1. Field of the Invention
This invention relates to the generation of electric power. More particularly, it is concerned with a unique method for operating an electric power plant, involving the conversion of coal to liquid and gaseous fuels which are subsequently utilized for generating electric power.
2. Discussion of the Prior Art
Solid carbonaceous materials are well known for their utility in the generation of power, particularly electrical energy. Generally, coal is combusted with air and the exothermic heat of reaction is used to produce high-pressure steam. The steam in turn is expanded through a turbine to generate mechanical or electrical energy. Similarly, natural gas and other gaseous fuels have been combusted and the heat generated used to form high-pressure steam for the generation of electrical power.
The electrical industry has developed highly efficient, large generators driven by expanding steam. However, one problem in the generation of electrical power from steam results from the greatly varying demands for electrical energy. Steam generators are not well suited for producing greatly varying amounts of steam, but rather are designed for base-load or constant-load types of operation. To provide for peak-load and reserve-load demands, the open-cycle gas turbines have generally been utilized because of their quick-startup capability and relatively low capital cost. Open-cycle turbines, however, require special fuels which are noncorrosive to the turbine blades. Generally it has been found to be uneconomical to combust coal or residual oils directly in the combination chamber of a gas turbine, because the fuel contains high amounts of ash and sulfur. Due to the incomplete combustion, such high-ash solid fuels generally produce solid particles which are abrasive and corrosive to the metal turbines. When such particles are entrained in the flue gas that is passed through the turbine, they deposit on the blades and erode the blade surfaces. When this corrosion occurs the blade is damaged, reducing the effiency of the unit, and the passages in the turbine become clogged. Further, the fine particles may deposit down-stream in heat-exchange surfaces and impair thermal efficiency. Similar problems are encountered when burning ash-producing liquid petroleum products.
Previous methods in which the fuel gas was cleaned prior to being introduced into the gas turbine were either impractical, unduly costly, or both.
The aforesaid problems of base-load and peak-load demand, combined with the special fuel requirements for gas turbines, are substantially avoided by the present invention, which integrates the production of a base-load power generation capability combined with the production of a clean, normally liquid fuel which is particularly useful for the generation of supplemental peak-load power.
Because coal and other solid carbonaceous materials often contain sulfur compounds, the combustion of coal for power production can also cause serious air pollution problems. Also, because of the very large volumes of gases produced by combustion, it is very expensive to remove the polluting sulfur compounds after combustion. These sulfur-removal problems and air-pollution problems have led to processes for the gasification of coal to produce a clean fuel gas wherein the sulfur is removed from the fuel prior to combustion. One problem, however, with such gasification processes is that only a low heat value gas is produced, and it is generally not economical to transport a low heat value gas over great distances. This has led to proposals for large on-site or "mine-mouth" gasification and power generation plants where the low heat value gas is immediately converted to electrical power for transmission. Such on-site gasification and power generation processes solve the problem of low heat gas transportation and sulfur-removal problems, but are not economical for producing greatly varying amounts of fuel as is needed for peak-load generation of power, either because it is too expensive to store gaseous products for subsequent use in gas turbines or because the capital expense of providing for greatly increasing the gas production rate and gas cleanup rate for peak-load demand is uneconomical.
The aforesaid problems are substantially avoided by the subject invention, which process provides for satisfying base-load power demand. The process integrates the production of a combustible fuel gas for meeting base-load power generation requirements with a process for producing easily storable fuels which are substantially free of sulfur and other impurities. These readily liquified fuels are suitable for storage and, when needed, for supplemental peak-load power generation.
U.S. Pat. No. 3,868,817 discloses a process for the generation of mechanical and electrical power from a purified fuel gas produced from solid carbonaceous fuels. The purified fuel gas is used to generate power using gas turbines.
Another patent, U.S. Pat. No. 3,986,349, discloses a process for generating electrical power from solid carbonaceous material in open-cycle gas turbines to meet variable power demands. The process involves the conversion of coal to a combustible synthesis gas or "syngas" by reaction with steam and oxygen. The synthesis gas is then divided into two portions, one of which is contacted with Fischer-Tropsch and hydrogenation catalysts to produce synthetic hydrocarbons ranging from methane and ethane to C.sub.22 or higher. The normally gaseous portion of the product is separated and recombined with the second portion of the synthesis gas stream and the combined streams are combusted and utilized to operate an electricity-generating gas turbine. The normally liquid hydrocarbon product (C.sub.5 -C.sub.22) is stored and utilized as fuel for a gas turbine to produce supplemental power for peak-load demand.
One of the more recent developments with regard to clean burning coal fired power plants has been the utilization of oxygen fired coal gasifiers coupled with combined-cycle power generators. In this type of plant a clean gaseous fuel obtained from the coal gasifier is burned in a gas turbine and the hot off-gases are used to generate steam. The steam is then used in a steam turbine to generate power. Heat recovered from the gasifier is used to generate additional steam, both for purposes of providing steam needed in the gasifier and as additional feed to the steam turbine.
One disadvantage to such plants has been that most coal gasifiers presently available are such that their output is neither easily nor economically varied to match the sometimes dramatic variations in power demand. The same problem results with regard to combined-cycle power plants with low efficiency if output must be reduced by more than about 20%. Many combined-cycle power plants are therefore operated in an on/off control mode.
A potential solution to this problem would be to use the syngas during times of low power demand to produce storable fuels. The oxygen-blown coal gasifiers have the advantage that they are similar to the gasifiers used for preparation of liquid fuels from coal. There are at present two processes available for use with gasifier-generated syngas for production of storable fuels: co-production of methanol and production of liquid hydrocarbons by the Fischer-Tropsch synthesis. Co-production of methanol suffers from the fact that almost all gasifiers presently contemplated for that purpose yield a synthesis gas with a low ratio of hydrogen to carbon monoxide (e.g. 0.5 to 1.0), which severely limits the amount of methanol obtainable without an additional and expensive water-gas shift. Production of hydrocarbon liquids via the Fischer-Tropsch route has the disadvantage that significant amounts of methane and ethane are produced and these are expensive to condense or otherwise separate. Also, as thermal energy is lost in the production of non-storable products from the syngas, the efficiency of the total process is greatly reduced. Our invention provides a system which overcomes these shortcomings in a way which results in an overall power generating scheme which is superior to anything previously suggested.