Entrained Bed Gasifier

Currently, large-scale commercial coal gasification technology is dominated by high-temperature, high-pressure entrained flow gasifiers. In fact, about 85% of existing commercial coal gasification reactors are entrained bed reac­tors. As shown in the previous section, recent successful tests with crops and biomass [6] indicate that this type of gasifier is reasonably well suited to the gasification of mixed feedstock. They eliminate tar formation and are not greatly affected by ash content differences with coal/coke; however, the Buggenum plant [6] noted a tendency to foul the syngas cooler when using sewage sludge. The primary drawback of this type of gasifier for the process­ing of mixed feedstock is the handling of feed materials that are difficult to pulverize such as switchgrass and straw, among others, as well as some woody biomass and high-pressure feed into the gasifier using either a slurry or dry feed system. The dry feed system requires pulverized particles of about 100 mesh which may be difficult to achieve with certain types of bio­mass. The reactor operates at temperatures in excess of about 1250-1300°C. Such a high temperature results in the production of syngas (CO and H2) with no methane.

The entrained flow gasifier (a) is able to gasify all coal regardless of coal rank, caking characteristics, or amount of coal fines; (b) has uniform temper­ature and short residence time; and (c) slagging operation with some entrain­ment of molten slag in the raw gas. Although all new IGCC plant reactors will be entrained flow reactors, the design of various entrained flow reactors may vary depending on whether feed is dry or slurry, the internal design to handle the hot reaction mixture, and heat recovery configuration. One of the key technical issues is the method for cooling the product gases. Recent studies have shown [105 ] that this type of reactor is very suitable for mixed feedstock up to about 30% of biomass. Due to the high oxygen content of the biomass, the mixed feedstock will produce more water and carbon dioxide and less carbon monoxide, lower syngas heat content, and in general lower sulfur and ash content. In processing some mixed feedstock that contains biomass such as straw and other herbaceous biomass which contains chlo­rine can cause corrosion and biomass that contains calcium and sodium can alter ash melting point.