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