part1ch2 state of art review of cogeneration

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State of art review of cogeneration 13

CHAPTER 2: STATE OF ART REVIEW OF COGENERATION

2.1 Technological Advances in Cogeneration

Cogeneration plants benefit from many of the energy efficiency improvements that arebrought about in utility power generation because the same basic technology is employed inboth cases. However, cogeneration being more attractive for small-scale decentralizedapplications, significant technological progress has been made in the development of modular and packaged cogeneration systems of lower capacities. Moreover, as suchsystems are being adopted in industrial zones and city centres, the stringent laws andregulations put in place for protecting the local environment has obliged the cogenerationtechnology providers to innovate incessantly. The greater availability of natural gas in manyparts of the world has helped in the maturing of gas turbine technology. In addition, the

possibility of using alternative fuels such as wood, agro-industrial residues, biogas, etc., for powering small-scale cogeneration systems has led to further technological progresses bytaking the specific characteristics of the fuels into consideration. This section brieflydescribes some of the developments in this domain.

2.2 Reciprocating Engines

Reciprocating engines are mostly employed in low and medium power cogeneration units.The lower and upper limits of engine sizes are often a function of the fuel in use; these canrange from 50 kW to 10 MW for natural gas, from 50 kW to 50 MW for diesel, and 2.5 MW to50 MW for heavy fuel oil. One of the major advantages of reciprocating engines is their higher electrical efficiency as compared to other prime movers.

The two main types of internal combustion engines employed in cogeneration systems arediesel engines and Otto engines. The characteristic feature of the Otto engine is that anelectric spark from a spark plug ignites a mixture of fuel and air, and this is thus known widelyas a spark-ignition engine. In power generation applications, the Otto engine may be either agasoline engine or a diesel engine converted to have spark-ignition operation. Gasolineengines have the ratings ranging from 20 kW to 1.5 MW. The spark-ignition enginesconverted from diesel engines and running on natural gas are available in ratings from 5 kWto 4 MW. The Otto engines operate at speeds between 750-3,000 rpm and have the electricalefficiencies of 25-35 per cent. These engines can run on different fuels such as gasoline,natural gas, producer gas, and digester gas.

As opposed to Otto engines, fuel is injected into the diesel engine cylinders in which it mixeswith air and is ignited by the heat generated when the pistons compress the fuel/air mixture,and this engine is often known as a compression-ignition engine. Diesel engines cangenerally be classified into two main categories, i.e. two-stroke and four-stroke engines. Thetwo-stroke engine is also known as a low-speed engine, and is characterized by ignitiontaking place once every revolution, and by the engine running at a speed below 200 rpm anddelivering an output of 1-50 MW at a high electrical efficiency of 45-53 per cent. In a four-

strike engine , ignition takes place during every other revolution, and this engine can bedivided into two categories. Medium speed engines are those running at speeds between400 and 1,000 rpm and can be designed for ratings between 0.5 and 20 MW with electricalefficiencies of 35-48 per cent. High-speed engines are those operating at speeds between

1,000 and 2,000 rpm and with ratings between a few kW and about 2 MW with electricalefficiencies of 35-40 per cent.



14 Part I: Overview of cogeneration and its status in Asia

Diesel engines can run on a variety of fuels such as diesel, heavy fuel oil, light fuel oil, LPG,natural gas, producer gas, digester gas, etc. The diesel engines that are converted to gasengines are also known as dual-fuel engines. In their operation, the main fuel is gas, which isignited by a small quantity of pilot oil, usually diesel oil. The pilot oil is used to make sure thatthe gas in the cylinder will ignite. The gas/oil ratio is normally controlled so that the proportionof pilot oil at full engine power will be around 5 per cent of the fuel quantity supplied. Dieselengines running in gas engine mode can be classified in another way into two groups: low-pressure dual-fuel engines and high-pressure dual-fuel engines.

Typical heat balance diagram of a gas engine is shown in Figure 2.1. About 25 per cent of theheat recovered from the engine cooling system (cooling water, oil cooler and inlet air cooler)is low grade at a temperature of about 95°C. Considering the same power output, the amountof heat recoverable at high temperature is lower than that for the gas turbine. That is whycogeneration with reciprocating engine is more commonly used for producing hot water/hotair or low pressure steam. However, medium pressure steam can be generated byemploying supplementary firing since exhaust gases from gas engines have an O2 content of

about 15 per cent.

