BASICS
The BIOMASS term biomass applies to a wide range of materials, but the main resources are: (i)
forest crops ;
(ii) wood wastes;
(iii) sugar crops (e.g., bagasse);
(iv) starch crops;
(v) herbaceous lignocelluloses crops;
(vi) oil crops;
(vii) agricultural wastes (e.g., rice hull);
(viii) municipal solid wastes
(ix) industrial wastes.
These materials are converted into fuels which are used to run the engines that generate
electricity. The conversion involves three main processes: thermochemical (combustion,
gasification, pyrolysis, liquefaction), chemical (esterification) and biochemical (acid
hydrolysis, enzyme hydrolysis, fermentation) processes.
Today, biomass provides us a little over three percent of the energy we use. It has been
replaced by coal, natural gas, petroleum, and other energy sources. Today, most of the
biomass energy we use comes from wood. It accounts for two-thirds of biomass
consumption. Other biomass sources include biofuels (alcohol fuels), crops, garbage, and
landfill gas.
Biomass and the Environment
Biomass can pollute the air when it is burned, though not as much as fossil fuels.
Burning biomass fuels does not produce pollutants like sulfur, which can cause acid rain.
Growing plants for biomass fuel may reduce greenhouse gases, since plants use carbon
dioxide and produce oxygen as they grow. Carbon dioxide is considered an important
greenhouse gas.
Biomass gets its energy from the sun. Plants absorb sunlight in a process called
photosynthesis. With sunlight, air, water, and nutrients from the soil, plants make sugars
called carbohydrates. Foods that are rich in carbohydrates are good sources of energy for
the human body. Biomass is called a renewable energy source because we can grow more
in a short period of time.
THERMAL CONVERSION
Thermal conversion processes can be applied to rice hull, considered one of the more
abundant biomass resource. There are three major processes involved:
(i)
direct combustion;
(ii)
gasification;
(iii)
pyrolysis.
In the direct combustion process, rice hull is burned in a furnace to produce steam
in a boiler. The steam is used to run a steam engine or a steam turbine, which, in
turn, drives an electric generator.
In gasification and pyrolysis, rice hull is converted into combustible gas to fuel
internal combustion engines (diesel or gasoline types). The thermal conversion
occurs inside a reactor containing the rice hull. In gasification, air is utilized as an
oxidant medium of conversion in order to facilitate the production of combustible
gas.
Pyrolysis is done without an oxidant. It is a more energy intensive process, and
the quality of gas produced is better. The latter process produces liquid and solid
(charcoal) by-products. The liquid portion contains methanol, acetone and other
organic acids.
CHEMICAL CONVERSION
Esterification is the chemical modification of vegetable oils into oil esters that can be used
as biofuels in engines. Oils are extracted from oil crops, e.g., rapeseed, coconut, sunflower,
and made to undergo esterification to adapt the vegetable oil to the requirements of diesel
engine. Methyl esters are formed when methanol is used while ethyl esters are formed if
ethanol is used. It is estimated that 1 ton of methyl ester can be produced from 3 tons of
rapeseed.
Esterification is the general name for a chemical reaction in which two reactants (typically
an alcohol and an acid) form an ester as the reaction product. Esters are common in organic
chemistry and biological materials, and often have a characteristic pleasant, fruity odor.
BIOCHEMICAL CONVERSION
Biological processes include anaerobic digestion, acid and enzyme hydrolysis and
fermentation. Methane is produced during the anaerobic digestion of wastes. It can be
used for direct burning or for internal combustion engines. A kilo of dry wastes can
produce 0.2 to 0.3 cu m of methane. The main product from acid and enzyme hydrolysis,
fermentation and distillation is ethanol. Ethanol can be used as fuel for engines, either in
its pure form or in mixture with gasoline.
Anaerobic_Flow_Diagram.flv
Using Biomass Energy
Burning is not the only way to use biomass energy, though. There are four ways to release
the energy stored in biomass: burning, bacterial decay, fermentation, and conversion to
gas/liquid fuel.
Burning
Wood heated homes and fueled factories. Today, wood provides only a little of our countrys
energy needs. Wood is not the only biomass that can be burned. Wood shavings, fruit pits,
manure, and corn cobs can all be burned for energy.
Garbage is another source of biomass. Garbage can be burned to generate steam and
electricity. Power plants that burn garbage and other waste for energy are called waste-toenergy plants. These plants are a lot like coal-fired plants. The difference is the fuel.
Garbage doesnt contain as much heat energy as coal. It takes about 2,000 pounds of garbage
to equal the heat energy in 500 pounds of coal.
