UTILIZATION IN MEDIUM SIZE (200 KW-1 MW) BOILERS

Fortum Oil & Gas (presently Neste Oil) has been developing combustion related equipment for the past several years together with Oilon OY of Finland.
In the project, the equipment developed by Oilon has been utilized without further modifications. It was found that in order to achieve clean combustion, a special burner retention head is required. This is because pyrolysis liquids which contain significant amounts of water have much lower heating value than hydrocarbon oils. This means that the amount of air to combust the pyrolysis liquid is approximately half compared to conventional fuels. A burner head was developed by Oilon taking this into consideration. In addition, the control system and the pump were changed.

The main aim of the performed tests has been to assess the technical feasibility of the designed combustion system during long duration tests of pyrolysis liquid. Special attention has been paid to material durability and deposit formation in valves, heaters, nozzles, burner retention heads and on boiler walls and to check the changes in the emission levels.

Different grades of pyrolysis oils were tested. Forestera™ produced from wood chips, having a solid content varying from 0.1 wt% to 0.04 wt% and water content 22-26 wt%, was tested.

Field trials were carried out in a heating oil boiler in Saarijärvi at the Thermia Oy plant. A total of more than 12000 liters of Forestera™ was combusted of which 4000 litres were for the COMBIO project and the boiler ran more than 1500 on-off cycles giving valuable feedback to further process improvements.
Start up and shut down operations were run with light fuel oil and the system was totally automated with the boiler being under the control of a thermostat.

erosion in nozzle

Early tests pointed out the importance of achieving very low levels of solids in the fuel. This resulted by both high levels of emissions and by erosion in the nozzles. Further development at the pilot plant reduced the solid levels to values <0.05 wt% which gave the emission levels presented in the table below.

It was found in these field trials that any inclusion of sand or any other hard solid in the liquid would cause erosion in the pump and in the nozzles. Nozzles had to be replaced every 1000 litres of fuel combusted. Emissions were also effected with higher particulates found in the flue gas. The erosion is due to the high pressure, 17-20 bar, required to obtain small drop size and clean combustion.

The emissions from the prototype burner in the Saarijärvi boiler were very low, approaching those of a light fuel oil boiler:

CO	30 ppm
O2	4,5 %
Particulate	no organic, only ash

The tests were run with pyrolysis oil having a solid content reduced by centrifuging to a value lower than 0.05 weight % and the nozzles showed no signs of erosion.

Technically the combustion system was a success and could be used for long term heat production. The cost of the combustion system was however higher than conventional systems due mainly to the cost for the industrial pump required but also to the increased automation.