Alkali, Chlorine, SOx and NOx Release During Combustion of Pyrolysis Oils and Chars

Biomass-derived pyrolysis oils must contain very low levels of alkali to become competitive fuels for use in turbine combustors for power generation. Based on standards set for traditional petroleum fuel oils, turbine manufacturers require total alkali levels in the sub-ppm range. Controlling NOx and SOx release during pyrolysis oil combustion must be considered to alleviate environmentalconcerns. Chlorine release from the combustion of pyrolysis oil will have important implications concerning corrosion of heat transfer surfaces in boilers and turbine blades in direct-pyrolysis oil-fired facilities. The transformation of nitrogen, sulfur, and chlorine during the production of biomass-derived pyrolysis oils is unknown. When pyrolysis oil is produced, most of the alkali present inthe starting biomass material remains sequestered in the char fines. The alkali content of the oils, therefore, is thought to be predominantly a function of the amount of char fines suspended in the oil. Biomass- derived oils and chars were screened to investigate the fate of nitrogen, sulfur, chlorine, and alkali metal sequestered in the char during the production of pyrolysis oil from biomass.We have used the molecular beam mass spectrometer system, constructed and operated at NREL, to directly sample the hot gases liberated during the combustion of small pyrolysis oil and char samples in a variable temperature quartz tube reactor. This system is ideal for studying high-temperature, ambient- pressure environments such as those encountered during these screening studies. Chemicalreactions are quenched and condensation is inhibited during the free-jet expansion of the high-temperature combustion gases. As a result, reactive and condensable species remain in the gas phase at temperatures far below their condensation point. We have successfully used this experimental technique to identify alkali species released during the combustion of selected biomass feedstocks and haveextended this work to monitor the release of nitrogen, sulfur, chlorine, and alkali metal vapors during the combustion of pyrolysis oils and chars. Four of the pyrolysis oils screened in this study (two switchgrass oils, an oak oil, and a poplar oil) were produced in the NREL vortex reactor. The other three oils screened (a switchgrass oil, a poplar oil, and a corn stover oil) were produced in alaboratory-scale (2-in. diameter) fluidized bed combustor. The biomass chars were recovered from the hot filters on the NREL vortex reactor during pyrolysis oil production. This presentation contains results describing the release of NOx ,SOx , chlorine, and alkali metal vapors during the combustion of biomass-derived pyrolysis oils and chars and compares these results with the combustion of theparent biomass feedstock where applicable. Comparisons will be made between the oils made by the two processes. The relative amounts of NO, SO2, and hydrochloric acid released during pyrolysis oil combustion will be explored. The presentation also compares the amount of alkali metal released as potassium chloride during the combustion of switchgrass, switchgrass chars, and switchgrass-derivedpyrolysis oils. Contact (e-mail): [email protected]