The primary objective of this project was to develop fundamental methods to maximize delivery of usable waste chemical energy to the combustion chamber of an internal combustion engine (ICE) in the simplest and most robust method possible. The work is novel in that it evaluated potential delivery of the chemical energy to the engine via the liquid fuel injection system as captured tars in addition to the traditional gas phase as syngas. The delivered chemical energy must: be combustible in the engine, not foul air and gas intakes, and not corrode or foul fuel injection systems. This work looked at pilot-scale production of tars and evaluated plasma and catalytic cracking of the tars to the minimal extent possible (to benzene, toluene, and xylene as opposed to hydrogen and carbon monoxide) to allow ICE utilization.
Waste materials and biomass are potentially a valuable energy resource if they can be gasified and the products combusted in electricity generation. Co-produced tars and gas cleanup have hindered utilization and reduced conversion efficiency. This study used an updraft gasifier to generate a tar rich gas stream and then attempted to evaluate plasma and catalytic reformation of the tars in a pilot plant configuration to make a higher percent of the tars usable as fuel. The feedstock used was a blend of wood chips, paper, plastic (halide free), and dry dog food. Gas was sampled at several process steps with techniques that allowed analytical and gravimetric analysis of the gas and tar stream. The sampling techniques simulated a scrubber using methyl chloroform, isopropyl alcohol, and a renewable JP-8 (R8) engine fuel as the working fluid in impinger tubes. The purpose of the simulated scrubber aspect of this work was to investigate capture of the tars in a fluid that could later be burned via the liquid fuel injection system of existing generator sets to enable maximum conversion of the waste to electricity. The captured gas and tar liquids were analyzed using gas chromatography (GC), flame ionization detection (FID), thermal conductivity detection (TCD), and mass spectroscopy (MS). The non-volatile constituents were evaluated for total mass. The numbers of experiments performed were limited due to the difficulty in getting the numerous systems and sampling systems to work together for the first time.
The gasification system, tar cracking experiments, and analytical techniques were put in place and shown to perform as expected to some degree. Hydrogen was produced by the gasifier at 16% maximum concentration (10% average); carbon monoxide was produced at 6% maximum and 4% average. Tar was generated from the gasifier at a minimum concentration of 150 g/m3. It is estimated that the usable portion of this tar is more than 100 g/m3. The tar was decreased by the catalyst experiment but was not used to generate statistically significant data due to technical startup problems that consumed time and funding availability. The plasma system had flow and arc instability problems coupled with a hard failure (melted insulator) but did show a decrease in hydrogen, carbon monoxide, and methane concentration in the limited data obtained. The plasma system has been repaired and is functional, but meaningful data has not yet been generated. Scrubbing with R8 removed all aromatic and heavy tar from the syngas. An amber ~30 carbon long straight alkane material was found running out from the flare that is mostly soluble in R8, implying the soluble portion would be an acceptable engine fuel additive and significant energy source depending on the amount of plastic in the waste stream.
Converting waste to energy cleanly and efficiently through gasification has long been desired to address waste disposal incineration limitations and exposures while providing an energy source at forward operating bases (FOBs). This project determined that a plasma or catalytic reactor system is probably too complex for a FOB waste to energy system and the focus for future work should fall back to the simplest method of getting the waste energy into the engine. This is likely gas scrubbing with a fuel, filtration of the fuel, then passing the fuel and the cleaned gas to the engine. With the data from this project, researchers can narrow down a design for a successful waste to energy unit for FOBs.