15.5. Syngas technology: composition, conditioning and upgrading to valuable products
15.5.1. Main contaminants
Although H2 and CO are the major components in syngas, there are other components present in the gaseous stream whose distribution depends on the process conditions and the technology used. In the case of gasification, these components can be divided into three groups (Huber et al., 2006; Hannula, 2009; Liu et al., 2009; Bridgwater, 2003; Xu et al., 2010; Yung et al., 2009; Tijmensen et al., 2002; Abu El-Rub et al., 2004; Babu, 2002; Boerrigter et al., 2003; Torres et al., 2007):
1. Major components. In this group CO2, N2, or CH4 can be included. Normally, they are present in quantities higher than 1–2%.
2. Minor components. These gases are normally light hydrocarbons (C2, C3, and C4) present in quantities lower than 1–2%.
3. Contaminants. These species act as pollutants and give rise to big difficulties for subsequent uses of syngas. Sulfur compounds (H2S and carbonyl sulfide, COS), ammonia, chlorine, tars, and ashes (mainly alkali metals such as K and Na) are grouped in this category. The concentration of these components in the syngas will vary a lot depending on the feedstock and operating conditions.
The contaminants can cause different negative effects on the gasification process and the downstream upgrading technologies and need to be removed (Huber et al., 2006; Hamelinck and Faaij, 2002; Luque et al., 2012). The removal targets for each contaminant depend on the final use of the syngas (Basu, 2010a,b; Tijmensen et al., 2002; Boerrigter et al., 2003; Higman and van der Burgt, 2008). For example, for the use of syngas in an internal combustion engine, tar concentration should be less than 100 mg/m3 whereas for methanol synthesis it is required to be less than 0.1 mg/m3.
15.5.1.1. Sulfur
Most of the sulfur content in the biomass is converted to H2S, although depending on the gasification temperature and moisture content, up to 10% can lead to COS formation (Liu et al., 2009; Gasifipedia). Sulfur is removed from the gas stream by removing H2S, so COS must be previously hydrolyzed to H2S (Gasifipedia; Basu, 2010a,b).
H2S is a highly corrosive gas and it is a major concern in the downstream upgrading processes (Huber et al., 2006). Most of these processes use catalysts or membranes that can be seriously affected by H2S (Huber et al., 2006). H2S needs to be removed from the gas stream in order to extend the catalyst's lifetime and to avoid corrosion.
15.5.1.2. Ammonia
The nitrogen content in the feedstock appears in the form of N2, HCN, or NH3 in the syngas (Xu et al., 2010; Torres et al., 2007). Between 60–80% of this nitrogen is converted to ammonia (Xu et al., 2010; Zwart, 2009). In comparison with coal gasification, the concentration of NH3 in the syngas from biomass is much higher (about 40 ppmv in coal gasification and up to thousands of ppmv in biomass gasification), whereas HCN production is similar (about 200 ppmv) (Zwart, 2009). Similar to sulfur compounds, the presence of ammonia in the syngas gives rise to problems such as corrosion or catalytic deactivation. Ammonia is also a precursor of NOx emissions, which are generally capped by environmental regulations (Xu et al., 2010; Torres et al., 2007).
15.5.1.3. Chlorine
Chlorinated compounds are formed when the feedstock contains Cl (Torres et al., 2007). Cl forms HCl when dissolved in water, which is very corrosive. In addition, Cl combined with ammonia can form ammonium chloride (NH4Cl) which solidifies under 250°C, resulting in fouling (Zwart, 2009).
15.5.1.4. Tars
Formation and removal of tars is one of the main challenges for the industrial implementation of biomass gasification technology (Navarro et al., 2007). Tars are formed from condensable volatiles released during biomass devolatilization which are not cracked and reformed into gas (Bridgwater, 2003). Due to the higher volatile content, the production of tars in biomass gasification is much higher than in coal gasification (Huber et al., 2006; Hannula, 2009; Devi et al., 2002; Xu et al., 2010). Tars are a complex mixture of organic compounds of high molecular weight and aromaticity that can condensate in pipes, valves, filters, and other equipment leading to fouling. Moreover, these tars tend to be refractory due to the high temperatures achieved in the gasification, which complicates their removal by thermal, catalytic, or physical processes (Huber et al., 2006; Bridgwater, 2003).
