SYNTHESIS OF METHANOL FROM BIOMASS/CO 2 RESOURCES. M. Specht*, A. Bandi*, F. Baumgart*, C. N. Murray**, and J. Gretz***

Transcription

1 SYNTHESIS OF METHANOL FROM BIOMASS/CO 2 RESOURCES M. Specht*, A. Bandi*, F. Baumgart*, C. N. Murray**, and J. Gretz*** * Center for Solar Energy and Hydrogen Research (ZSW), Hessbruehlstr. 21 C, D Stuttgart, Germany ** Environment Institute, Joint Research Centre, Ispra (VA), Italy *** Hydrogen Association Hamburg, Hamburg, Germany ABSTRACT The utilisation of biomass for methanol production via gasification faces the problem of a large excess carbon in the produced synthesis gas. The stoichiometric adjustment can be accomplished either by adding hydrogen or by removing carbon in form of carbon dioxide. The addition of hydrogen allows a nearly complete utilisation of the carbon contained in the biomass, with a high methanol production rate. But hydrogen admixture to the syngas requires supplementary investments for an electrolysis unit. The removal of carbon dioxide is less investment intensive, but due to the extremely low carbon conversion efficiency of about 20 % of the biomass carbon content, the methanol production costs become very high. An acceptable way is a partial compensation of the carbon excess by adding electrolytic hydrogen (using the oxygen for the gasifying process), saving about half of the carbon from the biomass and avoiding extremely high investment and electricity costs. INTRODUCTION Today's energy systems are not closed cycles and are therefore ecologically harmful. The recycling of carbon dioxide helps to reduce the level of climate relevant emissions as well as the reduction of fossil fuel consumption. The recycling of carbon dioxide aimed to produce liquid fuels, e. g. methanol, is of special interest for road transportation and for intercontinental renewable energy transport. Besides carbon dioxide various carbon sources can be considered for methanol synthesis, e.g. biomass and waste. Renewable methanol may be used as a fuel in an easily adaptable combustion engine today, or in fuel cell powered cars in the future. As vehicle transport produces an important part of the total CO 2 emissions, the substitution of petroleum fossil fuels with renewable methanol results in a significant reduction of emissions in this sector. The present paper investigates methanol generation concepts via synthesis gas production from biomass and a subsequent CO/CO 2 hydrogenation. As the biomass gasification processes generally lead to a synthesis gas with a carbon excess (mostly CO 2 ), the paper analyses different pathways for meeting the required stoichiometry for the methanol synthesis. Technical requirements, energy efficiency, methanol yield, and economic aspects are examined. These results are compared with renewable, non-biomass routes for methanol generation with CO 2 as a feedstock. TECHNICAL ASPECTS OF METHANOL GENERATION FROM BIOMASS In contrast to gasification processes for electricity production, the syngas for the methanol generation process is limited by inert gas components (CH 4, N 2 ), which are not converted during methanol synthesis. A second reqirement for the syngas composition is a high hydrogen content, because a main part of the biomass carbon is converted to CO 2 in the gasification step (CO 2 needs 3 moles of H 2 for hydrogenation to methanol). The preferable H 2 /CO ratio in the gasifier raw gas has to be > 2. In this case a shift reactor can be avoided. There are principally two ways for adjusting the desired stoichiometric factor (ratio of the components H 2, CO and CO 2 ) of the biomass derived synthesis gas for methanol production: a) Carbon Dioxide Separation and b) Addition of Hydrogen. A balanced synthesis gas is the critical requirement for high carbon conversion. Excess carbon in the form of CO 2 can either be removed by an acid gas separation process or compensated by admixture of an adequate H 2 amount for CO 2 hydrogenation. In the present analysis an allothermic steam gasifier with a biomass input of about two tons per hour (technology available today) is considered [1]. The gasifier size of about 10 MW was chosen as it meets the goal of decentralised fuel production from biomass with a capacity of < 50 tons of methanol per day. Different ways of oxygen production for gasification and stoichiometric adjustment of the synthesis gas are examined. 1

