BIOMASS GASIFICATION


Biomass gasification means incomplete combustion of biomass resulting in production of combustible gases consisting of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4).  This mixer is called producer gas.  Producer gas can be used to run internal combustion engines (both compression and spark ignition), can be used as substitute for furnace heat in direct heat applications and can be used to produce methanol.

History of gasification

The fortunes of coal and biomass gasification have waxed and waned in the world rich in petroleum, cola and other fossil fuels reserves.  The classical age of gasification (primarily coal) could be considered to extend from 1800 through 1940.  By 1920 most cities in the world had a “gasworks” where producer gas (a mixture of CO, CO2, H2, CH4 and N2) was made, stored in a gasholder and delivered to the households and industries in town.  

World War II was the golden age of the biomass gasification.  It was not practical to operate small gasifiers on coal; so most of the countries relied on the gas made wood for civilian transport.  Over a million gasifiers were in use in Europe.

In the first reaction to the oil embargoes of the 1970’s many new gasification projects were ill conceived and where based on inappropriate coal technology.  Most of these 1970 gasifier projects and manufactures no longer exist for a lot of reason.  These were the dark ages of biomass gasification.  However some of these early projects have moved from planning to pilot testing to full scale commercial.

The drop of oil prices in the mid 1980’s caused the death of many projects based on the high price of oil.  

During World War II, biomass gasification was used primarily for civilian transport in cars, trucks, boats and buses, but now gasification is seen as a potential source of Electric Power.  When biomass gasification used at a small scale it can provide power to millions of villages around the world, especially in India because India having abundant of biomass resources.  At a large scale it can make biomass industries such as sugar and paper making independent of fossil fuels and fossil fuel prices.

Biomass Gasification

Wood and other biomass has been burnt to produce heat.  Producer gas contains CO, CO2, H2, CH4 and N2 variously as unwanted diluents.  If oxygen is used for gasification instead of air, a higher energy called “synthesis gas” is made.  

Problems in gasification system

i)    Biomass drying
A major hidden issue in biomass thermal conversion is the importance of drying biomass before use and reducing it to a size suitable for gasification.  Biomass can easily contain 50% water without looking any different, but it feels cooler and heavier.

ii)    Tar formation
A major concern in the gasifier design is the amount of “tar” produced.  Tar is a black color liquid formed during gasification. Tar also known as creosote, is a sticky, condensable vapor that can be clog engine valves and turbocharger leading to decreased performance and increased maintenance.

iii)    Agglomeration in high ash fuels
Agricultural residues, such as rice hulls, nut hulls, straws and husks typically have a 5%-20% mineral content, mostly dispersed in molecular level.  Agricultural residues are potentially more important than woody biomass for energy, because they occur where food is produced and people live.  They are often burned off the field to make way for the succeeding crop.  Recovery of the minerals can recycle them for the next crop.

Applications of the gasification system

Early city gas gasifiers were always developed at large scale, since they required a distribution network to serve thousands of customers and could use coal, available at any scale required.  City gas disappeared as natural gas pipelines spread across the country.  During World War II small-scale gasifiers were developed using wood for energizing transport since military commandeered all the gasoline supplies for planes and tanks.

Current gasifiers developed focuses on the heat and power.  It is moving on the three parallel tracks.

•    Large gasifiers (> 50 MW) for industries that have large concentrations of
biomass fuels.

•    Medium gasifiers (10-50 MW) for shaft power of various rural industries like saw mills, small rice mills etc.

•    Small gasifiers (< 10 MW) for research purposes.

•    Very small gasifiers (2-4 kW) thermal application for domestic cooking