The British Gas Lurgi gasifier (BGL), which is a moving bed design type, is currently the only large-scale (>50 MWe) gasification technology with sig­nificant operational experience in the processing of mixed feedstock of coal and waste materials. BGL has the most operational experience in process­ing fuels of widely differing fuel properties. This technology is now owned by Advantica, a company leader in the large-scale gasification of variable property feedstock. Global Energy (part of ConocoPhillips since 2003) is the U. S. leader in mixed feedstock gasification. The BGL technology creating new plants in the United States and Europe for moving bed gasification of the materials which include coal, municipal refuse, and sewage sludge feed­stock. Global Energy also operates the SVZ plant in Germany and a plant in Scotland alongside its natural gas-fired power plant for co-gasification using the BGL gasifier [6].

In a moving bed gasifier (often called fixed bed gasification), large par­ticles (about 2 in.) of feedstock move down while gases move upward (or sometimes downward). The acceptability of fines is limited for a dry ash moving bed, although it is better for a slagging moving bed reactor. At the top of the bed, solids get heated by hot gases which get cooled before leav­ing the reactor. This is considered a drying zone. The solids then go through carbonization, gasification, and combustion zones as they move downward [6]. At the bottom of the reactor, oxygen reacts with the remaining char. In a dry-ash version (i. e., Lurgi dry ash gasifier) the temperature is moderated to below ash slagging temperature by reaction of char with steam. The ash below the combustion zone is cooled by the entering steam and oxidant. In a slagging version, less steam is used which maintains the temperature above the ash slagging temperature. The temperature in the reactor may vary from 300-650°C depending on the nature of the feedstock. The dry ash moving bed reactor prefers low-rank coal and slagging moving bed reactor prefers high-rank coal [6]. Both the dry ash (with some modifications) and slagging moving bed accept caking coals. The moving bed reactor technology has (a) a low oxidant requirement which produces hydrocarbon liquids such as oils and tars, (b) high "cold gas" thermal efficiency, when the heating values of the hydrocarbon liquids are included, and (c) a high steam requirement for the dry ash moving bed and low steam requirement for the slagging moving bed [6]. The key technical issue with moving bed technology is the utiliza­tion of fines and hydrocarbon liquids coming from the product gas. Two existing commercial processes using a moving bed reactor are the Lurgi dry- ash gasifier, which is used for town gas production and chemicals from coal in South Africa, and the BG Lurgi (BGL) slagging gasifier, which is currently used to process solid waste and a mixture of coal and waste.

In summary, although updraft and downdraft gasifiers have been used for small-scale applications, fluid bed, circulating fluidized bed, and entrained bed gasifiers are extensively used for large-scale operations. Updraft gener­ates a high amount of tar, whereas the fluid bed and CFB generate a medium amount of tar and downdraft and the entrained bed generates a low amount of tar. Syngas can be produced by fluid bed, CFB, and entrained bed opera­tions. Fluid bed and CFB have higher particle loading and can use larger particle sizes whereas the entrained bed requires a large amount of carrier gas and it has a particle size limit [6].