## 2 Method
### Conceptual Process
The gasifier is divided into two zones, as shown in Figure 1. Combustion (oxidation) and hydro-pyrolysis (thermal degradation and reaction with steam) of the biomass take place in Zone-I, which represents a typical gasifier as the one depicted in Figure 2. In this zone, O\({}_{2}\) with some steam is injected. If air were used as gasifying agent, large amounts of nitrogen (N\({}_{2}\)) would have to be heated up, decreasing energy efficiency. Additionally, this N\({}_{2}\) would have to be removed from the gaseous mixture prior to synthesizing methanol. Using pure O\({}_{2}\) instead of air means less sensible heat has to be provided for the process. Products from Zone-I include gaseous components, tars, and a carbonaceous solid called char that contains the ashes. We consider that gaseous components may include, in addition to H\({}_{2}\), CO, CO\({}_{2}\), and steam, some hydrocarbons (namely CH\({}_{4}\)), and some nitrogen- and sulphur-based impurities [7]. The char is recovered and separated from Zone-I. The gasifier must be designed and operated in such a way that most of the solids and particles out from the pyrolysis process end up in the char. The gaseous components flow into Zone-II of the gasifier. In this zone, O\({}_{2}\) and some steam are again injected. The purpose of injecting O\({}_{2}\) is to increase the temperature (up to ~1200 \({}^{\circ}\)C) of the gaseous mixture through its very incomplete combustion, while steam allows for steam reforming of tars and hydrocarbons into small molecules such as H\({}_{2}\), CO, and CO\({}_{2}\). As was explained for the gasifier inlet, injecting pure O\({}_{2}\) instead of air in Zone-II also leads to higher energy efficiency. Moreover, oxygenated tars typically require a low O\({}_{2}\) to fuel ratio for complete reaction, compared to aromatics-containing tars. The gas exiting the gasifier is expected to be tar-free and with most of its hydrocarbons thermally reformed. This type of process can be referred to as partial oxidation (POX) of producer gas. Figure 2: Schematic diagram of an updraft gasifier with flow patterns for gases and solids. At the exit of Zone-II, the gaseous mixture is referred to as a \"hot raw syngas.\" This mixture is fed into a heat recovery steam boiler (HRB), where the gas is rapidly cooled down to 150 \({}^{\circ}\)C while producing low-pressure steam. At the exit of the recovery boiler, impurities such as sulphur components are separated. We consider this separation step (SEP) to be realised through a physical adsorption process (e.g., using zinc oxide (ZnO) as an adsorbent). The required impurity removal rate is expected to be very low, considering that the biomass feed contains very low amounts of sulphur and other impurities. The gas is then fed into a water-condensing unit (COND), where it is dehumidified while recovering water as a condensate. At the exit of the condensing unit, the syngas has not yet been balanced. This balancing is achieved through mixing with electrolytic H\({}_{2}\) in order to have suitable syngas that fulfills the criteria of Equation (3). As shown in Figure 1, required electrolytic H\({}_{2}\) comes from a water (steam) electrolysis system (ELECTROLYSIS). This system must be specified to generate the amount of H\({}_{2}\) required for the process. Water electrolysis involves the co-production of O\({}_{2}\) as follows:  
\[\mathrm{H_{2}O}\rightarrow\mathrm{H_{2}}+1/2\;\mathrm{O_{2}} \tag{6}\]  
The quantity of coproduced O\({}_{2}\) is expected to exceed the amount of pure O\({}_{2}\) required for the gasification process. Part of the produced O\({}_{2}\) is vented or stored. The other part is mixed with steam. The mixture (O\({}_{2}\)/steam) is split into two streams--the first one is injected in Zone-I, while the second one is injected in Zone-II of the gasifier. Essentially, with the process concept hereby presented, the electrolysis system can be considered as a utility device supplying both H\({}_{2}\) and O\({}_{2}\). This dual function makes the process independent of the type of electrolyser chosen.