Production of synthetic fuels from biomass

Transcription

1 Production of synthetic fuels from biomass Igor Vlassiouk Department of Chemical Engineering, Lund University, P. O. Box, SE- Lund, Sweden This Master Thesis was realized in cooperation with and at Tekniska Verken AB in Linköping with the purpose to design a plant for production of synthetic fuels from biomass. After the initial literature survey the decision was made regarding choices on what fuel to produce and what kind of equipment that will be used in the plant. A major task in this study was to perform a complete technical and economical analysis for the plant. FT-liquids were chosen as a fuel produced in the plant mainly depending on that existing distribution systems and vehicle engines can be used. Investment cost for the plant was calculated to MSEK, and operation cost to MSEK. The production cost of FT-liquids under these circumstances is, SEK/l.. The results from this study show that the economic performance of the plant is relatively insensitive to variations of different parameters under assumption that FT-products can be sold to a minimum price of 8 SEK/l. A combination of environmental benefits with high cost- and energy efficiency makes biomass to fuel through gasification and FT-synthesis concept very promising for future applications. The study s positive results regarding overall performance of the plant creates a need for a further more accurate technical-economic analysis before a final decision can be made whether the plant should be build in the future. Introduction The study was initiated by Tekniska Verken AB and carried out during fall at the company s head office in Linköping. Fuels produced from biomass are an alternative to satisfy future need for renewable fuels. Since the supply of fossil fuels is limited new cost- and energy efficient processes that utilizes biomass must be developed. Several of the most promising routes that transform biomass to fuels are based on thermal processes as pyrolysis or gasification. Different fuels as Methanol, FT-diesel and DME can be produced depending on how the process is designed after the gasification step. The aim of this thesis is to identify and solve a number of technical hitches during design of the BTL-plant. Different tasks that were performed in this study are: A literature survey where state of the art technologies that utilises biomass are evaluated An investigation on different types of raw materials that can be used as feed in the plant and their potential in Sweden A literature survey where various process solutions for a BTL-plant are evaluated Energy and material balances for the plant designed in this thesis Economical analysis of the plant Process design During literature survey a number of conclusions were made which contributed to creation of an initial process flow sheet for the plant. Another factor that affected the design was TVAB s requirements considering the scale of the plant. Complete process flow sheet including all the equipment used in the plant is presented in Figure. Gasifier The choice of gasifier influence also the design of pre-treatment, gas cleaning and gas processing sections. Depending on several factors as fuel flexibility, capacity, scale etc. a CFB pressurized gasifier with oxygen and steam as gasification medium was chosen. Figure shows a typical CFB gasifier

2 Figure. CFB gasifier [] Pre-treatment Before the biomass (logging residues) can enter the gasifier it must be properly sized and dried. The feedstock is transported from the storage to a grinder where biomass is size reduced to approximately mm. To increase the efficiency of the gasifier the biomass is dried with hot air from %w to %w in a rotary dryer (Figure ). [] Figure. Rotary dryer [] Feeding system Biomass is compressed by piston feeder and subsequently fed by screw into the gasifier. The inert gas consumption reduces when piston feeders are used instead of lock hoppers. Furthermore, less inert gas will enter the gasifier and gasification efficiency will hardly be influenced. Another advantage is that less electricity is needed to pressurize the inert. The piston presses the fuel through a tapered opening, the dense plug falls into the pressurized tank that contains a disintegrator and a mechanism (screw) to transport the disintegrated briquettes into the gasifier. [] Gas cleaning Produced gas from the gasifier contains several impurities that must be removed. Impurities that are dealt with are: tars, BTX (benzene, toluene and xylene), H S, HCN, and COS, NH, HCl, metals, particles, dust and soot. The product gas contains also small hydrocarbons, H O, CH and CO. The small hydrocarbons, tars and methane are ether combusted or reformed in a tar cracker that follows immediately after gasifier. The cracker is fed by the same medium as gasifier and operates at the outlet temperature of o C. The reforming of light hydrocarbons and methane is essential when maximum FT-fuel production is desired sense a large additional amount of H and CO is produced during reforming reactions. [] In this study wet gas cleaning is used to clean the product gas from contaminates. Particles, dust, soot and fly ash are removed after the tar cracker by cyclones and a bag filter. The next cleaning step consist of a catalytic hydrolysis reactor where COS and HCN are transformed to H S and NH. Following step consist of a several scrubbers that absorb most of the NH, HCl, H S and CO. Traces of impurities are removed by ZnO and active carbon beds. Finally the cleaned syngas is preheated and compressed to a proper pressure prior FT-reactor. Water from scrubbers consist of several impurities, chemicals and salts and must be treated in order to separate by-products and chemicals that can be sold or reused from contaminants that must be further processed. Water treatment has not been investigated in details in this study but taken to account in economic evaluation of the plant. A complete gas cleaning train is presented as a part of the complete process flow sheet (Figure ). FT-reactor After gas cleaning syngas is fed to the reactor. The reactor contains slurry of catalyst dispersed in waxlike hydrocarbons produced in the synthesis. The synthesis is carried out at a temperature of oc and pressure of bar with external cooling. A gas fraction is obtained in the reactor top, consisting of lighter hydrocarbons produced in the synthesis and un-reacted feed gas. Heavier hydrocarbons are mixed into the slurry and separated in a separation step developed by SASOL. [] Figure. Slurry bed FT reactor [] In the catalytic FT-reactor one mole of CO reacts with two mole of H to form mainly paraffin straight-chain hydrocarbons (C x H x ) with minor amounts of branched and unsaturated hydrocarbons, and primary alcohols. In the exothermic FT reaction

