A process and apparatus for gasifying coal to produce carbon monoxide and hydrogen in which a first stream of coal is burned without bed formation in a combustion zone in the presence of water under oxidation conditions to produce gases containing carbon dioxide and steam. A second stream of coal is maintained as a fluid bed in a separate gasifier zone by upflowing carbon dioxide and steam from the combustion zone while being gasified under reducing conditions to produce carbon monoxide and hydrogen. Char produced in the fluid bed is elutriated overhead and material in the fluid bed is prevented from direct entry into the combustion zone. The ratio of carbonaceous material to ash in the char removed overhead is lower than the average ratio of carbonaceous material to ash in the solids in the fluid bed.
This invention relates to a process for gasifying coal, coke, or other carbonaceous solids to produce a gaseous mixture which, after removal of carbon dioxide and hydrogen sulfide, is composed mainly of carbon monoxide and hydrogen. The gaseous product may be utilized as a moderate Btu-content fuel; as a reducing gas for metallurgical and chemical purposes; and as an intermediate for conversion to hydrogen for use in chemical processes, in petroleum refineries, in coal conversion plants for manufacture of coal liquids or high Btu-content gas.
In accordance with the present invention, coal is converted to carbon monoxide and hydrogen by a process which exhibits a minimum potential for polluting. Essentially no water effluent is produced. Water makeup for use within the process as steam for gasification or as wash water may include polluted, solids-containing water from other processes. As a result, process requirements for fresh water are greatly reduced, and conventional requirements for purification and discharge of process waste water are similarly reduced.
Ash, entering as part of the coal feed, is removed from the process in the oxidized form as solidified slag, suitable for landfill or for additional processing to recover valuable minerals. Noncombustible solids introduced in water makeup from other processes or in raw water are also removed as part of the oxidized, solidified slag. Essentially no ash or other solids is rejected to the atmosphere.
Gaseous impurities, having a potential for pollution, which are generated within the process are treated within the process and converted into acceptable forms for sale or disposal, or the impurities are destroyed within the process. For example, sulfur compounds entering the process are converted to hydrogen sulfide directly, or to sulfur dioxide and then to hydrogen sulfide; the hydrogen sulfide is recovered by known processes; and the recovered hydrogen sulfide is converted to elemental sulfur for sale or storage by use of known processes. Nitrogen compounds entering the process are converted mainly into ammonia, or to nitrogen gas, or to nitrogen oxides and then to ammonia or nitrogen gas; the ammonia is recovered and purified by known processes for sale. Gas streams before venting are first water scrubbed within the process to remove all dust and particulate contaminants.
Any traces of oils and tars which may be formed within the process are treated at high temperature to cause thermal cracking and are thereupon converted to gaseous or solid materials which are further reacted to form the desired gas product. At the same time, the improvements of the present process enhance process economy, especially in water usage, in process heat utilization, and in reliability.
Most water is consumed within the process by the chemical reaction: C + H.sub.2 O .fwdarw. CO + H.sub.2, and is thereby converted to the desired gaseous product. Only small amounts of water are lost as moisture vapor contained in vented nonpolluting gas streams. Makeup process water does not need to be treated, and, in fact, solids-containing and polluted water from other processes may be used.
A high degree of process heat economy is achieved by virtually complete gasification of the carbonaceous portion of the feed. All fines and dusts are recovered within the process and then burned within the process in oxygen to generate the heat needed for gasification and for process steam generation. Process steam is generated internally with no heat transfer surfaces interposed between the source of heat and the vaporizing water, thereby avoiding most of the inefficiencies which are associated with steam generation in conventional boilers.
High temperature sensible heat is supplied for coal gasification; intermediate level sensible heat and latent heat generates high pressure steam for use in other processes; low level sensible heat and latent heat is rejected to the atmosphere by air coolers; therefore, a minimum of water cooling is needed.
Some of the advantages of process water economy and process heat economy are achieved interdependently. Water is used at many locations throughout the process to scrub particulates from gas streams and to cool hot particulates. The resulting slurry contains substantially all the ash from the process plus associated combustible material and dissolved pollutants. After settling, clarified water is recycled for additional scrubbing and cooling duties; the thickened, concentrated slurry is pumped at a controlled rate to the combustion chamber of the process where the combustibles are burned with oxygen to supply process heat; the slurry water is vaporized and superheated for reaction with coal; and the ash is melted to form slag which is easily separated from the process. In this manner, essentially no combustible carbonaceous matter is withdrawn from the process as byproduct or waste, and the process can accept and usefully burn undesirable high-sulfur, high-ash combustibles which are byproducts or wastes from other processes, such as the high-sulfur, high-ash solid wastes of a solvent coal liquefaction process.
The process is economical from a reliability basis because the hot, pressurized parts of the process contain a minimum of moving mechanical equipment, which may be subject to occasional failure. Mechanical equipment is used sparingly throughout the process.
The process is designed especially to assure safe operation. Coal gasification generates highly combustible gases and these gasification reactions can proceed only by application of high temperature heat which is supplied by combustion of carbon with oxygen. Safe operation requires that the possibility of oxygen mixing with generated gas will not occur even if the process is badly upset or if coal feed flow is interrupted. Design of the present process assures this safety by interposing a substantial fluidized bed of coal char between the oxygen injection zone and the combustible gas.