15.5.1.5. Ashes
Ashes are present in the syngas from gasification in the form of small particles. The accumulation of ashes can lead to fouling and corrosion. Their composition is very complex depending on the metals present in the feedstock and they normally contain a high amount of alkali metals. Operating temperature of the gasifiers is usually set to be lower than the ash melting point (around 1000°C) in order to avoid ash sintering and slagging (Navarro et al., 2007; Ni et al., 2006; Huber et al., 2006).
15.5.2. Cleaning technologies
Due to the presence of the contaminants previously described, syngas must be conditioned prior to its use. Cold and hot conditioning routes can be followed (Fig. 15.12) (Huber et al., 2006; Hamelinck and Faaij, 2002; Tijmensen et al., 2002). The cold route is the most extended at industrial level, since the hot route has not been completely developed (Hamelinck and Faaij, 2002). However, the hot route presents better overall energy efficiency when the syngas is to be used downstream at high temperature. For this reason intense efforts are being made to implement the hot route treatments (Xu et al., 2010; Hamelinck et al., 2004).
15.5.2.1. Tars
The removal of tars is one of the biggest challenges of biomass gasification (Navarro et al., 2007; Huber et al., 2006; Hannula, 2009; Devi et al., 2002; Zwart, 2009). Its cost can knock down a whole project (Balat and Kirtay, 2010). Primary and secondary approaches can be used to deal with the problem. Primary measures are taken in the gasifier: improvement of gasifier design, optimization of operation conditions, and use of additives (Navarro et al., 2007; Ni et al., 2006; Devi et al., 2002). Secondary measures are taken after the gasification process. In this section only the second approach will be described.
Figure 15.12 Routes for removal of contaminants from syngas (Huber et al., 2006; Hamelinck and Faaij, 2002; Milne et al., 1998; Zwart, 2009).
Tar removal starts with tars cracking in both hot and cold routes, since it is done before cooling down the gas stream due to the high temperatures needed (Hamelinck and Faaij, 2002). In the cold route, gas is cooled down and the remaining tars are removed by wet scrubbing (in water or oils), filtering, cyclones, or electrostatic precipitators (Hannula, 2009; Basu, 2010a,b; Han and Kim, 2008; Zwart, 2009). Alkali metals and particulates are also removed in wet scrubbers along with tars. These technologies are simple and cheap, although the final production tar-laden products need further treatment (Hannula, 2009; Han and Kim, 2008).
Hot tars can be removed by thermal or catalytic cracking (Huber et al., 2006; Devi et al., 2002; Abu El-Rub et al., 2004). Thermal cracking comprises very high temperatures, even higher than gasification, leading to energy penalties (Huber et al., 2006). Moreover, it gives rise to soot production, which can be a technical problem downstream (Huber et al., 2006). For these reasons catalytic cracking is the preferred process (Huber et al., 2006).
Catalytic cracking consists of the reaction of the tars with steam (steam reforming, Reaction [xv]), CO2 (dry reforming, Reaction [xvi]), O2 (partial oxidation, Reaction [xvii]), or H2 (hydrocracking, Reaction [xviii]) (Huber et al., 2006).
[xv]
[xvi]
[xvii]
[xviii]
These reactions affect the H2/CO/CO2 ratio, which will determine the final use of the syngas (Huber et al., 2006). Similarly to catalytic gasification, natural minerals and synthetic catalysts can also be used (Huber et al., 2006; Abu El-Rub et al., 2004; Zwart, 2009). Natural minerals include low-price dolomite, olivine, limonite, and clays that work at temperatures from 700 to 1000°C and contact times from 0.007 to 7 s (Turner et al., 2008; Huber et al., 2006; Xu et al., 2010; Yung et al., 2009; Abu El-Rub et al., 2004).
From the synthetic catalysts, noble metal-based catalysts are the best, especially from the coking resistance point of view (Holladay et al., 2009; Han and Kim, 2008; Yung et al., 2009). However, as in the oil and gas industry for naphta cracking, their high price makes Ni the most common catalyst for tar cracking (Huber et al., 2006; Holladay et al., 2009; Han and Kim, 2008; Yung et al., 2009). Ni-based catalysts normally work at temperatures in the range of 600–900°C and contact times around 0.01–3 s (Huber et al., 2006). Other metals that can be used are Co or Fe (Huber et al., 2006; Yung et al., 2009). Regarding the supports, alumina is normally used for Ni-based catalysts due to its high surface area and good mechanical properties (Huber et al., 2006; Yung et al., 2009). In addition, it provides acid sites, which promote cracking. However, supports like SiO2, ZrO2, activated carbons, zeolites, or natural minerals have also been used (Han and Kim, 2008; Xu et al., 2010; Yung et al., 2009; Abu El-Rub et al., 2004).
