## 1 Introduction  
Synthesis gas, commonly referred to as syngas, is a mixture that contains mainly hydrogen (H\({}_{2}\)), carbon monoxide (CO), and some carbon dioxide (CO\({}_{2}\)). When derived from solid fuels such as coal or wood, syngas may also be called producer gas. Syngas is used for the production of a broad range of products, namely, hydrocarbon fuels such as gasoline, diesel, or commodity chemicals such as methanol. Methanol is a versatile molecule. This alcohol is a basic commodity chemical used as a solvent, a starting material for a large spectrum of chemicals, and a liquid fuel that can be safely handled and stored. In addition, methanol can easily be converted into dimethyl ether (DME), another basic chemical widely viewed as a substitute for diesel in internal compression engines. From the mid-19 th century up until the early 20 th century, methanol was produced commercially through destructive wood distillation, which was a pyrolysis process conducted in a distillation apparatus to separate the products [1]. In 1923, the German BASF company started producing methanol, using fossil fuel derived H\({}_{2}\), CO, and CO\({}_{2}\) as reactant syngas. A catalytic, high-pressure synthesis of methanol was then achieved. Methanol (CH\({}_{3}\)OH) production is characterised by the well-known reaction as follows:  
\[\text{CO}+2\;\text{H}_{2}\rightarrow\text{CH}_{3}\text{OH} \tag{1}\]  
For this reaction, an ideal syngas should be a mixture of H\({}_{2}\) and CO with a molar H\({}_{2}\)/CO ratio of 2. As mentioned earlier, syngas always contains some CO\({}_{2}\) which can contribute to methanol formation, alongside water (H\({}_{2}\)O), according to the following reaction:  
\[\text{CO}_{2}+3\;\text{H}_{2}\rightarrow\text{CH}_{3}\text{OH}+\text{H}_{2}\text {O} \tag{2}\]  
In order to consider the amount of CO\({}_{2}\) that a given syngas may contain, and according to Reactions (1) and (2), the most widely used criteria for methanol synthesis is the following molar concentration balance ratio:  
\[\text{M}=(\text{H}_{2}-\text{CO}_{2})/(\text{CO}+\text{CO}_{2})=2 \tag{3}\]  
Because of concerns regarding fossil greenhouse gas emissions, biogenic materials such as forest residues are nowadays considered as a valuable source of carbon atoms to produce syngas. When biomass is used as a feedstock for the production of methanol, gasification with pure oxygen (O\({}_{2}\)) and steam is widely considered as a well-suited technology for the production of syngas. Due to its intrinsic atomic composition, woody biomass cannot directly produce syngas with a molar composition that satisfies the Equation (3) criterion. Generally, gas at the outlet of a gasifier will present an M number closer or even less than 1. In effect, with steam gasification of carbon from biomass, only one H\({}_{2}\) molecule is generated alongside each CO molecule in the following illustrative reaction:  
\[\text{C}+\text{H}_{2}\text{O}\rightarrow\text{CO}+\text{H}_{2} \tag{4}\]  
In order to obtain a particular syngas according to the criterion of Equation (3), composition adjustment or \"balancing\" of raw gas from a gasifier is required. Syngas balancing usually involves the use of the well-known water-gas shift (WGS) reaction as follows:  
\[\text{CO}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{H}_{2} \tag{5}\]  
This reaction leads to the production of extra H\({}_{2}\), but at the expense of a loss of CO. Extra CO\({}_{2}\) is produced, and must be separated and wasted, thus decreasing significantly the efficiency of carbon conversion into CO. In other words, a significant amount of carbon is lost, meaning a net decrease in carbon conversion yield. Theoretically, starting from Reaction (4), in order to obtain an H\({}_{2}\)/CO ratio of 2 for a given syngas, 1/3 of a mole of CO must react according to Reaction (5), producing 1/3 of a mole of CO\({}_{2}\) in the process. The theoretical carbon conversion yield is then 66.7%. In practice, the real, verified carbon conversion yield is lower than 60% when this WGS approach is taken.