Gasification

Gasification is a process that converts carbonaceous materials, such as coal or biomass, into carbon monoxide and hydrogen.

In a gasifier, the carbonaceous material undergoes three processes:

The pyrolysis (or devolatilization) process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.
The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. Pyrolysis and combustion are very rapid processes.
The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen. The resulting gas is called producer gas or syngas (or wood gas when fueled by wood) and may be more efficiently converted to energy such as electricity than would be possible by direct combustion of the fuel, as the fuel is first combusted in a gas turbine and the heat is used to produce steam to drive a steam turbine. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing high temperature combustion of the gas from otherwise problematic fuels.

The gasification process was originally developed in the 1800s to produce town gas for lighting and cooking. Natural gas and electricity soon replaced town gas for these applications, but the gasification process has been utilised for the production of synthetic chemicals and fuels since the 1920s. It is now recognised that gasification has wider applications; in particular the production of electricity using Integrated Gasification Combined Cycles (IGCC), with the long-term aim of producing hydrogen for fuel cells. IGCC demonstration plants have been operating since the early 1970s and some of the plants constructed in the 1990s are now entering commercial service.

(Within the last few years gasification technologies have been developed that use also plastic-rich waste as a feed. In a plant in Germany such a technology - on large scale - converts plastic waste via producer gas into methanol. [1] [2])

Gasification relies on chemical processes at elevated temperatures >700°C, contrary to biological processes such as anaerobic fermentation (digestion) which produces biogas.

Breakdown of hydrocarbons into syngas is done by carefully controlling the amount of oxygen present while heating the hydrocarbons to extreme temperatures.

Gasification processes

Four types of gasifier are currently available for commercial use: counter-current fixed bed, co-current fixed bed, fluid bed and entrained flow.

The counter-current fixed bed ("up draft") gasifier consists of a fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the "gasification agent" (steam, oxygen and/or air) flows in counter-current configuration. The ash is either removed dry or as a slag. The slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent. The throughput for this type of gasifier is relatively low. Thermal efficiency is high as the gas exit temperatures are relatively low. However, this means that tar and methane production is significant at typical operation temperatures, so product gas must be extensively cleaned before use or recycled to the reactor.

The co-current fixed bed ("down draft") gasifier is similar to the counter-current type, but the gasification agent gas flows in co-current configuration with the fuel (downwards, hence the name "down draft gasifier"). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources. The produced gas leaves the gasifier at a high temperature, and most of this heat is often transfered to the gasification agent added in the top of the bed, resulting in an energy efficiency on level with the counter-current type. Since all tars must pass through a hot bed of char in this configuration, tar levels are much lower than the counter-current type.

In the fluid bed gasifier, the fuel is fluidised in oxygen (or air) and steam. The ash is removed dry or as heavy agglomerates that defluidise. The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly suitable. The agglomerating gasifiers have slightly higher temperatures, and are suitable for higher rank coals. Fuel throughput is higher than for the fixed bed, but not as high as for the entrained flow gasifier. The conversion efficiency is rather low, so recycle or subsequent combustion of solids is necessary to increase conversion. Fluidised bed gasifiers are most useful for fuels that form highly corrosive ash that would damage the walls of slagging gasifiers. Biomasses generally contain high levels of such ashes.

The entrained flow gasifier is operated with fuel and oxygen (or air) in co-current flow. Most coals are suitable for this type of gasifier because of the high operating temperatures and because the coal particles are well separated from one another. The high temperatures and pressures also mean that a higher throughput can be achieved, however thermal efficiency is low as the gas must be cooled before it can be cleaned with existing technology. The high temperatures also mean that tar and methane is not present in the product gas; however the oxygen requirement is higher than for the other gasifiers. All entrained flow gasifiers remove ash as slag as the operating temperature is well above the ash fusion temperature, so fuels that form highly corrosive slags or have high ash fusion temperatures are unsuitable. The fuel particles must be much smaller than for other gasifiers, this means the fuel must be pulverised, which requires more energy.

Gasification processes

See also