Deactivation of the catalysts is a great issue in the catalytic cracking of tars. It can be caused by carbon deposition (due to the high C/H ratio of the tars) or poisoning by sulfur, chloride, and alkali (Huber et al., 2006; Yung et al., 2009; Trimm, 1999). Different strategies have been developed in the design of the catalyst to avoid this problem (Huber et al., 2006; Yung et al., 2009; Abu El-Rub et al., 2004; Xie et al., 2010; Nikolla et al., 2006; Laosiripojana et al., 2014):
1. Use of bimetallic catalysts: a second metal (Sn, Co, Rh, Cu) can give rise to different structures like alloys, more resistant to carbon deposition (by decreasing solubility and diffusion of carbon atoms in metal particles) and sulfur poisoning (by sacrificing the second metal which is more easily sulfided, whereas the main metal can maintain its activity).
2. Use of promoters: basic oxides (Na2O, K2O, MgO, CaO, ZrO, La2O3) are able to reduce carbon deposition due to their ability to adsorb CO2 that can gasify the carbon deposits. La2O3 can form carbon-resistant spinels with Ni and alumina. Ce2O3 has also been shown as a very promising promoter since its high oxygen storage capacity helps gasifying the carbon deposits and its redox capacity increases resistance to sulfur poisoning.
The catalysts can be regenerated by oxidizing agents (O2, air, H2O) that can remove coke and sulfur from the catalyst surface (Huber et al., 2006; Argyle and Bartholomew, 2015). However, repeated regeneration cycles lead to irreversible deactivation due to sintering, phase transformations, or loss of active metal particles (Huber et al., 2006; Argyle and Bartholomew, 2015).
15.5.2.2. Sulfur
In the cold route, H2S is removed by scrubbing the syngas using amines (normally methyl diethanol amine, MDEA). Methanol or liquid solutions of NaOH and physical sorbents like polyethylene glycol are also widely used (Hannula, 2009; Liu et al., 2009; Higman and van der Burgt, 2008; Zwart, 2009). This technology, although well established, presents some disadvantages like the need for large scrubbers or difficulties handling and disposing of amines and byproducts (Ma et al., 2009). After the scrubber, a metal oxide bed guard (Zn, Cu, or Fe at low temperatures, 300–500°C, and Ca or Mn at higher temperatures) and/or activated carbon filters can be included to keep the sulfur concentration below 0.1 ppm (Van der Drift and Boerrigter, 2006; Hamelinck and Faaij, 2002; Torres et al., 2007; Higman and van der Burgt, 2008; Zwart, 2009). In the hot route, H2S is absorbed on different oxides (Zn, Fe, Co, or Mn) guard beds (Liu et al., 2009; Torres et al., 2007).
15.5.2.3. HCl
HCl is commonly removed in the cold route by wet scrubbing using basic solutions (normally NaOH in water), or absorption in CaO, MgO, Na carbonates (Na2CO3, NaHCO3), or activated carbons (Basu, 2010a,b; Xu et al., 2010; McKendry, 2002; Tijmensen et al., 2002; Boerrigter et al., 2003; Zwart, 2009). Normally H2S is removed at the same time, giving rise to stable salts like NaCl, NaHS, or Na2S. CO2 can also be removed by these basic solutions, but the kinetics of absorption of HCl and H2S are faster, so these gases can selectively be separated while almost no CO2 is removed (Zwart, 2009). In the hot route, guard beds or in-stream sorbents can be used (Hamelinck et al., 2004). As for H2S, ZnO guard beds and activated carbon filters are needed to achieve the desired degree of removal (Van der Drift and Boerrigter, 2006; Boerrigter et al., 2003).
