Gasification is a technological process that can convert any carbonaceous (carbon-based) raw material such as coal into fuel gas, also known as synthesis gas (syngas for short).
Gasification occurs in a gasifier, generally a high temperature/pressure vessel where oxygen (or air) and steam are directly contacted with the coal or other feed material causing a series of chemical reactions to occur that convert the feed to syngas and ash/slag (mineral residues). Syngas is so called because of its history as an intermediate in the production of synthetic natural gas.
Composed primarily of the colorless, odorless, highly flammable gases carbon monoxide (CO) and hydrogen (H2), syngas has a variety of uses. The syngas can be further converted (or shifted) to nothing but hydrogen and carbon dioxide (CO2) by adding steam and reacting over a catalyst in a water-gas-shift reactor. When hydrogen is burned, it creates nothing but heat and water, resulting in the ability to create electricity with no carbon dioxide in the exhaust gases. Furthermore, hydrogen made from coal or other solid fuels can be used to refine oil, or to make products such as ammonia and fertilizer. More importantly, hydrogen enriched syngas can be used to make gasoline and diesel fuel. Polygeneration plants that produce multiple products are uniquely possible with gasification technologies. Carbon dioxide can be efficiently captured from syngas, preventing its greenhouse gas emission to the atmosphere and enabling its utilization (such as for Enhanced Oil Recovery) or safe storage.
Gasification is a form of renewable energy Solutions where raw material can be used to generate electrical power.
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Types of Gasification
In direct gasification, the thermal energy required for the reaction is produced by initial partial combustion of feedstock with air or oxygen supplied into the reactor. Due to the high content of nitrogen in the air, in the case of air-blown gasification the resulting product gas contains a lot of nitrogen, which reduces the calorific value2 of the product gas. On the contrary, if oxygen is used, the resulting product gas contains less nitrogen and thus has a higher calorific value, however the drawback of the oxygen-blown gasification is that it is much more expensive than the air-blown process, due to high cost of oxygen production.Oxygen blown gasification produces gas rich in hydrogen and CO and therefore is of interest for applications with higher added value such as FT Diesel or DME production, whereas it is not directly relevant for power and heat generation at present.
In indirect gasification the heat required for the gasification process is supplied by additional process, which takes place outside the main gasifier (the heat can be supplied for example by gases or bed material circulated through the heat exchanger or by combustion of char or ash from the main reactor). Steam is the most commonly used indirect gasification agent, because it is easy and cheap to be produced and increases the hydrogen content and consequently the calorific value of the combustible gas.
Indirect gasifiers are grouped as char- and gas indirect gasifiers, depending on the type of internal energy source. An indirect gasifier using gas as a heat source (gas indirect gasifier) is a fluidized bed gasifier, equipped with heat exchange tubes. Part of product gas is burned with air as oxidizing agent in a pulse combustor. The resulting heat is used for gasifying the fuel that is fed into the reactor. An indirect gasifier using char as a heat source (char indirect gasifier) consists of two separate reactors: a CFB steam gasifier that converts fuel to product gas and a CFB combustor that burns residual char to provide heat, which is needed to gasify the fuel. Silica sand, which commonly is the bed material, is circulated between the two reactors to enable heat transfer. In such a way, all gaseous and condensable products are reserved for gas production. This process is also known as “fast fluidized process” and generates high product gas yields.