What is Biomass Gasification?
The Old Method: Conventional Combustion
We know very well that the direct burning/ combustion of wood / biomass, is a complete oxidation process, which primarily produces heat (thermal energy). The by-products are solid waste, and carbon dioxide (CO2), NOx, CO, and particulate emissions most of which are potentially hazardous to the environment..
Combustion is an inefficient conversion process when generating power alone-some advanced designs are improving efficiency. It requires water if generating power with a steam turbine
Combustion is of limited value, it's dirty, and it makes the control and capture of CO2 very difficult, if not impossible.
Because of the relentless rise in oil and gas prices worldwide, We believe that gasification offers significant economic and environmental advantages as an unconventional source of energy in a carbon constrained world.
The New Technology: Gasification
Gasification is a flexible, reliable, and commercial technology that can turn a variety of low-value feedstocks into high-value products, help reduce a country's dependence on imported oil and natural gas, and can provide a clean alternative source of energy.
The gasification process renders the use of biomass relatively clean and acceptable in environmental terms. The gasifier does not generate emissions: engines exhaust gas is cleaned in a catalytic converter and then, in case that the concerning heat is used for biomass conditioning, sent to the dryer where it is further bio-filtered.
In this process organic or fossil fuel based carbonaceous materials converted into producer gas ,a low calorific power (1000-1200 kCal/m3 ) mix of Carbon monoxide 18%-20%, Hydrogen 15%-20% Methane 1%-5%, Carbon dioxide 9%-12%, Nitrogen 45%-55%.
This is achieved by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of air/ oxygen.
Gasification process
It includes four stages, each occurring in one of the four separate zones within the gasifier: drying, pyrolysis, oxidation and reduction zone.
Drying
In the drying zone, moisture in the feedstock is evaporated by the heat from the lower zones at a temperature of 150-200 degrees C. Vapours move down and mix with vapours originating in the oxidation zone. A part of the vapours is converted into oxygen under the following chemical reaction: ? + H2 O CO +H2 with the remainder being retained in the producer gas.
Pyrolysis
Located below the Drying Zone, temperatures here reach 400 to 650 degrees C. Pyrolysis reactions occurt temperatures above 250 degrees C. During pyrolysis, large molecules such as cellulose, polycellulose and lignin, are broken down into medium-chain hydrocarbons and carbon (charcoal). Pyrolysis products then move down towards hotter areas within the gasifier, some of these burn while the others break down further into smaller molecules and atoms such as hydrogen, methane, carbon monoxide, ethane, ethylene, and others.
Oxidation
At this level, air is injected into the gasifier, leading to the following chemical reactions:
C + O2 CO2
H + 1/2O2 H2 O
These reactions release large amounts of heat (401.9KJ/mol and 241.1 KJ/mol) that increase the temperature inside the gasifier to 900 - 1,200 degrees C. This assists with burning off a substantial part of tars that would otherwise condense further downstream.
Reduction
In the reduction zone, the thermal energy generated in the Oxidation zone is converted into chemical energy in the following reactions:
C + CO2 2CO C + H2 O CO + H2
CO + H2 CO + H2 O
C + 2H2 CH4
CO + 3H2 CH4 + H2 O
These reactions are endothermic and reduce the temperature of gas exiting the Oxidation zone as it passes through the bed of charcoal at the bottom of the gasifier. Producer gas in its final chemical composition is formed in this zone. Upon leaving the gasifier this gas has temperatures of 250-500 degrees C and can be fed to burners or, after additional cleaning and cooling, fired in internal combustion engines.