2 1. Generation from Biomass with CO 2 removal and Electrolytic Hydrogen/Oxygen Supply (V1) In this process (variant V1) the oxygen demand for the gasifier is generated by an electrolysis unit. The combined use of electrolytic hydrogen/oxygen and biomass for methanol synthesis have also been proposed by other authors [e.g. 2, 3]. The hydrogen produced in the electrolyser can be used to partly adjust the stoichiometric factor. The hydrogen produced in the electrolysis is, however, not enough for a complete C-conversion and a part of the carbon has to be removed in form of CO 2. A flow diagram of this process and the corresponding synthesis gas composition is represented schematically in Fig. 1. As can be seen from Fig. 1 more than 60% of the carbon dioxide from the product gas is released to the atmosphere. This leads to a relatively low carbon-to-methanol conversion yielding only a small methanol production of 1.2 t/h. Raw Gas Biomass Gasifier H % CO 15.8 % CO % CH % H 2 O 0.5 % N % CO 2 Separation 61% of total CO 2 Steam El. Energy O 2 Electrolysis H 2 MeOH Synthesis Balanced Synthesis Gas H % CO 13.6 % CO % CH % H 2 O 0.4 % N % Fig. 1: Flow diagram for a methanol synthesis plant with electrolytic hydrogen/oxygen supply and CO 2 separation (V1) 2. Generation from Biomass without CO 2 removal and Electrolytic Hydrogen/Oxygen Supply (V2) Raw Gas Biomass Steam El. Energy Gasifier O 2 Electrolysis H % CO 15.8 % CO % CH % H 2 O 0.5 % N % H 2 MeOH-Synthesis Balanced Synthesis Gas H % CO 7.8 % CO % CH % H 2 O 0.25 % N % O 2 Excess Fig. 2: Flow diagram for a methanol synthesis plant with electrolytic hydrogen/oxygen supply and no CO 2 separation (V2) 2

3 The electrolyser can be designed to produce enough hydrogen for the adjustment of the synthesis gas stoichiometry. No CO 2 separation is required in this case. The electricity demand will increase by about three times, compared to case V1 discussed above. An excess of oxygen is produced, which can be used for sewage-treatment plants and other applications. The large amount of CO 2, which is separated in process V1, is hydrogenated in V2 by additional electrolytic hydrogen supply. Due to the high carbon-to-methanol conversion the process yields twice as much methanol as in process V1. A flow diagram of this process and the corresponding synthesis gas composition is represented schematically in Fig Generation from Biomass with CO 2 removal and PSA Oxygen Supply (V3) In this process the oxygen supply for the gasifier is produced by a pressure swing adsorption (PSA) process (Fig. 3). To meet the stoichiometric requirement a CO 2 separation is included in the process. The electric energy demand is much lower than in each of the other two processes described above, but the methanol production is substantially reduced due to the separation of 95 % of the CO 2. This is corresponding to a separation of more than 50 % of the total carbon. A further disadvantage of process V3 is the high methane concentration of 17 % in the "balanced" syngas, which contains about 40 % of the gas heating value. This high amout of inert gas is not converted in the methanol synthesis unit and has to be removed as purge gas. In processes V1 and V2 the purge gas is fed to the burner to produce heat and steam for the allothermic gasification process. The purge gas in process V3 can not be completely used for gasification and has to be converted in an additional combined head and electricity generation unit. Raw Gas Biomass Gasifier H % CO 15.8 % CO % CH % H 2 O 0.5 % N % CO 2 Separation 95 % of total CO 2 Steam El. Energy O 2 PSA System MeOH-Synthesis Balanced Synthesis Gas H % CO 23.6 % CO % CH % H 2 O 0.75 % N % Fig. 3: Flow diagram for a methanol synthesis plant with oxygen supply by a PSA system and CO 2 separation (V3) COMPARISON OF METHANOL PRODUCTION PROCESSES A summarised comparison of the three methanol synthesis variants from biomass are given in Table 1. The calculations are based on the following assumptions. The feedstock for the gasifier is a wood input of 2 t/h with a water content of 10%. The efficiencies are related to the lower heating value of methanol. All costs are given in Deutschmark (1 DM = 0.56 US-$; Aug. 1998). The costs are mainly determined by the energy and biomass input and the capital costs of the production plant. Operating and maintenance costs were estimated as 2.5 % per year of the investment costs. For all vectors it was assumed that hydroelectricity is available at 0.05 DM/kWh e. Capital costs are calculated on a real interest rate of 8 % and depreciation periods of usually 15 years. The plant capacity was calculated to produce about tons of methanol per day. To evaluate investment costs and methanol production costs, a raw gas stream of 3000 m 3 /h is considered. The feedstock costs for natural waste wood are 50 DM/t and proceeds for contaminated wood (coated timber) 150 DM/t (both 50 % of the input material). Further assumptions are mainly taken from [4,5]. Table 1 shows that the methanol production in V1 and V3 is substantially lower than in V2. In process V2 the high hydrogen admixture to the gasifier raw gas, where the synthesis gas is completely balanced by the addition of hydrogen, 3

