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SPECIAL THEME COGENERATION The cogeneration process is made up of a set of operations aimed at the combined production of mechanical energy (electric) and thermal energy (usable heat), from a single primary energy source.

Cogeneration is characterised by the recovery of part of the heat which in the traditional production of electricity is discharged into the environment; thus the process achieves a more rational use of primary energy than separate

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U. DESIDERI

systems.

plant designed to produce electricity has an energy conversion efficiency which can vary from 20% for low performance steam plants to about 60% for the most recent gas-steam combined cycles. 40 to 80% of the primary energy used is discharged into the environment in the form of heat.

Instead, heat produced by combustion in a cogeneration plant is not dispersed but recovered for other uses. Thanks to this process, conversion efficiency can reach up to 90% resulting in savings in primary energy, and reductions in pollution emissions and energy production costs.

Schematically, the differences between energy flows in traditional systems of separate production of electricity and heat and flows in a cogeneration system are shown in figure 1.

Cogeneration saves energy and provides environmental advantages over the separate production of the same quantities of electricity and heat. By uniting the production of electricity and heat in a single plant, cogeneration optimally exploits the primary energy of fuels. The fraction of energy at a higher temperature is converted into valuable energy (electric) and energy at a lower temperature, instead of being discharged into the environment, is made available for the appropriate heat applications.

The reduction in environmental impact is due to both lower atmospheric pollution by greenhouse gases and lower heat pollution.

Cogeneration can be applied in both industrial and civil fields. As regards heat, it can be used in the form of steam or hot water for in-

Figure 1: Primary energy conversion for separate and combined production of electricity and heat.

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THEME COGENERATION

The use of wood as fuel in cogeneration Umberto Desideri Department of Industrial Engineering - University of Perugia

The cogeneration process is intended to carry out a more rational use of primary chemical energy than separate systems of generation of the two forms of energy. In this case production is interconnected.

We analyse cogeneration’s advantages and limits, the main types of plants and the potential of the use of wood as a fuel.

dustrial or civil uses (e.g. district heating through teleheating1 networks, as well as cooling through absorption refrigerating systems) or in the form of hot air (e.g. in industrial drying processes).

Cogeneration plants must be near heat consumers due to the difficulty of transporting heat, which can only be transported in the form of high temperature fluid and needs appropriately sized fluid networks. Presently available infrastructures do not exist. Transporting electricity is much easier because it can be done along the electricity lines which are very widespread and in practice reach all consumers who need electricity. Distribution of energy production plants across the country does

however have the advantage of generating less transmission and distribution losses in the national electricity grid.

The procedures for drawing the two components (electricity and heat) of the energy produced are often characterised by independent profiles which vary over time (continuous or discontinuous processes on a daily or seasonal basis). In some industrial sectors cogeneration represents a widely consolidated production option. It may be of even greater importance, both in terms of its contribution to national electricity demand and energy savings, by virtue of the effects induced by the technological change in the field of electricity generation.

ADVANTAGES AND LIMITS OF COGENERATION The main advantages of energy production using cogeneration systems are the following: • economic: with cogeneration the energy contained in the fuel is exploited better or rather, for the same electricity and heat used, less fuel is consumed.

For every kWhe produced, a cogeneration plant reduces CO2 emissions by 450g compared to the separate production of electricity and heat.

The photo shows a cogeneration plant.

KARKAN

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The heat produced by the cogeneration plant can in fact be carried across a heat distribution network known as a teleheating network in which the heat transfer medium is pressurised water at a temperature of 120°C. In this way it is possible to satisfy the thermal requirements of several buildings or whole neighbourhoods, subject to the substitution of traditional boilers with heat exchangers for each consumer served.

There are no doubts about the advantages of cogeneration in terms of energy efficiency compared to the separate production of electricity and heat. However, because these advantages derive from a combined production, it is appropriate that the available heat be used in the production cycle of the plant in which it is generated, since investments are not required for the implementation of a teleheating network nor for all the necessary connections to distribute the heat to the many consumers around the country.

On average a methane fired cogeneration plant reduces CO 2 emissions by 450g per kWhe produced when compared with the separate production of electricity (thermoelectric power station) and heat (conventional boiler).

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U. DESIDERI

THEME COGENERATION

Figure 2: Model of a steam plant.

