Diesel engines can be applied in stationary applications to provide heat and electricity. An interesting feature of these engines is that high electrical efficiency (> 40%) can be obtained even at relative low capacities. So far, this application has not been demonstrated.
Engine development and testing by Wärtsilä and VTT
The first work on using pyrolysis liquid in diesel engines was carried out in Finland by VTT and Wärtsilä. Engine performance and emissions were studied in a 4.8 kW single-cylinder test engine, in a 60 kWe four-cylinder Valmet 420 DS-engine and in a 410 kW Vasa 18V32 engine using one of its 18 cylinders on pyrolysis liquid. It was observed that:
- Pilot injection of diesel oil is needed.
- Fast heat release of pyrolysis liquid is observed.
- Encouraging thermal efficiency of 44.9% was achieved
- Specifications laid down on the properties of the pyrolysis liquid have to limit the solids content to a very low level and must provide tight heating value control
- Water content of pyrolysis liquid evens out the temperature gradient and is beneficial for NOx reduction
- The high density and low heating value of pyrolysis liquid has to be carefully considered when designing the fuel feeding system.
- Severe material wear occurs.
Wärtsilä’s 1.5 MWe medium-speed diesel power plant was modified for pyrolysis liquid use based on the experience gained in the earlier studies. A pyrolysis liquid feeding tank and feeding unit was constructed. The injection rate – injection system had been developed to 1450 bar and less than 30° injection period (6.7 ms) using the extreme fuel properties of 15 MJ/kg heating value and 1.2 kg/L density. Handling, quality control, feeding, and behaviour of a large amount (100 tonnes) of pyrolysis liquid were studied. The main results were as follows:
- All standard gaskets in the feeding system and seals in pumps could not tolerate the low pH of the pyrolysis liquid.
- The day/feed tank should be equipped with efficient mixing and temperature control to avoid segregation of the pyrolysis liquid.
- No direct heating of pyrolysis liquid allowed, preheating <90°C needed for viscosity reduction, minimum pyrolysis liquid re-cycling.
- In order to avoid lacquering at pistons or nozzles, the lubricating properties of the pyrolysis liquid should be improved and/or the lacquer forming compounds removed from the liquid.
- To prevent problems with the leakage of pyrolysis liquid to the lubricating oil side, a centrifuge in the lubricating oil line should be considered.
- Before carrying out large-scale diesel-engine tests, more R&D on the fuel feeding line and the injection nozzle system in a test-rig was recommended.
Wärtsilä stopped the development work mainly due to poor quality (i.e. high solids content) of pyrolysis liquids of that time.
Jay, D. C.; Rantanen, O. A.; Sipilä , K.; Nylund, N.-O. Wood Pyrolysis Oil for Diesel Engines. Proceedings of the ASME Fall Technical Conference, Milwaukee, WI, September 24-27, 1995; American Society for Mechanical Engineers (ASME): New York, 1995.
Chiaramonti, David; Oasmaa, Anja; Solantausta, Yrjö; Peacocke, Cordner. 2009. The use of biomass derived fast pyrolysis liquids in power generation: engines and turbines. 26 p. Publication No. 561 http://www.vtt.fi/inf/julkaisut/muut/2009/P561_2007_Chiaramonti.pdf.
Oasmaa, Anja, Peacocke, Cordner, Gust, Steven, Meier, Dietrich, McLellan, Richard. 2005. Norms and Standards for Pyrolysis Liquids. End-User Requirements and Specifications. Energy & Fuels, Vol. 19, Nr. 5, Pp. 2155-2163 doi:10.1021/ef040094o.
Engine development and testing at Pytec
|German company Pytec is developing CHP systems based on the use of fast pyrolysis oil in diesel engines. Their fast pyrolysis process is applying the so-called ablative pyrolysis principle. Instead of a heat carrier, the biomass is contacted with a hot metal surface. The bio-oil is combusted in a modified 450 kWe diesel engine (Mercedes-Benz).
Prior to injection the oil is preheated to 70°C to lower its viscosity and about 4% of diesel fuel was mixed with the pyrolysis oil to ensure lubrication and smooth combustion. The engine was started on diesel and then the bio-oil portion was slowly increased to 96%. In total 10 h of operation have been achieved. On average 120 l/h of bio-oil were consumed achieving an electrical output of 305 kWe and a reduction in exhaust gas temperature of 150°C compared to diesel combustion (derating).
Practical results from Pytec’s biomass to oil (BTO) process with ablative pyrolyser and diesel CHP plant, Dietrich Meier, Stefan Schöll, Hannes Klaubert, Jens Markgraf, Success & visions for Bioenergy, ISBN 978-1-872691-28-2.
Engine development at BTG
|BTG’s development started with a standard, 1-cylinder CI-engine, which has been adapted to run on bio-liquids including pyrolysis oil (see www.bioliquids-chp.eu). Obviously, some modifications are needed to run the engine on pyrolysis oil, and the most important one concerns the fuel injection pump and fuel injector; both have been made from a special stainless steel.
To overcome the poor ignition properties of pyrolysis oil a higher temperature is required in the cylinder when the fuel is injected. This can be achieved by increasing the air inlet temperature or the compression ratio. For pyrolysis oil fuelling an air inlet temperature of around 100°C is required at a compression ratio of 17.6. By increasing this ratio to 22.4 the air inlet temperature could be reduced with 40°C.
Adapting the fuel injecting timing might have some advantages for pyrolysis oil fuelling. Early injection means that more time is available to ignite the fuel and to achieve complete combustion. It appears that the optimal timing for pyrolysis oil is very comparable to sunflower oil and biodiesel. Early injection of pyrolysis oil resulted in severe operational problems.
Besides pure pyrolysis oil, a number of upgrading techniques have been applied to improve the properties of pyrolysis oil, like blending, hydrotreatment, emulsification and esterification. All these ‘upgraded’ pyrolysis liquids have been tested in the engine. Generally, the upgraded liquids were easier to ignite resulting in lower CO emissions and higher NOx, whereas the effect on overall efficiency was less pronounced. However, the liquids remained acidic, and therefore a modified injection system will still be required in all cases.
Long-term testing of pyrolysis oil fuelling will become necessary. CO emissions and pyrolysis oil fuel consumption were followed for a period of 40 h without a clear change in engine performance and emissions. Obviously, for further development and commercialization real long duration testing will be important (> 400 h).
The use of pyrolysis oil and pyrolysis oil derived fuels in diesel engines for CHP applications, L. van de Beld, J. Florijn, E. Holle, Applied Energy 102, (2013), pp. 190-197.
An experimental study on the use of pyrolysis oil in diesel engines for CHP applications, L. van de Beld, E. Holle, J. Florijn, 19th European Biomass Conference and Exhibition, 6-10 June 2011, pp. 1181-1187.