A typical Ranheat Engineering wood burning installation – cooler for summer use, or electrical generation, already on boiler room roof


Many have tried, and many have failed. Is the Organic Rankine Cycle the answer to producing electricity from wood-waste? Chris Franklin MD at Ranheat Engineering, a leading UK manufacturer of wood combustion equipment-continues his series of articles exclusively for Furniture & Joinery Production. In this issue Chris looks at the possibility of producing electricity using your wood-waste.

So, what is the ORC I hear you say? Well, it is the Organic Rankine Cycle. Still none the wiser? The basic theory is that using a conventional biomass boiler with relatively low water temperatures (85-110°C) the hot water passes through a turbine that produces the electricity the water is then cooled via a heat exchanger and then returned back to the boiler for re-heating.

I would point out that Ranheat do not make such a generator but are looking at projects with several manufacturers that do. Ranheat continue to be at the forefront of industrial wood fired systems but believe the construction and design of the ORC systems remains a very specialised process. The key seems to be the design and manufacture of the turbine.

Key points given by ORC technology providers are:

  • High efficiency even with low temperature heat sources
  • Reduced ownership running costs due to free fuel and low maintenance costs
  • Reliable and compact

Historically to generate electricity you needed to make high pressure steam and then pass it through a steam turbine. Generally, systems needed to be on a large scale as the amount of electricity was relatively small (20%) and a lot of waste heat was produced. This led to CHP plants (combined heat and power) these worked well if say attached to a district heating scheme that could use the waste heat. So not ideal for the UK where district heating is not common.

Separate combustion system below open bottom boiler, with removable grate system

Manufacturers of the ORC systems claim to have small systems available from 10kW to 180kW able to work with hot water from 80-120°C. Boilers in this temperature range need to be made to the new UKCA mark as the CE mark is being replaced in the UK following BREXIT – up until the end of 2024 the CE mark will still be accepted.

The success of an installation seems to require large water contents within the boiler, Ranheat systems have always had large water content boilers. This was to protect installations from power cuts. Ranheat systems incorporate separate combustion systems and boilers.

The combustion systems contain large amounts of refractory, necessary to reduce emissions of Carbon monoxide and other flue gas analytes.

The boilers are to a classic ‘horse shoe’ design with the open bottomed boiler sitting on top of the combustion system built into a boiler foundation which is lined with a combination of fire bricks and refractory cement.

Following discussions with local authorities the generation of electricity is seen as a good use of wood-waste particularly in the summer when heating for the factory is not required.

The combustion process will need to be permitted by the local authority and the type of permit required will be dependent on the mass of wood-waste the plant is capable of burning.

Plants up to 90kg per hour will need a part B permit for a SWIP (small waste incineration plant) plants over 90kg/hr but less than 220kg/hr will need a part B permit under PGN 602.

Plants over this size and up to 3000kg per hour will come under the jurisdiction of the Environment agency and all sizes  will come under the MCPD (medium combustion plant directive)

Ranheat continue to investigate these small-scale electrical generators. In theory they appear to be a good ‘bolt on’ to many of the existing Ranheat installations around the UK.

For further information contact Ranheat Engineering.

01604 750005