62%

Rad ia t i onlosses

3 6 . 5% 4 9 . 5 %

100 %

M ec ha n i c a l T h e r m a l

38%

5 %

7.5%

Exhaus t gaslosses

1.5%

G e n e r a t o r

losses

Elect r ica l T h e r m a l

3 6 . 5 % 25 . 0 % 24 . 5 %

E x h a u s tga s

EngineCoo l i ng

S y s t e m

Overa l l e f f ic iency

86%

Figure 2.1 Typical heat balance of a gas engine

In the operation of low -pressure dual-fuel engines , gas at low pressure, i.e. 3-5 bar, ismixed with the engine combustion air during the induction cycle. The gas/combustion air mixture is compressed in the cylinder and is ignited at the top dead centre by a small amount(approximately 5 per cent) of diesel oil being injected into the cylinder and ignited in the usualmanner. Low-pressure dual-fuel engines have relatively low ratings and efficiencies. Thesystem is sensitive to variations in gas quality.

Gas is compressed outside the engine in a separate compressor in a high -pressure dual-

fuel engine up to 250 bar and is injected into the cylinder with a minor amount of pilot oilwhen the piston is in the vicinity of the top dead centre. High-pressure dual-fuel engines have




higher ratings and efficiencies and they are not sensitive to the gas quality. High-pressuredual-fuel engines are available in both two-stroke and four-stroke versions.

2.3 Gas Turbines

Gas turbines used for cogeneration are usually designed for continuous duty because gasturbines for stand-by use normally have low efficiencies and are most suitable for applications where the operating periods are short.

Gas turbines for continuous duty are traditionally divided into two groups on the basis of differences in design philosophy (there is now some convergence in their design).

The aero-derivative gas turbine, as its name indicates, is more or less derived from anaircraft propulsion engine. The characteristics of aero-derivative gas turbines are low specificweight, low fuel consumption, high reliability, etc. The major advantages of aero-derivativegas turbines are high levels of efficiency and a compact and modular design with easy

access for maintenance. However, because skilled service personnel are required, gasturbines of this type are often taken off the site for maintenance. Aero-derivative gas turbinesrequire a relatively high specific investment cost ($/kWe), high quality fuel and mayexperience a lowering in output and efficiency after a long period of operation.

The industrial gas turbine, also referred to as the heavy duty or heavy frame gas turbine, is arobust unit constructed for stationary duty and continuous operation. It has a somewhat lower efficiency than the aero-derivative type, but usually maintains its performance over a longer period of operation. Maintenance can be easily carried out on site, and maintenance costsare low. The industrial gas turbine usually has a lower specific investment cost than its aero-derivative counterpart. Furthermore, it has the ability to make use of low quality fuel.

The performance of a gas turbine depends on the pressure and temperature of ambient air that is compressed. Since the ambient conditions vary from day-to-day and from location-to-location, it is convenient to consider some standard conditions for comparative purposes.The standard conditions used by the gas turbine industry are 15°C, 1.013 bar (14.7 psia) and60 per cent relative humidity, which are established by the International StandardsOrganization (ISO). The performance of gas turbines is expressed under ISO conditions.

The actual power output of a gas turbine varies with ambient conditions. The power output of a gas turbine decreases when the ambient temperature rises. In contrast, the power outputincreases with the ambient pressure. The variations in power outputs of a typical gas turbinewith ambient conditions are shown in Figure 2.2 as a percentage of ISO power output.

The heat recovery steam generator (HRSG) is one of the major components of the gasturbine cogeneration system. Since the energy content of the exhaust gas rejected to theatmosphere is considerably high, HRSGs are designed to produce process steam (or hotwater) by recovering a large share of the energy contained in the exhaust stream. Theexhaust gas at 500-550°C is cooled in the HRSG to about 150°C to extract useful heat. Atemperature of 150°C is recommended at the outlet of the HRSG to avoid condensation of exhaust gases. At lower temperature levels, gases such as SOx and NOx would form acidsalong with the condensation and corrode the materials of HRSG.




60

70

8090

100

110

120

10 11 12 13 14 15

Ambient Pressure (psia)

60

70

80

90

100

110

120

-5 5 15 25 35

Ambient Temperature (°C)

% o

f I S O P

o w e r O u t p u t

% o

f I S O P o

w e r O u t p u t

Figure 2.2 Power output variation of a gas turbine with the ambient conditions

The basic heat-to-power ratio of a simple gas turbine cogeneration system is about two.

