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Combined Heat and Power Plants | Steam, Gas, Micro Turbine, Fuel Cell

Combined heat and power (CHP) or cogeneration plants allow the simultaneous production of both electricity and heat. Heat, which is an inevitable by-product of generating electricity by burning fuel, can be used directly in an industrial process or for district heating.

The capacity of CHP plants varies from a few kW to hundreds of MW depending on the source of energy and the prime mover technology.

Prime movers of different technologies used for CHP are capable of burning a variety of fuels.

Table 1 summarizes the technologies commonly used for large CHP plants. Small CHP units are based on gas turbines, reciprocating engines or fuel cells and are used for small local generation in houses and commercial buildings and are usually fueled by natural gas.

Table 1 CHP Technologies and Their Common Fuels

Technology Fuel Type
Steam turbine Any fuel
Single-cycle gas turbine Natural gas, gas oil, landfill gas, LPG or Naphtha
Combine cycle gas turbine As single cycle gas turbine
Reciprocating engine Natural gas, gas oil, heavy fuel oil

1 Steam Turbines

Steam turbines are broadly categorized into three types: fully condensing, back-pressure or non-condensing and pass-out condensing or extraction. Back-pressure and pass-out condensing turbines are used for CHP.

1.1 Back-Pressure Turbines

In a back-pressure steam turbine, a fuel that may be solid, liquid or gaseous is burned in a boiler, and the resulting high-pressure, high- temperature steam is then passed through a turbine.

A set of blades, forming the turbine rotor, are driven by the steam. In this process, the steam expands and its pressure drops. In a back-pressure turbine, all of the energy in the steam that is unused for power generation is exhausted as potentially useful heat.

The pressure of the exhaust steam is arranged to be at the value required by the site. This arrangement is shown in Figure 1. In the Figure, Pe is the electrical power and Qh is the useful heat.

Back-pressure CHP plants are very reliable and have a high availability. CHP plants using back-pressure turbines are available with electrical power outputs of 0.5 MW upwards.

As can be seen from Figure 1, the CHP cycle involves many processes. Inside the boiler, a combustion and heat transfer process takes place where high-pressure steam is generated.

If the specific enthalpy of the steam at A is hA and that of the feed water at D is hD, then disregarding any pressure drops and heat losses, the heat energy added in the boiler is:

Elements of back-pressure steam turbine cycle

Figure 1 Elements of back-pressure steam turbine cycle

\[\begin{matrix}   {{Q}_{h}}(in)=\overset{.}{\mathop{m}}\,\left[ {{h}_{A}}-{{h}_{D}} \right] & {} & \left( 1 \right)  \\\end{matrix}\]

Where $\overset{.}{\mathop{m}}\,$ is the mass flow rate. Similarly, for the other processes, the following equations apply:

\[\begin{matrix}   {{P}_{e}}(out)=\overset{.}{\mathop{m}}\,\left[ {{h}_{A}}-{{h}_{B}} \right] & {} & \left( 2 \right)  \\\end{matrix}\]

\[\begin{matrix}   {{Q}_{h}}(out)=\overset{.}{\mathop{m}}\,\left[ {{h}_{B}}-{{h}_{C}} \right] & {} & \left( 3 \right)  \\\end{matrix}\]

\[\begin{matrix}   {{P}_{e}}(in)=\overset{.}{\mathop{m}}\,\left[ {{h}_{D}}-{{h}_{C}} \right] & {} & \left( 4 \right)  \\\end{matrix}\]

From Equation 1, it can be seen that the electrical power output can be increased if the steam input is of high specific enthalpy (hA), i.e. at high pressure and temperature, and the heat output is of relatively low enthalpy (hB).

If a higher steam pressure is selected at the turbine input that will increase the capital costs of the boiler and plant running costs.

 There is a value of steam pressure that results in a given heat/power ratio. The ratio of usable heat to power in a back-pressure steam turbine CHP plant is typically in the range of 3:1 to 10:1. This makes them unattractive for industries which have a high electricity but low heat demand.

Elements of pass-out condensing steam turbine cycle

Figure 2 Elements of pass-out condensing steam turbine cycle.

1.2 Pass-Out Condensing Steam Turbines

In a pass-out condensing steam turbine, some steam is extracted at an intermediate pressure for the supply of useful heat. The rest of the steam is expanded and cooled in the condenser. This process is shown in Figure 2. Qh1 is the useful heat output whereas Qh2 is the heat rejected by the cycle.

Similar to back-pressure steam turbines, in these CHP units for a given fuel input the electric power output decreases with the heat extraction; thus there is an optimum heat-to-power ratio.

For a CHP unit of rating less than 5 MWe operating with a steam export pressure of 15–20 bar, a 1 MW change in electrical output causes, approximately, a 5 MW change in heat output.

2 Gas Turbines

Gas turbines are widely employed in CHP plants that have power outputs from 1 to 200 MWe. Gaseous or liquid fuels can be used in a gas turbine. The most popular fuel used is natural gas (methane).

Figure 3 shows the components of a gas turbine–based CHP plant. Air is first compressed by the compressor. Then heat is added through combusting fuel in the compressed air. The products of combustion at a temperature in the range 900°C–1200°C pass through the power turbine.

Simple cycle, single shaft gas turbine

Figure 3 Simple cycle, single shaft gas turbine.

Inside the power turbine, the gas is expanded to atmospheric pressure rejecting heat at 450°C–550°C.