15.5.2.4. Ammonia
In the hot conditioning route, ammonia can be removed from syngas by means of catalytic decomposition (Reaction [xix]) or selective oxidation (Reaction [xx]) (Xu et al., 2010; Torres et al., 2007; Zwart, 2009; Dou et al., 2002). In catalytic decomposition, some components of the syngas (CO, CO, or H2) can compete with NH3 for the adsorption in the active sites of the catalyst (Xu et al., 2010; Simell et al., 1997). The catalysts used are similar to those used in the tar catalytic cracking: Ni-based, noble metals (mainly Ru), Fe-based, and natural oxides (limonite or dolomite) (Xu et al., 2010; Torres et al., 2007; Zwart, 2009; Dou et al., 2002).
[xix]
[xx]
In the cold route ammonia is removed by scrubbing with water or acid aqueous solutions (normally H2SO4), or activated coal beds (Basu, 2010a,b; Tijmensen et al., 2002; Boerrigter et al., 2003; Zwart, 2009; Dou et al., 2002). At the end of the process, ZnO guard beds and activated carbon filters remove the last traces of NH3 (Van der Drift and Boerrigter, 2006; Boerrigter et al., 2003).
15.5.2.5. Ashes
Pretreatment of the biomass, such as fractionation and leaching, has been employed with the aim of minimizing ash production. Leaching seems to be the best option, since it can remove the inorganic fraction of biomass and improve the quality of the remaining ash (Navarro et al., 2007; Ni et al., 2006; Arvelakis and Koukios, 2002). However, even using pretreatments, some particles will be present in the syngas and need to be removed. In cold conditioning, cyclones, bag filters, scrubbers, or electrostatic precipitators can be used; whereas granular beds and ceramic candle filters are the preferred technologies applied in hot conditioning (Huber et al., 2006; Hamelinck and Faaij, 2002; Han and Kim, 2008; McKendry, 2002; Boerrigter et al., 2003; Zwart, 2009).
15.5.3. Upgrading technologies: from syngas to hydrogen, biofuels, and high-value chemicals
Although H2 is the most demanded product that can be obtained from syngas, there is a wide range of syngas-derived products, as illustrated in Fig. 15.13 (Navarro et al., 2007; IEA, 2006a,b; Wender, 1996; Van der Drift and Boerrigter, 2006; Huber et al., 2006; Higman and van der Burgt, 2008).
The final use of the syngas produced will affect the type of gasification chosen. Thus, air gasification leads to syngas diluted in high amounts of nitrogen that is only attractive if the final use of syngas is the production of ammonia (Hamelinck and Faaij, 2002; Bridgwater, 2003). Otherwise, steam or oxygen gasification is more suitable, since N2 will not act as a diluent in subsequent processes and downstream equipment size will be smaller (Hamelinck and Faaij, 2002; Bridgwater, 2003). In addition, light hydrocarbons can still be present, representing an important part of the heating value of the syngas. These hydrocarbons are converted to H2 and CO by means of steam reforming (Reaction [xxi]) (Huber et al., 2006; Quaak et al., 1999).
[xxi]
15.5.3.1. Production of H2
In the case of hydrogen production, H2:CO ratio needs to be shifted to the maximum value possible, with water gas shift being the most widespread option (Hamelinck and Faaij, 2002). However, the production of sponge iron is an alternative that has gained interest. Once hydrogen content has been maximized, it is separated from the syngas by means of pressure swing adsorption or membranes.
Water gas shift reaction
The WGS reaction (Reaction [i]) is a well-established process for the production of H2 from syngas (Navarro et al., 2007; Hannula, 2009; Higman and van der Burgt, 2008). This technology has been extensively used in refineries. The process involves the reaction of CO with steam to produce CO2 and H2 (Huber et al., 2006; Holladay et al., 2009). Thermodynamically this reaction is favored at low temperatures. However, to achieve high reaction rates and conversions, two stages are normally needed (Navarro et al., 2007; Huber et al., 2006; Hannula, 2009):
1. Firstly, a high-temperature reactor (350–500°C) using a Fe oxide-based catalyst (Fe2O3/Cr2O3), where CO concentration is reduced to levels around 2–3%.
2. Secondly, a low-temperature reactor (200°C) where a Cu-based catalyst is used (Cu–ZnO/Al2O3). This second reactor can reduce CO concentration to values lower than 0.1%.
