Combined Heat and Power (CHP) Generation

The energy losses from power generation and from heat production can be reduced by combining heat and power generation. While the fuel utilization efficiency in conventional power plants is around one third, cogeneration increases this to 88-92% - depending on the technique used. Cogeneration significantly increases the overall energy recovery efficiency and helps reducing fuel consumption and air emissions. In most CHP applications the power to heat ratio is approximately 0.30 and this is an appropriate level for most mills. Moreover, high electricity and heat demands at a relatively stable ratio over the whole year makes paper manufacturing particularly well-suited for CHP generation. There can, however, be a need to balance power production towards mill's het demand. Moreover, high electricity and heat demands at a relatively stable ratio over the whole year makes paper manufacturing particularly well-suited for CHP generation (BREF, 2010. p. 67, 542).

There are several schemes for CHP and the capacities for production differs from < 1 MWth for smaller mils up to > 500 MWth for recovery boilers in large mills. In cases where a gaseous fuel can be used, the combined cycle gas turbine (CCGT) is an option that offers very high efficiency and is able to produce significantly more electricity for the same amount of heat required in the paper mill. Some key figures for different CHP plant types are given in Table below. 

Key figures for CHP plant types (BREF, 2010. p.67)
CHP plant type Total efficiency (LHV)
Fuel utilization rate
Power to heat ratio
Gas-fired boiler with extraction turbine 88 - 90% 0.20 - 0.35 
Biomass boiler and extraction turbine 88 - 90% 0.20 - 0.35
Combined cycle gas turbine with steam turbine 85 - 92% 0.40 - 1.1

In the figures below main types of CHP configurations used in the industry are depicted. Whie the boiler shown in the figure is a bubbling fluidized bed type, the boilers can be of any type and can be run on any fuel. An extracting back pressure (option A in the figure) or a condensing turbine (option B in the figure) can be used alternatively or at the same time (BREF, 2010. p.68)

An example CHP configuration

Typical CHP production at a pulp/paper mill (BREF, 2010. p.67)

CHP with CCGT

Example of a CCGT unit as CHP producing unit at a mill with both a back pressure steam turbine and a saturated steam circuit (BREF, 2010. p.68)

Development Status Products
Commercial

Combined Heat and Power (CHP) GenerationCosts & Benefits

Parent Process: Chemical Recovery
Energy Savings Potential

While in conventional power plants only less than 40% of the energy input is converted into electricity – and the rest of the energy is wasted –in cogeneration facilities 80-93% of the energy input is converted into 40-70% heat and 20-45% electricity.

CO2 Emission Reduction Potential

The CHP can reduce CO2 release rate by 50% compared to conventional power generation systems (BREF, 2010. p. 542).

Costs

The specific investment cost for transforming existing steam back-pressure units into combined cycle cogeneration plants is estimated to be EUR 1000/kW. The achievable savings and the payback time depend mainly on the price of electricity and fuels within the country.

Combined Heat and Power (CHP) Generation Publications