Formic Acid-Based FischerâTropsch Synthesis for Green Fuel

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Formic Acid-Based Fischer-Tropsch Synthesis for Green Fuel Production from Wet Waste Biomass and Renewable Excess Energy Jakob Albert, Andreas Jess, Christoph Kern, Ferdinand Pöhlmann, Kevin Glowienka, and Peter Wasserscheid ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01531 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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ACS Sustainable Chemistry & Engineering

Formic Acid-Based Fischer-Tropsch Synthesis for Green Fuel Production from Wet Waste Biomass and Renewable Excess Energy Jakob Albert1, Andreas Jess2, Christoph Kern2, Ferdinand Pöhlmann2, Kevin Glowienka,2 Peter Wasserscheid1,3 * 1

Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany; *[email protected]

2

Lehrstuhl für Chemische Verfahrenstechnik, Zentrum für Energietechnik (ZET), Universität Bayreuth, Universitätsstr. 30, 95447 Bayreuth, Germany

3

Forschungszentrum Jülich GmbH, Helmholtz-Institut Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstr. 3, 91058 Erlangen, Germany

Abstract While the production of hydrocarbons by Fischer-Tropsch synthesis (FTS) is a widely recognized, yet technically quite complex way to transform biomass via syngas (mostly from biomass gasification) into liquid fuels, we here present an alternative route transforming biomass first into formic acid (FA) followed by syngas formation by decomposition of FA and finally FTS using regenerative hydrogen (or if needed H2 from the stored FA) to balance the C:H ratio. The new method builds on the recently developed, selective oxidation of biomass to formic acid using Keggin-type polyoxometalates of the general formula (H3+x[PVxMo12-xO40]) as homogeneous catalysts, oxygen as the oxidant and water as the solvent. This method is able to transform a wide range of complex and wet biomass mixtures into FA as the sole liquid product at mild reaction conditions (90 °C, 20-30 bar O2). We propose to convert FA with hydrogen from water electrolysis - the electrolysis step producing also the oxygen for the biomass oxidation to FA – to green hydrocarbon fuels using a typical Co-based FT catalyst.

Keywords: Chemical energy storage, formic acid, biomass oxidation, Fischer-Tropsch synthesis, electrolysis

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Introduction Growing concerns about global warming have created a world-wide interest in new concepts for sustainable and biomass-based future energy supplies. A main interest of related research efforts is to provide liquid transportation fuels from biomass waste or non-edible biomass resources1 as competing use of agricultural products for feeding people or animals against its conversion to fuels is increasingly inacceptable in the light of a still growing global population. One intensively studied approach for converting biomass into fuels involves its conversion to syngas followed by catalytic Fischer-Tropsch-synthesis.2 Traditional processes to convert biomass into syngas include processes like biomass gasification,3 reforming in supercritical water4 or aqueous phase reforming.5, 6 All these processes are characterized by harsh reaction conditions in complex process equipment and by the production of a variety of gaseous, liquid and solid products. For example, biomass gasification takes place in either fluidized bed reactors (800-900 °C) or in entrained gas flow reactors (about 1300 °C) by contacting the solid biomass with air, oxygen and/or steam. Nitrogen-free synthesis gas is only obtained if pure oxygen is used as gasification agent. The raw gas product contains a mixture of CO, H2, CO2, CH4 and H2O in variable ratios depending on feedstock composition and process parameters.7-10 Solid inorganic impurities in the biomass have to be discharged as ash. The raw product gas has to be cooled, filtered and scrubbed with physical or chemical absorption agents like methanol or amines to remove unwanted components (e.g. H2S) and particles. Furthermore, the H2/CO ratio has to be adjusted prior to FT-synthesis. Of similar complexity is biomass reforming in supercritical water where a water phase loaded with typically 10 wt% biomass is converted at about 600 °C and 300 bar in the presence of a catalyst, typically Ru on alumina.11 While the excess of water reduces greatly the formation of tar and coke in the reactor, a significant amount of methane and CO2 is formed and requires elaborate downstream processes to obtain a suitable syngas for FT-synthesis. Dumesic and co-workers investigated intensively aqueous phase reforming (APR).12-16 These authors described the reaction of alcohols or polyols with water using heterogeneous catalysts at temperatures between 200 and 250 °C to produce primarily H2 and CO2. The temperature level of the reaction allows generating hydrogen with low amounts of CO in a single reactor. The process typically forms 35% of hydrogen, 40% of CO2 and 25% of combined alkanes. The high amount of alkane formation originates eventually from CO hydrogenation and 2 ACS Paragon Plus Environment

