64 MAKALE / ARTICLE Additional firing in the heat recovery system is another balance possible technique - to increase flexibility. This is also energy efficient due to combustion at elevated air temperatures. in some processes, recovered low-grade heat is used in a heat pump (absorption ehiller) to produce chilled water for local cooling, further increasing efficiency compared to local, electric chilling. This - the production of heal, power and cooling - is sometimes referred to as 'trigeneration'. treatment facilities, and increased operation and maintenance costs. Diesel engines are able to lake load within 2 or 3 minutes from cold-start. Exceptionally, some generating plants include low speed, marine diesel engines for lower fuel costs, but the initial capital cost is higher. Medium-speed and, increasingly, high-speed units are now the norm, but require a larger number of relatively small units. Despite the widespread use of gas, availability or price TYPES OF PRiME MOVER issues may justify large (>15 MWe per engine) heavy fuel The selection of prime mover is determined by a number of diesel engines in a number of applications. factors, including size of planı, heat-to-power ratio, and fuel availability and cost. The key parameters are compared in Table 1 for three example prime movers. Diesel engines Diesel engines have evolved over a number of years with upgrades in specific power output. They run typically at 500-1800 rpm. Some bum heavy (residual) fuels for economy, albeit at the cost of additional fuel storage and Turnkey 'capex' costs• Operation and maintenance costs* Fuel costs* Lubricating oil Heat rate Sub-15 MWe diesel engine US$640-800/kW . (f:400-500/Kw). Boiler adds only 3-4% of capital cost 1.3 c/kWh (0.8 p/kWh) Fueı based on liquid fuel cost at heat rate below Add lubricating oil cost at manufacturer's stated consumption and local cost 7260 kJ/kWh Gas engines Gas engines have been produced for some time, of two types: O variants of mainstream diesel engines, using distillate oil as a pilot fuel to initiate combustion as a gas/air mixture is fed into the cylinders. Generally, these are also able to run on distillate, as 'dual fuel' engines. This type of engine is rugged but generally requires a constant presence of skilled maintenance staff 15 MWe gas reciprocating engine US$640-800/kW. (f:400-500/Kw). Boiler adds only 34% ofcapital cost 1.1 c/kWh (0.7 p/kWh) Fuel based on gas cost at heat rate below, and an additional 1% for distillate oil as pilot fuel Add lubricating oil cost at manufacturer's stated consumption and local cost 7660 kJ/kWh 30 MWe gas turbine Mean ofsix models: combined cycle US$750/kW (f470kW) 0.3-0.6 C/kWh (0.2-0.4 p/kWh) Assumes base load operation. Higly dependent on gas turbine model and balance of planı Fuel based on gas cost at heat rate below Lubricating oil not significant as a consumable item Mean of six models: Normally represented for liquid fuel as 170 g/bkWh on 42,700 kJ/kg fuel 93420 kJ/kWh (epen cycle), 6830 kJ/kWh (combined cycle) Fuel efficiency Fuel efficiency with heat recovery (CHP model) NOx emissions Heat to process Normal life 49,60% Up to 90% with heat recovery, but lowgrade heat <1000 ppm at 15% oxygen. May require selective catalytic reduction in seme markets, but not yet a standard fitment 0,4 ton/h per MWe installed, typically 8 barg 20-25 years * Cost estimates are based on European installations ENERJi & KOJENERASYON DÜNYASI 47,00% Up to 90% with heat recovery, but low-grade heat 500 mg/m3 at 5% oxygen 0,4 ton/h per MWe installed, typically 8 barg 20-25 years 38,6% epen cycle, 53,7% combined cycle Up to 90% w i th heat recovery <25 ppm. Dry low NOx generally meets prescribed limits 1 ,0 ton/h per MWe installed, typically 15 barg 20-25 years
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