The use of these catalysts presents some drawbacks such as slow kinetics or the pyrophoric nature of the Cu–ZnO/Al2O3, which can hinder the application in small devices like fuel cells attached to small-scale gasifiers. Different alternatives to the conventional catalysts have been proposed, including Co-V binary oxides and noble metals supported in CeO2 for low-temperature WGS, and Co–Mo and Ni–Mo sulfides for high-temperature WGS (Navarro et al., 2007; Huber et al., 2006; Gonzalez Castaño et al., 2014; Reina et al., 2014, 2015).
Sponge iron
Direct reduction iron or sponge iron is an old method for producing hydrogen (Milne et al., 2006; Peña et al., 2010; Biljetina and Tarman, 1981) that was replaced by more efficient and economic processes. Recently, the interest in sponge iron as a hydrogen production process has grown again, although the technology still has some major technical and economic challenges to overcome (Milne et al., 2006; Peña et al., 2010; Sime et al., 2003). The main reasons for this renaissance are the simplicity of the process and the high purity of the H2 produced (Navarro et al., 2007). The concept is based in a cyclic process. Firstly, Fe oxides are reduced to metallic iron and/or Fe(II) oxide with the syngas (Reactions [xxii] and [xxiii]). The metallic iron is then oxidized to iron oxide using steam (Reaction [xxiv]) (Milne et al., 2006; Peña et al., 2010). After oxidation, H2 is recovered and the Fe2O3 is recycled to the initial step (Sime et al., 2003).
[xxii]
[xxiii]
[xxiv]
Separation
The final step for hydrogen production is its separation from the syngas with the aim of obtaining a product with the required purity. The main techniques for this operation are pressure swing adsorption (PSA) and membrane separation (Huber et al., 2006; Balat and Kirtay, 2010; Hannula, 2009; Higman and van der Burgt, 2008; Spath and Dayton, 2003).
The leading technology to efficiently separate H2 from syngas is PSA (Bermúdez et al., 2013; Yang et al., 1997; Ahn et al., 1999). PSA is a cheap, low-energy and efficient technology for gas separation, based on the different adsorption behavior of the molecules present in the syngas. Adsorption of gases like CO, CO2, CH4, and other contaminants is stronger than H2 adsorption, which leads to high-purity hydrogen. Carbonaceous materials, alumina oxides, or zeolites are the most commonly used adsorbents in PSA (Yang et al., 1997; Wiessner, 1988; Schröter, 1993).
A promising alternative to the widely used PSA is membranes (polymer, metallic, or ceramic) (Hannula, 2009; Bermúdez et al., 2013). Membrane gas separation is a partial pressure-driven process in which a gas mixture is forced to pass across the surface of a membrane through which some components selectively permeate (Hamelinck and Faaij, 2002; Hannula, 2009). Membrane gas separation entails several advantages like easy operation, low CAPEX-OPEX, and low-energy requirements (Han and Kim, 2008). In addition, ceramic membranes can work at high temperatures, so they can be implemented in the hot conditioning process (Hamelinck and Faaij, 2002).
15.5.3.2. Ammonia
Ammonia is an important chemical used in several industrial processes like the production of fertilizers, disinfectants, or nitric acid. It is produced by a catalytic reaction between N2 and H2 (Reaction [xxv]) (Ni et al., 2007; Barreto et al., 2003; Stahl et al., 2004):
[xxv]
The production process is the Haber–Bosch method based on the use of an Fe-based catalyst working at high pressures (100–200 bar) and temperatures higher than 450°C (Basu, 2010a,b; Higman and van der Burgt, 2008; Spath and Dayton, 2003). High pressures are needed for thermodynamic reasons, since the conversion per pass is extremely low and the equilibrium needs to be shifted toward the products. In the case of temperature, a trade-off occurs between equilibrium and reaction rate (Spath and Dayton, 2003). Thermodynamics establish that lower temperatures favor the direct reaction, while kinetics are reduced. Even under optimum conditions and with good catalysts, the conversion per pass is about 15% and the unreacted gas needs to be recirculated (Stahl et al., 2004).