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Fischer-Tropsch (FT) reaction.17-20 In order to get better hydrogen selectivities, the authors have focused on platinum and palladium catalysts for their APR development. Other metals like Ru, Ni, Ir and Rh are known to be more active catalysts for the C-C cleavage21 and for the water-gas shift (WGS) reaction.22 However, as Ru catalysts show also much higher FT activity its use leads to alkane selectivities of up to 65%.20 However, there are severe limitations concerning the feedstock selection as for some important substrates, such as e.g. glucose, the process can only be operated in very diluted systems to avoid rapid tar formation.23-25 A very different approach to produce fuels from biomass is followed by researches of the Cluster of Excellence “Tailor-Made Fuels from Biomass” (TMFB) at the RWTH Aachen University.26 Herein, chemists and engineers practice an intense collaboration to develop selective bio- and chemocatalytic process steps to convert lignocellulosic biomass into molecularly well-defined fuel molecules. Goals are to preserve nature’s synthetic power to the largest possible extent by providing efficient and short defunctionalization sequences. Leitner et al. have proposed, for example, elegant routes to produce γ-valerolactone (GVL), 1, 4pentanediol (1,4-PDO) and 2-methyltetrahydrofuran (2-MTHF).27,28 However, to produce these fuel target molecules from largely oxygen containing biomass feeds a huge amount of hydrogen is necessary. Only if this hydrogen origins equally from biomass (which is complex and expensive as described above) or is provided from water electrolysis with excess renewable electricity (a requirement that would imply an unsteady fuel production process according to the availability of wind and sun) this way of biofuel production can be regarded as fully sustainable. In this contribution, we present a novel approach to produce green fuels from wet biomass for future mobility requirements. The approach is based on a combination of our recently developed OxFA technology with water electrolysis and a conventional Fischer-Tropsch synthesis that uses formic acid, CO and hydrogen as feedstock.

Fuels from biomass-derived FA and renewable electricity The overall proposed process is illustrated in Figure 1. As a whole, the proposed process scheme opens a feasible technical path to produce hydrocarbon fuels from wet waste biomass and renewable electricity in a Fischer-Tropsch process.

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Figure 1: Novel scheme to produce hydrocarbon fuels from biomass, water and renewable electricity in a combination of the OxFA process and Fischer-Tropsch synthesis. The proposed process consists of four main steps: 1) oxidation of biomass to formic acid in aqueous solution under mild conditions (OxFA process); 2) syngas formation by selective decomposition of formic acid to CO (and H2O) and to H2 (and CO2 - in case hydrogen from water electrolysis is not available); 3) water electrolysis and 4) Fischer-Tropsch synthesis to liquid fuels and fuel gas. These process steps are subsequently discussed in more detail. The so-called OxFA process (details are given below) produces formic acid (FA) by selective catalytic oxidation of biomass in aqueous solution using air or oxygen. As the only byproduct, pure carbon dioxide is formed that can be separated and used for further biomass production, e.g. for algae growth. For the next step, namely the decomposition of FA to syngas for the Fischer-Tropsch synthesis, two different operation modes are possible: The first option, herein called the “energy-rich scenario”, reflects times where excess electricity from renewables (wind, solar) is available. In this scenario we can produce all 4 ACS Paragon Plus Environment

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hydrogen that is necessary for Fischer-Tropsch synthesis (FTS) by water electrolysis according to Equation 1. 2 H2O → O2 + 2 H2

(1)

Therefore, FA-decomposition is uniquely carried out to produce CO for the FT-synthesis: HCOOH → CO + H2O

(2)

In the subsequent FTS step, higher hydrocarbons (mainly high quality diesel oil) are formed: 2 H2 + CO → -(CH2)- + H2O

(3)

Thus, the overall reaction of this “energy-rich scenario” reads:

HCOOH → -(CH2)- + O2

(4)

In the “energy-lean scenario” (no renewable electricity from renewables; thus, no hydrogen from electrolysis available), we propose to provide both reactants for FT-synthesis from FA decomposition. While thermal FA decomposition results in CO and water (see Equation 2), catalytic decomposition of FA forms CO2 and hydrogen: 2 HCOOH → 2 CO2 + 2 H2

(5)

The combined generation of both carbon monoxide and hydrogen in the right stoichiometry leads to 3 HCOOH → CO + H2O + 2 H2 + 2 CO2

(6)

The overall reaction of the “energy-lean scenario” reads therefore:

3 HCOOH → -(CH2)- + 2 H2O + 2 CO2

(7)

Process step 1: Oxidation of biomass to formic acid (OxFA process) The key element of the here proposed new process scheme is the selective oxidation of biomass to formic acid (FA) using air or oxygen as oxidant. This reaction has been developed by some of us in the recent five years and has become known as the “OxFA process”.29-33 The reaction proceeds under mild temperature conditions (< 100 °C) in aqueous reaction media and applies precious metal-free homogeneously dissolved POM catalysts of the general type HPA-x= H3+x[PVxMo12-xO40].29-31 Compared to reductive biomass conversion processes, the process avoids all formation of solid or gluey by-products. Heteroatoms are not converted to the catalyst poisons NH3 or H2S but are oxidized to nitrates or sulfates and thus remain in the 5 ACS Paragon Plus Environment


