Opra Turbines develops new combustor for biofuels

An R&D project is underway to develop a gas turbine combustor by Opra Turbines for efficient combustion of pyrolysis oil. The higher viscosity, lower energy density and limited chemical stability of the pyrolysis oil require different fuel handling compared to conventional liquid fuels. As a fuel for the experimental work, pyrolysis oil from pine wood, provided by BTG Bioliquids BV, was used.

In a test, due to the higher viscosity of the pyrolysis oil, a pintle airblast nozzle, designed by OPRA, was selected as the fuel injector. The pintle airblast nozzle can handle liquid fuels with higher kinematic viscosity than a standard pressure nozzle. A major advantage of the pintle airblast nozzle is that it works with the pressure difference caused by the pressure loss across the combustor flame tube. This means that a pintle airblast nozzle does not need any external source of air or steam.

The OP16-3A gas turbine conventional combustor working in diffusion mode was used for the experiments. A benefit of this combustor is that the flame tube is flexible for changes related to the effective flame tube area and the air split across the combustor.

The OP16-3A conventional combustor was adjusted in several steps to find the optimal configuration for pyrolysis oil burning.

The first test was performed with the standard flame tube and without fuel pre-heating. With the standard configuration it was found to be possible to keep a stable combustion process with a mixture of 25% pyrolysis oil and 75% ethanol. The residence time is related to the combustor volume. Due to the differences in fuel composition, especially the large amount of dilutants associated with the pyrolysis oil, the residence time for pyrolysis oil is longer than for fossil fuels. From the initial tests it became clear that the required combustor outlet temperature could not be reached.

The unburned fuel resulting from incomplete combustion formed sediment on the flame tube inner wall and in the exhaust duct. Based on the test results from this configuration, the combustor geometry was adjusted and an optimized geometry was found.

To simulate the effect of a larger volume the effective area of the flame tube and the amount of air entering the combustor were decreased.

The pressure loss of the combustor is an important parameter governing the mixing process and the function of the airblast atomizer. Therefore, it was

important to decrease the effective combustor area and air mass flow to maintain the same pressure loss as the original combustor. By decreasing the effective flame tube area and the inlet air mass flow, a significantly longer residence time was achieved. A significant improvement of the flame stability was achieved and no unburned fuel in the exhaust duct was observed. Due to the decrease of the combustor loading the full load condition was reached.

Furthermore, it was found that with between 70% to 100% load, it is possible to burn 100% pyrolysis oil without the need of mixing it with ethanol. Based on this research, OPRA has been able to design a new combustor for burning pyrolysis oil and other low-calorific fuels. The new combustor is large enough to provide sufficient residence time for complete combustion of the pyrolysis oil. 

(Above are excerpts from a paper prepared by Martin Beran and Lars Uno-Axelsson of Opra Turbines)