4 leads to high carbon conversion and methanol production rates of more than 2 t/h. The low methanol production rate and the poor C-conversion in the case of V1 and V3 can be explained by the CO 2 removal for stoichiometric reasons. Without any additional electrolytic hydrogen supply and a CO 2 separation of 95 % of the raw gas CO 2, the carbon conversion drops to about 20 % with a methanol production of only 0.6 t/h. The energetic efficiencies (lower heating value of methanol versus raw material input and electricity) for the hydrogen/oxygen supply variants are 46 % (V1) and 51 % (V2), respectively (Fig.4). The efficiency of V3 with only 32 % is due to the high inert CH 4 concentration in the synthesis gas, which requires a high rate of purging. Therefore a large amount of energy is lost for the methanol synthesis. In the efficiency of 32 % the energy content in the purge gas, which could be used for electricity production, is not considered. The investment costs are the highest in V2 with high electrolytic hydrogen production and the lowest when a PSA system for oxygen generation and CO 2 removal is considered (V3). However the methanol production costs in the three discussed cases show a different picture. The lowest methanol production costs can be achieved with a methanol plant corresponding to variant 2 (766 DM/t). This is due to the much higher methanol production rate compared to the other two processes. The methanol production costs are presented in Fig. 5. For comparison: the average methanol spot markt price in 1997 was 335 DM/t and the gasoline equivalent (taxed) methanol price in Europe was about DM/t. The methanol costs are calculated without considering revenues for electricity sale. Especially in V3 using the purge gas for electricity production can reduce the specific costs. Even though the lowest cost can be obtained with V2, the high investment costs represent a higher economic risk. The clear disadvantage of V1 and V3 are the low C-conversion and the dissipation of a large amount of CO 2. Table 1: Comparison of different methanol synthesis variants Variants production [t/h] C-Conversion [%] Efficiency 1) [%] El. Power 2) [MW] Investment [10 6 DM] Costs [DM/t] V Electrolysis - O 2 V Electrolysis - H 2 V PSA - O 2 1) refers to the Lower Heating Value of methanol versus biomass and electricity input 2) electrical energy for electrolysis, synthesis gas compression and for oxygen production (PSA) Two other methanol generation processes using CO 2 sources as feedstocks are compared to the methanol production from biomass: capture of CO 2 from the flue stack of a coal fired thermal power station and absorption from the atmosphere [4-6]. For the CO 2 recovery from power stations the calculation is based on a pulverised coal fired power station with MEA (monoethanolamine) scrubbing. The energy of the flue stack CO 2 separation is assumed to be provided by extra fossil fuel burnt in a 500 MW(e) station such that the nominal electrical production remains 500 MW(e). Electrical power production efficiency declines from 40 to 29 % at a CO 2 recovery rate of 90% [7, following the IEA base case scenario]. If atmospheric CO 2 and renewable energy is used for the methanol production, this system is almost climate neutral. As shown in Fig.4 the energy efficiency for the methanol generation is 46 % (conc.-co 2 ) and 38% (air-co 2 ), respectively. The costs of the CO 2 -methanol vectors are mainly determined by the energy input and the capital cost of the production plants (Fig.5). The methanol production are in the order of about (conc.-co 2 ) and DM/t methanol (air-co 2 ) untaxed. 4

