MICROWAVE-INDUCED PYROLYSIS OF CORN STOVER: THE INFLUENCE OF POTASSIUM ON THE GAS FRACTION

Open Access
Author:
Hartwell, Bradley J.
Graduate Program:
Energy and Geo-Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
February 01, 2010
Committee Members:
  • Sarma V Pisupati, Thesis Advisor
Keywords:
  • catalysis
  • catalyst
  • gas fraction
  • potassium
  • corn stalk
  • corn stover
  • pyrolysis
  • microwave
  • hydrogasification
Abstract:
At the turn of the twenty-first century, climate change became a hot topic in the mainstream media, and as a result, a worldwide concern arose about the possibility of human activity causing recent global temperatures to increase. This concern, coupled with fossil fuel resources becoming increasingly scarce, has caused a push for alternative fuel research. This work focuses on converting a renewable feedstock, corn stover, to produce a useful gaseous fuel. Corn stover is the portion of the corn plant that is left over after the corn has been harvested from the stalk. Corn stover was selected for this study due to its relatively low ash content, which in turn allows for removal of catalytic inorganic content that would interfere with the investigation of potassium’s effects on conversion. The conversion process carried out in this study is pyrolysis, which is the thermal decomposition of organic matter in an oxygen-free environment. Different pyrolysis conditions determine whether bio-oil or biogas will be a major byproduct. This study’s pyrolysis conditions are designed to maximize the biogas fraction, and more specifically, to produce a high quality syngas consisting of mainly H2 and CO. Syngas can be efficiently burned in a small gas turbine to generate electricity, or utilized in a Fischer-Tropsch process to produce liquid fuel. The objective of this study is to investigate the effects of potassium between conventional and microwave pyrolysis. This objective was met by carrying out laboratory scale pyrolysis experiments in both an electrical tube furnace, and a multimode microwave operating at 2.45 GHz. Three different corn stover samples were investigated: stripped (S), stripped with 0.7 percent potassium addition (K0.7), stripped with 3.5 percent potassium addition (K3.5). All feedstocks were brought to 700 °C in the conventional pyrolysis experiments. All feedstocks were brought to both 700 and 1000 °C in the microwave experiments in order to get a qualitative trend on the influence of microwave pyrolysis temperature. To get a quantitative understanding of potassium’s response to microwave pyrolysis temperature, K3.5 was also brought to 800, 900 and 1100 °C in the microwave. It was observed in both microwave and conventional pyrolysis that potassium’s main effects on pyrolysis in order of descending influence are: inhibiting volatile release, promoting secondary cracking reactions of heavy hydrocarbons (oil) and promoting the decomposition reaction of methane at 700 °C. Potassium did not however appear to promote the carbon/CO2 heterogeneous reaction which would have rejuvenated catalyst sites for further methane decomposition, as proposed in a previous study [1]. Potassium was shown to significantly inhibit the release of oxygen containing volatiles during pyrolysis at temperatures 700 to 1100 °C, even with a loading of 0.7 percent. The result from this is a much larger char fraction when potassium was added. This inhibiting effect of potassium was found to be important at higher temperatures. A consistent trend among all temperatures tested in this study was that in microwave pyrolysis, potassium addition decreased the oil fraction produced. This means that potassium promoted the secondary reactions that broke down the heavy hydrocarbons into non-condensable gases. A potassium loading between 0.7 and 3.5 percent was found to promote the decomposition of methane reaction at 700 °C, but not at 1000 °C. The reason why the decomposition of methane was not observed at the higher temperature is because at higher temperatures, potassium’s pyrolysis inhibiting affect is the dominant effect. As a result of this, there was no increase in H2 at the expense of CH4. Potassium’s promoting of the decomposition of methane was 60 percent more pronounced in microwave pyrolysis compared to conventional pyrolysis at 700 °C. A likely reason for this difference is that in microwave heating, the potassium could have achieved temperatures much higher than the rest of the sample, because in microwave heating, microwave receptive materials reach much higher temperatures than adjacent materials. This anisothermal heating has been demonstrated to enhance reactions taking place within a sample [2]. Anisothermal conditions were the most likely reason for the microwave pyrolysis’ increased methane decomposition reactions. Methane decomposition was promoted with a potassium loading between 0.7 and 3.5 percent. The experiment that showed the best syngas production was for sample S brought to 1000 °C in the microwave. This experiment yielded 88 mL/g (ash free) of H2 and 120 mL/g (ash free) of CO.