Pyrolysis oil combustion in a horizontal box furnace with an externally mixed nozzle

Authors: LUJAJI, FRANK - Nelson Mandela African Institute Of Science And Technology; Boateng, Akwasi; Schaffer, Mark; Mullen, Charles; MTUI, PETER - Nelson Mandela African Institute Of Science And Technology; MKILAHA, IDDI - Nelson Mandela African Institute Of Science And Technology

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2016
Publication Date: 4/28/2016
Publication URL: https://handle.nal.usda.gov/10113/62550
Citation: Lujaji, F.C., Boateng, A.A., Schaffer, M.A., Mullen, C.A., Mtui, P.L., Mkilaha, I.S. 2016. Pyrolysis oil combustion in a horizontal box furnace with an externally mixed nozzle. Energy and Fuels. 30:4126-4136.

Interpretive Summary: Fast pyrolysis is a thermochemical process to convert biomass such as forestry and agricultural waste into biofuels by rapidly heating them in the absence of oxygen. The fuel properties of pyrolysis oil (bio-oil) produced from the fast pyrolysis of biomass presents challenges when used as fuel in burner systems. These challenges include high acidity, high water content, high oxygen content, wide range of volatility, high viscosity, and the presence of low levels of char in the bio-oil. We recently conducted a spray-atomization study with a high viscosity bio-oil and ethanol blend using an externally mixed nozzle and achieved good spray characteristics. In this study, we sought to demonstrate and study the combustion characteristics of 100% bio-oil in a burner equipped with externally mixed nozzle type. We designed a combustion chamber, instrumented and installed a modified burner nominally rated at 100 KW (350,000 BTU/h) for testing. We found out that, the externally mixed nozzle allowed for a successful stable combustion of 100% bio-oil. The combustion chamber wall temperatures for bio-oil were slightly lower when compared to diesel at the same energy input indicating furnace derating. The bio-oil flame was longer with slightly higher emission of hydrocarbons as compared to diesel. We can conclude that, bio-oil can successfully be used as fuel in a commercial burner equipped with externally mixed nozzle. This provides an opportunity for utilization of bio-oil as a bio-resource replacement of fossil fuels in heating energy applications.

Technical Abstract: Combustion characteristics of neat biomass fast-pyrolysis oil were studied in a horizontal combustion chamber with a rectangular cross-section. An air-assisted externally mixed nozzle known to successfully atomize heavy fuel oils was installed in a modified nominal 100 kW (350,000 BTU/h nominal capacity) burner to explore full utility for pyrolysis oil (bio-oil) combustion in a furnace. Combustion experiments were conducted at an equivalence ratio of 0.46, 0.53 and 0.68 (117, 88 and 47% excess air, respectively) and compared with diesel fuel flames (control) at the two higher equivalence ratios. In these experiments, the fuel flowrate was maintained at a constant energy input (equivalent of 24 kWth). The results revealed that while the externally mixed nozzle could effectively atomize and ensure stable combustion of neat bio-oil at the set heat rate, this comes with a penalty associated with a lower peak flame temperature and hence heat flux. Formation of carbon monoxide (CO) decreases with increasing equivalence ratio for bio-oil combustion. The levels of carbon dioxide (CO2) and nitrogen oxides (NOx) increase with increasing equivalence ratio for bio-oil combustion and were slightly higher than that generated by diesel. Hydrocarbon emissions do not follow any defined trend with increasing equivalence ratio for bio-oil as is typically observed for diesel fuels due to the oxygenated nature of the bio-oil mixture.