However, supplementary firing can double the heat-to-power ratio. The HRSG withsupplementary firing option contains an additional burner to increase the heat output of thewhole system. This is made possible due to the high oxygen content of the exhaust gases,typically 14 to 17 per cent, as a result of the need for high excess air in the combustionchamber (for avoiding very high hot gas temperature that can affect the turbine). By addingsupplemental firing, fuel consumption increases slightly, however the steam productionincreases significantly. Addition of supplemental firing is quite common in gas turbinecogeneration systems.

In a gas turbine cogeneration cycle, the power output can be increased by steam injection.High-pressure steam produced in HRSG can be injected into the combustion chamber sothat the mass flow rate through the turbine is increased. Steam injection allows the flexibility

of matching with the process steam demand and can increase the power output by about 15per cent.

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

ISO Power Output (MW)

E l e c t r i c a l E f f i c i e n c y ( % )

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

ISO Power Output (MW)

E l e c t r i c a l E f f i c i e n c y ( % )

(i) Aero-Derivative (ii) Industrial

Figure 2.3 Power generation efficiency ranges of gas turbines

The power generation efficiency ranges of aero-derivative and industrial gas turbines arecompared in Figure 2.3. The overall efficiency of the gas turbine cogeneration system is goodwithout post-combustion (70 to 85 per cent), which can be further boosted to between 83 and

89 per cent with post-combustion. When the system is opted as a retrofit in a facility alreadyhaving boilers, it is at times possible to make use of the existing boilers.




Recuperators are used to increase the power output of gas turbine cogeneration systems if the heat demands are low. The recuperator is in fact only a heat exchanger that is employedto heat the air leaving the compressor. The exhaust stream from the turbine is passedthrough the recuperator before going into the HRSG so that a part of the energy contained inturbine exhaust is utilized in the recuperator. The gas turbine cogeneration system withrecuperator is sometimes known as the heat exchange cycle.

2.4 Steam Turbines

Steam turbines are the most commonly employed prime movers for cogenerationapplications, particularly in industries and for district heating. The technology is well proven insugar and paper mills having demand for both electricity and large quantity of steam at highand low pressures. Some steam turbine manufacturers are over 100 years old and haveproducts ranging from a few kW to 80 MW. However, turbines below two MW may beuneconomical except where the fuel has no commercial value.

A cogeneration system using a backpressure steam turbine (see Figure 1.2) consists of boiler, turbine, heat exchanger and pump. In the steam turbine, the incoming high pressuresteam is expanded to a lower pressure level, converting the thermal energy of high pressuresteam to kinetic energy through nozzles and then to mechanical power through rotatingblades. Thermal energy of the turbine exhaust steam is then transferred to another fluid,water, air, etc., in a heat exchanger, providing heat to the processes. For instance, the air heated by heat exchanger can be used to dry products in food processing industries.

Depending on the pressure (or temperature) levels at which process steam is required,backpressure steam turbines can have different configurations. The most common types of backpressure steam turbines are shown in Figure 2.4. In extraction and double extractionbackpressure turbines, some amount of steam is extracted from the turbine after beingexpanded to a certain pressure level. The extracted steam meets the heat demands atpressure levels higher than the exhaust pressure of the steam turbine.

The backpressure steam turbine has a higher heat to power ratio and higher overallefficiency. Furthermore, back pressure turbine cogeneration systems need less auxiliaryequipment than condensing systems, leading to lower initial investment costs.

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 the

load patterns are highly fluctuating.

In the reheat cycle, steam is extracted from the turbine and reheated in the boiler during theexpansion process. Reheat cycles improve the overall thermal efficiency and eliminate anymoisture that may form as the steam pressure and temperature are lowered in the turbine.Steam turbines may also include a regenerative cycle where the steam is extracted from theturbine and used to preheat the boiler feedwater.

The efficiency of a backpressure steam turbine cogeneration system is the highest. In caseswhere 100 per cent backpressure exhaust steam is used, the only inefficiencies are gear drive and electric generator losses, and the inefficiency of steam generation. Therefore, withan efficient boiler, the overall thermal efficiency of the system could reach as much as 90 per

cent.