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acidic aqueous reaction system. Most important, as water is the solvent of the OxFA process, wet biomass can be applied without problems. Water-insoluble feedstocks can be converted best in the presence of oxidation-stable acidic additives, such as p-toluenesulfonic acid (TSA), that act as solubilizer and depolymerisation agent. Recently, we could demonstrate that the process is able to convert a very wide range of biomasses of the first, second and third generation including contaminated biomasses.30,32 For example, waste from the food and forest industries, effluent sludge and even railway sleeper can be effectively converted to FA. Only in case of heavy metal ion contaminated biomass streams, a limitation has been found as these heavy metal ions form catalytically inactive, water-insoluble complexes with the polyoxometalate ion. Very important for the here proposed new process scheme is the fact that the OxFA process does produce from all these mentioned biomass substrates only two products if the residence time in the reactor allows for complete conversion: Formic acid and CO2. The ratio of these two products is mainly dependent on the substrate with sugar-containing feedstock resulting in the highest and lignin as substrate in the lowest FA yields.30,31 Remarkably, it has been recently demonstrated that in-situ extraction of the formed FA from the aqueous reaction mixture using long-chain primary alcohols has a very positive effect on the FA yield. This is probably due to a pH shift during FA formation in the aqueous catalyst phase that lowers the catalyst selectivity. By using a biphasic reaction system, the pH value of the aqueous catalyst phase is kept constant as the formed FA is partially extracted into the organic alcohol phase. This keeps the high activity of the POM catalyst and improves the FA yield. Using in-situ extraction with 1-hexanol, FA yields from carbohydrate biomasses were boosted up to 85% while from the complex substrate beech wood still more than 60% of the carbon could be converted into FA.33 All reported specific advantages of this biomass valorisation technology are maintained (mild reaction conditions, precious metal-free catalyst, broad range of applicable biomasses, simple product spectrum) while two of the main obstacles for this technologies, insufficiently low FA yields and problematic FA isolation, are elegantly solved. For the subsequent decomposition of FA to either CO and H2O or CO2 and H2, the azeotropic mixture of FA and H2O can be used as this contains a very high amount of FA with about 77 wt%. Another option could be to use pure formic acid from the biphasic liquid-liquid approach after an additional distillation step to separate pure FA from the linear alcohol. 6 ACS Paragon Plus Environment

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Table 1 compiles and summarizes key results from our recent papers on the development of the OxFA process to highlight in the context of this contribution the fact that various types of biomasses can be used as substrates even if the latter are wet, water-insoluble, have a recalcitrant carbon-framework or are poisoned with organic or inorganic pollutants. Successful closing of the carbon mass balance of the OxFA process could be proven by NMR and gas chromatography. The yield of FA was determined by means of 1H-NMR using benzene as an external standard. FA yields were calculated as YFA = n(FA)/n(C-atoms feedstock). The yield of CO2 was determined by means of GC-analysis as YCO2 = n(CO2)/n(Catoms feedstock). The combined yield of formic acid and CO2 was calculated by n(FA) + n(CO2)/n(C-atoms feedstock), respectively. This value was applied to describe the degree of substrate oxidation to FA and CO2 after a given reaction time. Note that feedstock conversion analysis by quantification of unconverted feedstock is not feasible as some of the substrates (see entries 3-10 in Table 1) are very heterogeneous in nature and the obtained intermediate mixtures are very difficult to analyse with respect to unconverted feedstock.



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Table 1: Oxidative conversion of divers biogenic feedstocks including wet, complex, water-insoluble and chemically contaminated biomass of 1st, 2nd and 3rd generation – some feedstocks have been converted in presence of the additive toluene sulfonic acid (TSA) using the HPA-x POM-catalyst in monophasic, respectively biphasic reaction mode. Combined yieldb FA+CO2 [%]

FAyieldb [%]

FA:CO2selectivityb [%]

Entry

Substrate

Molecular compositiona

1

Glucosec

C1.00H2.00O1

100

84.7

85:15

2

Sucrosed

C1.09H2.00O1

96.1

76.1

79:21

3

Beech woode

C1.21H1.86O1

90.1

61.4

68:32

4

Cellulosef

C1.07H1.88O1

76.2

31.1

37:63

5

Ligninf

C1.70H1.84O1

100

31.8

32:68

6

Hemicellulosef

C1.17H2.05O1

100

58.2

58:42

7

Pomaceg

C0.24H1.59O1

97.3

54.6

56:44

8

Chlorellag

C1.69H2.92O1

53.7

21.6

40:60

9

Effluent sludgeh

C6.31H11.35O6.63N1

46.1

15.7

34:66

10

Railway sleeperh

C1.25H1.90O1

39.5

11.2

29:71

a

b

Reaction conditions: Determined via C, H, N, S elemental analysis; Yield and selectivity determined by means of 1H-NMR using benzene as an external standard according to n (FA)/n(C-atoms feedstock); c 1.80 g glucose, 0.91 g (0.5 mmol) HPA-5 as catalyst dissolved in 100 g H2O, 100 g 1-hexanol, 363 K, 20 bar O2, 1000 rpm, 48 h in 600 ml autoclave; d 3.44 g sucrose; e 1.63 g beech wood and 1.72 g (10 mmol) TSA as additive; f 2.70 g substrate, 1.74 g (0.9 mmol) HPA-2 as catalyst, 3.0 g (17 mmol) TSA as additive dissolved in 100 g H2O, 30 bar O2, 90 °C, 24 h, 1000 rpm; g 3.30 g substrate, 1.74 g (0.9 mmol) HPA-2 as catalyst, 1.9 g (11 mmol) TSA as additive dissolved in 100 g H2O; h100 mmol substrate (1.85 g for entry 1; 5 g for entry 2 depending on molecular weight).


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Process step 2: Syngas formation from formic acid While formic acid (FA) is a basic chemical that is widely used in chemical, textile, leather, pharmaceutical, rubber and other industries,34,35 its further conversion to fuels would open a much broader utilization. Therefore, this contribution deals with strategies to couple biomass oxidation according to the OxFA process with a FA-based FT-synthesis with the help of additional hydrogen from water electrolysis or from catalytic FA decomposition. FA can be selectively decomposed to hydrogen and CO2 through metal catalysed processes under very mild conditions. In addition, thermal or catalytic decomposition of FA (above 373 K) forms CO and water.

