(co-) Combustion

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Introduction

Combustion of pyrolysis oil [1] is considered to be the easiest application although the ignition of the PL is more difficult than e.g. fuel oils or gas. Due to its properties some modifications are required on the combustion system and operating conditions. Typically, all devices, piping, storage and burner should at least be constructed from stainless steel to avoid rapid corrosion. Co-combustion with fossil fuels is a simple way to introduce this fuel in the energy market. Also 100% pyrolysis oil combustion has been demonstrated. Contrary to solid biomass, pyrolysis liquid can easily replace fuel oils or can be co-fired with natural gas.


Replacing heavy fuel oil at Fortum’s district heating plant

Significant amounts of bio-oil have been combusted in Fortum Power and Heat’s 1.5 MW district heating plant in Masala, Finland since 2010. The biooil was produced at Metso’s pilot plant, and it was whole oil including extractive-rich top phase [2]. No additives were used. The existing burner was replaced with a new bio-oil burner consisting of a modified mono block heavy fuel oil burner originally designed for high-pressure atomization. The modifications to the existing burner included, for example, burner head configuration. Also the piping, pumping and valve systems including pre-heating of the oil were specially designed for bio-oil. The total amount of bio-oil combusted is currently above 40 tons. Two main topics for the tests were the overall functionality of the bio-oil receiving, storing, and pumping system and the function of the burner. Both topics were successfully addressed. The receiving system and oil tank were located outside the boiler building. The system worked well, despite the outside temperatures, which varied from -20 to +10 °C during the test periods. As a result, good reliability and a satisfactory turn-down ratio of 1:3 were achieved. The unit has even been operated unmanned for 1 night. Flue gas emissions were close to those of heavy fuel oil. No odour emissions occurred.


Co-combustion with natural gas

DSCO3116.jpg
A large co-combustion test pyrolysis liquid with natural gas was carried out in 2003 by BTG and Electrabel [3]. In Harculo, Electrabel owns and operates a gas-fired power station of 350 MWe. Specifically for this test BTG produced roughly 15 ton of pyrolysis liquid from woody biomass in their pilot plant. A couple of months after production, the oil was co-fired in the power plant at a rate of about 1.5 t/hr. The test was successful and no operational problems were encountered. During the combustion campaign, the power station control system maintained a constant power output of 251 MWe and the resulting reduction of the natural gas flow also corresponded to 8 MWth. During pyrolysis oil co-firing an increase of 3 ppm in NOx emission was observed due to the nitrogen in the fuel.







Combustion tests of PL at Stork Thermeq

Stork Thermeq (Hengelo, the Netherlands) does have a test boiler with a performance representative for large scale units. In that specific unit the combustion of pyrolysis oil made from pine wood was compared to a reference case of heavy fuel oil (HFO) in the 9 MWth Stork test boiler using a Stork Low NOx Double Register gas- and oil burner [4] [5] . For atomization of the liquid fuels, an optimized Y-jet steam-assisted atomizer was used. The pyrolysis oil was preheated to a temperature of 60 ˚C in order to lower the viscosity and thereby enhance the atomization. The heavy fuel oil was preheated for the same reason to a temperature of 100 ˚C.

Pyrolysis oil was successfully fired at 2.6 MWth while HFO was fired at a capacity of 4.7 MWth. The reason for the lower capacity on pyrolysis oil, was the limited amount of available pyrolysis oil in combination with the minimum time required for reliable dust emission measurements. The flame of the pyrolysis oil stabilized at a larger distance from the impeller than the HFO flame. This is most likely due to the water content of the pyrolysis oil in combination with the lower heating value. It was found that a small natural gas pilot flame of 0.6 MW is required for flame stabilization when firing pyrolysis oil. It is believed that this pilot flame can be reduced or even omitted when preheating the combustion air. Besides natural gas, a liquid fuel may also be used for the pilot flame. The combustion of the pine oil was homogeneous and no abnormalities were visible. The combustion of the pine oil gave a significant lower NOx emission when comparing it to the HFO emission, which is due to the reduced flame temperature and low fuel nitrogen content.

Besides wood-derived PL also PL from Empty Fruit Bunches (EFB) was tested in the same unit. This liquid has a higher viscosity and a higher nitrogen content resulting in higher emissions of NOx.

Stork comb.jpg
Emissions from pyrolysis oil combustion [Stork Thermeq / BTG Bioliquids]
component Heavy Fuel Oil Wood Pyrolysis Liquid EFB Pyrolysis liquid
Density [kg/m 3 1,050 1,150 1,150
Kinematic viscosity [cSt at 40 C] n.d. 18 55
LHV [MJ/kg] 40 15 20
Moisture [wt%] 0.7 22 8
Nitogen Content [wt%] 0.4 0.1 0.4
Sulphur Content [wt%] 0.94 0.01 0.08
COMBUSTION
O2 [vol%] 4.0 3.0 3.9
NOx (mg/Nm3 at 3% O2) 550 133 395
CO (ppmv) < 5 16 <50
Dust [mg/Nm3] at 3% O 2 30 13-20 n.d.




References

  1. Lehto, Jani; Oasmaa, Anja; Solantausta, Yrjö; Kytö, Matti; Chiaramonti, David. 2013. Fuel oil quality and combustion of fast pyrolysis bio-oils. Espoo, VTT. 79 p. VTT Technology; 87 ISBN 978-951-38-7929-7 (Soft back ed.); 978-951-38-7930-3 http://www.vtt.fi/inf/pdf/technology/2013/T87.pdf
  2. Solantausta, Y., Oasmaa, A., Sipilä, K., Lindfors, C., Lehto, J., Autio, J., Jokela, P.,Alin, J. & Heiskanen, J. 2012. Bio-oil production from biomass: Steps toward demonstration: ACS. Energy & Fuels, Vol. 26, No. 1, pp. 233–240.
  3. Bio-oil as a coal substitute in a 600 MWe Power Station, BM Wagenaar, RH Venderbosch, W Prins, F Penninks, 12th European Conference and technology Exhibition on Biomass for Energy, Industry and Climate Protection, 17-21 June 2002, Amsterdam, The Netherlands
  4. 9TH EUROPEAN CONFERENCE ON INDUSTRIAL FURNACES AND BOILERS (INFUB-9), Estoril, Portugal, 26th - 29th of April 2011
  5. Experience with firing pyrolysis oil on industrial scale, Maarten Rinket, Ardy Toussaint, PyNe newsletter No. 31, pp 3-4