High pressure steam Extracted steam Exhaust steam

(i) Simple backpressure (ii) Extraction backpressure (iii) Double extraction

backpressure

Figure 2.4 Different configurations for back pressure steam turbines

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, extraction condensingcogeneration systems have higher electricity generation efficiencies.

The techniques available for energy generation from fossil fuels are well established. In order

to make greater use of alternative fuels, efforts have been made to take the specificity of fuelcharacteristics into account in order to overcome the technological constraints. The physicalproperties of agro-industrial residues vary considerably and can affect the conversionefficiency. Some areas where technological progresses have been made include fuelhandling, combustion system and pollution abatement equipment.

Fuel handling and transformation is important for appropriate functioning of the installation.Handling biomass residues depends mainly on the fuel granulometry and moisture content.Coarse residues can be transformed into homogeneous mass by crushing and chipping.Reduction of the moisture content by drying represents two main advantages: increases inthe fuel heating value, and decrease in the fuel losses through fermentation during storage.Suitable technologies are available in the market to cover the handling, drying and storage

requirements of different types of fuels.

The selection of combustion system using alternative fuels depends on parameters such asthe size of the unit, energy required, fuel characteristics, etc. Though grate-fired systems(Dutch-oven type or spreader-stokers) have been widely used because of the flexibility theyoffer, suspension burners and fluidized-bed combustors are emerging as relevanttechnologies because of their high conversion efficiencies and improved performance inmeeting the environmental constraints. In suspension burners, ash is dragged out with theexhaust gases or it falls to the furnace bottom. Fluidized-bed combustors control thecombustion better and make use of an inert material capable of absorbing energy, thusmaximizing the heat transfer from the fuel. These units are capable of burning fuels with verylow calorific values. Modern designs of furnaces offer staging combustion and good control of air-fuel ratio.




2.5 Trigeneration and Vapour Absorption Cooling

Trigeneration is the concept of deriving three different forms of energy from the primaryenergy source, namely, heating, cooling and power generation. Also referred to as CHCP

(combined heating, cooling and power generation), this option allows having greater operational flexibility at sites with demand for energy in the form of heating as well as cooling.This is particularly relevant in tropical countries where buildings need to be air-conditionedand many industries require process cooling. A typical trigeneration facility consists of acogeneration plant, and a vapour absorption chiller which produces cooling by making use of some of the heat recovered from the cogeneration system (see Figure 2.5).

Generator

HRSG

SteamTurbine

Steam

ELECTRICITY

Steam

Generator

FUEL

HeatExchanger

HotGases

Chiller

AIR

GE Frame 6Gas Turbine

CHILLEDWATER

HOTWATER

STEAM

Figure 2.5 Schematic presentation of a gas turbine based trigeneration facility

Although cooling can be provided by conventional vapour compression chillers driven byelectricity, low quality heat (i.e. low temperature, low pressure) exhausted from thecogeneration plant can drive the absorption chillers so that the overall primary energyconsumption is reduced. Absorption chillers have recently gained widespread acceptancedue to their capability of not only integrating with cogeneration systems but also because they

can operate with industrial waste heat streams. The benefit of power generation andabsorption cooling can be realized through the following example that compares it with apower generation system with conventional vapour compression system.

A factory needs 1 MW of electricity and 500 refrigeration tons (RT)1. Let us first consider thegas turbine that generates electricity required for the processes as well as the conventionalvapour compression chiller. Assuming an electricity demand of 0.65 kW/RT, thecompression chiller needs 325 kW of electricity to obtain 500 RT of cooling. Hence, a total of 1325 kW of electricity must be provided to this factory. If the gas turbine efficiency has anefficiency of 30 per cent, primary energy consumption would be 4417 kW. A schematicdiagram of the system is shown in Figure 2.6.

1 Refrigeration ton (RT) is defined as the transfer of heat at the rate of 3.52 kW, which is roughly therate of cooling obtained by melting ice at the rate of one ton per day.

ELEC-TRICITY

Gas Turbine




Figure 2.6 Schematic diagram of power generation and cooling with electricity

However, a cogeneration system with an absorption chiller can provide the same energyservice (power and cooling) by consuming only 3,333 kW of primary energy. A schematicdiagram of the system is shown in Figure 2.7.

