Pyrolysis of sugarcane bagasse

[摘要] ENGLISH ABSTRACT: The world's depleting fossil fuels and increasing greenhouse gas emissions have given rise to muchresearch into renewable and cleaner energy. Biomass is unique in providing the only renewable source offixed carbon. Agricultural residues such as Sugarcane Bagasse (SB) are feedstocks for 'second generationfuels' which means they do not compete with production of food crops. In South Africa approximately 6million tons of raw SB is produced annually, most of which is combusted onsite for steam generation. Inlight of the current interest in bio-fuels and the poor utilization of SB as energy product in the sugarindustry, alternative energy recovery processes should be investigated. This study looks into thethermochemical upgrading of SB by means of pyrolysis.Biomass pyrolysis is defined as the thermo-chemical decomposition of organic materials in the absence ofoxygen or other reactants. Slow Pyrolysis (SP), Vacuum Pyrolysis (VP), and Fast Pyrolysis (FP) arestudied in this thesis. Varying amounts of char and bio-oil are produced by the different processes, whichboth provide advantages to the sugar industry. Char can be combusted or gasified as an energy-dense fuel,used as bio-char fertilizer, or upgraded to activated carbon. High quality bio-oil can be combusted orgasified as a liquid energy-dense fuel, can be used as a chemical feedstock, and shows potential forupgrading to transport fuel quality. FP is the most modern of the pyrolysis technologies and is focused onoil production. In order to investigate this process a 1 kg/h FP unit was designed, constructed andcommissioned. The new unit was tested and compared to two different FP processes atForschungszentrum Karlsruhe (FZK) in Germany. As a means of investigating the devolatilizationbehaviour of SB a Thermogravimetric Analysis (TGA) study was conducted. To investigate the quality ofproducts that can be obtained an experimental study was done on SP, VP, and FP.Three distinct mass loss stages were identified from TGA. The first stage, 25 to 110°C, is due toevaporation of moisture. Pyrolitic devolatilization was shown to start at 230°C. The final stage occurs attemperatures above 370°C and is associated with the cracking of heavier bonds and char formation. Theoptimal decomposition temperatures for hemicellulose and cellulose were identified as 290°C and 345°C,respectively. Lignin was found to decompose over the entire temperature range without a distinct peak.These results were confirmed by a previous study on TGA of bagasse.SP and VP of bagasse were studied in the same reactor to allow for accurate comparison. Both theseprocesses were conducted at low heating rates (20°C/min) and were therefore focused on char production. Slow pyrolysis produced the highest char yield, and char calorific value. Vacuum pyrolysis produced thehighest BET surface area chars (>300 m2/g) and bio-oil that contained significantly less water comparedto SP bio-oil. The short vapour residence time in the VP process improved the quality of liquids. Themechanism for pore formation is improved at low pressure, thereby producing higher surface area chars.A trade-off exists between the yield of char and the quality thereof.FP at Stellenbosch University produced liquid yields up to 65 ± 3 wt% at the established optimaltemperature of 500°C. The properties of the bio-oil from the newly designed unit compared well to bio-oilfrom the units at FZK. The char properties showed some variation for the different FP processes. At theoptimal FP conditions 20 wt% extra bio-oil is produced compared to SP and VP. The FP bio-oil contained20 wt% water and the calorific value was estimated at 18 ± 1 MJ/kg. The energy per volume of FP bio-oilwas estimated to be at least 11 times more than dry SB. FP was found to be the most effective process forproducing a single product with over 60% of the original biomass energy. The optimal productions ofeither high quality bio-oil or high surface area char were found to be application dependent.

[发布机构] Stellenbosch University