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Description of the biomass CHP technology based on a vegetable oil combustion engine

BIOS BIOENERGIESYSTEME GmbH, Graz

Combustion engines convert the chemical energy of fuels into mechanical energy. The engine is attached to the shaft of the generator and provides the mechanical energy to drive the generator to produce electricity. Combustion engines have one or more cylinders in which fuel combustion occurs.

Combustion engines can generate electricity almost immediately upon start-up. For this reason, internal combustion generators based on fossil fuels are often used to provide emergency power and as solutions for base-load or peak-load electricity supplies in remote or inaccessible areas throughout the world. During combustion, the pressure and temperature increase is very high and this allows high a conversion efficiency for small units.

Most engine systems for power generation use diesel oil or heavy fuel oil as liquid fuel. But the commitment to the continuous growth of renewable energy production is giving increasing room for the use of liquid biomass in combustion engines.

In the last time therefore some potential supplier of engines developed solutions to burn liquid fuels such as crude vegetable oils (rape seed oil, olive oil and palm oil) and some waste and recycled biofuels (animal fat, waste cooking oil, recycled frying fat, etc).

The physical and chemical properties of liquid biofuels are different compared to most commonly used mineral fuels. Biofuel has for instance the following identifying features when compared to mineral fuels: lower net calorific value, higher viscosity and density, lower stöchiometric air-to-fuel ratio because of higher oxygen content. Therefore a special adapted fuels storage and supply system as well as a fuel injection equipment is required for operation with biofuels. Some higher viscosity biofuels need to be heated up sufficiently to reduce viscosity to injection levels.

But in general vegetable oils are within the minimum quality requirements for operation in combustion engines. Animal fat, waste cooking oil and recycled frying fat can be used after an advanced fuel treatment.

Working principle and integration in a biomass CHP plant

In principle diesel engines are fuel flexible and can use the mentioned vegetable oil and biofuels. In a diesel engine, air is forced into the cylinder and is compressed by the piston. Fuel is injected into the cylinder and ignited by the heat of the air compression. The burning mixture of fuel and air expands, pushing the piston. Finally the products of combustion are removed from the cylinder, completing the cycle. The energy released from the combustion of fuel is transferred to the engine flywheel via the moving piston. An alternator is connected to the rotating engine flywheel and produces electricity.

All combustion engines in this field of application are typically equipped with a turbocharger. A turbocharger is an exhaust gas driven supercharger. All superchargers have a gas compressor in the intake tract of the engine which compresses the intake air above atmospheric pressure, greatly increasing the volumetric efficiency beyond that of naturally-aspirated engines. A turbocharger also has a turbine that powers the compressor using waste energy from the exhaust gases. The compressor increases the pressure of the air entering the engine, so a greater mass of oxygen enters the combustion chamber in the same time interval. Cold air has an higher density than hot air. Therefore compressed air from a turbo is cooled to increase the volumetric efficiency before it is fed into the cylinders, using a charge air cooler.

For the starts and stops of the combustion engine due to the bad injecting characteristics of the vegetable oils (higher viscosity at lower temperatures) and to wash away impurities from the fuel pipes conventional mineral fuels or rape seed methyl ester are used (dual fuel system).

In vegetable oil operation, the fuel is first pre-cleaned and heated in the fuel treatment system before injection into the engine. The filters and separators in the fuel treatment system remove impurities and water in the fuel.

Fig. 1

As a basic principle at combustion engine plants heat can be provided via heat exchangers for lube oil, engine cooling water and charge air cooling as well as from the flue gas (see also Figure 1). In hot water production the cascade connection of the particular heating sources depends on the requirements of the heat consumers on site (e.g. necessary outgoing hot water temperature). If there is a demand certain amounts of hot water above 100 °C or low pressure steam can be produced by utilising the energy in the flue gas.

Another interesting utilisation of the flue gas energy is the implementing of an ORC module downstream of the combustion engine to increase the electricity production.
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To reduce emissions from vegetable oil combustion engines for energy generation depending on the power range different secondary measures have to be implemented. The installation of catalytic systems (SCR, oxidation) and particle filters allow the abatement of relevant emissions down to levels which are in compliance with respective environment regulations (e.g. German TA-Luft).

Relevant technical data and electric efficiencies of CHP plants based on a vegetable oil combustion engine

  • Engine cycle: four-stroke
  • Number of cylinders: 1 – 16
  • Cylinder configuration: inline engine, V-engine
  • Turbo charging system: with two stage charge air cooler
  • Starting method: electrical (small engines) or compressed air
  • Electric capacity (at the generator terminals): 5 – 8,000 kW
  • Electric efficiency based on the net caloric value of the fuel: 20 – 45 %

Realised projects and proposals under design based on a vegetable oil combustion engine

  • Kraft-Wärme-Kopplungsanlage auf Basis Pflanzenöl mittels Motorenblockheizkraftwerk und nachgeschaltetem ORC-Prozess – Pflanzenöl BHKW New Energy (Deutschland)
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