Figure 2.7 Schematic diagram of power generation and absorption cooling

It can be seen that the cogeneration system incorporating an absorption chiller can saveabout 24.5 per cent of primary energy in comparison with the power generation system andvapour compression chiller. Furthermore, a smaller prime mover leads to not only lower capital cost but also less standby charge during the system breakdown because steamneeded for the chiller can still be generated by auxiliary firing of the waste heat boiler.

Since many industries and commercial buildings in tropical countries need combined power and heating/cooling, the cogeneration systems with absorption cooling have very high

potentials for industrial and commercial application.

1000 kW

Fuel Input

325 kW

1325 kW

Process

4417 kW

Gas Turbine

Generator Compression

Chiller 500 RT Cooling

Fuel Input Generator

3,333 kW Gas Turbine 500 RT Cooling

2.25 Tons/hr of Steam AbsorptionRecovery Chiller Boiler

Exhaust Heat

1,000 kW

Process




2.6 Working Principle of Absorption Chillers

Like the vapour compression chiller (VCC), the vapour absorption chiller (VAC) extracts heatin the evaporator which is placed in the space to be cooled and rejects this heat in thecondenser. However, VAC needs a heat source as the driving force while VCC requiresmechanical power or electricity for the same duty. Figure 2.8 shows the schematic diagramsof VCC and VAC.

Figure 2.8 Comparison between vapour compression and absorption cycles

The improved version of the VAC, commonly known as the double effect type, is designedsuch that it utilizes the vaporized refrigerant as an extra heat source. The generator is dividedinto high and low temperature sections. The refrigerant vapour produced in the hightemperature generator gives up its latent heat to the partially refrigerant-rich solution in thelow temperature generator that operates at a low pressure, hence the lower boiling point of the refrigerant. The energy consumption of a double effect VAC is approximately half that of the single effect VAC for the same cooling effect. Moreover, heat rejected in the condenser isalso reduced, resulting in smaller condenser and cooling tower.

The performances of absorption chillers strongly depend on the thermo-physical properties of

the working pair, i.e., the refrigerant and absorbent. Binary working pairs such as ammonia-water (NH3-H2O) and lithium bromide-water (LiBr-H2O) have been employed commercially inabsorption chillers for a long time and these are in commercial use. A single effect LIBr-H2Oabsorption chiller requires about 0.8 m3/h of hot water at around 90ºC or 8.3 kg/h of steam at1.5 bar to provide 1 RT. On the other hand, a double effect chiller requires only 4.5 kg/h of steam, though at a higher pressure between 6 and 8 bar.

2.7 District Heating/Cooling Network

Individual buildings and industries may lack economies of scale when setting up cogenerationfacilities and it may not be always possible to optimize the design parameters due to thepeculiarity of the energy demand patterns. In such cases, one may think of developing afacility that caters to several user-groups with varying demand patterns that can becomplimentary. In the building sector, for instance, offices are active during the daytime

High PressureHigh Pressure Vapour

Vapour Refrigerant Refrigerant

Heat

Mechanical InputHeat

Power/Electricity Exchanger Vapour

Compressor

Low Pressure Low PressureVapour Refrigerant Vapour Refrigerant

(i) Vapour Compression Chiller (ii) Vapour Absorption Chiller

Condenser Condenser Generator

Evaporator Evaporator Absorber




whereas hotels may have high loads at nights. When the two loads are combined, a uniformcomposite curve may be obtained with very small amplitude.

Besides, there are a number of justifications for grouping together several buildings andindustries in order to meet their different energy services, such as:

- larger cogeneration system and the economies of scale associated with it;

- system expansion to users for whom individual facility cannot be justified;

- improvement in the overall generation efficiency;

- increased reliability and availability of utility services;

- pooling of maintenance personnel and reduction in manpower cost;

- saving of mechanical room space in the user buildings;

- purchase of fuel at more competitive rate;

- better negotiation power for power purchase/sale to the electric utility, etc.