Some limits to the adoption of cogeneration systems do exist and must be considered, particularly during the stage of the feasibility study: • The main limit of cogeneration concerns, above all, the correspondence of production to demand for both electricity and heat. A cogeneration plant is as efficient as its U. DESIDERI

Figure 3: A pass-out steam and condensation plant.

• environmental: lower fuel consumption leads to lower quantities of harmful emissions into the environment and therefore a reduction in the social costs of pollution.

• protection of primary energy resources: cogeneration permits a more efficient use of traditional energy resources (oil, coal, natural gas) reducing wastes.

• financial: cogeneration is considered to be a comparable source of energy to renewable sources and therefore enjoys several incentives and facilitations provided by law and by the National Energy Plan.

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ability to time and quantify electricity and heat requirements in line with the capacity of the plant to produce the required amounts.

• Electricity and heat absorption follow essentially independent rules. Given that electricity cannot be practically accumulated and heat can, but only for short periods, cogeneration is feasible and advantageous when the demands for electricity and heat are contemporaneous.

• Heat and electricity users must be near the energy generating system, in particular with the heat distribution network.

• The spread of cogeneration systems has so far been obstructed, above all, by a plant’s high initial costs which can be attributed to the greater complexity of such a system, compared to the costs of traditional systems. From this point of view the complexity of the plant is increasingly unsustainable as the potential of the plant itself decreases.

• The need, in order to have better economic returns from investments, to generate cold air during the summer months using absorption machines: water and lithium bromide single-stage cycles produce chilled water at 5-7°C using hot water at 80-95°C or super heated water at 110-140°C or saturated vapour at 1,5-2 bars in the generator.

TYPES OF PLANTS Technologies used for the cogeneration of electricity and heat basically derive from technologies for the production of electricity, with the addition of heat recovery equipment from the main engines (steam turbines, gas turbines and internal combustion engines).

This means that for an equal quantity of electricity, the cogeneration plant requires greater unitary investments, as well as greater management and running complexity compared to non cogenerative plants.

From a plant engineering point of view, the cogeneration technologies are classified on the basis of the typology of the main engine.

Each one shows different ratios between the quantities of electricity and usable heat produced, just as the flexible nature of such ratios with regard to demand are very different.

Historically thermoelectric production developed with the thermodynamic Rankine cycle which is the basis for closed steam cycles. The simplest Rankine cycle is made up of four basic components: the steam generator, the turbine, the condenser and a pump (Figure 2).

The steam generator is the component in which the fuel combustion process takes place and in which the heat developed by this process is used to heat, vaporise and superheat steam at high pressure. The turbine produces the usable work which is usually transformed into electricity, while the condenser is a heat exchanger in which the heat subtracted from the condensation of the steam is released into the atmosphere. The pump is needed to close the cycle and bring the condensed fluid back to the pressure level at which it is introduced into the steam generator.

The thermodynamic efficiency increases in these plants both by increasing the temperature and maximum steam pressure and by reducing the temperature of the cold source. However, if we wish to use part of the heat which is normally released into the atmosphere as heat for an industrial process or for a teleheating network, we will have to raise the condensation temperature. Industrial processes which require usable heat at average temperatures (120-150°C) recover heat by changing the steam discharge conditions, causing a reduction in the production of electricity but increasing the global efficiency of conversion of the fuel’s primary energy. It is clear, therefore, that the greater the temperature at which heat is subtracted from the thermodynamic cycle, the less electricity produced.

Two typologies of steam plants are used for cogeneration: • back pressure steam plants, in which the steam turbine discharges into the condenser at a pressure greater than atmospheric pressure; • condensation steam plants with extraction, in which the removal of steam for technological or heating uses is partial and carried out during the expansion stage in the turbine (Figure 3).

The former are characterised by high energy recovery values compared to separate production and

THEME COGENERATION tercooler at temperatures varying from 70° to 90°. Should the engine be supercharged, the heat from the exhaust gases (about 400500°C) can also be recovered. The retrievable heat from the cooling of the engine and oil can be quantified at about 35% of the primary energy inserted and the retrievable heat from exhaust gases at about 30% of the primary energy inserted.

In gas turbines heat released into the atmosphere is in the form of burnt gases in large volumes and at high temperatures (450-550°C) which can be used to generate steam, hot water at a high temperature and hot air used in drying processes.

The efficiency of conversion of the primary energy in internal combustion engines and in gas turbines is about 60-70%. Since the recovery of heat neither influences nor reduces electricity production, there may be rather wide ratios between usable heat and electricity produced.

