Different carbon sources are used, or are being considered, as feedstock for gasifiers; including natural gas, coal, petroleum coke, and biomass. Biomass has been used with limited success because of issues such as ash impurity interactions with the refractory liner, which will be discussed in this paper.
Gasifiers were first used in industry around 1800; but the modern high temperature, high pressure units currently used by the chemical, petrochemical, and power industries were first developed and put into commercial service in the 1950's and 1960's; and are greatly improved technologically over those of the past [1]. An example of an air cooled slagging gasification system, with the ability to produce a variety of products, from electricity to chemicals, is shown in figure 1. Complex issues determine the type of gasification process selected for a given carbon feedstock, and a number of technical issues remain to be resolved before this technology will be widely adopted for carbon feedstock such as biomass or waste materials. Among these issues are improved lining materials with adequate service life for the gasification chamber.
In its simplest form, a gasifier is nothing more than a containment vessel used to react a carboncontaining material with oxygen and water (steam) under reducing conditions (shortage of oxygen) using fluidized-bed, moving-bed, or entrained-flow technology. The gasification process produces CO and H2 as the primary products (also called synthesis gas or syngas), along with by-products of CO2 and minority gases according to the following simplified equation:
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Gasification is considered a non-catalytic process that involves a number of endothermic and exothermic reactions, with the overall process being exothermic [1]. In the reducing environment of gasification (oxygen partial pressure can be as low as 10-10 atm.); excess carbon from the feedstock becomes a by-product of the process. Ideally, the amount of excess carbon should be small, about 1.0 wt pct; but is dependent upon variables such as the gasifier type, carbon feedstock, O2/C ratios, and the level of carbon beneficiation [2, 3]. By-products of gasification depend on process variables and impurities in the carbon feedstock; and include excess carbon, sulfur, ash, soot, metal oxides, tars and low levels of impurity gases (CO2, H2S, CH4, NH3, HCN, N2, and Ar). The formation of tars is a serious limitation to the use of biomass as a carbon feedstock. Tars, defined as a complex mixture of condensable hydrocarbons that includes single ring to 5-ring aromatic compounds [4-6], are viewed as an environmental hazard, and can adversely impact processing equipment, including the fouling of heat exchanger tubes. Depending on the application for the syngas, impurities can be removed at the gasification facility using a variety of chemical processing techniques that are located downstream from the gasifier vessel.
Types of Commercial Gasifiers
A number of different gasifier designs are used commercially, with the residence time for carbon feedstock in the gasifier varying from seconds to about 45 minutes, depending on the gasifier type. Three of the most commonly used types are shown in figure 2.