There are, however, a few drawbacks to district heating/cooling, the most important amongthem being the high initial investment on the system. The cost of steam/hot water and chilledwater transportation and distribution can also be high. Because of the down-sizing of thedifferent components installed at the central plant, capital investment cost can in fact bereduced by 10 to 20 per cent as compared to those which would have been required in theindividual buildings. This takes into account the piping distribution network cost that is notrequired in conventional decentralized systems. For instance, a district cooling network isinstalled in Paris which includes three chiller plants with a total of 25,500 RT to supply to amuseum, shopping complex, exhibition centre and offices having a total equivalent areaexceeding one million m2. Decentralized plants would have required a total capacity of approximately 34,100 RT to be installed. The district-cooling network has thus helped toachieve an investment saving of over US$ 8 million for the reduced installed cooling capacity.2

2.8 Evolution of Package Cogeneration

Cogeneration systems traditionally constituted various components which were ordered andassembled at the site according to the client’s requirements, mostly matching the thermalenergy needs. The minimum power generation capacity was of the order of a few MW due tothe limited products available in the market, some of the reasons being:

1) Investment cost per kWe is considerably higher for smaller units;

2) Limited financing capabilities of small and medium scale enterprises;

3) Additional investment needed by smaller units to cope with environmental regulations;

4) Unavailability of guarantee for the overall system.

2 R. Caillaud, “District cooling with thermal storage for shifting power loads in south-east Asia”, APECDemand-side Management Inter-Utility Liaison Group Meeting, Chiang Mai, 26-29 March 1996.




However, trends have changed considerably with the introduction of modular concept whichconsists of cogeneration units packaged as “of-the-shelf" products and whoseperformances, both electrical and thermal, are guaranteed by suppliers who act as the soleresponsible for the design of the overall system and all its interfaces. This has led towidespread propagation of cogeneration plants with power generating capacities less than aMW. Many of these adopted by enterprises that are located at the end of electric networksand are faced with the problem of getting reliable and uninterrupted power. Moreover, theexpansion of the natural gas network has made it possible to employ gas engines of smaller capacities in urban areas without violating the environmental regulations. For example, over 2,500 units have been installed in the Netherlands alone in the range between 100 and 300kW, the main clients being hospitals, community buildings, sports centres, teachingestablishments, commercial buildings, small and medium enterprises, etc.

A typical module of less than one MWe capacity presents itself as a mono-bloc, compact andsoundproofed packaged unit, consisting of the following:

−

engine for mechanical energy generation;

− alternator for electrical output;

− heat recovery unit for thermal energy generation;

− component for evacuation of combustion products;

− control system, electrical protection and low voltage connection box;

− soundproofing insulation.

These modules are designed for being installed within a few days with very little structural or engineering work at the site. Moreover, as the components are well matched, high efficiencyis guaranteed for the overall system. Some of these cogeneration facilities are designed for “trigeneration” at sites with process or space cooling needs.

The strength of the package units lies with their high overall efficiency and system availability.Manufacturers propose cogeneration systems whose overall efficiency can be between 84and 92 per cent (with a mechanical efficiency between 30 and 35 per cent) and 95 per centavailability. Variations in their performances are a function of the type of prime mover, thelevel at which heat is required, and the quality of heat recovery devices.

The package cogeneration plants are well suited for intermittent operations and variable

loads. The nominal power can be delivered within a few seconds after starting (typically 90seconds) and the loading can be modulated between 50 and 100 per cent without muchreduction in the efficiency. When supplied in soundproof casing, the unit may limit the noiselevel to only 65 dB at a metre.

The supplier defines a well-defined maintenance schedule to guarantee long-term operationwithout unscheduled breakdowns. Use of the same core prime mover for numerousapplications allows to have improved availability of the spare parts at a lower cost. A wellmaintained package cogeneration unit can have a life span of over 60,000 hours. Themaintenance cost on small size engine-based units still remains relatively high comparedwith units with capacities exceeding 600 kW.




2.9 Innovation in Exhaust Gas Heat Recovery

Sites requiring more thermal energy than that is available at the exhaust of reciprocating

engine or gas turbine have the option of adopting post-combustion of oxygen-rich exhaustgases. For this, either the fuel required by the prime mover or an alternate cheaper fuel maybe employed. New types of burners have been designed in the recent years that can beoperated efficiently to provide the varying thermal energy demand of the site.

The “GRC Induct” type of burners has been specially designed by EGCI Pillard for combustion of either liquid or gaseous fuels, by making use of the gas turbine exhaust gas(leaving at around 500°C and 13 per cent of O2 content) as the oxidizing air. Located at theinlet of the heat recovery boiler, it helps to increase the temperature of the gas turbineexhaust gas, and thus the overall efficiency of the cogeneration installation. In case the gasturbine is out of operation, these burners can assure steam generation by making use of coldinlet air from the surrounding. The heat output per burner can range from 4 to 50 MW.