The heat added by the combustion of fuel (QF) is given by

\[\begin{matrix}   {{Q}_{F}}={{h}_{C}}-{{h}_{B}}=\overset{.}{\mathop{m}}\,c\left[ {{T}_{C}}-{{T}_{B}} \right] & {} & \left( 5 \right)  \\\end{matrix}\]

Where

$\overset{.}{\mathop{m}}\,$ is the mass flow rate of the air

c is the specific heat of air

T is the temperature of the air and gas

The useful heat from the heat exchanger Qh (out) is given by

\[\begin{matrix}   {{Q}_{h}}(out)=\overset{.}{\mathop{mc}}\,\left[ {{T}_{D}}-{{T}_{A}} \right] & {} & \left( 6 \right)  \\\end{matrix}\]

Neglecting losses, the electrical output is given by

\[\begin{matrix}   {{P}_{e}}(out)={{Q}_{F}}-{{Q}_{H}}(out)-{{P}_{c}} & {} & \left( 7 \right)  \\\end{matrix}\]

Where Pc is the power required to run the compressor. The ratio of heat to power ranges from 1.5:1 to 3:1.

3 Combined Cycle

In combined cycle systems the heat exhausted from a gas turbine is used to drive a steam turbine. The exhaust heat from the gas turbine allows steam to be generated at high pressure so as to run the steam turbine.

Figure 4 shows the components of a combined-cycle CHP system. Electric power is generated from generators connected to both the gas and steam turbines. The useful heat is extracted at the exhaust of the steam turbine through a heat exchanger.

4 Reciprocating Engines

The reciprocating engines used in CHP systems are diesel or Otto engines.

In a diesel engine, air is compressed in the cylinder and fuel is injected at high pressure to self-ignite and burn.

In Otto engines, a spark plug is used to ignite a pre-mixed charge of air and fuel after compression in the cylinder.

CHP units based on diesel engines are available up to ratings of 15 MWe and can utilize gas oil and heavy fuel oil.

Otto engine-based CHP units are available up to 4 MW and usually operate on gaseous fuel.

CHP units are based on existing engine models used for power generation or automotive engines and are less expensive than small gas turbines. The usable heat to power ratio ranges from about 1:1 to 2:1.

Combined cycle gas turbine

Figure 4 Combined cycle gas turbine.

The components of CHP based on an internal combustion engine are shown in Figure 5.

These types of CHP units are mainly used for on-site heat and power generation. Their economics depend on the effective use of the thermal energy in the exhaust gas and cooling system.

 Heat in the cooling system can produce hot water at a temperature of around 80°C–90°C. The engine exhausts gases at around 400°C.

Components of an Otto engine-based CHP unit

Figure 5 Components of an Otto engine-based CHP unit.

5 Micro-Turbines

Micro-turbines are small gas turbines especially designed for domestic and commercial applications.

When used as a CHP unit, the control system of the micro-turbine aims to follow the heat load and any deficit or excess of electricity is exchanged with the grid.

Figure 6 shows a commonly available micro-turbine. In this unit, the compressed air from the compressor is first heated in a heat exchanger called the recuperator before the fuel is added. This heating process uses the heat in the exhaust air.

The rotational speed of the turbine is very high and is in the range of 60,000–100,000 rpm. Two back-to-back electronic power converters are used to obtain 50 Hz of power output.

Micro-turbines with two shafts are also available. In that arrangement, a separate power turbine is used to drive the generator through a gearbox.

Internal combustion engine–based micro-CHP units are also commercially available. Their electrical power rating varies from 1 to 5 kWe. These CHP units generally allow three operation options: heat-led, electricity-led or a combination of the two.

The most commonly used mode is heat-led, where the CHP unit is controlled to meet the heating demand in the installation. In an electricity-led operation, the CHP system is controlled to meet the building electrical power demand.

6 Fuel Cells

Figure 7 shows the components of a fuel cell–based CHP unit. The fuel such as H2 or CH4 enters the anode and is combined with the ions exchanged across an electrolyte.

Exhaust gas from the electrochemical reaction at the anode is mixed with air and oxidized in a catalytic oxidizer en route to the cathode.

At the cathode O2 from the air, exhaust gases from the anode and electrons react and form ions. These ions migrate through the electrolyte to the anode.

By-products of this process are water and heat, where water may be released as steam, depending on its temperature.

Components of an internal combustion–based CHP unit

Figure 6 Components of an internal combustion–based CHP unit.

Fuel cell-based CHP

Figure 7 Fuel cell-based CHP.

Fuel cell-based CHP units are of different sizes and types depending on the fuel, electrolyte, and membrane used. The following are common fuel cell types for distributed generation:

  1. Proton exchange membrane (PEM): This uses a solid polymer electrolyte and operates at a low temperature (60°C–80°C). PEM units have a high power density and their power output can be varied.
  2. Alkaline fuel cell (AFC): This technology uses alkaline potassium hydroxide as the electrolyte. AFC units operate at low temperature (90°C–250°C).
  3. Molten carbonate fuel cell (MCFC): This uses an alkali metal carbonate as the electrolyte. It operates at 600°C–700°C.
  4. Solid oxide fuel cell (SOFC): This uses a non-porous metal oxide membrane and operates at 750°C–1000°C. However, it is a less mature technology.

Fuel cell-based CHP units are available in different sizes. The largest commercially available CHP unit is MCFC based, with an electrical output of 1.2 MW. The exhaust of this type of CHP unit in the form of a humid flue gas has a high temperature, approximately 300°C–400°C, and a flow rate of 7000 kg/h.

About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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