Another advantage of the present process is its flexibility in using a variety of conventional fuels, combustible wastes, and potential pollutants as a source of heat for gasification of coal. These combustible materials, may have high sulfur content, high ash content, high moisture content but still would be useable. Such fuels are injected into the combustion zone where oxidation occurs. Sulfur oxides and nitrogen oxides which may be formed initially are ultimately reduced to hydrogen sulfide and nitrogen gas or ammonia within the process for easy separation and conversion to acceptable forms. Ash is melted and the slag withdrawn from the process with coal ash slag. Associated moisture is vaporized, superheated, and is reacted with coal to form the desired gas product.
In the present process, gasification is performed in a single reactor vessel which is divided into three zones including a fluidized bed gasification zone, a combustion zone and a slag quench zone. The boundary between the gasification and combustion zones is a grid or perforated partition which acts to support the fluidized bed and distribute gas flow to it. The coal particulates in the fluidized bed in the gasification zone comprise a large excess of carbonaceous material. Therefore, above the grid, within the fluidized bed and in the vapor space above the bed, there exists a reducing zone where chemical reactions occur which form hydrogen and carbon monoxide. At the same time, formation of a bed of carbonaceous material is avoided below the grid in order to produce an oxidation zone in which combustion takes place by burning carbonaceous fuel with oxygen forming carbon dioxide, carbon monoxide, and steam. Heat evolved in the exothermic combustion zone, or combustor, is transferred to the fluidized bed zone, or gasifier, as sensible heat in the gas to support the endothermic gasification reactions.
A solid hydrocarbonaceous feed such as coal, char, or coke is passed through a crusher and subdivided into particles which are introduced by a dry solids feeding device to the fluid bed gasification zone. In the gasifier the particulate coal is maintained as a fluidized bed, a pseudo liquid state of finely divided solids, by upward flowing hot combustion gases and steam from the combustion zone. These gases flow through a perforate material such as a screen, grate, or grid which supports the fluidized bed and which prevents downward solids flow from the gasifier to the combustor. The gases flow at a sufficient velocity to maintain particles in the gasification zone in a highly agitated, disperse, fluidized condition while maintaining a pseudo liquid level at the top of the particles. Essentially no solid or gaseous flow of material occurs downwardly through the grid so that material and heat flow through the grid is entirely in an upward direction and there is essentially no downflow directly from the gasifier zone to the combustion zone.
The preferred position of the combustion zone is immediately beneath the fluidized bed gasification zone, although the combustor may be positioned beside or even above the gasifier so long as combustor gases are introduced beneath the gasifier grid. Feed to the combustor is comprised primarily of the fine coal or high-ash-content char slurried in water, although liquid or gaseous fuels may also be used. The aqueous slurry is pumped into the combustor at a controlled flow rate, is suitably atomized, and the carbonaceous content is burned with oxygen. Heat of combustion vaporizes and superheats slurry water, and causes ash and other normally solid inorganic substances contained in the slurry to melt, forming a liquid slag. The slag collects on the surfaces of the combustor and drains by gravity to a slag quench container and is thereby separated from the upward flowing combustor gas.
In the preferred apparatus embodiment of the present process, an upper fluidized bed gasifier, an intermediate combustion zone, and a lower slag quench drum are arranged in a single vertically coaxial reactor arrangement. In this arrangement, the only downward flowing material is molten slag which flows by gravity from the combustion zone to the slag quench drum beneath. Aside from downward flow of molten slag, all other primary flows in the combustor and gasifier are upward, including steam produced in the slag quench pot, the combustion gases and superheated steam produced from water and/or steam charged to the combustion zone, the gasifier gases, and the fine carbon-containing ash and char particulates which are formed within the gasifier as a reuslt of gasification and inter-particle impacts occurring within the fluidized bed. Elutriated ash-containing char from the gasifier cyclone is separated from the raw gas outside of the reactor vessel, is scrubbed, cooled, and slurried in water, thickened to a slurry or paste, and pumped or injected as fuel to the combustor by a path outside of the reactor apparatus.
The gasification zone is maintained at as high a temperature as possible in order to achieve the highest reaction rates, but temperatures are avoided that promote excessive agglomeration of fluid bed particles caused by ash in the particles softening, becoming sticky, and thereby by agglomerating with others as a result. Such temperatures vary depending on composition of coal ash, but may be approximately 2000.degree.F. (1093.degree.C.) and higher. If temperatures are below about 1400.degree.F. (760.degree.C.), gasification reaction rates for high carbon conversions are too low for practical purposes. The gasifier temperature range, therefore, is about 1400.degree. to 2000.degree.F. (760.degree. to 1093.degree.C.), and typically may be about 1700.degree.F. (927.degree.C.). The gasifier pressure is in the range of 10 to 500 psi (0.7 to 35 Kg/cm.sup.2). The lower limit provides sufficient pressure to cause the raw gas product to flow through simple processing for particulate cleanup without requiring intermediate compression; the higher limit is based entirely on the current commercially demonstrated limit for dry solids injection into a pressurized system and, otherwise, could be substantially greater than 500 psi (35 Kg/cm.sup.2). Higher pressures are desirable because they make possible higher flows through a vessel's internal cross-sectional area, and process investment costs are thereby reduced. Typically, a pressure of 450 psi (31.5 Kg/cm.sup.2) is desirable. Average residence time of a particle in the fluidized bed depends on the particle composition and size, pressure and temperature, and the composition of the fluidizing gas. Usually temperature is varied to change average residence time which may typically be 20 to 30 minutes. A residence time greater than about 60 minutes is undesirable because unusually large and costly gasifier volumes would be needed. A residence time less than about 5 minutes is undesirable because of difficulty in control of fluid bed level as a result of the minimal carbon capacity of the bed.
Following are the principal chemical reactions which occur within the gasifier fluidized bed: