TESTS WITH PO IN A LARGE BOILER

The aim of the test was to verify the combustion behaviour and the emissions level with pyrolysis oil (PO) in a large scale boiler.

The combustion test was carried out in a 9 MW boiler at the Årsta District Heating Plant in Stockholm. The test was performed 1st and 2nd of April 2004 by Fortum Värme.

Årsta District Heating Plant is one of several production plants in the south district heating net of Stockholm. The plant is used as peak load and stand by plant within the net.
It is situated in a built up area and dwelling houses are on a distance less than 50 m.
Årsta District Heating Plant is equipped with four hot water boilers and one boiler producing steam for soot blowers. The used fuels are fossil and bio oils. The plant is operating from October until the middle of May.

The test was carried out in boiler 1 in Årsta District Heating Plant. The 9 MW boiler was originally designed for heavy fossil fuel but it is now adapted to different bio oils. The boiler has forced circulation with temperature regulated at 160 °C and no flue gas cleaning equipment.
The burner is a rotating cup burner manufactured by Petrokraft AB. The fuel is added inside a rotating cone (5000 rpm) and the rotation together with the primary air added outside the cone, atomises the fuel. When using different fuels it helps in mixing them.

The fuel was produced in the pilot-scale production plant in Porvoo, Finland, during the autumn 2003, shipped to Stockholm September 2003 and stored for 8 months in 1 m3 plastic containers.
When the test was run the fuel was pumped from the containers to a storage tank. There were no sediment layer and viscosity was good at room temperature. The liquid was pumped easily, but a problem was the characteristic smell.
Having the PO a low heating value, 21.10 MJ/Kg, it was necessary to co-fire it with a support fuel. In this test fatty acid was used but it could also have been tall pitch oil or heavy oil. Its heating value was 45 MJ/Kg.
The support fuel was delivered to the mass flow meter, filtered and preheated in the normal fuel system. The PO was handled in a separate test unit - an air tight, stainless, 10 m3 tank with stirring device. The PO was lead from the tank to a loop - rotor pump with controlled rpm and further to a 400 micrometer filter, a pre heater and through a mass flow meter.

The boiler was started up with 100 % support fuel. After a while PO was added. During the test burning PO was mixed with a decreasing amount of fatty acid. When the combustion began to be unstable the flame started to pulsate and the CO-level increase. The amount of PO was then reduced until stable conditions appeared again. The emissions measurement was started up.
It was noticed that the combustion was stabilized from an higher O2 level.
During the two days the boiler was operating with PO approximately six hours and the total consumption of PO was about 4 m3.

The result of the measurements in the flue gas is:

Parameter	Unit	Average
O2 content	vol % tg	4,5
CO2 content calculated	vol % tg	12,7
CO content	ppm tg	13
Dust content	mg/m3ntg	158
Spec dust emission	mg/MJ	68
NOx content	ppm	71
Do	mg/m3ntg	145
NOx emission	mg/MJ	62
SO2 content	ppm	<2
Spec gasflow*	m3ntg/MJ fuel	0,43

* Theoretically from oil analysis and measured O2 content.

Results:

· Handling of the PO was easy and the test worked out well;
· Emissions measurements indicated a dust content higher than expected while the NOx level was more or less the same than with pure fatty acid;
· Further tests are needed.

In order to have an industrial use of pyrolysis oil his heating value has to be increased or the combustion conditions have to be modified. The latter could be done adding some refractory bricks at the bottom of the boiler and close to the burner to create a radiating shield. Another option could be air preheating.