GAS CONDITIONING FOR POWER GENERATION USING GASIFIER

Need for gas cleaning
Biomass gasification is a process of converting biomass to a combustible gas in a reactor, known as gasifier, under controlled conditions. During gasification, biomass is subjected to partial pyrolysis under sub-stoichiometric conditions. A typical gasifier plant consists of a reactor, which receives air and solid fuel and converts them into gas, followed by a cooling and washing train where the impurities are removed. The clean combustible gas at a nearly ambient temperature is a must for running diesel-generator sets in dual fuel mode or gas engine generator sets suitable for running on producer gas alone. In thermal applications, the cooling and cleaning of the raw gas is limited to the requirements of the thermal process.
The gasification system development has also included extensive research and development on product gas cleanup. Raw gases contain particulates, tars, ammonia, and other impurities that interfere with equipments and downstream processes and components or create emission problems. For low-technology thermal applications where the product gas is simply burned to provide heat, such as a cement kiln, the gas clean-up requirements may be minimal. However, high technology systems such as fuel cells and gas turbines or systems using synthesis gases require clean fuel gases. Extensive clean up may also be required to meet the environmental regulations even in relatively undemanding applications. There are examples of failure due to non-adherence of critical fuel quality requirements with end-use.
In 1993, the International Energy Agency (IEA), Bioenergy Gasification Activity repared a document summarizing the status of gas clean-up systems. This document identified primary gas contaminants, discussed unit operations that could potentially remove such contaminants, and discussed gas-cleaning strategies in relation to the end-use of the product gas. Since then, progress has been made in developing a better understanding of the chemistry and mechanisms of hot gas cleanup and conditioning as evinced by recent gasification systems which have incorporated technologies to improve gas cleanup.
Gas ontaminants
Gasification processes, particularly updraft gasification produces a dirty gas that has a relatively low level of purity and includes toxic chemicals and other contaminants besides carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen and water vapor. Pollutants such as tars, dust, and ash are also present, in relatively high concentrations, in these crude gases. Following a gasification process the crude gas must normally be cleaned of tar, filtered, purified and cooled before it is used in many energy-producing applications.
Cleanup systems
Gas cleanup systems may contain several components individually or in combination viz., cyclones, scrubbers, or filters; each of which removes one or more contaminants. Systems producing gas must deal with the cleanup of five primary contaminants including:
· Particulates
· Alkali compounds
· tars
· nitrogen-containing components
· Sulphur
Various gas-conditioning systems employed are:
Barrier Filters – Rigid, packed bed filters, bag filters, sieve-plate scrubbers, wet scrubbers, cyclone filters, electrostatic precipitators, sand bed filters, water wash towers and catalytic thermal tar destruction beds.
The application of particulate removal technologies to biomass gasification systems in general has matured over the past decade. Cyclones are routinely being used for bulk particulate removal, and technologies including bag filters and wet scrubbers are being used in large-scale systems. While this progress is incremental in nature, it is required for wide- spread deployment of gasification technologies. Based on the available data except for the catalytic tar crackers, none of the gas cleaning systems can meet a targeted cleaning objective exceeding 90 %.