Combined Heat and Power using Turbines
In most CHP units of larger size, turbine technology is almost exclusively used. It consists of a steam turbine or a gas turbine or a combined cycle of steam and gas turbine. Turbines can also be applied for midsize or small power levels. Steam turbines are commercially available for all power sizes; gas turbines are available above about 1 MWe.
The steam turbine is one of the oldest and most familiar technologies to drive generators or other rotating machinery. Hence this is the favoured technology for electricity production. Steam turbines can also be selected for CHP applications and have been applied intensively in industry. They exist under different forms such that they can be tailored to a multitude of specific demands and applications.
The Rankine cycle is the basic thermodynamic cycle for steam turbines. By means of heat produced by the combustion of fuel, one produces steam at high pressure in a steam boiler. The steam expands in a turbine creating a rotation of the turbine blades and axis and driving a generator for electricity production. After expansion in the turbine, the steam is condensed and pumped back as water to the steam boiler. This is illustrated in figure 1. The combustion takes place outside the steam boiler such that there is no contact between the combustion gases and the process fluid (steam). This means that there are no particular quality requirements imposed on the fuel, and that almost any fuel can be used. The drawback is the unavoidable energy loss by the heat transfer between the combustion gases and the water/steam.
Figure 1: Basic flow diagram for CHP based on steam turbines
Steam turbines for CHP applications can be classified in two large types: The backpressure steam turbine and the condensing steam turbine with steam extraction. In the backpressure steam turbine the steam is leaving the turbine at a certain pressure (and temperature) imposed by the heating requirements by the user. Hence the steam expansion is limited to a pressure larger than atmospheric pressure.
In a condensing type steam turbine, the required quantities of steam are extracted at the required pressure (and temperature) to satisfy the external user heat demands. The remaining steam in the turbine expands to very low pressure (below atmospheric pressure) and is subsequently condensed to water in the condenser. The water is then fed back to the steam boiler. It is furthermore possible to provide several extraction points in the turbine. If it is possible to design the cycle for different steam extraction pressures and steam flow rates, then the process becomes very flexible and the ratio between heat and power can vary.
For a steam turbine for CHP applications, one has to give priority to the heat production process, with the logical result that the thermal efficiency is rather high while the electrical efficiency is rather low. The steam turbine proper (excluding the combustor/boiler) can reach very high availability of more than 99%.. The downtime periods for maintenance and inspection can be programmed more than a year apart.
The gas turbine technology is historically well known for its applications in airplane motors. It’s only in the last decennia that gas turbines for CHP applications came into full development for the power range beyond 1 MWe.
The Brayton cycle is the basic thermodynamic cycle for gas turbines. Contrary to steam turbines, this cycle is based on internal combustion, which means that the choice of the fuel becomes important for CHP applications. Natural gas is the favoured fuel, but also light fuel oil can be used. The cycle goes as follows. Atmospheric air is ingested and compressed in a compressor, from where the compressed air flows into the combustion chamber where it is mixed with fuel and ignited. The hot exhaust gases at high pressure expand into a turbine producing work to drive on one hand the air compressor and on the other hand a generator to produce electricity. The exhaust gases leaving the turbine still have considerable heat content (high temperature). This heat can be recovered in heat recovery steam generator (HRSG). This steam can further be used for different processes where heat is required. To satisfy larger heating demands, the possibility exists for producing extra steam by injecting fresh fuel in the HRSG. This process is illustrated in figure 2.
Figure 2: Basic flow diagram for CHP based on gas turbines.
When speaking about “gas turbine”, one usually means the combination “compressor-combustion chamber-turbine” and not only the gas turbine proper. There exist two important types of gas turbines: de heavy duty and the aeroderivative. The last type is derived from airplane motors as the name suggests. This type has the highest efficiency since it is designed as a multi-axes configuration. The high pressure turbine drives the high pressure compressor, while the low pressure turbine drives the low pressure compressor and the generator. The high pressure turbine and compressor can run at higher speeds than the low pressure section. The aeroderivative gas engines are very lightweight but limited in size. The bigger gas turbines are of the heavy duty type. They are heavier but also more robust. The maintenance intervals are larger which makes them more suitable for baseload applications.
The electrical efficiency of a CHP with gas turbine varies between 25 and 40% depending on the power level. Also the frequency and quantity of the additional fuel injection in the HRSG influences the efficiency. The total fuel utilisation, in this course defined as the sum of the electrical and thermal efficiency is between 70 and 85. The gas turbine availability is also high between and in excess of 85%. The frequency of the maintenance stops strongly depends on the number of starts and stops the unit was subjected to.
Gas turbines and steam turbines can be combined in so called combined cycle plants. As described above, for gas turbines plants, steam can be produced in heat recovery steam generators (HRSG) with added option for extra fuel injection. The available heat in the exhaust gases from the gas turbine can be recovered to produce steam, which in turn can drive a steam turbine if process heat is not fully needed. By coupling the steam turbine to a generator, additional electricity can be generated, while heating needs can be met by extracting steam from the turbine.
It is evident that such plants can reach very high electrical efficiencies, what automatically leads to lower thermal efficiencies. They are comparable to combined cycle plants that are used for electricity production only without steam demands.
Figure 3 illustrates the flow diagram of a CHP based on a combined cycle concept.
Figure 3: Flow diagram of a CHP based on a combined cycle concept
There is basically little difference in concept between microturbines and well known gas turbines used in the airplane industry or for large CHP applications. They function according to the same principle: The air sucked in, is compressed and heated in a combustion chamber where it is mixed with fuel-often gas- and ignited. The exhaust gases expand through a turbine. This turbine powers the compressor as well as a generator for the production of electricity.
The main difference between microturbines and large turbines is evidently their size as the name implies. Microturbines are available in sizes from 25 to 250 kWe. In order to reach higher electrical efficiencies, the microturbine is equipped with a recuperator, where the compressed air leaving the compressor section is heated by the exhaust gases leaving the turbine. This is shown in figure 1.
Figure 4: Flow diagram for a microturbine
Compared to the small gas motor technology for low power levels, the microturbines offer some interesting advantages. All heat in a microturbine is available in the exhaust gas. Hence only one heat exchanger is needed to recover the heat which simplifies the layout. A gas motor on the contrary provides heat at different locations and different temperatures, which makes the heat recovery and usage much more complex. The polluting emissions from a microturbine are lower than those from a gas motor due to the catalytic burning process in a microturbine. Another advantage of microturbines is lower operating and maintenance costs since the number of rotating parts is very small. Indeed, the compressor, the turbine and the generator are all mounted on a single high speed rotating axis. Some microturbines are equipped with air bearings for oil free operation. The vibration level of microturbines is very low and the noise spectrum can be attenuated easily. Qua investment cost, both the gas motor and microturbine are not cheap but comparable in cost.
The microturbine presents an interesting option for biogas feeding since microturbines, in contrast to gas motors, can easily cope with variations in the specific heat content of the biogas feedstock.
Compared to gas motors, microturbines have some disadvantages such as lower electrical efficiency and slightly lower total efficiency. This disadvantage becomes even more important when the fuel has to be compressed before injection in the combustion chamber. Microturbine technology is a relative new technology lacking experimental evidence for large scale implementation in the Flemish region. On the other hand, evidence from results abroad is quite encouraging.