Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 2
Apart from the described gasifiers, there are several other models that have been
developed to improve the quality of the produced gas, such as: twin fluidized bed and
entrained bed (Bridgwater, 1995). The first one aims to increase the heat value of the
produced gas by promoting the production of H 2 . The second operates at high
temperatures 1200 to 1500°C in order to eliminate tar and condensable gases in the
produced gas. In fact, it has conversion efficiencies close to 100%. However, some
problems arise in the materials selection to withstand the high temperatures and
liquefaction of ash. Readers interested in deepening this topic may consult the cited
reference.
2.1.3.4 Syngas conditioning
tel-00623090, version 1 - 13 Sep 2011
Gases formed by gasification are contaminated by some or all of the constituents listed
in Table 2.2. The level of contamination varies, depending on the gasification process
and the feedstock. Gas cleaning must be applied to prevent erosion, corrosion and
environmental problems in downstream equipment. Table 2.3 also summarizes the
main problems resulting from these contaminants, and common cleanup methods.
Table 2.2 - Syngas contaminants (Bridgwater, 1995)
Contaminant Examples Problems Cleanup method
Particulates
Ash, char, fluid bed
material
Erosion
Filtration, scrubbing
Alkali metals
Sodium and potassium
Cooling, condensation,
Hot corrosion
compounds
filtration, adsorption
Nitrogen
Scrubbing, Selective
NH 3 e HCN
NO x formation
components
catalytic reduction
Clog filters, difficult to
Tars
Aromatics
burn, deposit
Tar cracking; Tar removal
internally
Sulfur, Chloride H 2 S , HCl Corrosion, emissions
Lime or dolomite scrubbing
or absorption
Tars are mostly heavy hydrocarbons (such as pyrene and anthracene) that can clog
engine valves, cause deposition on turbine blades or fouling of a turbine system
leading to decreased performance and increased maintenance. In addition, these
heavy hydrocarbons interfere with synthesis of fuels and chemicals. Conventional
scrubbing systems are generally the technology of choice for tar removal from the
product syngas. However, scrubbing cools the gas and produces an unwanted waste
stream. Removal of the tars by catalytically cracking the larger hydrocarbons reduces
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