Since the end of the 1970s combined cycle plants have been developed in which a gas turbine and a Rankine cycle coexist. The exhaust gases from the gas turbines generate steam which powers a steam turbine..

Generally the combined cycle plants are designed to produce only electricity and represent one of the most efficient solutions, with nominal efficiencies for larger sizes of about 55% and with medium-term objectives of 60%. The spread of combined cycles determines a radical change in the application models of cogeneration – while production of heat prevails over electricity in traditional steam cycles with back pressure turbines and, in the case of internal combustion engines or turbogas with heat recovery boilers, such productions are relatively independent, in the combined cycle plants the production of electricity is decidedly predominant, thus generally creating problems of transfer of the electricity to the network.

The photo shows a cogeneration plant (Alholmens Kraft Finland) with a 240MW steam turbine fired by various types of wood fuels. It produces about 100 MW of process steam for the UPMKymmene Pietarsaari paper mill and about 60 MW for the Pietarsaari local heating network.

MR HANNU VALLAS / LENTOKUVA VALLAS OY

by a high ratio between the quantities of heat produced and electricity. The latter show primary energy conversion efficiencies of fuel similar to those of a conventional thermoelectric plant.

In 1986 more than 97% of electric power installed in Italy with cogenerative plants (4505 MW) was based on steam cycles, the rest being represented by internal combustion engines, both eight-stroke and Diesel cycles and gas turbines.

The Rankine cycles give off heat at a temperature only a few degrees above room temperature, while internal combustion engines and gas turbines dispel heat at a temperature greater than room temperature. For this reason the recovery of heat from such plants does not lead to reductions in electricity production but makes it possible to use a quantity of heat which would otherwise be lost.

In eight-stroke and diesel internal combustion engines, heat is recovered from the engine and oil cooling circuit and from a possible in-

THEME COGENERATION

MR HANNU VALLAS / LENTOKUVA VALLAS OY

Plant types vary from several dozen to several thousand kW, as in the case of internal combustion engines, up to several hundred MW with gas turbines.

APPLICATION FIELDS OF THE PLANT ENGINEERING TYPOLOGIES Internal combustion engines are used when quantities from several tens to several thousand kW (2000-5000kW) and heat transfer fluids at a temperature of less than 100°C are required. The heat is recovered from the cooling water, from the lubricating oil, from the supercharging air (at temperatures below 90°C) and from the exhaust gases.

Internal combustion engines are, in particular, characterised by the fact that heat is recovered at a low temperature and they are therefore suitable for the use of heat transfer mediums such as water at 90°C. It is important to note that, for the integrity of the engine, it is necessary to ensure the recovery of heat from the engine and from the oil when the engine is working.

Gas turbines are available in sizes between 30-80 kW (microturbines) and several hundred MW

per individual machine and are adopted for applications with heat transfer mediums at temperatures over 100°C. In fact, the recovery of heat from exhaust gases at high temperatures (up to 500°C) is stored in an appropriate heat recovery boiler.

The main characteristics of the various cogeneration systems mentioned above are summarised in the following table Table 1.

The average efficiency values in electric power, if related to the burnt fuel, are on average included in the following areas in the field of small cogeneration Table 2.

Consideration of the system’s global efficiency (heat and electricity produced compared to energy inserted as fuel) gives the following Table 3.

The fuels used in cogeneration are usually liquid or gaseous hydrocarbons. At present the use of gaseous hydrocarbons such as methane is preferred for various reasons including moderate costs and lower environmental impact.

The steam turbines can also be driven by steam produced by the combustion of solid fuels, such as coal, biomasses and solid urban waste.

The choice of the main engine most suitable to the individual case depends on several factors.

The most important are: the entity of the powers in question the ratio between electricity and heat required by consumers the temperatures of use and the type of hot fluid required by heat consumers In order to compare the performance of the various types of cogenerators, a parameter Z (elec-

TABLE 1 Technology

Combustion engines Turbogas Steam turbine Combined cycles TABLE 2 Technology

Steam turbine Turbogas Alternative engines

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Power (MW)

18 - 20% 23 - 33% 32 - 40%

STEAM TURBINE The use of steam plants means: possibility of obtaining reasonably high values of global heat efficiency, at the same time supplying recovered heat at high temperatures. Given that these machines use steam from an appropriate generator, it is possible to adopt any fuel, particularly solid and less valuable ones.

