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The following measures are also typically taken during major outages to guarantee the continued integrity of the HRSG through its design life (which may range between 15 and 30 years): - • Sample header / drum internal inspections for corrosion/debris, thermal fatigue, blockage and/or FAC of orifices. • Measurement of creep pips (header diametrical measurements). • Sample header end-cap and butt weld inspections. • Pipework butt-weld inspections. • Tube sampling/thickness checks (for flow accelerated corrosion and offload corrosion). • Valve casting inspections. 3.9 Industrial HRSG Applications HRSG applications are more diverse at the industrial scale and can be broadly classified as below. 3.9.1 Industrial Gas Turbine HRSGs At the small scale, gas turbines may be used in smaller CHP schemes or to provide shaft power e.g. for pumping stations on gas pipelines. In CHP schemes the demand is usually for process steam (e.g. for enhanced oil recovery) and the generation of electricity by using a GT to burn the fuel and generate electricity rather than just burning it in a boiler is an economic bonus. Because the provision of steam to the process is usually the paramount concern, an auxiliary burner is usually fitted to allow continuation of boiler operation even when the GT has tripped. In this case a fresh air inlet duct is needed. Units may also be installed for marine use in gas turbine driven ships, floating production storage and offloading vessels (FPSO) and offshore platforms. GT based CHP schemes typically achieve an electrical efficiency of around 23% (GCV) and a heat efficiency of around 49% (GCV) [7] . In recent years a new breed of microturbines has been introduced, based on turbocharger rather than aero-derivative technology. These are usually in the range up to 0.5MWe, at which scale the aero-derivative type becomes more economic. At present a typical microturbine unit from Bowman Power Systems has an output of 80kWe and a thermal output of 130 – 260 kWth in an exhaust gas stream at a temperature of around 600°C [58] . Most units installed so far have recovered heat as hot water, but in some specialised applications steam has been generated. 3.9.2 Reciprocating Engine Exhaust Gas Boilers Internal combustion engines may be used on a small scale for electricity generation and HRSGs may be added to run in combined heat and power mode. Low grade heat is usually recovered as hot water from the engine cooling circuit. Higher grade heat may be recovered from the exhaust as steam. Normally smoke tube design package units are used to generate saturated steam. Engines may be run on liquid or gaseous fuels. Increasingly, alternative fuel sources are being used, such as landfill gas, coal mine methane (75)
and bio-gas from anaerobic digestion of sewage sludges, agricultural wastes and industrial wastes. Reciprocating engine based CHP schemes typically achieve an electrical efficiency of around 27% (GCV) and a heat efficiency of around 42% (GCV) [7] . 3.9.3 Heat Recovery from Other Industrial Exhaust Gases HRSGs are used to recover energy from the hot exhaust of other industrial processes such as: - • Glass and metallurgical furnaces • Kilns (e.g. sponge iron plants): Coal based high temperature reduction of iron ore produces a flue gas with a temperature in the range of 1000- 1200°C and a dust load as high as 40gNm -3 . The first stage of the heat recovery system is a radiant section with water membrane panel walls where the gas is cooled to around 750°C to reduce the risk of slag deposition on downstream heat transfer surfaces. This is followed by a plain tube superheater fitted with soot blowers and evaporator and economiser sections. • Roaster based plants: a typical application is in roasting of pyrite ores. Pyrite ores are oxidised in a fluidised bed to produce the oxide required for manufacture of the primary metal. The flue gases contain sulphur dioxide and are at a temperature of around 900°C with a high dust load. The Thermax design [59] uses a water tube boiler to recover heat. A vertical tube alignment and a wide tube pitch are used to minimise problems of dust deposition. A hammering device dislodges dust from the tubes into hoppers below from which it is continuously removed. • Smelters and converters: for example in copper and zinc smelting. The high temperature waste gas has a very high dust load. The heat is recovered in two stages. In the first stage the gas passes through a large water membrane walled radiant section where some of the dust and slag is allowed to settle. The partially cleaned and cooled gas then passes through a conventional convection section with vertical bare tubes, again fitted with hammering devices to dislodge dust [59] . • Coke ovens • Solid / liquid / gas waste incinerators • VOC thermal oxidisers 3.9.4 Process Integrated HRSGs Many process industries use HRSGs to recover heat from the necessary cooling of process gases. Industries include: • Petrochemicals (e.g. in sulphur recovery units) • Sulphuric acid plants: The double conversion double absorption process for the manufacture of sulphuric acid from elemental sulphur generates considerable heat in the exothermic conversion of sulphur to SO2 and SO 3 gases. The optimum working temperature for the V2O5 catalyst is about 440°C, so it is essential to have a process integrated HRSG in the system to cool the gas to this temperature. (76)
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Page 45 and 46: Detailed studies of CCTs within the
Page 47 and 48: knowledge, expertise and experience
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