5 % DM/t V 1 V 2 V 3 CO 2 from atmosphere CO 2 from flue gases V 1 V 2 V 3 CO 2 from atmpsphere CO 2 from flue gases Fig. 4: Energetic efficienies for different concepts of methanol synthesis Fig. 5: production costs for different concepts of methanol synthesis CONCLUSIONS The only possibility to achieve high carbon conversions for methanol synthesis from biomass feedstocks is hydrogen admixture to the gasifier syngas. The conversion of biomass to methanol by addition of external hydrogen requires high investment costs for an electrolysis unit, but provides high methanol production rates. Nevertheless, hydrogen admixture is senseless, if electrolysis electricity is expensive or creates additional CO 2. Using conventional gasification processes without hydrogen admixture but CO 2 removal, the investment costs are lower, but carbon conversion is limited due to the stochiometric adjustment and the high purge gas rate. In this case a co-production of methanol and electricity is necessary with an additional conversion of the purge gas. Comparing the biomass methanol production costs with the two CO 2 pathways, the methanol production from biomass is less cost intensive than the production via CO 2 except for case V3. V3 is attractive, however, due to its lower investment costs and the avoidance of additional hydrogen supply compared with either to the other two biomass-methanol production concepts or to the CO 2 -methanol production paths. The methanol production from CO 2 from flue gas may be considered as a competitive alternative to methanol production from biomass from the economic and from the energetic point of view. Depending on the spot market price, methanol from renewable resources is at least 2-3 times more expensive than fossil based methanol. Comparing the untaxed renewable methanol costs with taxed gasoline, the costs difference may be rather small. If electrolytic hydrogen were to be used for the methanol generation, biomass is more cost effective than the CO 2 /H 2 vectors. In the case where renewable hydrogen were not available for the synthesis process, the requirement for a future - methanol adapted - gasification process would be a pre-balanced raw gas with a high hydrogen and a low methan content combined with a CO 2 separation process. Processes combining a reduction of costs and an incease of the energetic efficiency are presently under development. in "Greenhouse Gas Control Technologies", B. Eliasson, P.W.F. Riemer, A. Wokaun (Eds.), pg. 723, Pergamon, Amsterdam (1999) 5

6 REFERENCES [1] H&C Engineering GmbH, "Gas Generation from Biomass", Gummersbach, Germany (1998) [2] N. Ouellette, H.-H. Rogner, D. S. Scott; Int. J. Hydrogen Energy, Vol. 20 (11), (1995) [3] S. Stucki, T. Schucan; VDI Berichte 1129, 175 (1994) [4] M. Specht, F. Staiss, A. Bandi and T. Weimer; Int. J. Hydrogen Energy, Vol. 23 (5), (1998) [5] M. Specht, A. Bandi, M. Elser, F. Staiss; in "Advances in Chemical Conversions for Mitigating Carbon Dioxide", T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Ya maguchi (Eds.), p. 363, Elsevier, Amsterdam (1998) [6] C.N. Murray, J. Gretz, M. Specht, A. Bandi in "Technologies for Activities Impemented Jointly", P.W.F. Reimer, A.Y. Smith, K.V. Thambimuthu (Eds.), p. 259 (1998) [7] Ormerod, B., "The disposal of carbon dioxide from fossil fuel fired power stations", IEA Greenhouse Gas R&D Programme (1994) 6