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Apr-2009

Clean coal technology for fuel gas

The high prices of oil and natural gas, coupled with ease of carbon 
capture, favour coal gasification for fuel gas supplies

Andrew Williams
Advantica

Viewed : 4685


Article Summary

After a period of decline, when coal was supplanted by oil and gas, particularly as a feedstock for chemical plants, its usage is rapidly increasing. This is because reserves of coal are much greater than those of oil and gas combined. As oil and gas reserves struggle to meet the increasing demand for energy and feedstocks, so coal has become increasingly competitive. Coal firing is thus widely used in existing power plants and is now coming to the fore in a new generation of power and chemical plants, where security of supply is important or coal supplies are the cheapest option.

Over the past 25 years, technologies have been developed that enable coal to compete more effectively against gas in terms of efficiency and environmental cleanliness. Many of these technologies are based on gasification. As well as providing an economically viable alternative to gas-fired combined cycle gas turbine (CCGT), gasification technologies offer an environmentally friendly and efficient means of using coal for a number of applications.

Gasification can work for other feedstocks as well. In some markets, gasification of low-value or waste materials is an attractive option. In the refining industry, gasification is widely used as a means of adding value to refinery wastes such as heavy hydrocarbon residues and petroleum coke through the generation of hydrogen.

Gasification involves the conversion of a feedstock, usually coal or heavy oil, to syngas, by reacting it with a limited quantity of oxygen. The syngas formed is mostly carbon monoxide (CO) and hydrogen (H2). Syngas can be used as a fuel or as a feedstock for many chemical processes. In most gasification process, 70–90% of the coal energy is converted to syngas energy.

Applications
Gasification is the cleanest of 
all commercially available coal technologies. All of the impurities and pollutants formed during the conversion of coal are concentrated in the product syngas and can easily and safely be removed using commercially available gas treatment processes.

Gasification uses only 30–40% as much water as other coal-based power generation technologies, while processes have been operated since the 1930s and more than 400 commercial-scale gasifiers are currently in operation worldwide. 

Gasification is becoming cheaper as more experience is gained and designs are developed and optimised. It is the lowest-cost route to capturing carbon dioxide (CO2). Nearly all of the carbon present in coal ends up in the syngas and, if a high-pressure oxygen-fed gasifier is used, the CO2 is present at high concentrations, making its removal relatively easy and efficient compared to removal from low-pressure flue gases, where the CO2 is diluted by large amounts of excess combustion air. 

Coal conversion technologies have been used commercially to produce natural gas, liquid fuels and bulk chemicals. Gasification is the best route currently available for producing hydrogen from coal.

BGL development
Advantica’s coal gasification offering is centred on the British Gas Lurgi (BGL) slagging gasifier. As the direct descendant of British Gas Research and Technology, Advantica is a licensor of the BGL technology and licenses it jointly with Envirotherm. The BGL process was developed from 1974 to 1992 at the Westfield Development Centre in Fife, Scotland, as the first stage in a process to produce substitute natural gas (SNG). The SNG project was motivated in part to meet security of supply issues as part of British Gas’s role as the UK’s national gas supplier following the formation of OPEC and the quadrupling of crude oil prices in the 1970s. The aim was to have a proven design for a commercial coal-to-SNG plant by the 1990s. 

The BGL gasifier is a development of the Lurgi dry ash gasifier and was developed to increase efficiency with low reactivity coals. The increase in efficiency was achieved by reducing the steam/oxygen ratio and allowing the ash to melt to form a molten slag. This required the bottom half of the gasifier to be redesigned to handle higher temperatures and to remove the molten slag produced. The process has several advantages over the original design:
—   Higher conversion efficiency of coal to gas
—   Significantly higher plant throughput
—   Significantly lower steam usage
—   Lower oxygen consumption
—   Less CO2 in the product gas
—   Ability to use low ash fusion temperature coals
—   Coal minerals exit the process as non-leachable glassy solid rather than ash.

Principles of operation
Like the Lurgi gasifier, the BGL gasifier has a moving bed design. Coal is fed to the top of a bed of coal contained in a vertical cylindrical pressure vessel. The gasification agents, steam and oxygen are fed to the base of the vessel and the resulting syngas passes up through the coal bed. This has the advantage of drying and devolatilising the coal using the excess heat generated in the gasification reactions and makes the BGL one of the most efficient gasification technologies available.

Other slagging gasifiers have also been developed and several of these are now in operation commercially. All of them differ from the BGL technology in being entrained flow designs. Compared to an entrained flow gasifier, the BGL process uses 30–40% less oxygen and can have a 10–20% greater coal-to-syngas conversion efficiency.

Development programme
The Westfield site was built in the early 1960s for 
town gas production and originally had four Lurgi dry ash gasifiers, each 2.7m in diameter. Town gas production ceased in the mid-1970s as the use of North Sea natural gas became widespread in the UK. However, the site was kept open as the Westfield Development Centre for development and demonstration of the BGL technology.


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