Medium-high potentiality, between 1 - 250 MWe of electric power.

Electric index Z= 3-14 (Z= 14 means the electric power produced by the machine is 14 times greater than thermal power) High initial investment costs Plant complexity (long delivery times) Limited ability to adapt to speed conditions other than those of the project.

Permanent presence of specialised personnel.

HE

0.05 - 10 0.08 - 180 0.5 - 250 >5 Average efficiency in electric power

tric index) is defined as the ratio between electric power and thermal power (Z = Pe/Pt). For the cogenerator, Z is fixed and depends on the rating of the plant itself. For consumers, Z is variable depending on the time of year, day of the week and time of day.

To choose the optimum cogenerator, it is necessary to compare the characteristic electric index of each machine with the average of the consumption to be served.

The closer these two figures, the better the plant’s energy production efficiencies and as a result, its economic results. Below is a summary of the main characteristics and the most common application fields for the cogenerative systems used.

0.25 0.20 0.10 0.40

HT

- 0.40 - 0.38 - 0.35 - 0.55

TABLE 3 Technology

Steam turbine Turbogas Alternative Engines

0.25 0.35 0.60 0.10

- 0.45 - 0.50 - 0.75 - 0.45

Global efficiency

80 - 90% 70 - 85% 65 - 90%

The figures are average values provided only to give a general overview

THEME COGENERATION GAS TURBINE

The use of renewable fuels such as wood may represent an important incentive for the spread of cogeneration to areas where such fuel is readily available on the market and found at reasonable prices.

TURBODEN

The use of a gas turbine means: Reduced weight/power ratio and limited encumbrance.

Functioning does not require cooling systems if heat consumers do not require heat.

Available thermal power is largely independent of electric power, which means operations are more flexible.

Structural simplicity and relatively short construction and delivery times.

Electrical power from 80 kWe 180 MWe. Electric index Z= 0,2-4.

Need to use valuable fuels (gas, light oils) to avoid situations of dirtying and corrosion of the turbine blade.

INTERNAL COMBUSTION ENGINE The use of internal combustion engines in a cogeneration systems means: High efficiencies even when only partially operating.

Wide range of usable fuels – both liquid (heavy naphtha) and gaseous (methane, GPL), including poor gases, blast furnace gases, synthetic gases from the gasification of biomasses, waste gases and coal gases.

Supercharged modular structure facilitates maintenance, reducing the risks of complete interruptions of the service, at the same time guaranteeing flexibility of installation and operation.

Wide field of application, electrical power from 15 kWe - 10 MWe. Electric index Z= 0,4-2,2 Poor capacity to satisfy heat consumption requiring heat at high temperatures (over 140 °C).

Significant emissions of NOx need to be limited using the appropriate devices according to current regulations.

Increase in the occurrence of maintenance costs (particularly for lubrication oils) which can represent 2 to 4% of investment costs.

TURBODEN

One use of the gas turbine is the combined cycle, i.e. the coupling with a heat recovery boiler to generate saturated steam.

WOOD AS FUEL FOR COGENERATION PLANTS As illustrated previously, cogeneration plants can use all fuels available on the market.

Wood, as a solid fuel, can be used directly in steam generators using various processes of combustion: grate, fluidised bed, hot bed, etc..

For this reason steam plants represent the typology of cogeneration plant in which it is easiest to use wood as fuel. Numerous studies concerning external combustion of biomasses in gas turbines have been presented in recent years.

However, commercial use of this technology does not yet exist.

Numerous wood fired steam cogeneration plants are, however, operating in northern European countries and in the Alps.

The use of wood as fuel in internal combustion engines or gas turbines has instead been applied after a chemical-physical transformation using the processes of gasification and fast or slow pyrolysis. Such processes transform

solid fuels into gaseous or liquid fuels which can be used in internal combustion engines and in gas turbines.

CONCLUSIONS Cogeneration represents the most rational method for using primary energy sources. A correct analysis of feasibility based on the electricity and heat needs of consumers and on the economic situation of the electricity and fuel markets is the basis for the development of cogeneration.

The use of renewable fuels such as wood may represent an important incentive for the spread of cogeneration in areas where such a fuel is available on the market.

Greater use of wood as fuel for cogeneration plants is strongly linked to the development of a network to gather and commercialise wood which can satisfy the needs of consumers and guarantee definite market prices comparable to other energy sources and guarantee a constant, satisfactory quality of fuel.

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