FA dehydrogenation to form H2 and CO2 Homogeneous and heterogeneous catalysts were subjects of various studies for selective decomposition of formic acid to H2 (and CO2). In both cases mainly noble metal catalysts have been used.36-41 Beller and co-workers investigated the continuous dehydrogenation of FA with [RuH2(dppe)2] (dppe=1,2-bisdiphenylphosphinoethane) as catalyst.39 Using dimethyl-octylamine as co-catalyst, hydrogen was produced for 45 days at 25 °C (TOF = 1700 h-1) with a CO-content below 2 ppm. Under optimized conditions, a much higher activity was achieved (60 °C, 1 atm, TOF = 6400 h-1) and the CO-content was even below 2 ppm. However, under these conditions the stability was limited to 6 h which can be explained by the loss of the amine by evaporation. The observed loss in activity might be suppressed by recycling of the amine. Using heterogeneous catalysts, higher temperatures (close to or above 100 °C) are generally required to achieve a relevant activity.38 A promising catalyst for gas-phase formic acid dehydrogenation is gold. Ojeda and Iglesia reached a TOF of 25600 h-1 and a CO2-selectivity of almost 100% (< 10 ppm CO) using highly dispersed gold on alumina at 80 °C.40 In recent measurements with Au on TiO2 as catalyst, the selectivity of gas phase decomposition of FA exceeded 99% within the investigated temperature range of 80 to 180 °C.41 Pd, Ir, Mo and Ag catalysts on various supports have also been investigated in gas and aqueous phase but activity has been found way behind the Au/Al2O3 catalyst mentioned afore (Ir/C; TOF = 960 h-1 at 100 °C; SH2 ≈ 99%).38

Dehydration of FA to CO and water As noted already in 1977 during the investigation of different metal oxides that the selectivity of FA decomposition is influenced by the acidity of the catalyst.42 Also more recent studies 9 ACS Paragon Plus Environment


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showed that the CO formation is preferred by acidic catalysts, e.g. a CO-selectivity of more than 99.6 % has been achieved using commercially available zeolites (H-ZSM5).43 Already in 1951, Barham determined CO and H2O to be the only products of thermal formic acid decomposition with activity being a strong function of the water content.44 Mars and Scholten explained this by the drop in acidity with higher water content.45 This is consistent with the measurements of some of use in which thermal and sulfuric acid catalysed liquid phase FA decomposition was studied. For the gas phase FA decomposition, β-zeolites were found as promising catalysts (1 atm, 140 - 200 °C, SH2 ≈ 100%).41

Process step 3: Water electrolysis The topic of water electrolysis will not be elaborated here and we refer to recent reviews and books on the topic.46-49 As it is highly attractive for the here proposed process scheme to not only use the hydrogen under pressure in FT synthesis but also the electrolysis by-product oxygen as pressurized gas in the biomass oxidation step (Step 1, see Fig. 1) we would recommend to apply in this context a PEM electrolyser operating at an output pressure of 30 bar and higher for both gases. PEM electrolysers are characterized by high dynamics, high efficiencies and excellent over-load capacities. Commercial PEM electrolysers are available from many companies in a wide range of power classes.50

Process step 4: Fischer-Tropsch synthesis With regard to Fischer-Tropsch synthesis (FTS) we have to consider that the composition of the syngas depends on the mode of operation (“energy-rich” or “energy-lean”) of the upstream FA decomposition process, i.e. whether hydrogen is produced by decomposition of formic acid or by water electrolysis. If possible, the reaction rate of the Fischer-Tropsch synthesis as well as the product selectivities should be constant in both cases to allow continuous and steady state-operation of the FT-downstream units, for example the distillation units to separate gases and light products from the desired fuel fractions (see Figure 1). Subsequently, the impact of a changing syngas composition is discussed for cobalt as FTcatalyst; compared to iron, cobalt shows no water gas shift activity, and thus carbon (CO) is only converted to hydrocarbons and not lost via CO2 formation. Hence, a cobalt catalyst was selected for the proposed process. The respective kinetic data for a FT-cobalt-catalyst were previously determined by some of us (Bayreuth team) in dedicated experiments.51-54 Based on these kinetic data, the main aim of the following discussion is to show that an almost steady10 ACS Paragon Plus Environment

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state and safe operation of a FA-based Fischer-Tropsch unit is indeed possible under conditions switching between the before described “energy-rich” and “energy-lean” scenarios. In order to get a first idea of the dimensions of such a flexible Fischer-Tropsch application, the total number of individual tubes of a multi tubular fixed bed reactor in such a unit is also estimated. For this purpose, we consider the following case: The applied FT reactor runs under typical low-temperature conditions (230 °C, 25 bar) with cobalt as active metal, yielding predominantly longer-chain paraffinic hydrocarbons (diesel oil, jet fuel and waxes). Cobalt is dispersed homogeneously on a γ-alumina support (10 wt% cobalt). For our application a multi-tubular fixed bed reactor without gas recycle (to simplify the process assuming that the here proposed FA-based FT-technology will especially be interesting for decentralized, relatively small units) is applied. With respect to the reactor performance, we assume ideal radial heat transfer within the fixed bed and to the cooling medium (boiling water). As a consequence, isothermal conditions are guaranteed. Furthermore, we only consider the intrinsic reaction, that means the influence of internal mass transfer on local reaction rates and selectivities inside the catalyst particle are excluded. The reaction rate of the consumption of carbon monoxide (CO) is calculated by a LangmuirHinshelwood rate approach:

