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Integration of hot water extraction in biomass based CHP plants: possibilities for green chemicals and increased electricity production
2010 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

The main objective of this thesis was to investigate the possibility to increase the steam temperature in the superheater in biomass (hard wood) fired CHP plants by the integration of a hemicellulose hot-water extraction process. This was motivated due to the cleaner fuel (extracted wood chips) obtained after the extraction process. One important result from the chemical analysis of the extracted wood-chips was the removal of a large share of the inorganic constituents, except Chlorine and Sulphur, from the lignocellulosic biomass. Some of those inorganic constituents (alkalis) are believed to be responsible for the fireside heat exchanger and boiler walls corrosion problems during combustion. A large portion of the current work aimed to investigate the possibility to increase the superheated steam temperature in biomass fired boilers. Thermodynamic equilibrium calculations were performed for the extracted as well as for the original birch wood chips combustion using FactSage 6.0. The results showed significant differences in equilibrium concentrations between the fuels, which could be relevant initial step towards increased temperature operation in biomass fired boilers. Converting those results into possible operational steam temperature increment is something that needs further research. A techno-economic analysis was performed for a new integrated process that produces ethanol and acetic acid from hardwood (Birch) in addition to electricity and heat. The process involves the extraction of hemicellulose content of hardwood using water at a temperature of 160 - 180℃ and a pressure of 7 bars. The extraction process is integrated into a biomass fired CHP plant. The unit operations considered in this work include a boiler/turbo-generator which operate on Rankine cycle, extraction and hydrolysis pressurized vessel, liquid-liquid separation process (for separation of acetic acid and fermentable sugars), and fermentation and ethanol up-grading processes. The hot-water extraction process at this stage is assumed to operate batch wise and heat was supplied from boiler exhaust gases. Results show low ethanol yield rate from hardwood through the hot-water extraction pathway, 0.0356 kg/ kg drywood, as compared to the current SHF (Separate Hydrolysis and Fermentation) technologies from hardwood conversion rate. This is due to the fact that hot-water extraction process dissolves the hemicellulose portion of the lignocellulosic biomass and the cellulose remains intact in the extracted chips. In addition to ethanol the process yields 0.0276 kg/kg drywood of acetic acid. Under the assumed economic conditions, ethanol can be produced to a cost in the range of SEK 3.4 to SEK 11 per litre depending on the plant size and annual operational hours. Electricity can be generated to a cost in the range of SEK 450, for the case with 100MW plant size and 8400 annual operational hours, to SEK 1120 per MWhel, for the case with 10MW plant size and 2000 operational hours per year.

Place, publisher, year, edition, pages
Keyword [en]
Technology, High temperature corrosion, biomass, ethanol, CHP
Keyword [sv]
URN: urn:nbn:se:ltu:diva-43415ISRN: LTU-PB-EX--10/034--SELocal ID: 14c8162d-3428-4c4e-b373-de9dfa15a680OAI: diva2:1016647
Subject / course
Student thesis, at least 30 credits
Educational program
Energy Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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