These burners function equally well on natural gas as well as liquid fuels (light or heavy fueloil, residual fuel) or in simultaneous mixed mode. Steam or compressed air assurespulverization of the liquid fuel. The design based on the GRC LONO x FLAM technology,assures perfect flame stability, a low-pressure drop and an excellent combustion with lowemissions of unburnts and NOx, thus well within the environmental pollution thresholds set bythe regulation. When there is a combustion zone in the boiler, it is possible to reduce theoxygen level in the exhaust gas to around three to four per cent for further increasing theefficiency, while still maintaining the emission of pollutants lower than the norms.3

For its operation with cold ambient air, the control flaps close a part of the recovery section.While using heavy fuel oil, a suitable adaptation is necessary for limiting emissions. One of

the main features of the system is the mechanism for quick dismantling which allows tochange the burners during operation by opening the whole frame laterally within 15 minutes.

2.10 Research and Development on Cogeneration Technologies

There has been a steady rise in the efficiency of gas turbines and diesel engines. The inlettemperature of a large size gas turbine has risen to 1,350ºC and can be expected to reach1,500ºC in the near future. The thermal efficiency of gas engines has been increasing thanksto an increase in compression ratio, and the application of pre-chamber lean burntechnologies. These improvements have been made possible mainly due to the progressesmade in cooling, heat-resist materials, turbo machinery and combustion technologies.

Various projects are ongoing to achieve rapid efficiency improvements by the year 2000.4

These include development of ceramic gas engine and gas turbine that require advancedtechnology related to ceramic science. To prove the concept, the Miller cycle gas enginesystem is being developed which has a unique intake and exhaust timing mechanism thatallows to power generation efficiency exceeding 35 per cent.

In a ceramic gas engine, ceramic is used as the materials of the combustion chamber toallow an advanced combustion. Similar to a thermos structure, air gap is provided andgaskets with low thermal conductivity are placed between the ceramic and metallic parts to

3 Energie Plus, “Lumières et ombres sur la cogénération, No.197, pp. 6, 15 December 1997.4 M. Motokawa, “R&D efforts for cogeneration technologies with high efficiency”, Proceedings of theConference on Natural Gas Technologies: A Driving Force for Market Development, International Energy

Agency, pp. 627-636, Berlin, 1-4 September 1996.




enhance the effect of insulation. The wall temperature of the combustion chamber ismaintained above 1,000ºC, which helps to reduce the heat transfer from the combustion gasto the wall. Such a structure eliminates the need for a cooling system and renders the enginevery compact. High efficiency is achieved by both diesel cycle combustion and the energyrecovery unit where exhaust energy from the heat insulation is recovered and converted intoelectricity by a turbo compound system, an ultra high speed generator, and a highly efficientconverter. As for the ceramic gas turbine, the target is to develop units having efficiencies of 42 per cent or more.

The thermal efficiency of an Otto cycle engine is a function of the difference between themaximum combustion temperature and the exhaust gas temperature. The maximumcombustion temperature in an engine increases with a higher compression ratio while theexhaust gas temperature decreases with a lower expansion ratio. But the compression andexpansion ratios of an Otto cycle engine are the same and the engine is adjusted for a lower compression ratio to avoid knocking. In a Miller cycle, the expansion ratio can be set larger than the compression ratio by adjusting the intake timing, and this results in an improved

efficiency as well as improved durability due to the lower exhaust temperature.

The gas injection diesel engine can now attain an electrical efficiency of 45 per cent, which isthe highest among commercialized gas engines. The engine no longer requires pilot oil andglow plugs be used to ignite natural gas ignited into the cylinder at 25 MPa.

R&D efforts are also on going to develop solid oxide fuel cells to exploit the excellentproperties of ceramic materials and achieve efficiencies in the range of 50 per cent. Oncethese technologies are commercialized, cogeneration promotion can get a further boost asan energy saving and environmentally sound technology.

2.11 Cogeneration and the Environment

The high efficiency of cogeneration and efficient use of fuel guarantee a significant reductionof CO2 emission. However, cogeneration can have environmental implications in the form of CO, SO2 and NOx emissions to the atmosphere. The quantity of each of the pollutantgenerated depends largely on the type of fuel used and the characteristics of thecogeneration technology adopted.