3 about % of the chemical energy is released as heat: [] CO H ( CH ) + H O + () The polymerisation-like Fischer-Tropsch chaingrowth reaction results in a range of products, comprising light hydrocarbons (C and C ), LPG (C -C ), naphtha (C -C ), diesel (C -C ), and wax (>C ) fractions. The theoretical chain length distribution can be described by means of the Anderson-Schultz-Flory (ASF) equation, which is represented as: log W n () n ( α) = n logα + log α Where W n is the weight fraction of a product consisting of n carbon atoms and α the chain growth probability factor. The maximum amount of liquid products that can be produced from syngas is approximately % (overall syngas to fuel energy efficiency). Table. Syngas to fuel efficiency [] The FT-reactor used in this study is optimized towards maximum productions of liquid fuels. Cobalt catalyst is used in the synthesis. Cobalt catalysts have the advantage of a higher conversion rate and longer life (over years). Another advantage of cobalt catalysts is that they are in general more reactive for hydrogenation and produce therefore less unsaturated hydrocarbons and alcohols compared to other catalysts that can be used. A value of α =,9 for the reactor and 9 % conversion of syngas is assumed in this study. [] Product upgrading The product upgrading consists of two operations; distillation of different hydrocarbons fractions and wax cracking. The gas fraction obtained at the top of the reactor consists of light hydrocarbons (C -C ), un-reacted syngas and water. This gas is burned with the purpose to generate heat to the biomass dryer. The liquid fraction from the FT-reactor is first separated in two streams (C -C, C + ) in a distillation column. The wax fraction (C + ) is fed to a hydro cracker. Long hydrocarbons chains are converted to shorter chains such as diesel and naphtha in a hydro cracker in the presents of catalyst and additional hydrogen. After cracker the final separation step is taken place in a distillation column where diesel and naphtha fractions are separated. Steam production The gas after the tar cracker has a temperature of o C and must be cooled before it enters wet gas cleaning section. A two-step heat exchanger is therefore installed directly after the cyclone following the tar cracker where gas is cooled to 9 o C. The feed water entering heat exchangers will be transformed to HP steam and used for electricity and district heat production. Low pressure steam is produced from feed water in the FT-reactor and contributes to additional production of electricity and district heat. Parts of the produced steam in the plant are used to provide the gasifier and distillation columns. Almost all of the electricity produced in the plant is used for internal needs (8, MW e ). The energy that can be delivered as district heat is MW th. Technical results The overall energy efficiency of the plant is % when district heating is utilized otherwise 8 % on LHV basis. The plant consumes ton/year of biomass and produces, million liter/year of FT-products., TWh/year of district heat and 8, GWh/year electricity. Figure. Weight fractions of FT-products at α =,9 Figure. Total mass balance for the BTL-plant

4 Figure. Total energy balance for the BTL-plant Figure. Process flow sheet for the BTL-plant Economical evaluation Total capital investment The total capital investment (TCI) cost is based on cost data at the component level, which were obtained from a literature survey. The capacity or scale of each component is derived from the energy and mass balances obtained in technical calculations. The specific costs of most system components are affected by their capacity. The general relation is: C = C S S R () C = cost of a component at known scale S = scale of a component in this study S = scale of a component for which the cost is known R = scaling factor After scaling, the TCI of each component is found by multiplying the scaled base cost (C ) by an overall installation factor which includes auxiliary equipment, installation labor, engineering and contingencies. The TCI for the whole plant is found by adding together all the TCI s for different components. In Table the TCI for each component and the entire BTL-plant is shown. Table. Total capital investment for the plant Component TCI M Storage.8 Grinding. Dryer Band conveyer. Feeding. CFB gasifier.8 Oxygen plant 8. Oxygen compressor. Tar cracker. Cyclones. Heat exchanger.9 Bag filter. Hydrolysis reactor Scrubbers. Active carbon + ZnO. beds Compressor. FT-reactor. Product upgrading Steam turbine Water + steam system. Other CO adsorption (Selexol) Total TCI for BTLplant The TCI is calculated to MSEK () Operational cost Total operational cost is estimated by using data from technical calculations and rules of thumbs obtained from literature. Table. Operational cost for the plant Capital cost. MSEK Direct variable cost 99 MSEK Indirect variable cost. MSEK Total operational. MSEK cost Investment calculations The plant s economical performance was evaluated with TVAB s own software used by the company for economic calculations.

