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Prime Movers For Polygeneration Plants
The typical polygeneration systems are based mainly on two types of prime movers: reciprocating heat engines (diesel and NG engines) and gas turbine engines (micro, mini, industrial and aeroderivative turbines). Below is presented a brief comparative analysis of these prime movers, offering a greater electric output per unit of input energy (as compared to steam turbine plants), a higher overall efficiency, and acceptable reliability and availability in the range of 90-95%.
Diesel engines represent the prime mover with the lowest installed cost, very high reliability, minimal maintenance and excellent electric load-following characteristics. However, unless the installations include particulate traps and selective catalytic reduction (SCR) systems, intended to capture the NOx emissions from diesel exhaust, local air quality standards may limit the system’s use to less than 300 hours per year (only for standby purposes). Diesel polygeneration systems are suitable for applications in the unit range of 300 kWe to 30 MWe electrical output and 450 kWth to 24.5 MWth thermal output. These systems require on-site fuel storage.
Learn-burn reciprocating gas engines are characterized by excellent performance and very low NOx emissions (1-1.5 g/kWe-hour). Without exhaust aftertreatment, these engines are suitable for even the most environmentally sensitive areas of the Russian Federation. The installed costs of the gas engine-based cogeneration plants are about one-half that of CHP systems based on gas turbines. Practical polygeneration systems range in unit size from 300 kWe to 15 MWe electrical output and 450 kWth to 14 MWth thermal output.
Gas turbines (from micro and mini to large industrial and aeroderivative engines) have the advantage of greater thermal output per unit of input energy. Although costing considerably more per kWe installed, and having significantly lower electric efficiency than reciprocating engine-based polygeneration systems, the industrial and aeroderivative gas turbines are favored for large industrial cogeneration systems, where high-pressure steam is a required output for industrial processing, or for energy supply systems, where a high (more than 1.8-2.0) thermal-to-electric load ratio is expected. The size of industrial and aeroderivative gas turbines ranges from 1.5 MWe to hundreds of megawatts, however a time required for start-up of large turbines may achieve 15-20 minutes. Despite their lower electric efficiency, micro and mini turbines are often favored for their compact size, low noise, clean operation and where fuel may be of low or variable quality. The size of such turbines ranges from 30 kWe to 1 MWe. Thermal output of gas turbine generator depends mainly on its electric efficiency and exceeds an electric output by a factor of 1.5-2.5. NOx emissions are similar to those of a learn-burn gas engine generators. A fuel efficiency and power output of all gas turbines significantly decrease with increase in ambient air temperature above 15oC and pressure losses in inlet and exhaust ducts. In addition, the turbine performance are markedly deteriorated due to often start-ups and shut-downs. Finally, operation of any gas turbine is impossible without a special compressor increasing a pressure of natural gas upstream of turbine’s combustor(s) up to required 5-30 barA.
Summary and conclusions from prime movers comparative analysis
Comparative analysis of two main prime movers being used in the polygeneration plants revealed the following advantages of the reciprocating engines, as compared to gas turbines:
Much higher electrical efficiency at the full (100%) load
Much less marked decrease in electrical efficiency at the partial loads between 50% and 100% of full load
Keeping the rated (nominal) fuel efficiency and power output at the ambient air temperature up to 27°C and with the higher pressure losses in the inlet and exhaust ducts
Practical independence of engine performance on the frequency of startups and shutdowns
Much shorter time-period (1-3 min) required for engine loading
Markedly lower capital investments due to lesser specific installed cost in the unit power range up to 15-20 MWe and the absence of necessity for installing a special NG compressor
Much longer time-period between the major overhauls
With regard to results of comparative analysis presented above, the MEEC Group - NEWPOLYGEN 's approach to autonomous energy supply is based on preferable use of the reciprocating engines as prime movers for the polygeneration systems. Thereby, gas engine generator is a logical economical choice for the CHP systems, taking account the excellent environmental chracteristics of gas engine.