CO is a poisonous gas produced due to incomplete combustion and can be reduced tonegligible levels by assuring satisfactory air-fuel ratio control. SO2 is an acidic gas producedwhen sulphur-containing fuels such as oil or coal are burned. Its emissions cause acid rain.Sulphur-containing exhaust gases are the main cause of corrosion of heat recovery devices

when the SO2 in the gas is cooled below its condensation temperature. NO x is a mixture of nitrogen oxides produced due to the combustion of a fuel with air, and its formation is afunction of the combustion condition, characterized by the air-fuel ratio, combustiontemperature, and residence time. It also causes acid rain and can result in ozone and smogafter undergoing several chemical reactions in the atmosphere.

Technologies which have undergone rapid development are those based on spark andcompression ignition engines and gas turbines, primarily using natural gas as the fuel.Natural gas is considered the cleanest among the fossil fuels as it does not practicallycontain any sulphur, nitrogen and is free of dust particles. However, the emission of NO x isgreater, particularly for the prime movers operating at high temperatures.




Appropriate designing of the combustion chambers and control of the flame characteristichelp to reduce NOx formation in engines and turbines. Engine design alone cannot eliminateNOx formation. Moreover, efforts to reduce NOx emission can lead to increase in COemissions while adversely affecting the power output and efficiency. Therefore, end-pipe NOx

abatement technologies such as those based on catalytic reduction systems must beapplied to assure very low emission.

2.11.1Gas engine

Technical options adopted to minimize emissions from gas engines are optimal combustionprocess and flue gas cleaning. Lean-burn techniques are used for self-igniting engines usingnatural gas as fuel. With high load pressure and excess air (typically, 35 to 60 per cent), NO x

emission can be reduced to 200 mg/m3, below the standards set by many industrializedcountries.

Flue gas can be cleaned with a 3-way catalyst; as its name implies, NO x, CO andhydrocarbon emissions are reduced. In order for it to function efficiently, a constant NO x-CO

ratio needs to be maintained by proper control of air-fuel ratio and ignition.

2.11.2Gas turbine

Three commonly employed methods for eliminating NOx emissions from gas turbines arewater or steam injection, use of dry low NOx burners, and selective catalytic reduction.

Water or steam injection are well established techniques which boost the power output dueto increased mass flow rate in the turbine. These also help to lower the flame temperatureand the partial pressure of oxygen, thus inhibiting NOx formation. There is an upper limit toNOx reduction by this method without affecting gas turbine performance. Beyond a certaininjection rate of water or steam, there is greater flame instability that leads to formation of CO

and emission of unburned hydrocarbons.

More modern gas turbines make use of dry low-NO x systems instead of water or steaminjection in order to avoid the costs of treating and pressurizing water or producing highquality steam. The fuel is mixed with combustion air to a homogeneous mixture in a mixingchamber before being sprayed into the flame; this reduces the peak flame temperature andassures less NOx generation. Such systems are effective at high loads but perform poorly atpartial loads. Where the cogeneration system is required to have a wide range of operatingconditions, a hybrid design of low NOx burners is employed which incorporates a smalldiffusion pilot flame for stabilizing flame at low loads.

At sites where stringent environmental standards are applied, selective catalytic converterscan be adopted as an end-of-pipe technique. A reducing agent, normally ammonia, is used toconvert NOx to nitrogen and water in the presence of a catalyst, the most common beingvanadium oxide.

2.11.3Steam turbine

In steam turbine cogeneration systems, sulphur and nitrogen oxide emissions are importantin oil-fired boilers whereas particulate and nitrogen oxides have to be considered in wood-fired boilers.

As far as the boilers are concerned, technologically advanced equipment has beendeveloped to meet increasingly stringent environmental requirements. A significant

development is the use of a secondary combustion chamber where complete combustion of the unburned gases occurs. Better monitoring of combustion parameters through adequateinstrumentation has allowed the operator to better regulate the combustion.




Four types of emission control devices widely used in boiler systems are electrostaticprecipitation, fabric filters, multi-tube cyclones and wet scrubbers. Chemical agents such aslime, magnesium oxide, etc., are used for flue gas desulphurization. Commonly usedtechniques employed for NOx emission abatement in steam turbine cycles include low NO x

burners, selective catalytic reduction, flue gas recirculation, ammonia injection, etc.

part1ch2 state of art review of cogeneration

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