15.5.3.3. Methanol production
Methanol is one of the most important chemicals in industry due to its high versatility and several applications (Wender, 1996; Bermúdez et al., 2013; Olah et al., 2011). Methanol can be used as an H2 carrier, fuel, or platform chemical for the production of a wide range of chemicals (olefins, dimethyl ether, acetic acid, formaldehyde) (Huber et al., 2006; Spath and Dayton, 2003; Olah et al., 2011). Its synthesis is carried out at temperatures in the range of 220–300°C and pressures about 50–100 bar using a catalyst of Cu–ZnO supported on alumina (Wender, 1996; Olah et al., 2011). Methanol synthesis takes place through two different reactions (Reaction [xxvi] and [xxvii]) (Wender, 1996; Olah et al., 2011; Tjatjopoulos and Vasalos, 1998):
[xxvi]
[xxvii]
As both CO and CO2 react with hydrogen in the process, the ratio between these gases should be optimized according to the R parameter (Eq. [15.1]), which should be in the range of 2.03–2.05 (Wender, 1996; Olah et al., 2011; Tjatjopoulos and Vasalos, 1998; Bermúdez et al., 2013):
[15.1]
Normally, the conversion per pass is quite low, so recycling is needed (Wender, 1996; Olah et al., 2011; Bermúdez et al., 2013). One of the main challenges for this process is the efficient removal of the heat generated in the methanol synthesis reactor, which is usually a fixed-bed reactor. The integration of these reactors with other process streams can play a key role in the global efficiency of the process (Hamelinck and Faaij, 2002; Bermúdez et al., 2013; Olah et al., 2011).
As an alternative to the fixed-bed reactor, the liquid phase methanol (LPMEOH) synthesis was developed (Wender, 1996; Hamelinck and Faaij, 2002). In LPMEOH, syngas dissolves in the liquid phase and diffuses until the reactants reach the catalyst, where they react to produce MeOH. An example of this technology is the slurry bubble column reactor developed by Air Products and Chemicals Inc., which can reach conversions higher than 50% (Air Products, 1998, 2015).
15.5.3.4. Production of liquid fuels: Fischer–Tropsch
Fischer–Tropsch (FT) is a technology that transforms syngas into high-quality liquid fuels (alkanes free from S or other contaminants) that followed a similar development to gasification. It was mainly developed by Germany during the Second World War and SASOL in South Africa after that. It is a very mature technology currently used by leading companies like SASOL from coal gasification–derived syngas or Shell from natural gas–derived syngas (Wender, 1996; Luque et al., 2012; Higman and van der Burgt, 2008; Dry, 2002; Khodakov et al., 2007).
Currently, FT can be carried out using two different options: high temperature in fluidized bed reactors (300–350°C, for gasoline production) and low temperature in slurry bed reactors (200–240°C, for wax production) (Wender, 1996; Tijmensen et al., 2002; Luque et al., 2012; Dry, 2002; Khodakov et al., 2007).
Only metals from group VIII (Ni, Fe, Co, and Ru) present good catalytic properties for FT synthesis; among them Ru and Ni have been discarded due to the excessive price and the high production of CH4 respectively (Wender, 1996; Dry, 2002; Khodakov et al., 2007; Iglesia, 1997). Thus, only Fe- and Co-based catalysts are used in FT synthesis (Luque et al., 2012; Dry, 2002; Tavakoli et al., 2008). Fe presents the advantage of low cost and that yields high olefinic content in the product distribution. However, it is also active for the WGS reaction what decreases the yield of the overall process (Huber et al., 2006; Tijmensen et al., 2002; Boerrigter et al., 2003; Dry, 2002). Co is more expensive but has higher stability, higher activity, and lower yield of oxygenated products. However, it is only used in low-temperature FT because at high temperature it gives rise to high production of CH4 (Luque et al., 2012; Dry, 2002; Tavakoli et al., 2008).
Several reactions (Reactions [xxviii], [xxix], and [xxx]) take place in FT synthesis to give rise to paraffins, olefins, and alcohols (as byproducts) (Wender, 1996). As a result of FT process, a range of straight chain alkanes from C1 to C50 are obtained regardless of operating conditions (Wender, 1996; Huber et al., 2006; Basu, 2010a,b; Dry, 2002; Iglesia, 1997). The product distribution is governed by the Anderson–Schulz–Flory (ASF) polymerization model, which predicts the selectivity of the different products on the basis of a chain growth probability parameter (Fig. 15.14(a)) (Huber et al., 2006; Tijmensen et al., 2002; Luque et al., 2012; Dry, 2002; Iglesia, 1997; Tavakoli et al., 2008; Martínez and López, 2005). Based on this product distribution, it is not possible to selectively synthesize diesel or gasoline without obtaining byproducts (light hydrocarbons or waxes) (Huber et al., 2006). Nevertheless, the catalyst design can influence the selectivity, maximizing different target products (gasoline, kerosene, diesel, fuel oil, waxes) (Fig. 15.14(b)) (Huber et al., 2006; Tijmensen et al., 2002; Luque et al., 2012; Dry, 2002; Iglesia, 1997; Martínez and López, 2005).
