The thermal conversion processes are separated principally by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature) and divided in to four main processes; combustion, torrefaction, pyrolysis, and gasification.

• Combustion: Is the direct burning of solid biomass and the simplest process (combustion of wood)
• Torrefaction: A thermo chemical treatment of biomass at 200 to 320 °C under atmospheric conditions in the absence of oxygen (a mild form of pyrolysis). During torrefaction the biomass properties are changed to obtain a much better fuel quality for combustion and gasification applications. During the process, the water contained in the biomass is removed. This process produced solid, dry, blackened material known as biocoal.
• Pyrolysis: Is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible. This process also produces biocoal and liquid biofuel.
• Gasification: Is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures (700°C), without combustion, with a controlled amount of oxygen and/or steam. This process is used to produce biogas (methane).

2) Chemical Conversion

A range of chemical processes may be used to convert biomass into other forms, such as to produce a fuel that is more conveniently used, transported or stored, or to exploit some property of the process itself.

3) Biochemical Conversion

Biochemical conversion makes use of the enzymes of bacteria and other microorganisms to break down biomass. Most microorganisms are used to perform the conversion process; anaerobic digestion, fermentation, composting and transesterification:

• Anaerobic digestion: Is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. This process usually uses for waste management system and convert these products to methane (biogas) and carbon dioxide.
• Fermentation: Is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. The product of this process usually in the form of alcohols (use in biofuel) but hydrogen and methane (biogases) are also produced.
• Composting: Is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Only for the agriculture industry use. The product from this process is not used in generating electricity.
• Transesterication: Is the process of exchanging the organic group R″ of an ester with the organic group R′ of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. This process is the most widely used in Malaysia to turn palm oil into biofuels like biodiesel.

After the biomass is turned into a more manageable form (biofuel & biogas), its time for the electric generation for the nation! The most common way to capture the energy from biomass was to burn it to make heat. Since the industrial revolution, biomass fired heat has produced steam power, and more recently this biomass fired steam power has been used to generate electricity. Burning biomass in conventional boilers can have numerous environmental and air-quality advantages over burning fossil fuels.

For generating electricity, biomass power plants have similar operations like any fossil fuel power plant. The only noticeable difference is the fuel used. Because of this, biomass can be use directly to existing fossil fuel power plant (with few minor adjustments) according to their fuel types; coal can be replaced with biocoal or wood and natural gas with biogas like methane. Below are several method for thermal biomass power generation applications:

1) Direct combustion

The oldest and most common way of converting biomass to electricity is to burn it to produce steam, which turns a turbine that produces electricity. The problems with direct combustion of biomass are that much of the energy is wasted and that it can cause some pollution if it is not carefully controlled. Direct combustion can be done in a plant using solely biomass or in a plant made to burn another fuel, usually coal.

2) Co-firing

Biomass can be mixed with coal and burn it at a power plant designed for coal. Biomass in the form of gas (biogas) can also be co-fired at natural gas-powered plants. The benefits associated with biomass co-firing can include lower operating costs, reductions of harmful emissions like sulphur and mercury, greater energy security and, with the use of beneficial biomass, lower carbon emissions. Co-firing is also one of the more economically ways to increase biomass power generation today, since it can be done with modifications to existing facilities.

3) Repowering

Coal plants can also be converted to run entirely on biomass. Similarly, natural gas plants could also be converted to run on biogas.