Steam turbine cogeneration: Combined heat and power (CHP) systems

A steam turbine by definition is a mechanical device that extracts thermal energy from pressurized steam and converts it into useful mechanical work.

What is cogeneration:

By definition, a cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.

A typical fossil-fuel power plant has an average thermal-to-power efficiency of 35–45%, because its steam turbine can only extract so much electrical energy from high-pressure steam, and because the low-pressure steam that exits the turbine cannot be used. Therefore, the steam exiting the turbine must be condensed, and its potential energy is lost.

Industrial plants, however, have a competitive advantage over power plants because they have several process units that can use the lower-pressure outlet steam, which eliminates the need for inefficient steam condensing.

In a cogeneration system, the steam that passes through the turbines is maintained at an outlet pressure high enough to be used by process units. Because energy at the turbine outlet is re-used, cogeneration systems have overall efficiencies that can be higher than 85%, which allows industrial plants to generate electrical power at lower-than-market costs.

In a cogeneration plant, the two types of steam turbines most widely used are the backpressure and the extraction-condensing types

Types of turbines: General outline

Non condensing

Noncondensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, pulp and paper plants, and desalination facilities where large amounts of low-pressure process steam are required.

Condensing

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially saturated state, typically of a quality greater than 90%, at a pressure well below atmospheric to a condenser. Steam is extracted at suitable stages of the turbine and sent to boiler feedwater heaters to improve overall cycle efficiency. In this case, the extraction quantity is adjusted automatically to suit the requirement of the feed water to the boiler.

Reheat

Reheat turbines are also used almost exclusively in electrical power plants with capacities generally greater than 200 MW. In a reheat turbine, steam flow exits from a high-pressure section of the turbine and is returned back to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.

Extraction

Extracting type turbines are common in process industries. In an extracting type turbine, steam is extracted in considerable quantities from suitable stages of the turbine, and used for industrial process needs. Extraction flows may be controlled with a valve or left uncontrolled.

The thermodynamics of cogeneration

An understanding of the basic thermodynamic terms and concepts is needed to analyze a turbine’s energy conversion efficiency.

Specific enthalpy (h) is the amount of energy per unit mass of steam. It is usually expressed in BTU/lb, MWh/kg, or GJ/kg.

Specific heat of vaporization (?hlg) is the amount of energy per unit mass required to effect a change of state from water to steam at constant pressure. It is usually expressed in units of BTU/lb, MWh/kg, or GJ/kg.

Specific entropy (s) can be thought of as the potential energy of steam, where a lower entropy value means higher potential energy and a higher entropy value means lower potential energy. For a closed system with no losses, the overall system entropy can only increase or stay constant (sfinal ≥ sinitial).

Steam turbine efficiency (ηeff) is the ratio of the actual work produced by the turbine to the maximum amount of work that the turbine could extract if the process were ideal (i.e., a no-loss isentropic expansion).

Backpressure turbines: Different scenarios

Many industrial processes require electrical power and heat. This heat is often provided from large quantities of low-pressure steam. In a back pressure turbine, the pressure is raised to above atmospheric pressure so that the turbine exhaust steam can be transported to the process heat load then the steam will give up its latent heat usefully rather than reject this to the condenser cooling water. Although the steam turbine output is reduced, the overall efficiency is increased significantly as the generated steam is used to provide both heat and electrical power.

The two types of steam turbines most widely used are the backpressure and the extraction-condensing types. The choice between backpressure turbine and extraction-condensing turbine depends mainly on the quantities of power and heat, quality of heat, and economic factors. The extraction points of steam from the turbine could be more than one, depending on the temperature levels of heat required by the processes.

Image A: Back-pressure turbo-alternator operating in parallel with the grid supply

Image: A The simple back-pressure turbine provides the maximum economy with the simplest installation. An ideal backpressure turbogenerator set relies on the process steam requirements to match the power demand. However, this ideal is seldom realized in practice. In most installations, the power and heat demands will fluctuate widely, with a fall in electrical demand when steam flow, for instance, rises. These operating problems must be overcome by selecting the correct system. The image above shows an arrangement that balances the process steam and electrical demands by running the turbo-alternator in parallel with the electrical supply utility. The turbine inlet control valve maintains a constant steam pressure on the turbine exhaust, irrespective of the fluctuation in process steam demand.

