Gasification is a flexible and versatile technology capable of converting a wide range of feedstocks into various useful products.
A variety of supporting or auxiliary technologies are needed to complement the core gasification technology in its applications. These technologies are commonly categorized into the following general processing areas or sections (click on the section name for more information on each supporting technology):
Coal Storage and Feed Preparation
Coal storage and feed preparation consists of a coal handling system to provide the means to receive, unload, store, reclaim, and convey coal to the storage facility, from which coal can be reclaimed and sent onto the feed preparation plant. While most of the technologies in this section involve solid handling equipment that has been widely used in pulverized coal industries, gasification requires special coal feed preparation that is gasifier technology specific. Coal can be pumped to the gasifier reactor either as a slurry, or transported as a dry feed via a lockhopper system. The GE and ConocoPhillips E-Gas™ slurry-fed gasifiers require wet grinding of coal into the right particle size and distribution in order to form a stable, coal/water slurry at optimum concentration. Dry fed gasifiers of both the entrained-bed (Shell and Siemens) and fluidized-bed types (KBR) require drying of the coal to reduce its surface moisture to prevent the small coal particles from sticking to each other during transport and feeding.
Gasification processes require an oxidant, be it air, oxygen, or oxygen-enriched air. Oxygen-blown systems have the advantage of minimizing the size of the gasification reactor and its auxiliary process systems. However, the oxygen for the process must be separated from the atmosphere. Commercial large-scale air separation plants are based on cryogenic distillation technology, capable of supplying oxygen at high purity and pressure. While a 95% oxygen purity is believed to be the economic optimum for integrated gasification combined cycle (IGCC) applications, a higher purity oxygen is usually required for the production of clean fuels and chemicals. Cryogenic air separation is recognized for its reliability, and it can be designed for high capacity (up to 5,000 tons per day). However, it is costly and energy intensive to operate, and any outages can disrupt the entire plant process. There is great incentive to develop a new approach or technology for air separation.
Syngas Cooling and Heat Recovery
It is essential to recover heat from the high-temperature gasification operation. Heat recovery can reclaim significant portions of the energy in the feedstock. The raw synthesis gas (syngas) leaving the gasifier is typically at a temperature of 2,400 to 2,800°F, and it can be cooled by radiant and/or convective heat exchanger(s), or by a direct quench system, wherein water is injected into the hot raw syngas. The syngas typically passes through a series of heat exchangers to recover heat, in the process making steam for either power generation or process heating.
Syngas Cleanup and Conditioning
The raw syngas leaving the gasifiers needs to be cleaned and conditioned in order to remove the contaminants in the feedstock. Typical contaminants include fine particulates, sulfur, ammonia, chlorides, mercury, and trace heavy metals, as well as carbon dioxide (CO2). This is done to meet environmental emission regulations and to protect downstream processes. The demands for cleanup and conditioning vary with application. Typical processes include filter and cyclone for bulk-solid fine removal, wet scrubbing to remove fine particulates, ammonia and chlorides, water-gas-shift (WGS) for carbon monoxide (CO) and hydrogen (H2) composition adjustment, an activated carbon bed to capture mercury, an acid gas removal (AGR) unit to remove sulfuric acid and CO2, followed by a sulfur recovery plant, and final tail gas treatment.
Power Train and Other Utility Facilities
Once the syngas is cleaned, if it is to be used to produce electricity, it is typically used as fuel in an IGCC power generation configuration. The combined cycle system utilizes a high-efficiency gas turbine that has been modified to burn syngas, with exhaust heat recovered to generate steam for use in a steam turbine. Both the gas turbine and the steam turbine drive an electric power generator. The combined cycle plant is supported by a host of utility processing facilities such as steam and condensation collection and distribution systems, as well as cooling water and waste water treatment systems.
Syngas Conversion Processes
Besides making power, syngas can be converted into a variety of liquid fuels, gases, and chemicals, depending on the catalyst system and reactor conditions employed. Examples include: WGS reaction to make H2, Fischer-Tropsch synthesis to make clean transportation fuels, methanation reaction to make synthetic natural gas (SNG), and methanol synthesis. In addition, hydrogen produced from the gasification process can be further processed into other valuable products. It can react with nitrogen to make ammonia and urea, a base for fertilizer production from coal. Using methanol as their intermediate, Eastman Chemical Company has produced a variety of acetyl chemicals, from their Kingsport, Tennessee, coal-to-chemicals facility since 1983.