Introduction to Combined Heat and Power (CHP)
Thermal power plants have been traditionally fired on fossil fuels, however because of the amount of CO2 and other greenhouse gasses produced and emitted to the atmosphere, various other less polluting fuels are now being used.
Waste heat from these power plants can be utilized for community heating, where it is used for heating water for both the domestic and central heating systems.
This is an article on community heating in which we will examine a combined heat and power system along with its operating principles and benefits, both to the local people and the environment.
We begin with an overview of a typical thermal power plant supplying community heating.
Combined Heat and Power Using Biomass– Overview
There are numerous Combined Heat and Power plants throughout Europe and the USA using biomass as a solid or gaseous fuel. Solid biofuels such as chipped or pelleted wood are used to fire the boilers, raising superheated steam to power turbines that drive the power generators.
Wood can also be gasified; the resultant bio-gas used to run gas engines or gas turbines is also used to drive the electricity generators.
We shall examine the operation of a thermal power station which we will presume uses biomass as a boiler fuel to produce superheated steam, in a combined heat and power system.
The superheated steam passes through three stages in the steam turbine; entering the High Pressure (HP) turbine before passing into the Intermediate Pressure (IP) stage, from where it is fed into the Low Pressure (LP) turbine, exiting from here as steam and condensate into a vacuum condenser.
Fresh water is circulated around the condenser which further condenses the steam and condensate before it is pumped into the boiler feed system.
The condenser cooling water would then normally be pumped to a cooling tower from where it is cooled and circulated back through the condenser in a closed circuit.
However, in a CHP plant instead of using a conventional cooling tower to cool the condenser circulating water, it is pumped through a heat exchanger.
The medium used for community heating is also fed through the heat exchanger; extracting heat from the condenser cooling water and raising its temperature to between 80 and 130 °C, before being pumped to the community heating system.
At the same time this heat exchange reduces the temperature of the condenser cooling water before it is pumped back around the steam condenser in a closed circuit.
There can be additional boilers in the community heating system to supplement the hot water output from the power plant at peak times, such as during the winter months.
Community Heating System
The hot water supply and return pipes from the power plant community heating process area are fully insulated before being buried underground and joined by welding, in much the same manner as gas or water mains.
Numerous variable-speed in-line pumps are incorporated in the system to ensure optimum circulation and distribution of hot water.
The hot water is circulated through large bore supply and return pipes from which each individual dwelling is connected by small-bore piping. It is fed into a control box where its flow and return temperature is monitored, controlled, and metered.
After passing through the domestic hot water tank coils and central heating system, the water is returned to the street mains to be pumped back to the community heating plant.
Objectives of Using Combined Heat and Power
- Plant Thermal Efficiency
The overall thermal efficiency of a power station (oil @ 38%, gas @ 45%, and oil @ 34%) can be improved by either by retrofitting combined heat and power, or integrated into new builds.
This can raise the average thermal efficiency of a power station from 39%, to beyond 80% with the addition of CHP.
The CO2 and noxious gas emissions, which add to greenhouse gases and cause acid rain, are drastically reduced if biomass is used either as the sole fuel, or mixed in with a fossil fuel such as coal. Biogas produced from wood is also being used as a fuel both to fire boilers and run gas engines, whilst reducing emissions.
All these methods of producing electricity from biomass are being currently used worldwide saving large amounts of air pollutants.
- Benefits to the Community
I have examined several community heating projects and found that the local people were very satisfied with this type of heating system. They were proud to be recipients of community heating, extolling its virtues as being efficient, reliable, controllable, non-polluting and, less expensive than the conventional heating systems.