=

(8)

For details on the kinetic parameters see.51,52 In this context it is important to note that for a given H2-to-CO ratio, a decrease of the concentration of CO (and H2) only leads to a marginally decrease of the reaction rate, at least if the pressure and CO-concentration, respectively, are not too low. Furthermore, the selectivity to longer-chain hydrocarbons, expressed by the chain growth probability α, shows only minor dependence on temperature at least in a range of 220 to 240 °C. The chain growth probability α is therefore considered as constant. In our scenario, the conversion level of carbon monoxide is set to 60% (by choice of the right residence time in the reactor). These conditions result in a typical α-value of 0.85, with selectivities (by weight) of 10% methane, 13% C2 to C4, and 77% C5+.51,52,54 Here we assume that the C5+-fraction, which is a mixture of liquid hydrocarbons ranging from gasoline to waxes - the main part is diesel oil - can be considered as liquid fuel, whereas methane and the 11 ACS Paragon Plus Environment


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C2 to C4-hydrocarbons remain in the off-gas of the Fischer-Tropsch process which are used in a gas engine for production of electricity and heat (see Fig. 1). In a real world scenario, the feedstock for syngas production will not be pure FA but the azeotropic mixture of FA and water (77 wt% formic acid (FA) and 23 wt% water (H2O), i.e. 0.76 mol H2O per mol FA) that is easily isolated from the OxFA process (Step 1) by distillation (using the exothermicity of the biomass oxidation or of the FT-process as source of heat). Within the proposed concept, the way to produce the CO feedstock for the FT step is always the same, namely thermal decomposition of FA. However, the hydrogen feedstock is produced in two optional ways depending on the availability of renewable electricity. In the “energy-rich scenario”, H2 is produced by water electrolysis from unsteady, renewable electricity (namely wind and solar power). Combining this step with thermal decomposition of formic acid the feed gas for FTS shows the following composition: 37 mol% H2O, 21 mol% CO and 42 mol% H2 (see Fig. 2).

Figure 2: Syngas production in the energy-rich scenario - H2 produced by water electrolysis. In the “energy-lean scenario” renewable power is not available, and consequently H2production by water electrolysis is not intended. Hence, syngas is provided by thermal and catalytic decomposition of formic acid (see Fig. 3). Besides the byproduct water, carbon dioxide (CO2) is formed during the catalytic decomposition step. Subsequently, in the energylean scenario syngas for FTS shows the following composition: 39.7 mol% H2O, 12.1 mol% CO, 24.1 mol% H2 and 24.1 mol% CO2.

Figure 3: Syngas production in the energy-lean scenario - H2 produced by catalytic FA decomposition. 12 ACS Paragon Plus Environment

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Note that in both scenarios the syngas for FT synthesis contains large amounts of steam (about 40%) and in the energy-lean scenario also significant amounts of CO2 (24%) are present. As a consequence, the influence of these components on the kinetics of the FT reaction has to be considered in order to demonstrate the feasibility of the presented concept. In terms of steam, irreversible deactivation of the catalyst has been reported for steam contents above 25% in the syngas.53 Our studies exhibit that an addition of 20% of steam (partial pressure of 5 bar) to the syngas (pH2O = 5 bar, pCO = 6 bar, pH2 = 12 bar, pN2 = 2 bar, 212 °C) results only in a slight deactivation with time on stream until steady-state and a constant activity is reached again (for details see 53,54). This phenomenon may be explained by the fact that the stability of cobalt catalyst exposed to water containing syngas is a function of cobalt cluster size. Cobalt clusters with diameters > 6 nm stay unaffected by the added amount of water steam, whereby smaller cobalt particles oxidize, a process that leads to a slight decrease of catalyst activity. It has not been studied yet whether a further increase of water steam up to 40 mol% (at 25 bar) in the syngas causes stronger deactivation behavior of the catalyst. Due to our previous experimental results, we propose to consider the water steam in the here relevant amounts as diluting gas, but we recommend that this assumption should be further confirmed using experiments with partial pressures of H2O of more than 10 bar. In cobalt-catalyzed FTS no or very little CO2 is formed so the effect of CO2 on the catalyst has not been studied extensively. However, in the here proposed scheme CO2 is a significant part of the feedstock and this provokes the question whether this may influence the conversion and the product selectivity. Based on our own previous work we can state that CO2 behaves indeed as inert gas in the reactor as long as a sufficient amount of CO is present in the syngas. However, at intrinsic reaction conditions (crushed particles, dp < 150 µm) the conversion of carbon dioxide was demonstrated at very low CO partial pressures (pCO < 1 bar) in the syngas.52 In these experiments, the partial pressure of carbon dioxide, hydrogen and the total pressure were kept constant, but carbon monoxide was substituted stepwise by nitrogen. The experiments clearly evidenced that the adsorption of CO and CO2 is competitive, and, due to the much stronger adsorption of CO on the inner surface of the catalyst, the conversion of CO2 is inhibited severely. Therefore, in this paper the influence of CO2 on the kinetics of FTS can be neglected. Nevertheless, at severe pore diffusion limited conditions (large particles of e.g. 5 mm size, high temperatures > 240 °C) CO is consumed completely and a core region free of CO is formed inside the particle.51 Herein, conversion of CO2 takes place 13 ACS Paragon Plus Environment


