New multiple fuel power plant

by P H Heyerdahl, Agricultural University of Norway

The Agricultural University of Norway has developed a multiple-fuel combustion unit that can burn many different types of biofuel, mixed in any proportion.


The author inspecting the prototype SK 200 combustion unit at the
Agricultural University of Norway.

Introduction
Developed under the programme "Small Scale Energy Technology", the combustion unit can burn logs, wood chips, pellets, bark, whole tree-roots, sawdust, straw, paper, wood from demolished buildings, slurry, solid waste fractions and biogas. These fuels can be mixed freely, in any proportions, and can even contain iron pieces, nails, sand, stones, etc. Fuel with a moisture content of up to 60%, depending on the fuel's calorific content, is accepted by the combustion plant and the power output can be continuously regulated from 25­100%. A 200 kWt combustion unit has been tested since February 1998 and a 1­2 MWt  unit is currently being set up in the town of Notodden. An Organic Power Combined Heat & Power (CHP) plant is made up of one or more combustion units and standard steam turbine(s). The maximum energy output of a plant depends on the number of combustion units combined. A 10 MWt plant would generate about 2 MW of electricity. The turnkey CHP plants are produced and sold by Organic Power A/S, Norway.

Programme aims
Established energy carriers such as coal, oil and gas have well-organised distribution networks, and utilisation equipment, like burners and engines, is easy to use and works well. If bio-fuels are to compete with these energy carriers we have to utilise their comparative advantages, namely: that bio-fuels often are found close to the energy consumer and that the raw fuel usually has the lowest price. Therefore the main goals of the programme "Small Scale Energy Technology" is to develop combustion plants meeting the following demands:

  • no need for costly pre-processing of the fuel;
     
  • can burn all fuels in any mix with varying moist content;
     
  • robust technology;
     
  • installation size can be economically fitted to energy needs;
     
  • meet the strictest emission requirements;
     
  • reduce transport;
     
  • solve a waste problem;
     
  • keep jobs and capital within the community.


The multiple fuel combustion unit.

Working principle
The body of the combustion unit is a fireproof well-insulated silo. The silo is filled manually or automatically from the top of a storage silo that may contain fuel for several days. The only fuel requirement is that pieces are small enough to allow the lid to be closed after refilling and that they are no larger than the silo opening.

Primary combustion occurs at the bottom of the fireproof silo, (see figure on previous page). Air is pushed in at the the bottom of the combustion chamber through a patent-pending manifold system. The air­fuel mixture starts a combination of incomplete combustion and gasification takes place. This causes the fuel to gasify above the lower part of the combustion chamber. Gravity forces the new fuel down the fuel column into the primary combustion zone, keeping it constantly supplied with new fuel even though the silo is refilled in batches.

The main body of ashes remains in the primary chamber and is pushed towards the apex of the V-shaped bottom. The ashes are sluiced out of the combustion chamber while unwanted air is prevented from entering the primary combustion zone. The ashes may later be utilised as an additive to fertilisers or be spread over suitable cultivation areas.

The hot mix of flue gases and low-calorimetric gases is led into the secondary combustion chamber where pre-heated secondary air is added. At this point bio-gas can be added for co-combustion with the gases from the solid fuel, reducing the solid fuel consumption correspondingly. The flue gases are then led into an insulated cyclone for further combustion. Here, remaining particles in the flue gas are separated. In both turbulent zones, the gases are completely combusted.

The flue gas is then led to a boiler or steam generator. The driving force of the gas flow through the combustion unit and heat exchangers is a slight under-pressure created by the flue-gas fan at the top of the chimney. This also directs any possible leakage into the combustion chambers.

 Contents of unfiltered flue gas: preliminary  results

 CO

 5­50 mg/Nm3

 NOx

 30­150 mg/Nm3

 Dust

 <100 mg/Nm3

A 1 MW combustion plant will be completed by February 1999. Performance and certified emission tests will then be carried out on different fuels.

The NOxcontent is sensitive to the temperature in the secondary chamber, but is below the strictest international limits.

For more information contact the Norwegian National Team at Rud.

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