Gasification systems

There is an extremely wide variety of gasifiers that have been implemented, particularly in the field of small-scale biomass and waste gasification. The number of processes operating at over 100 MW is however limited and only these are discussed here. 

The GE coal gasifier was developed out of the design for liquid and gaseous feed, which exhibits the same main characteristics except the radiant cooling feature of the coal gasifier. 

The Shell Gasification Process (SGP) for liquid and gaseous feeds has a down-flow, refractory-lined configuration. Syngas cooling is performed exclusively with a steam raising heat exchanger. 

Modifications of the process have been developed. The BGL is a slagging version that is able to recycle the tars to the lower part of the gasifier. 

Another modification is the MPG liquids gasifier, which was developed to process the tars into s mgas in a separate gasifier. 

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The importance of coal gasification as a means of produciag fuel gas(es) for iadustrial use caimot be underplayed. But coal gasification systems also have undesirable features. A range of undesirable products are also produced which must be removed before the products are used to provide fuel and/or to generate electric power (see Power generation) (22,41). 

Biomass gasification offers the potential for producing a fuel gas that can be used for power generation system or synthesis gas applications. The volatile matter contains a considerable amount of tar which is a complex mixture of aromatics. Despite extensive research efforts tar formation which causes the pipe plugging and the reduction of conversion efficiency is still a major problem in biomass gasification systems [1-6]. 

Fig. 3. Caloric value of the product gas from different fluidized bed gasification systems.

Westinghouse Electric Corporation, Advanced Coal Gasification System for Electric Power Generation, Quarterly Progress Report NTIS No. FE-1514-61 (1977)... 

Iyer M.V., Gupta FT, Sakadjian B.B. and Fan, L.-S., High Temperature C02 Capture and Hydrogen Production Using Calcium Oxide Process Development and Economics for Combustion and Gasification Systems, The 2005 Clearwater Coal Conference, Apr. 17-21, Clearwater, FL, 2005. 

Thermodynamic and equilibrium characteristics of gasification systems, if available, could help to determine conditions under which certain desired products may be... 

In the Battelle/Columbus gasification system biomass particles are surrounded with extremely hot sand, which converts it into gaseous form. The solid biomass is surrounded by sand heated from 1800 to 1900°F, which converts the biomass into gas and residual char in a fluidized-bed reactor at 1500 to 1600°F. Sand is used to carry the biomass and the char and to distribute the heat. Using sand as a heat carrier keeps out the air. This results in a better quality fuel gas. A second reactor combusts the char to heat the sand. Remaining traces of condensable matter formed during gasification are removed in a chamber where a catalyst cracks and converts them into fuel gas. The clean biogas is then pressurized before it reaches the gas turbine. 

Table 2.11 provides a summary of the commercial and semicommercial experiences for gasification systems. 

Figure 3.1 Ranges for particulate and tar levels for different biomass gasifiers. (After Neeft, J.P.A., Knoef, H.A.M., and Onaji, R, 1999. Behavior of Tars in Biomass Gasification Systems. NOVEM EWAB Program Report 9919. Available from MHP Management Services, PO. box 127, 3950 AC Maam, Netherlands. 75 pp.)...

Acceptable levels of ammonia in the gas stream are dictated by local regulations. Gasification systems must meet emissions regulations however, the concentrations of ammonia are relatively low with common feedstocks in most gasifiers. 

There are various technologies that can be used to remove unwanted components, including particulates, alkali, tars, sulfur, and ammonia, from the producer gas stream. The gas cleanup and conditioning technologies for gasification systems are outlined in this chapter. 

These filters are well proven for removing particulates in a variety of systems but have not previously been used in biomass gasification systems. For gasification systems these filters require hot product gas to be cooled prior to filtration. For that reason, they are most appropriate for applications where retaining the sensible heat of the product gas is not critical. In addition, the presence of tars in the product gas can cause potential problems since tar condensation on the filter cake or the filter itself can lead to plugging. Care must be taken to ensure either that tars are removed from the stream prior to the bag filter or that the temperature remains high enough to prevent tar condensation. 

Electrostatic filters have been used in many coal-fired power stations, and they have been used in some biomass combustion facilities. Their use in medium- or large-scale gasification systems is limited. Electrostatic filters are best suited for large-scale operation due to their physical size and cost, and the primary impediment to their use in current gasification systems is an economic one. 

Wet scrubbing requires that the water remain in the liquid phase, which requires that the product gas be cooled to below 100°C. This loss of sensible heat may be undesirable in some systems. Most gasification systems that currently use wet scrubbers do so primarily as a means to remove tars rather than particulates from the gas stream. Removing the particulates separately can prevent condensation of the sticky tars on the particulate surface, and that can prevent fouling and plugging of filter surfaces. 

The key to successful commercialization of gasification systems devoted to power production rests both on maintaining the government R D focused on industry needs and ensuring that the proper key technical issues are being addressed. The sidebar discussion summarizes those factors that have an impact on the future development of the technologies described here. 

A 2002 study for the National Energy Technology Laboratory found that coal gasification systems with C02 capture could reach efficiencies of 60% or more in cogenerating hydrogen and electricity using different configurations of turbines and solid oxide fuel cells (SOFCs). 

The numerous coal gasification systems available today can be reasonably classified as one of three basic types a) moving-bed, b) fluidized-bed, and c) entrained-bed. All three of these types utilize steam, and either air or oxygen to partially oxidize coal into a gas product. The moving-bed gasifiers produce a low temperature (425 to 650°C 800 to 1,200°F) gas containing... 

For example, a COS hydrolysis reactor needs to operate at about 180°C (350°F), the ammonia and acid scrubbers need to be in the vicinity of 40°C (100°F), while the zinc oxide polishers need to be about 370°C (700°F). Thus, gasification systems with cold gas cleanup often become a maze of heat exchange and cleanup systems. 

Coal Gasification Systems A Guide to Status, Applications and Economics, prepared by Synthetic Fuels Associates, Inc., EPRIAP-3109, Project 2207, Final Report June 1983. 

The relative simplicity of the gasification system enables its operation to be within the technical expertise of most operators who are experienced with conventional boilers and furnaces, and results in favorable project economics. Its modular design allows a wide range of scale-up or scale-down possibilities, so the systems can vaiy in size from about one ton per hour of residue to 20 tons per hour or larger, with the size being limited only by biomass availability. 

Most biomass gasification systems utilize air or oxygen in partial oxidation or combustion processes. These processes suffer from low thermal efficiencies and low Btu gas because of the energy required to evaporate the moisture typically inherent in the biomass and the oxidation of a portion of the feedstock to produce this energy. 

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