Abstract
The presented paper deals with the obstacles in the utilization of biodiesel in small stationary combustion units. The aim of the paper is to discuss the technical problems related to the combustion of biodiesel by means of two different types of atomizers, namely the pneumatic effervescent atomizer and the pressure spill-return swirl atomizer. The technical problems are described based on the comparative combustion tests that were carried out at the nominal heat duty about 1000 kW. The tests were carried out at the experimental burner testing facility, whose main apparatus is the water-cooled combustion chamber.
The first part of the paper describes the influence of the physical-chemical properties of the methylester of rape seed oil (RME) on the quality of combustion. The properties of RME are compared to the physical-chemical properties of the most commonly used standardized liquid fuel in small stationary combustion units, namely extra light heating oil (ELHO). There were identified two properties of the RME, namely the flash point and the shape of distillation curve, which cause the problems with the combustion stability by the application of both mechanisms of the atomization.
The results of the combustion tests showed that it is required to preheat the RME at the temperature higher than 60 °C and it is useful to use a small gas stabilization burner to stabilize the spray ignition in close proximity of the atomizer head. The next increase of the preheating temperature above 80 °C leads to the significant increase in nitrogen oxides formation, however simultaneously, the stability of the flame becomes better and the carbon monoxide formation decreases.
The negligible difference between ELHO and RME was observed in the heat transfer rate to the water-cooled chamber shell. On the other hand, the problems related to the RME spray drops evaporation causes that the maximum heat transfer shifts in the direction further from the burner mouth.
The second part of the paper is aimed at the assessment of the possibility of the substitution of the ELHO by RME or some of the vegetable oils from the general view. It may be concluded that the requirements on the RME combustion are evidently harder than for ELHO. It mainly concerns the quality of atomization and the quality of mixing of fuel drops with the oxidizer.
Generally speaking it would be better to focus the research on the development of the pressure atomizer and on the decrease of the burner nominal heat duty to the range between 200 and 500 kW. This fact is related to the possibility to utilize these fuels at the decentralized units and/or as the stabilization fuel during the combustion of renewable solid fuels or waste. It is not recommended to use renewable liquid fuels as the main fuel at the large stationary combustion units since it could cause their shortage in other areas of economy.