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leading predominantly to methane. Note that the selectivity to methane in the hydrogenation of CO2 using a cobalt catalyst is 95%. Further details regarding the influence of CO2 on the kinetics of the FT synthesis are found in our previous papers.51,52, 54 From the said we conclude that in our scenario both, water and carbon dioxide can be considered as diluting gases. The Fischer-Tropsch reactor shall be operated at a constant CO conversion of 60% resulting in a constant amount of liquid products. In order to guarantee almost steady-state operation, although syngas composition changes during the process (from the “energy-rich” to the “energy-lean” scenario and back), the reactor temperature has to be adapted in a way that the average reaction rate of CO consumption ̅ stays constant. Eq. (9) illustrates the relation between mass of catalyst, CO-conversion, (average) reaction rate of CO, and molar feed rate of CO:

=

,

≈ ∙

, ̅

(9)

The average reaction rate of CO is given in approximation by:

̅ =

!, ,"#$ %

(10)

&

In the “energy-rich” scenario, hydrogen production from water electrolysis takes place and the syngas for FTS contains 21 mol% CO. This results at 230 °C in an average reaction rate of CO of 0.053 mol kgCo-1 s-1 at a conversion of 60%. Without hydrogen from electrolysis, the additional catalytic decomposition of FA for hydrogen production leads to a syngas diluted by water and carbon dioxide containing only 12 mol% of CO in the syngas. According to our established kinetic data, this leads to a slight decrease (by about 25%) of the average reaction rate of CO to a value of 0.040 mol kgCo-1 s-1. Moreover, CO conversion would drop to 50%. In order to compensate this effect of lower reactant concentration, one would have to increase the reaction temperature from 230 °C to 234 °C to reach 60% conversion again. This little rise in reaction temperature has only a minor effect on the product selectivity and thus the total amount of liquid product would remain the same. The effect of changing syngas concentration on the reaction rate and its compensation by adapting the reaction temperature is summarized in Table 2.


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Table 2: Effect of unsteady availability of renewable electricity (e.g. a day-night cycle) on the syngas concentration and reaction temperature in order to conserve a steady average reaction rate of CO and CO conversion level (25 bar, H2/CO ratio of 1).

scenario “energy-lean” “energy-rich”

',(

̅ in mol kgCo-1 s-1

) in °C

in%

0.12

0.0398

230

50

0.12

0.0528

234

60

0.21

0.0528

230

60

The composition of the off-gas (fuel gas) leaving the FT reactor and its respective caloric value for both scenarios at constant conversion of 60% is presented in Table 3. The given concentration of methane also includes the gaseous hydrocarbons C2 to C4. Table 3: Composition, (lower) heating value Hv and density ρ (1 bar, 298 K) of off-gas (dry) from the Fischer-Tropsch reactor without syngas recycle ( = 60%, steam content in wet raw gas: 54% (“energy-lean” scenario) and 63% (“energy-rich” scenario)).

scenario

'

“energy-lean”

0.118

“energy-rich”

0.29

ρ in kg/m3

,- in MJ/kg

,- in MJ/m3

0.237 0.591 0.054

1.242

4.8

6

0.58

0.459

31.7

14.6

'*

'

-

'*+

0.13

For the assumed CO-conversion of 60% and a selectivity to liquid fuels of 77%, the production level is 0.54 t liquid fuels per ton of carbon (entered into the reactor as CO). A FT fixed-bed reactor is a wall-cooled multi-tubular reactor cooled by boiling water. Each individual tube has a typical length of 8 m and a diameter of 3 cm. If we suppose an average catalyst bulk density of 500 kgcat m-3, we can assume a mass of 2.8 kg (0.28 kg Co) catalyst per tube. For the target CO-conversion of 60%, a catalyst with 10 wt% cobalt, and the given average reaction rate of 0.0528 mol CO kgCo-1 s-1 (Table 2), Eq. (9) leads to an annual COfeed rate (per tube) of 777 kmol CO (= 9.3 t carbon). Consequently, the annual production rate of liquid fuels per single tube is 5 tons. It should be noted that this value is only a rough estimation; a more detailed modeling of multi-tubular reactors for Fischer-Tropsch synthesis taking into account pore diffusion limitations as well as radial and axial temperature profiles is described in the literature.54



