Information sheet: Combined heat and Power
Biomass Energy Centre, DEFRA, UK, Version 2.0 April 20, 2007 Biomass.Centre@forestry.gsi.gov.uk

Excerpt:

Combined heat and power (CHP), or co-generation, is the simultaneous generation of usable heat and electricity.
Conventional power plants usually dissipate the heat generation using cooling towers. With CHP it is used for domestic or industrial space heating or for process heat. Tri-generation also includes cooling using an absorption chiller.

Current biomass CHP technologies
Above about 1 MWe (around 3 MWth of heat output) conventional superheated steam turbine technology can be
used. Alternatively biomass gasification used with a gas turbine in a number of configurations may also be
appropriate at high output levels. However at lower output requirements neither of these options is suitable.
Organic Rankine Cycle (ORC)

Both reciprocating steam engines and steam turbine use the thermodynamic Rankine Cycle. At small scales
this becomes very inefficient and expensive owing to the high temperatures and pressures required. It is
possible to replace water as the working medium with an organic compound with a lower boiling point, such as a
silicone oil, freon or organic solvent. This allows the system to work more efficiently at much lower
temperatures, pressures and at smaller scale. As the working medium may be less corrosive to components
such as turbine blades, superheating is no longer necessary and also the turbine can operate at a lower speed,
potentially improving reliability.

Biomass CHP systems based on ORC technology are now commercially available from a number of
manufacturers. Electrical outputs are typically in the range 300 kWe–1.5 MWe with thermal to electrical output
typically around 5:1.

Internal combustion engine
Small scale biomass gasifiers exist that can convert biomass into a flammable product gas. Following suitable
clean up and cooling, this can be used to run an internal combustion engine such as a gas or modified diesel or
petrol engine.

Many commercial biomass CHP systems are available based on this technology, at scales from around 10 kWe
to 1 MWe, and typically give heat to electrical output ratio of 2:1, even down to 1:1. Gasifiers are however
inherently more complex than a simple combustion based system and there are a number of areas that can
potentially give difficulties.

There are a number of different designs of gasifier, however the majority of systems for use with an internal
combustion engine are of down-draft (fixed bed) design. These tend to produce low levels of tars, but they also
tend to require a high degree of consistency in the fuel supply, and cannot readily be scaled up above about
1 MWth. A fuel pre-processing unit of some kind can help ensure consistency, and can allow the use of a wide
range of feedstock, including dried sewage sludge. Good gas clean up is critical to reliable operation of the
engine, and this may include a particle filter and/or a cyclone as well as tar removal.
This tends to be the most widely available technology to meet biomass electricity generation needs at around
100 kWe output levels.

Stirling engine
The Stirling engine is an external combustion engine, using differential expansion of the working gas (such as
air, helium or hydrogen) in different regions of the engine drive the piston(s). As combustion is external and the
system is sealed, contamination can be less of a problem than with an internal combustion engine, and
maintenance accordingly eased.

Heat can be provided by direct combustion of biomass, or by combustion of product gas from a gasifier. Owing
to the lower purity requirements for the gas in the latter option, an up-draft gasifier can be utilized, potentially
easing the fuel supply constraints.

Stirling engines are available commercially with electrical output from 1 kWe to about 75 kWe. This is less well
proven, more experimental technology than a conventional internal combustion engine, and they tend to be less
efficient, giving a higher heat to electricity output ratio of 4:1 or greater.
Steam engine (reciprocating)

Although possibly viewed as rather old fashioned technology, a conventional reciprocating steam engine can be
used for electricity generation at a range of scales. It offers robust, well proven technology, and commercial
systems designed for woodfuel CHP applications are available.
Efficiencies tend to be low, however, with a thermal to electrical output ratio of perhaps 15:1, depending on the
system and scale.

Gas micro-turbine
A gas micro-turbine can be driven either directly by flue gas, or by air indirectly heated via a heat exchanger. A
compressor is required to raise the gas pressure to a suitable value, which requires significant power input.
There are few micro-turbine based biomass CHP systems in operation at present, and again this should be
regarded as technology still under development, however a relatively low heat to electrical output ratio of around
2:1 has been quoted by a manufacturer, suggesting good efficiency.