O lighter duty, smaller, spark-ignited gas engines. Spark plug life was a problem in some early examples. With the advent of low-cost electronics, more recent designs have higher specific power outputs and lean-burn tor fuel efficiency. Usage of pilot oil can be as low as 1 %. As with diesel engines; gas engines are able to lake load within 2 or 3 minutes from cold start-up. Where natura! gas is available, a modern reciprocating gas engine is a viable option. The largest reciprocating gas engine in the market develops 17 MWe with 47% fuel efficiency, and up to 90% with heat recovery in CHP mode. However, the recovered heat from the cooling system is relatively low grade (80°C), which is suitable tor district heating but probably not tor industrial or process use. Alternatively, it the waste heat in the exhaust is used in isolation, only 25% of the waste heat (13% of the heat value in the fuel) can be taken as steam, at around 350°C. NOx emissions from reciprocating gas engines are lower than tor diesel engines, as the unit burns with high air-fuel ratios (lean) thereby lowering exhaust temperatures and NOx production. However, NO x emissions are higher than those far gas turbines. Gas turbines Where gas is available and there is a high steam demand, the gas turbine is the preferred choice. Due ta their increased efficiency over open cycle units, combined cycle and CHP plants are used in a wide spectrum of power and heat generation applications in utilities, tood, paper and pulp, and chemical industries. Process steam may be passed out from a condensing steam turbine, exhausted from a back-pressure steam turbine or simply delivered to the plant directly from the heat recovery steam generator (HRSG). The gas turbines in the size range being considered tor industrial-scale CHP are either designed tor industrial use, or are derivatives of proven aero-engines. Each type has its own characteristics and the cost and pertormance properties will be a significant part of a feasibility study. Gas turbines typically have exhaust temperatures of around 500°C, which allows steam to be produced at high pressures and temperatures. Additional steam production is possible by supplementary firing of the HRSG. This alsa allows the steam flow to be varied without changing the gas turbine load. A modified version of duct firing, known as auxiliary firing, allows steam to be raised in the HRSG when the gas turbine is out of service. This may be desirable if security of process steam supplies is critical. MAKALE / ARTICLE Gas turbines require natura! gas to be supplied at higher pressures than tor gas-fired reciprocating engines, so gas compression may be required. This increases costs, parasitic loads and noise emissions of the planı. Emissions from gas turbines are substantially lower than from diesel and gas engines, with guarantees of 25 ppm NOx being available when firing gas. Distillate oil may be burned in most gas turbines. However, loca! legislation may require water injection to control NOx, which may need additional water treatment to be employed. Water injection increases operating and maintenance costs and reduces efficiency. However, recent trends in gas turbine design indicate that next generation machines will include dry firing of liquid fuel. Where gas is not available, oil firing may be feasible, but heavy fuel oil should not be considered tor aero-derivative gas turbines. This 200/240 MWe CHP plant serves a paper mil/ in the UK. Plant comprises two GE Frame 6FA gas turbines, each with a heat recovery steam generator (HRSG) and exhaust stack, and one condensinglpass-out steam turbine. The gas and steam turbines drive 100 MVA generators. Up to 136 ton/h of process steam interfaces with the adjacent paper mili via a tubular heat exchanger. The photo shows one gas turbine with its HRSG, and one of the two stacks. lnstalled price, longer start-up times and gas compression costs are such that the gas turbine has not entirely replaced the reciprocating engine option. Gas turbines are available with higher unit output than diesel engines, thereby requiring fewer machines tor a given duty, but giving reduced flexibility and reduced efficiency at low loads. in combined cycle mode, there would be a more significant impact it one unit of, say 30 MWe were to trip than would be the case with, say a 15 MWe diesel or gas reciprocating planı. lf a wide variation in load is not envisaged, the gas turbine fuel efficiency variation with load is not an issue. ENERJi & KOJENERASYON OÜNYASI 65
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