Methane and CO2 are constituents of liquid natural gas (LNG). According to conventional technologies, the two gases are separated and methane is used for the production of Syngas, an H2-CO mixture (Eq. 7.1) that has been used for more than 50 years for the synthesis of gasoline (Fischer Tropsch process). Methanol is also produced from Syngas (Eq. 7.2). The gas-to-liquid (GTL) conversion is currently a process of great interest as its implementation at the site of LNG extraction site would considerably reduce transportation costs:
The production of Syngas from methane and water (Eq. 7.1) is a strongly endother-mic process (methane is burned to furnish the necessary energy) and produces an H2-CO mixture with a higher H2/CO ratio than required for the synthesis of methanol (Eq. 7.2) or gasoline. The excess hydrogen is not recovered and therefore as a whole the process is not energetically optimized (Lange, 1997).
CO2 can, in principle, either partially or totally substitute CO in the synthesis of methanol. It can either be added to the steam reformer (Eq. 7.3) or to the methanol unit (Eq. 7.4) (an ICI process in use for several decades), resulting in a better utilization of the energy and hydrogen:
In the CO-CO2-H2 system the direct conversion of CO2 to methanol has been shown by Rozovskii (1989) by tracer analysis. In a standard technology based on methane wet-reforming and subsequent conversion of Syngas into methanol (Eqs 7.1 and 7.2), the energy consumption is 31 kJ/t methanol with a thermal yield (lower heating value or (LHV)) of 64.3% (Lange, 1997); the thermal yield based on natural gas is just 42.3% (M. Ricci and F. Podesta, Novara, Italy, 2004, personal communication). Substitution of 5% methane with CO2 increases the thermal yield to 66.5% (LHV) (Lange, 1997; Lemonidou et al.,
2003) as a consequence of the better use of hydrogen (Maroto-Valer et al., 2002; Song, 2002). This feed substitution can be further increased up to a maximum of 30%. In order to partially compensate the two endothermic processes of wet- (Eq. 7.1) and dry-reforming (Eq. 7.3) of methane, its partial oxidation is used (Eq. 7.5):
The combination of Eqs 7.1, 7.3 and 7.5 is known as 'tri-reforming' (Song, 2002) and it is this that produces the correct H2/CO ratio (1.7:2.0) for methanol or hydrocarbon synthesis (Fox, 1993; Rostrup-Nielsen, 1994; Ross et al., 1996). The tri-reforming has been recently reviewed by Halmann and Steinfeld (2005) and shown to be of practical interest for the treatment of flue gases from coal and gas-fired power stations within the framework of better energy management. Progress in catalyst development (Mirodatos et al., 2005) for dry-reforming may foster the further exploitation of tri-reforming and CO2 utilization.
Addition of CO2 to methane for the synthesis of methanol, despite being an endothermic process and the fact that CO2 consumes 1 mol of H2 in excess with respect to CO (compare Eqs 7.2 and 7.4), reduces
CO2 emissions by °.2 tCO2/tmethanol as a consequence of better energy and carbon management.
An interesting innovation is the coupling of cold plasmas with catalysts in methane dry-reforming (Eq. 7.3) for the synthesis of methanol (Eliasson and Kogelschatz, 1991) or oxy-fuels (Zhang et al., 2003). Interestingly, in 2002 a pilot plant for the conversion of natural gas into liquid fuel under plasma conditions began operation in Alberta, Canada (Czernichowski et al., 2002).
The catalytic hydrogenation of CO2 to methanol has attracted the attention of several research groups and remarkable progress has been made in the last 10 years in terms of catalyst development (reaching a 100% selectivity and high turnover frequency (TOF) ). Such excellent performances are due to a different reaction mechanism (Kieffer et al., 1997) of CO2 with respect to CO. The first methanol
synthesis from H2-CO2 at the demonstration pilot plant scale (50 kg/day) has already been achieved in Japan (Ushikoshi et al., 1998), using a Cu-ZnO-based catalyst at 250°C and 5 MPa. Direct methanol synthesis from an H2-CO2 feed has been reviewed by several authors (Saito et al., 1996; Arakawa, 1998; Halmann and Steinberg, 1999). Experimental results show a higher yield of methanol from H2-CO2 at 260°C, with respect to H2-CO, with a further improvement when Pd-modified Cu-ZnO (Inui et al., 2000) is used as catalyst. Nevertheless, if the H2 is produced from fossil fuels, the direct production of methanol from H2-CO2 is uneconomic and makes a very limited contribution to reducing net CO2 emissions. Conversely, integrated systems can also be used such as:
in its infancy, but it has been demonstrated that the three enzymes can be encapsulated and used together for an easy conversion of CO2 into methanol (Jiang et al., 2003; Xu et al., 2005). The current limitation is in the use of NADPH/NADP couple as an electron source. The hurdle could be overcome by coupling the reaction with the electrochemical re-activation of the reducing agent or with the use of other cheap reductants. An increased availability of enzymes, and the use of existing knowledge of chemical engineering of bioreactors, may allow further exploitation of this technology.
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