Figure 15.14 (a) Product distribution of the Fischer–Tropsch synthesis predicted by Anderson–Schulz–Flory (ASF) polymerization model. (b) Product distribution obtained in the Fischer–Tropsch synthesis with different catalysts. Reprinted with permission from Huber, G.W., Iborra, S., Corma, A., 2006. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chemical Reviews 106 (9), 4044–4098. Copyright (2006) American Chemical Society; Reprinted from Martínez, A., López, C., 2005. The influence of ZSM-5 zeolite composition and crystal size on the in situ conversion of Fischer–Tropsch products over hybrid catalysts. Applied Catalysis A: General 294 (2), 251–259 with permission from Elsevier.
[xxviii]
[xxix]
[xxx]
FT technology has been proven with syngas from biomass gasification in a demonstration plant in the Netherlands. The plant included a fluidized bed gasifier followed by wet gas cleaning and conditioning, WGS and FT synthesis to produce waxes that were then cracked to obtain a high-quality sulfur-free diesel fuel (Wender, 1996; Huber et al., 2006).
15.5.3.5. Production of synthetic natural gas: methanation
Under the adequate conditions, H2 and CO can be used for the production of CH4 (Reaction [xxxi]) (Wender, 1996; Kopyscinski et al., 2010). The process, uses a Ni catalyst (although other transition metals can be used) and works at temperatures in the range of 700–1000°C (Wender, 1996; Kopyscinski et al., 2010).
[xxxi]
Initially, this process was used for removing CO traces from H2-rich streams in ammonia production plants (Kopyscinski et al., 2010). However, nowadays it is considered as an interesting alternative for producing substitute natural gas (SNG), since it is free from contaminants like H2S and its production from biomass has a neutral carbon footprint (Kopyscinski et al., 2010).
15.5.3.6. Production of iso-C4: isosynthesis
Isosynthesis is part of the FT process, but involving the conversion of syngas to isobutane and isobutene (Wender, 1996; Spath and Dayton, 2003). Other differences compared to conventional FT synthesis are the catalyst used (ThO2- or ZrO2-based catalysts), the extreme operating conditions (450°C and 150–1000 bar) and that the process does not follow the ASF polymerization model (Wender, 1996; Huber et al., 2006). It was developed during the Second World War, since iso-C4 are raw materials for the synthesis of high-octane gasoline. However, the development of catalysts for the production of high-octane gasoline from petroleum stopped its commercial use (Wender, 1996; Huber et al., 2006).
15.5.3.7. Production of alcohols and aldehydes: oxosynthesis
Oxosynthesis (or hydroformylation) is a reaction between syngas and olefins to give rise to aldehydes or alcohols (Wender, 1996; Spath and Dayton, 2003). This process is the fourth largest use of syngas and involves the production of chemicals of high importance (eg, butanol, propanol, or isobutanol) (Wender, 1996; Huber et al., 2006; Higman and van der Burgt, 2008). Firstly, the olefin is hydroformylated to give rise to an aldehyde with one carbon more than the original olefin (Reaction [xxxii]). Then, the aldehyde is hydrogenated to give rise to the alcohol (Reaction [xxxiii]) (Wender, 1996):
[xxxii]
[xxxiii]
The reaction is highly exothermic and takes place in the presence of a homogeneous catalyst that is a soluble complex of Co or Ru (Wender, 1996; Huber et al., 2006).
15.5.3.8. Syngas fermentation
Some bacteria have the ability of metabolizing syngas to give rise to a wide variety of valuable products, depending on the bacteria and the fermentation conditions (see Table 15.5) (Huber et al., 2006; Beneroso et al., 2015a,b; Munasinghe and Khanal, 2011). These bacterial systems present a great potential for integration in biorefining schemes. They present the important advantage of being almost unaffected by the H2/CO ratio or the presence of CO2. However the production rates are low and further development is needed (Huber et al., 2006).
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