5 Table. Economical performance of the plant Total investment MSEK Profits MSEK Operational cost. MSEK Economical years lifetime Technical lifetime years Interest rate % Inflation % Tax 8 % Future cash flow 8 MSEK Capital value MSEK Income from. % capital Payoff time years The production cost for FT-products corresponding to the base case is, SEK/l. Sensitivity analysis In order to investigate the contribution from different parameters a sensitivity analysis was made. Table. Parameters used in sensitivity analysis Parameters Base case Range Biomass price SEK/MWh - SEK/MWh TCI MSEK - MSEK Operational cost MSEK - MSEK Incomes heat, MSEK - MSEK electricity FT-products price 8 SEK/l - SEK/l Produktionskostnad, FTvätskor 9 8 Känslighetsanalys, råvara Råvarukostnad (kr/mwh) Figure 8. Sensitivity analysis of the biomass price Produktionskostnad, FT-vätskor 9 8 Känslighetsanalys, investeringskostnad Investeringskostnad (MSEK) Figure 9. Sensitivity analysis of the TCI Produktionskostnad, FT-vätskor 8 Känslighetsanalys, driftskostnad Driftskostnad (MSEK) Figure. Sensitivity analysis of the operational cost Produktionskostnad, FT-vätskor Känslighetsanalys, el+cert+fjärrvärme 8 Inkomster, el+cert+fjärrvärme (MSEK) Figure. Sensitivity analysis on incomes from electricity and district heat sales Avkastning (%) Känslighetsanalys, FT-produkter 8 - FT-produkter, försäljningspris Figure. Sensitivity analysis on incomes from FTproduct sales Discussion and conclusions A number of assumptions were made in the study in order to evaluate the plants technical and economical performance. The assumptions were based on data obtained from the literature survey. One of the problems with use of data from other investigations was that they often use the same source which leads to uncertainty. No commercial BTL-plant exists in the world today, a fact indicating that the technology is still under development. The economical evaluation of the plant showed positive results despite a pessimistic base case scenario. The results from sensitivity analysis indicate that the plants economical performance is relatively stable under assumption that FT-products can be sold at the today s market price. The optimisation towards

6 maximum production of FT-products is essential for the total economy of the plant sense the income in SEK/MWh is larger for FT-products compared to electricity and district heat incomes. The environmental benefits combined with high energy- and cost efficiencies makes the BTL concept through gasification and FT-synthesis an interesting option that utilises biomass in the optimal way to satisfy future demand of renewable liquid fuels for the transportation sector. References [] H.Boerrigter; R.W.R Zwart: High efficiency coproduction of Fischer-Tropsch (FT ) transportation fuels and Substitute Natural Gas (SNG) from biomass ECN-C -, (February ) [] Carlo N Hamelinck; Andre P.C. Faaij; Herman den Uil; Harold Boerrigter: Production of FT transportation fuels from biomass; technical options, process analysis and optimization, and development potential Utrecht University, Energy research Centre of the Netherlands (ECN), ISBN 9-9--, (March ) [] J.P. Jansen; R.H. Berends: Large-scale production of biofuels through biomass (co) gasification and Fischer-Tropsch synthesis Sasol Process Development Twente, TNO-MEP-R / (August ) [] Keith R. Cummer; Robert C. Brown: Ancillary equipment for biomass gasification Department of Mechanical Engineering and Chemical Engineering, Centre for Sustainble Environmental Technologies, Iowa State University, Biomass and Bioenergy () -8 (March ) [] Kei Yamashita; Leonardo Barreto: Biomass gasification for the co-production of Fischer-Tropsch liquids and electricity International Institute for Applied Systems Analysis, IR--, (September ) [] Ingemar Olofsson; Anders Nordin; Ulf Söderlund: Initial Rewiew and Evuluation of Process Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels Energy Technology & Thermal Process Chemistry, University of Umeå, ISSN - ETPC Report -, (March ) Received for review December 9,