1983 ASPHALT 1983 COAL
3. THE SHELL COAL GASIFICATION PROCESS (SCGP)
3.1 SCGP - EXPERIENCE
The application of gas-fired combustion turbine-combined cycle systems has grown rapidly in the industry due to the lower cost, higher efficiency and demonstrated reliability of gas turbine equipment. There are already gas fired combined cycle power plants with overall efficiencies around 55% LHV. Developments in gas turbines and steam cycles point in the direction of 55 to 60% LHV efficiency for such plants in the near future.
Modern coal gasification technologies present a unique opportunity to combine the advantages of high efficiency combined cycle power generation with an environmentally friendly coal based process. The Shell Coal Gasification Process (SCGP) is especially well-suited to produce clean turbine fuel gas efficiently and it can be coupled with a combined cycle system in either an integrated or a non-integrated arrangement. This allows considerable flexibility in configuring the gasification plant.
Recent studies show that optimal integration of the three main building blocks of an ICGCC plant (air separation unit, SCGP and the combined cycle) could give an overall efficiency (LHV) of 46-48% based on commercially available gasturbines.
In such cases the air separation unit receives part of its air from a dedicated air compressor, with the remainder coming from die air compressor of the gas turbine. The nitrogen not required in the SCGP is used for dilution of the clean syngas fired in the gas turbine to reduce N Ox emissions and increase electrical output.
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Shell's operational experience with coal gasification started with a 6 t/d pilot plant in Amsterdam, followed by a 150 t/d unit in Harburg, Germany and a third unit in Houston with a capacity of 250-400 t/d. This experience has been the basis for the successful design of the 2000 t/d coal gasification unit of the Demkolec plant.
The basic concepts selected for the Shell Coal Gasification Process are:
• Pressurised: compact equipment;
• Entrained flow: compact gasifier;
• Oxygen blown: compact equipment, high gasification efficiency;
• Membrane wall, slagging gasifier: robustness, high temperature, insulation by slag layer;
• Opposed burners: good mixing, high conversion, scale-up possibility;
• Dry feed of pulverised coal: high gasification efficiency, feed flexibility.
The process can handle a wide variety of coals, ranging from bituminous to lignite, in an environmentally acceptable way and produces a high purity, medium-Btu gas as a fuel for power generation, as a chemical feedstock or as a source of hydrogen.
3.2 SCGP - DESCRIPTION
Raw coal is crushed and fed to a conventional bowl mill, similar to those used in a pulverised coal boiler. This mill grinds the coal to a size range suitable for efficient gasification (90% wt less man 100 microns). As the coal is being ground, it is simultaneously dried utilising a heated inert gas stream that carries the evaporated water from the system as it sweeps the pulverised coal through an internal classifier to collection in a baghouse.
The oxygen required in the SCGP gasification step (95% pure for IGCC) is supplied by an air separation plant.
Other oxygen purities can be used, depending on project premises. Nitrogen from the air separation unit is compressed to provide low-pressure and high-pressure nitrogen for use in the gasification plant, e.g. for transporting coal in the feed system.
Milled and dried coal from the coal milling and drying area is pneumatically transported to the coal pressurisation and feeding system. Pressurised coal, oxygen and, if necessary, steam enter the gasifier through pairs of opposed burners.
The gasifier operates in the range 20 to 40 bar. The gasifier consists of a pressure vessel with a gasification chamber inside. The inner gasifier wall temperature is controlled by circulating water through the membrane wall to generate saturated steam. The membrane wall encloses the gasification zone from which two outlets are provided.
One opening at the bottom of the gasifier is used for the removal of slag. The other outlet allows hot raw gas and fly slag to exit from the top of the gasifier.
Most of the mineral content of the feed coal leaves the gasification zone in the form of molten slag. The high gasifier temperature (over 1500°C) ensures that the molten slag flows freely down the membrane wall into a water-filled compartment at the bottom of the gasifier. High carbon conversions (above 99%) are obtained, and the high temperature ensures that no organic components heavier than methane are in the raw syngas. The insulation provided by the slag layer in the gasifier minimises heat losses, such that cold gas efficiencies are high and C O2 levels in the syngas are low. The recycle of fly slag enhances gasification efficiency.
Flux may be added to the coal feed to promote the appropriate slag flow from the gasifier at the preferred operating temperature. As the molten slag contacts the water bath, the slag solidifies into dense, glassy granules.
