Steam and Electric Power

Whereas during the past century the chemical industry - unlike power stations -still used medium-pressure systems and only slightly superheated steam, the need for the best possible efficiency has meanwhile forced chemical plants, too, to use steam pressures as high as 120 bar and temperatures up to 500 °C. Such systems are cost-effective above all if the plants consume large amounts of energy, as in the case of coal-based methanol plants, whose total installed output - steam plus electricity - already reaches the order of small power stations. 

Contrary to power stations which derive all the heat they need to generate and superheat steam from combustion, chemical plants often derive a considerable part of their entire thermal demand from process heat Processes whose reactions are strongly exothermic, such as the methanol synthesis reaction, may in some cases produce steam of as much as 50 bar. 

Frequently it will be appropriate to oversize the condensation stages of machines which have to be started up at a time when steam generated from process heat is not yet available, i.e. when the entire steam output has to be provided by fired steam boilers. This reduces the steam rate required to start up the plant and ensures that the steam boiler plant can be dimensioned so that it meets the normal continuous steam demand of the plant. However, it will be desirable as a rule to use bleeding and backpressure turbines which are not only much more cost-effective than condensation turbines, but which also improve the overall energy efficiency considerably owing to the combination of energy, process steam and heating steam. 

The steam boiler plants are preferably fired with sulfur-fiee or low-sulfur fuels from the plant itself, for instance purge gas from methanol synthesis or flash gas from coal gasification and gas scrubbers. Wherever this heat supply is insufficient, it may be supplemented by firing natural gas, if available, or coal. The latter often requires complex desulfurization systems so that in certain cases even the production of low-sulfur fuel gas by coal gasification with air and subsequent desulfurization of the gas may be considered as an alternative to coal-fired systems. 

A system particularly suited K provide both steam and power to a coal-to-methanol complex is the Circulating Fluid Bed Combustion Process together with conventional heat recovery and power generating systems. This technology, developed by LURGI in the seventies, combines high efficiency with low SO2 and NOx emissions. More than 90% of the sulphur contained in the coal is captured in die combustion zone already, and due to low combustion temperature, less than 200 mg of nitrous oxides per m of flue gas can be analysed. Coal burnout is between 98 and 99%, depending on the fuel. 

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Carbon dioxide is used in the manufacture of sodium carbonate by the ammonia-soda process, urea, salicyclic acid (for aspirin), fire extinguishers and aerated water. Lesser amounts are used to transfer heat generated by an atomic reactor to water and so produce steam and electric power, whilst solid carbon dioxide is used as a refrigerant, a mixture of solid carbon dioxide and alcohol providing a good low-temperature bath (195 K) in which reactions can be carried out in the laboratory. 

Alternatively, short-rotation hybrid poplar and selected grasses can be multicropped on an energy plantation in the U.S. Northwest and harvested for conversion to Hquid transportation fuels and cogenerated power for on-site use in a centrally located conversion plant. The salable products are Hquid biofuels and surplus steam and electric power. This type of design may be especially useful for larger land-based systems. 

All process plants require both steam and electric power in their operations. Power is cither purchased from local utility companies or generated at the plant site. Even if power is generated by the process plant, arrangements for standby power from the local utility must be made for emergency purposes. Steam is rarely purchased but is generated at the plant for use in the process and as a driving medium for pumps and compressors. 

Steam and electric power Water treatment HgS manufacture Site facilities... 

Gas cooled reactors use carbon dioxide under pressure as a recirculating heat transfer medium (coolant) between the hot nuclear reactor core and water in a secondary circuit in order to raise steam and electrical power in an otherwise conventional high pressure steam generator/turbine/condenser loop. The role played by ion exchange is denoted by systems A-D in Figure 8.22. 

The coke residue is the result of the hydrogen-deficient stoichiometry of the process. With a proximate analysis (determination of the compounds, moisture, ash, etc., present) of carbon 80%, volatiles 10%, sulfur 6%, and ash 4%, the coke has a fuel value near that of high rank coals. It is burned in the site power plants to provide steam and electrical power for oil sands processing. However, the high sulfur content detracts from its wider utility as a fuel. Any coke in excess of the current fuel requirement is finely powdered and incorporated into the dyke walls to help trap any hydrocarbons present in water seepage through the wall. 

Vacuum distillate undergoes hydrogenation in the VGO hydrogenation unit before being fed to the catalytic cracking (FCC), whilst vacuum residue passes to the visbreaker for further conversion. FCC residue is used as a fuel component in the refinery s own power station, and it supplies the refinery, in return, with steam and electrical power. It includes three oil- and one gas-fired boilers and is designed for 110 MW output. 

Energy self sufficiency The process is energy self-sufficient. There is sufficient energy in the char byproduct to produce all the steam and electric power needs of the process. At larger scale (greater than 300 dt/d), the process produces significant excess power that can be exported to the grid. 

Main distribution systems for water, steam, and electric power should be looped, with block valves (or disconnect switches on electric lines) at appropriate points so that, if any part of the system is damaged, supply can be obtained from another source. 

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