Unlike combustion, gasification is carried out with less than a stoichiomet­ric amount of required oxygen such that only partial oxidation of waste materials occurs. Gasification generates a producer gas of low, medium, or high BTU content depending on the operating conditions and amount of oxygen. The gasification temperature varies from 500°C to about 1,800°C depending upon the desired gas composition and the nature of the slag. The producer gas generally contains CO, CO2, H2, H2O, CH4, trace amounts of higher hydrocarbons such as ethane and ethylene, inert gases originat­ing from the gasification agent, and various contaminants such as small particles and others depending upon the impurities present in the waste [12, 17, 18]. The partial oxidation can be carried out using either air, oxy­gen, carbon dioxide, steam, or a mixture of these substances. Generally oxygen produces medium BTU producer gas whereas air produces low BTU producer gas. The producer gas can either be used for heating and the generation of electricity or it can be reformed to produce syngas that largely contains CO and H2 which can then be used as a raw material for second-generation biofuels via Fischer-Tropsch syntheses. Syngas can also be produced by operating a gasification reactor at a very high tempera­ture (greater than 1,400°C). Several types of gasification reactors have been developed to process MSW, hazardous waste, and dried sewage sludge. A stable and optimum operation of the gasification reactor with minimum generation of tar or slag formation requires feedstock of uniform size with some consistencies in its composition. This may require pretreatment of the feedstock which can be expensive.

The gasification process involves smaller gas volume (by a factor of 10 if pure oxygen is used) and smaller waste water from the producer gas cleaning process compared to incineration. High operating pressures applied in some gasification processes can also lead to smaller and more compact aggregates. Unlike incinerators, a gasification process mainly produces carbon monoxide instead of carbon dioxide. High-temperature

gasifiers capture inorganic impurities within the slag. Incinerators only produce heat (and thereby electricity), but syngas produced from a gasifier can be used to produce materials and transportation fuel along with heat and electricity.

Various types of gasification reactors (packed bed with upflow or down­flow mode of operation, bubbling or circulating fluidized bed, entrained bed, and cyclone) are used in commercial operations. The gasification tech­nology is very versatile and well developed, and it can process all types of waste. In recent years, new gasifiers tend to be entrained bed with both low — and high-pressure operational flexibility. The most preferred feed­stock for the gasifier is high-energy density solids and for efficient opera­tion of all gasifiers, waste materials must be finely ground before feeding into the gasifier. Hazardous waste may be gasified directly if it is liquid or finely granulated.

A typical simplified Texaco gasification process for conversion of MSW to a medium BTU gas is illustrated in Figure 6.5. Numerous other commer­cial processes for waste gasification are available, and they are described by Lee, Speight, and Loyalka [4]. SVZ Schwarze Pumpe GMbH operates both a packed bed gasifier for coal-waste mixtures (with waste up to 85%) and an entrained flow gasifier for hazardous waste. The entrained flow gasifier is operated at temperatures between 1,600°C and 1,800°C. The packed bed gasifier has a capacity of 8-14 tons per hour, and it operates between 800 and 1,300°C and 25 atm pressure and produces syngas using steam and oxygen as the gasification agents. A slag bed gasifier operates up to 1,600°C with a

FIGURE 6.6

Fluidized bed gasifier with high-temperature slagging furnace. (From EBARA. (2003). EUP — EBARAUBE. Process for gasification of waste plastics. Retrieved May, 2010 from http://www. ebarra. ch/) throughput rate of 30 tons per hour and slag is discharged as liquid [19, 20]. Recently, gasification technology has been used for numerous types of waste in addition to MSW. Among others, hazelnut shell, rice husk, salmon waste, and several other types of solids and liquid organic waste have been success­fully gasified to generate producer gas or syngas [21-25]. In all cases, gasifi­cation technology produced good quality producer gas. In some instances, producer gas was subsequently transformed into biomethanol.

A gasification process can also use two stages. An example of a two-stage waste gasification process using a fluidized bed and an entrained flow reac­tor (see Figure 6.6) is used in Japan for waste conversion to syngas. The fluidized bed gasifier operates at a lower temperature, and it converts hetero­geneous waste into syngas. The ash produced in this reactor is then passed onto a high-temperature cyclone gasifier where slag is collected. The syngas produced from this process is used for ammonia production and other appli­cations. Other modifications of this process for different types of wastes are described by Bridgewater [26].

A two-stage gasification system sometimes also uses a gasification reac­tor in combination with a combustion reactor. For example, a combina­tion of fluidized bed gasifier and a high-temperature combustor is used to process shredded MSW, plastics, and residues. In this process, the gasifier is generally operated at 580°C to produce gas and the combustor is oper­ated at 1,350-1,450°C for melting ash and other solid materials to further recover energy [7]. Generally particle size of 300 mm is preferred in such a process [20].