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Basic data for the proposed FA-based “biomass to liquid fuels” process For the following calculations we make the assumption that a farmer has a significant amount of photovoltaic cells on his roofs or fields that produce solar electricity at sunny times during the day. As he/she drops out of the support by the EEG (“Erneuerbare Energien Gesetz”, German law guarantying fixed compensation rates for feeding-in renewable electricity for a period of 20 years), he wants to use the then more-or-less worthless excess energy (electricity market prices are extremely low in Germany at sunny times) for hydrogen and oxygen production to produce his own diesel for his farm tractor, car or truck. We assume that the farmer can provide 50 kW electricity with 1000 peak-load hours (“energy-rich” time) via his photovoltaic facility (excluding his/her own-consumption of electricity). With an efficiency level of the applied electrolysis of 75% the hydrogen production amounts to 1.13 kg (0.56 kmol) hydrogen and 9.01 kg (0.28 kmol) oxygen per full load hour (heating value of H2 is 33.3 kWh/kg hydrogen). The carbon monoxide for FT-synthesis is always produced by thermal decomposition of formic acid (Eq. (2)). If we assume a typical syngas for FTS with a molar H2-to-CO ratio of 2, we need 0.28 kmol CO per hour and hence also 0.28 kmol/h (12.89 kg/h) formic acid, i.e. in total 280 kmol (12.89 t) for the assumed 1000 peak-load hours. For the rest of the year (7760 hours, “energy-lean” time), both CO and H2 are produced from FA. If we assume that still the same amounts of CO and H2 per unit time (0.28 kmol/h and 0.56 kmol/h, respectively) should be fed to the FT-reactor to assure almost steady-state conditions, we need 3 x 0.28 kmol/h (38.7 kg/h) of FA, i.e. 6518 kmol (299.8 t) for the 7760 hours. In total (8760 hours time-on-stream for both “energy-rich” and “energy-lean” times), 6798 kmol (312.8 t) of FA are consumed per year and 2453 kmol/a of CO (29.4 t carbon/a) are fed into the FT-reactor. The number of tubes of the reactor is only about three (2453 kmol CO per year/818 kmol CO per year and tube) and the resulting annual production rate of liquid fuels is 15.9 tons. The OxFA process produces product selectivities that are dependent from the substrate that is used. The estimation for the demand of biomass to produce the above estimated quantity of formic acid (6798 kmol/a) deals with an average water content of 80% for the biomass and a middle molar mass of one “carbon-unit” of 30 g/mol (assumption of H-C-OH). The average effectiveness factor for conversion of the preferred feedstock, non-eatable and wet biomass, 16 ACS Paragon Plus Environment

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such as e.g. wood, algae etc., is estimated to 60% (rest being CO2). With this numbers 6798 kmol formic acid correspond to 11330 kmol of “carbon-units” from biomass resulting in a demand of 340 tons/year pure biomass or 1700 tons/year of wet biomass. The main data of the proposed process are summarized in Table 4. Table 4: Basic data of the proposed FA-based Fischer-Tropsch synthesis for green fuel production from wet waste biomass and renewable excess energy (1000 hours PV)

Production of electricity (PV)

50 kW (during 1000 peak-load hours) = 180 GJ/a

Feed rate of wet biomass (water content 80%)

1700 t/a

Feed rate of dry biomass

340 t/a (approx. 5223 GJ/a)

Production rate of formic acid

312.8 t/a

H2-production (electrolysis)

1.13 t/a

H2-production (decomposition of FA)

8.69 t/a

CO-production (decomposition of FA)

68.6 t/a (29.4 t carbon/a)

Production rate of liquid fuels by Fischer Tropsch-synthesis

15.9 t/a (673 GJ/a)

Production rate of off-gas from the FT unit (used in gas engine for electricity and heat)

9.5 . 103 m3/a (1 bar, 25 °C) = 138 GJ/a (produced during 1000 peak-load hours) and . 3 3 180 10 m /a = 1081 GJ/a produced during rest of time (7760 h)

Ratio of heat content of liquid fuel to heat content of off-gas

1.1 / 1

Overall efficiency of process = heat content of liquid fuel and off-gas/(electrical energy and heat content of biomass)

35%

The overall input of energy (as biomass and electrical energy) is 5403 GJ/a compared to the output as liquid fuel (673 GJ/a) and off-gas (1219 GJ/a) for further use in a gas engine. Hence the overall efficiency is around 35% (= (673 + 1219) GJ/a/5403 GJ/a). This already seems to be a rather good value, as we have to consider that wet waste biomass is converted into valuable and clean gaseous and liquid fuels. Note that other processes using such wet biomass would require energy intensive drying of the biomass prior to its use e.g. in a gasification 17 ACS Paragon Plus Environment


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process. In addition, our estimation does not consider that energy is also released by the exothermic FTS (152 kJ per mol converted CO, i.e. 223 GJ/a in case of the process summarized in Table 4), which leads to the production of saturated steam of about 20 bar by the cooling of the reactor tubes. If this is counted as useful heat, the overall efficiency would increase to about 39%. The reaction heat of the exothermic biomass oxidation could form another valuable source of heat. However, due to the relatively low temperature level (< 100 °C) this heat is not taken into account here. A similar calculation can carried out for the case that renewable electricity from wind turbines is used instead of electricity from PV. This change in scenario has an implication on the number of hours per year that are considered as “energy-rich”. For the wind scenario, we can consider a typical value of 2500 energy rich hours. This means that for 2500 hours per year the produced FA serves only as source of CO while hydrogen is taken from water electrolysis during these energy rich hours. This more favorable ratio of “energy-rich” and “energy-lean” times leads to an overall process efficiency of 38% and 42%, respectively, if the reaction heat of the FTS is included. Liquid fuels based on biomass hold the promise of a closed carbon cycle. They can be stored, handled and moved around easily in existing infrastructure and provide very high energy densities. While battery or hydrogen technologies may develop into a serious challenge for hydrocarbon combustion vehicles in the field of individual mobility, it is hard to believe that these green options are available in the midterm to power commercial aircrafts or heavy duty trucks, trains or farm tractors. For these applications, the here proposed process is of special interest as it allows to produce green hydrocarbon fuels in a decentralized manner from wet, waste biomass with the help of renewable electricity. Formic acid can be regarded as a liquid syngas equivalent and conversion of wet biomass to FA can be regarded as up-grading an abundant natural resource into a green energy and synthesis equivalent. In the light of the excellent storability of liquid FA it is thus possible to operate a continuous (almost) steady-state FT-plant purely on biogenic feedstocks and without additional harvesting of CO2 or CO from exhaust gases or from the atmosphere. During energy-rich times (sun is shining, wind is blowing) the production of excess electric energy is expected at locations with a high coverage of sun and wind power units. This excess electricity enables the production of hydrogen via electrolysis. Converting this hydrogen with