Sometimes, fast-growing crops like sugar cane are grown especially for their energy value.
Scientists are also researching ways to grow aquatic plants like seaweed to use for their
energy value.
Bacterial Decay
Bacteria on feed dead plants and animals. As the plants and animals decay, they produce
a colorless, odorless gas called methane. Methane gas is rich in energy. Methane is the
main ingredient in natural gas, the gas we use in our furnaces and stoves. Methane is a
good energy source. We can burn it to produce heat or to generate electricity.
In some landfills, wells are drilled into the piles of garbage to capture methane produced
from the decaying waste. The methane can be purified and used as an energy source,
just like natural gas.
Fermentation
We can add yeast (a fungus) to biomass to produce an alcohol called ethanol. For
centuries, people have fermented crops to make alcoholic drinks like beer and wine. Wine
is fermented from grapes. Wheat, corn, and many other crops can be used to make ethanol.
Ethanol is sometimes made from corn to produce a motor fuel. Automobile pioneer Henry
Ford wanted to use ethanol to power his cars instead of gasoline. Ethanol is more
expensive to use than gasoline. Usually, it is mixed with gasoline to produce a fuel called
E-10, which is 90 percent gasoline and 10 percent ethanol. For cars to run on ethanol, their
engines would have to be changed. But cars can run on E-10 without changes. Adding
ethanol to gasoline lowers carbon dioxide emissions.
Conversion
Conversion means changing a material into something else. Today, we can convert
biomass into gas and liquid fuels. We do this by adding heat or chemicals to the biomass.
The gas and liquid fuels can then be burned to produce heat or electricity, or it can be used
as a fuel for automobiles. In India, cow manure is converted to methane gas to provide
heat and light.
Biomass energy
the use of organic matter for power and fuel - offers not only an alternative to dependency on
nonrenewable resources such as fossil fuels, but also offers benefits in the form of
environmentally sound energy, a potential for rural economic growth, and national energy
security benefits.
Consider the following:
As a renewable energy source, biomass energy produces fewer emissions than conventional
sources and can actually improve environmental quality by offsetting fossil fuel use and
related emissions and by using wastes that are creating land use problems.
Biomass energy growth can create new markets and employment for farmers and forest
workers, many of whom currently face economic hardship. It can establish new processing,
distribution, and service industries in rural communities.
Wood from sustainably managed forests can be replenished continuously,
leading to a dependable and renewable supply.
Because trees store carbon as a result of photosynthesis, there is no net
production of carbon dioxide (CO2), the major greenhouse gas from wood
combustion, as long as the volume of trees harvested is replaced by new growth.
In a sustainably managed forest, the CO2 generated during combustion of the
wood equals the CO2 sequestered from the atmosphere during the lifecycle of
the tree.
Wood fuel contains very small amounts of sulfur and heavy metals. The sulfur
and nitrogen in wood, yields SO2 and NOX in the combustion process.
Other agricultural industries use the plant waste of their industry as the biomass energy
source, such as rice husks in the rice industry. Green waste is another source of biomass
energy consisting of council garden waste and wood waste left over from pruning trees.
This type of biomass is usually transported to a processing plant, where it passes through
a grinding machine and over screens to filter out any dirt and other contaminants. The
different types of biomass can also be blended to create a fuel blend, and then placed in a
fuel storage bin where it is then supplied to the boiler.
What is cogeneration
Cogeneration involves the combustion of fuel sources to produce two forms of
energy, usually steam for both heat energy and electricity generation.
Cogeneration can be used to reduce energy costs for the host industry, such as a
sugar mill, because the mill uses some of the steam produced for heat energy
and electricity to operate the mill. The surplus steam produced is used to generate
electricity for export to the grid.
Biomass cogeneration plants make better use of waste biomass, which would
otherwise be dumped or burnt as waste.
Biomass and sugar mills
As the sugar cane season occurs for only part of the year, green waste can be used in
some cases for the rest of the year, to ensure a continuous supply of electricity.
On average, one tone of sugar cane yields 250 kg of bagasse.
The ash, which forms from non-combustible material in the biomass, is removed from
the sugar mill to be used as a soil conditioner in landscaping work or added to cane farms
to supplement fertilizer.
The waste water from the cogeneration facility cooling system is passed through
treatment systems before being utilized off site, such as for irrigation water for local cane
farms.
Some sugar mills have their own distillery, where alcohol (particularly fuel alcohol or
ethanol) is produced from molasses. This provides another use for molasses, which was
considered a waste product used only as stock feed for cattle.