7.7 Prime Movers for Cogeneration
7.7.1 Steam Turbine
Steam turbines (Figure 7.8) are the most commonly employed prime movers forcogeneration applications In the steam turbine, the incoming high pressure steam isexpanded to a lower pressure level, converting the thermal energy of high pressure steamto kinetic energy through nozzles and then to mechanical power through rotating blades.
Figure 7.8 Steam Turbine
 Back Pressure turbine
: In this type steam enters the turbine chamber at High Pressureand expands to Low or Medium Pressure. Enthalpy difference is used for generatingpower / work.Depending on the pressure (or temperature) levels at which process steam is required,backpressure steam turbines can have different configurations as shown in Figure
7.9. In extraction and double extraction backpressure turbines, some amount of steam isextracted from the turbine after being expanded to a certain pressure level. The extractedsteam meets the heat demands at pressure levels higher than the exhaust pressure of thesteam turbine.The efficiency of a backpressure steam turbine cogeneration system is the highest. Incases where 100 per cent backpressure exhaust steam is used, the only inefficiencies aregear drive and electric generator losses, and the inefficiency of steam generation.Therefore, with an efficient boiler, the overall thermal efficiency of the system couldreach as much as 90 per cent.
Extraction Condensing turbine
: In this type, steam entering at High / Medium Pressureis extracted at an intermediate pressure in the turbine for process use while the remainingsteam continues to expand and condenses in a surface condenser and work is done till itreaches the Condensing pressure.(vacuum).In Extraction cum Condensing steamturbine as shown in Figure 7.10, highPressure steam enters the turbine andpasses out from the turbine chamber instages. In a two stage extraction cumcondensing turbine MP steam and LPsteam pass out to meet the processneeds. Balance quantity condenses inthe surface condenser. The Energy
Figure 7.10 Extraction condensing turbine
difference is used for generating Power. This configuration meets the heat-powerrequirement of the process.The extraction condensing turbines have higher power to heat ratio in comparison withbackpressure turbines. Although condensing systems need more auxiliary equipment suchas the condenser and cooling towers, better matching of electrical power and heat demandcan be obtained where electricity demand is much higher than the steam demand and theload patterns are highly fluctuating.The overall thermal efficiency of an extraction condensing turbine cogeneration system islower than that of back pressure turbine system, basically because the exhaust heat cannotbe utilized (it is normally lost in the cooling water circuit). However, extractioncondensing cogeneration systems have higher electricity generation efficiencies
7.7.2 Gas Turbine
The fuel is burnt in apressurized combustionchamber using combustionair supplied by a compressorthat is integral with the gasturbine. In conventional Gasturbine (Figure 7.11), gasesenter the turbine at atemperature range of 900 to1000
C and leave at 400 to500
C. The very hotpressurized gases are used toturn a series of turbineblades, and the shaft onwhich they are mounted, toproduce mechanical energy.Residual energy in the form of a high flow of hot exhaust gases can be used to meet,wholly or partly, the thermal (steam) demand of the site. Waste gases are exhausted fromthe turbine at 450
C to 550
C, making the gas turbine particularly suitable for high-gradeheat supply.
Figure 7.11 Gas Turbine
The available mechanical energy can be applied in the following ways:
to produce electricity with a generator (most applications);
to drive pumps, compressors, blowers, etc.A gas turbine operates under exacting conditions of high speed and high temperature.The hot gases supplied to it must therefore be clean (i.e. free of particulates which woulderode the blades) and must contain not more than minimal amounts of contaminants,which would cause corrosion under operating conditions. High-premium fuels aretherefore most often used, particularly natural gas. Distillate oils such as gas oil are alsosuitable, and sets capable of using both are often installed to take advantage of cheaper
interruptible gas tariffs. LPGs and Naphtha are also suitable, LPG being a possible fuelin either gaseous or liquid form.
Gas Turbine Efficiency
Turbine Efficiency is the ratio of actual work output of the turbine to the net input energysupplied in the form of fuel. For stand alone Gas Turbines, without any heat recoverysystem the efficiency will be as low as 35 to 40%. This is attributed to the bladeefficiency of the rotor, leakage through clearance spaces, friction, irreversible turbulenceetc.Since Exhaust gas from the Gas Turbine is high, it is possible to recover energy from thehot gas by a Heat Recovery Steam Generator and use the steam for process.
Net Turbine Efficiency
Above efficiency figures did not include the energy consumed by air compressors, fuelpump and other auxiliaries. Air compressor alone consumes about 50 to 60 % of energygenerated by the turbine. Hence net turbine efficiency, which is the actual energy outputavailable will be less than what has been calculated. In most Gas Turbine plants, aircompressor is an integral part of Turbine plant .
7.7.3 Reciprocating Engine Systems
This system provides process heat or steamfrom engine exhaust. The engine jacketcooling water heat exchanger and lube oilcooler may also be used to provide hot wateror hot air. There are, however, limitedapplications for this.
As these engines can use only fuels like HSD,distillate, residual oils, natural gas, LPG etc.and as they are not economically better thansteam/gas turbine, their use is not widespreadfor co-generation. One more reason for this isthe engine maintenance requirement.
of 4