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FA derived CO to liquid fuels enables both hydrogen storage and transport whenever there is no hydrogen infrastructure or hydrogen consumer onsite the electrolysis plant. Note that the here presented combination of processes can make use of the oxygen from water electrolysis which is normally useless. Our approach combines in an ideal manner the advantages of the OxFA process (using wet, complex biomass, oxygen from water electrolysis and water) with the water electrolysis (using water and renewable excess energy) and the Fischer-Tropsch synthesis (using hydrogen from electrolysis or catalytic FA decomposition and the carbon source CO from thermal FA decomposition) in order to produce green fuels, diesel or waxes. The proposed process has an overall energy efficiency of at least 35% (1000 energy rich hours, photovoltaic, no utilization of heat of FTS); in case of 2500 energy rich hours (wind power) and utilization of the heat of FTS, even 42% are in reach. Additional experiments and considerations are still needed for a further improvement and a more detailed layout of the proposed process scheme:

o Higher temperatures in the biomass oxidation step would make the herein produced heat more valuable. However, today’s biomass oxidation to FA looses at temperatures above 100 °C some of its selectivity and significant amounts of acetic acid are formed making the here proposed process scheme significantly more complex.55,56 Thus, future work could be directed towards highly selective high temperature biomass oxidation to FA and CO. A higher temperature level would have the additional advantage of higher reaction rate increasing the volumetric efficiency of the biomass oxidation step;

o With regard to FT synthesis, it should be proven by respective kinetic experiments whether and to what extent cobalt catalysts are stable, if the partial pressure of steam is very high (here up to about 15 bar). If the stability is not given, the syngas (formed by formic acid decomposition) has to be cooled prior to the FT reactor to separate some of the water.

o Instead of operating separate reactors for FA decomposition (either to CO and H2O or to H2 and CO2), formic acid may be directly fed to the FT reactor. Hence, the activity and selectivity of a FT cobalt catalyst for a combined process of FA decomposition and FT synthesis should be studied in greater detail.



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Acknowledgements We like to thank Dr. Wolf Ibach and Dr. Gunthard Scholz from OxFA GmbH for fruitful discussions. The authors thank the German Science Foundation for support within the project WA 1615/14-1. In addition, J. A. and P.W. like to thank the Energie Campus Nürnberg and the Erlangen Cluster of Excellence “Engineering of Advanced Materials” for infrastructural support. J. A. acknowledges material donations by VCI.


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(53) Kaiser, P.; Pöhlmann, F.; Jess, A. Intrinsic and Effective Kinetics of Cobalt-Catalyzed Fischer-Tropsch Synthesis in View of a Power-to-Liquid Process Based on Renewable Energy. Chem. Eng. Technol. 2014, 37, 964-972. (54) Pöhlmann, F. Zusammenspiel von chemischer Reaktion und Porendiffusion bei der kobaltkatalysierten Fischer-Tropsch-Synthese unter Einsatz von CO2-haltigem Synthesegas. Ph.D. Dissertation, University Bayreuth, 2016. (55) Tsakoumis, N. E.; Rønning, M.; Borg, Ø.; Rytter, E.; Holmen, A. Deactivation of cobalt based Fischer-Tropsch catalysts: A review. Catalysis Today 2010, 154, 162-182. (56) Jess, A.; Kern, C. Modelling of Multi-Tubular Reactors for Fischer-Tropsch Synthesis. Chem. Eng. Technol. 2009, 32, 1164–1175. (57) Zhang, J.; Sun, M.; Liu, X.; Han, Y. Catalytic oxidative conversion of cellulosic biomass to formic acid and acetic acid with exceptionally high yields. Catalysis Today 2014, 233, 7782. (58) Wang, W.; Niu, M.; Hou, Y.; Wu, W.; Liu, V.; Liu, Q.; Ren, S.; Marsh, K. N. Catalytic conversion of biomass-derived carbohydrates to formic acid using molecular oxygen. Green Chem. 2014, 16, 2614-2618.



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Formic Acid-Based Fischer-Tropsch Synthesis for Green Fuel Production from Wet Waste Biomass and Renewable Excess Energy

Jakob Albert, Andreas Jess, Christoph Kern, Ferdinand Pöhlmann, Kevin Glowienka, Peter Wasserscheid

Synopsis

An alternative route for transforming biomass into green fuels is presented. It uses biomassderived formic acid (FA) to produce syngas for FT synthesis.


Formic Acid-Based FischerâTropsch Synthesis for Green Fuel

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Formic Acid-Based FischerâTropsch Synthesis for Green Fuel