Image: B back-pressure turbine with PRDs valve

Image: B

This process steam flow will dictate output generated by the turbo-alternator and excess or deficiency is made up by export or import to the supply utility, as appropriate. The alternative to the system in Image A is to use a back-pressure turbine with a bypass reducing valve and dump condenser, as shown in Image B. On this system, the turbine is speed controlled and passes steam, depending on the electrical demand. The bypass reducing valve with an integral desuperheater makes up any deficiency in the steam requirements and creates an exhaust steam pressure control. Alternatively, any surplus steam can be bypassed to a dump condenser, either water or air-cooled, and returned to the boiler as clear condensate.

Back-pressure turbines are inexpensive, thermally efficient, and compact, and usually the most economical proposition for partial generation schemes; but inflexible. Power generation is dependent on the steam flow required to meet process requirements and the pressure drop over the turbine. The running cost for electrical generation is therefore the marginally additional cost of generating steam at higher pressure and temperature than would be required for process usage only, plus the fuel equivalent of the heat drop across the turbine. Exhaust pressure from the turbine is determined by process steam pressure requirements. Inlet steam conditions, therefore, depend on the power generation required; seldom more than 4.5 MPa gauge, 400°C, although in large-scale chemical industry pressures may be up to 13 MPa. Thermal efficiencies in the range 75–85% are common, and this is virtually unaltered by varying the back pressure provided that all the steam can be used. Steam can be blown into the atmosphere or passed to a dump condenser to increase electrical generation in relation to process steam demand, but if this is practiced for more than short-term emergency occasions, the costs involved can cancel out the financial savings of the scheme.

Advantages and disadvantages of backpressure turbine

[1] Back-pressure turbines are inexpensive, thermally efficient, and compact, and usually the most economical proposition for partial generation schemes; but inflexible.

[2] Power generation is dependent on the steam flow required to meet process requirements and the pressure drop over the turbine. The running cost for electrical generation is therefore the marginally additional cost of generating steam at higher pressure and temperature than would be required for process usage only, plus the fuel equivalent of the heat drop across the turbine.

[3] Exhaust pressure from the turbine is determined by process steam pressure requirements. Inlet steam conditions, therefore, depend on the power generation required; seldom more than 4.5 MPa gauge, 400°C, although in large-scale chemical industry pressures may be up to 13 MPa. Thermal efficiencies in the range 75–85% are common, and this is virtually unaltered by varying the back pressure provided that all the steam can be used. Steam can be blown into the atmosphere or passed to a dump condenser to increase electrical generation in relation to process steam demand, but if this is practiced for more than short-term emergency occasions, the costs involved can cancel out the financial savings of the scheme.

Pass-out condensing turbines

Image C: The pass-out condensing turbine

If the process steam demand is small when compared with the electrical demand then a pass-out condensing turbine may provide the optimum solution. Image C illustrates a typical scheme, which consists of a backpressure turbine. This gives operational flexibility of the back-pressure turbine with improved power output.

Backpressure with double pass-out

Many industries require process steam at more than one pressure, and this can be done by use of a backpressure turbine supplying two process pressures Image: D

Image: D The double pass-out turbo-alternator

Image: D

Backpressure with double pass-out

Many industries require process steam at more than one pressure, and this can be done by use of a backpressure turbine supplying two process pressures

Extraction-condensing Turbines

Image: E The right-side image shows a typical extraction condensing turbine

Extraction-condensing turbines are employed when steady power generation and steam extraction at a fixed pressure is required. Extraction pressure is controlled internally in the turbine, allowing a wide range of extraction flow rates.

The extraction condensing turbine is able to change the electric power and the process steam flow independently by adjusting inlet steam flow and process steam flow. Adjustment of process steam flow is implemented by the extraction control valve. The turbine output is adjusted by the main control valve for inlet steam flow in conjunction with the LP turbine flow affected by the extraction control valve. That is to say, the extraction condensing turbine has both features of the condensing turbine and the backpressure turbine and has the capability to fulfill both the requirements of the electric power supply and the process steam flow.

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