The slag is washed, depressurised and then fed to intermediate storage.
The hot raw product gas leaving the gasification zone is quenched with cooled, recycled product gas to convert any entrained molten slag to a hardened solid material prior to entering the syngas cooler. The syngas cooler recovers high-level heat from the quenched raw gas by generating high-pressure steam.
The bulk of the fly slag contained in the raw gas leaving the syngas cooler is removed from the gas using commercially demonstrated equipment such as filters or cyclones. The recovered fly slag can be recycled back to the gasifier via the coal feeding system. The syngas then goes to a scrubbing system, where the remaining traces of solids and water soluble contaminants are removed, and thereafter to an acid gas removal system, where an amine-based solvent, such as Sulfinol, removes typically 99% of the sulphur species.
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A bleed from the scrubbing system is sent to a sour slurry stripper. The water is then clarified and can be recycled to minimise the volume of effluent to be bio-treated and discharged or evaporated. The acid gas from the acid gas removal system and from the sour slurry stripper is fed to a Claus plant, where saleable elemental sulphur is produced. For maximum sulphur recovery and minima! sulphur emissions, the Shell Claus Offgas Treating process (SCOT) is used.
THE SHELL COAL GASIFICATION PROCESS GASIFICATION & GASTREATING
Of the energy in the coal being fed to the gasifier, approximately 80-83% is converted into raw synthesis gas.
Over 99% of the carbon in the coal is converted. With a further 14% to 15% of the heating value of the coal being recovered as steam, the overall thermal efficiency of the Shell Coal Gasification Process is very high. The composition of the syngas is similar for most coals processed, with CO and H2 accounting for over 90% of the syngas on a molar basis.
Sixteen different coals, varying from bituminous to lignite, as well as petroleum coke were successfully run at SCGP-1 in Houston during some 15000 operating hours.
Coals can be switched during operation, with gasifier operating conditions being adjusted as the new coal
"breaks through". The use of automatic process control makes it possible to maintain optimal operating conditions even with variable coal quality.
To accommodate changing power demand from the grid, load-following was thoroughly tested at SCGP-1, including steady-state operation at 50% plant capacity. In response to demand changes, the control system is able to accommodate ramp changes in syngas demand at a rate of 3% per minute between 100% and 50% plant capacity.
3.3 SCGP - ENVIRONMENTAL
Integrated gasification combined cycle power generation based on SCGP has an excellent environmental performance. A priority throughout the development and commercialisation of me process has been the establishment of an environmental data base and also a number of design options for treating and handling effluents.
Gaseous Effluents
A key advantage of using coal gasification for power generation is that the gasification step converts the sulphur in the coal into hydrogen sulphide and some carbonyl sulphide, both of which can be removed down to extremely low levels. The resulting sulphur rich acid gas is normally sent to a Claus unit for conversion into sulphur, a readily saleable product
The nitrogen in me coal is converted to molecular nitrogen, except for small amounts of ammonia and hydrogen cyanide which are completely removed in the syngas clean-up sections.
Particulate removal is done in a dry solids removal section comprising cyclones and filters to produce a syngas that meets particulate specifications of typically 1-5 ppmwt. This is further reduced in subsequent scrubbing and treating steps.
Aqueous Effluent
Process water from SCGP has no detectable amounts of volatile or semi-volatile organics. Biological treatment of the stripped and clarified process water provides oxidation for the small amounts of inorganic nitrogen and sulphur species that remain. Biotreated effluent contains fully oxidised products and very low concentrations of trace metals. Stripped sour water and treated scrubber water can be recycled.
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Toxicity tests have been carried out on samples of biotreated effluent. The results showed no acute toxicity in the undiluted SCGP effluent and no chronic toxicity effects at a modest 3 to 1 dilution of the effluent. A "zero water discharge" configuration is also an option.
Solid By-Products
Depending on the coal and the gasifier operating mode, more than 90% of the ash in the coal ultimately leaves the process as bottom slag. Both bottom slag and fly slag are non-hazardous according to RCRA (Resource Conservation and Recovery Act, USA) requirements.
As part of a solids utilisation program, SCGP slag has been used as a major component in concrete mixtures to make roads, pads and storage bins. Other applications of SCGP slag and fly slag that have been demonstrated are asphalt aggregate, Portland cement kiln feed and light weight aggregate.