## 5 Thermochemical Conversion of Carbonaceous Feedstocks
### Pyrolysis  
Pyrolysis is a thermochemical process that transforms material such as wood, agroforestry, or plastic waste into gaseous, liquid, and solid compounds, without the presence of oxygen. Depending on the operating conditions, the pyrolysis process can occur at temperatures between 200 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C and at pressures of up to 100 bar. Outputs vary with process and feedstock conditions. In general, it can be said that there are three main outputs [129]:  
Figure 9: Diagram of down-draft reactor, adapted from [120]. * Biochar (10-30%);
* Bio-oil (20-50%);
* Syngas (20-30%). The pyrolysis process can occur under three major operating conditions: slow pyrolysis, intermediate pyrolysis, and fast/flash pyrolysis [130]. Table 5 presents a summary of the features of each operating condition. CSP technologies can be coupled to such processes, as through their concentration factor, such operating temperatures can be achieved and, eventually, can increase the Lower Heating Value of the produced syngas by a factor of 1.4-1.9 [131]. Operating costs are relatively difficult to identify in the literature, as they depend on the intended output and operating conditions. However, it is possible to state costs between EUR 75-300 per ton of oil (12-54 EUR/MWh), assuming a feedstock cost between 0-100 EUR/t (0-1.9 EUR/GJ) [129]. On the other hand, a recent NREL study for a 550-metric-ton/day dry wood fast/flash pyrolysis plant producing 106 million liters of bio-oil per year concluded that the capital cost is 46 M EUR and the operating cost will be 9.1 M EUR/year (based on a product value of 0.16 EUR/J) [132]. It is also important to note that the pyrolysis process can be combined with other thermochemical processes, as shown in Figure 10 [133]. The advantage of such combinations is to diversify and/or increase the efficiency of biomass waste conversion in multiple outputs, such as hydrogen, fuels, heat power, etc. Figure 10: Pyrolysis process can be combined with other thermochemical processes for the production of multiple outputs. Here, a flow-chart of a possible thermochemical conversion process for lignocellulosic biomass is presented, adapted from [133]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Operating Condition** & \
\begin{tabular}{c} **Operating** \\ **Temperature** \\ \end{tabular} & **Residence Time** & **Main Output** &
\begin{tabular}{c} **Compatibility with** \\ **Use of MS** \\ \end{tabular} \\ \hline Slow pyrolysis & 200–400 \({}^{\circ}\)C & 5–30 min & Biochar & Yes, if above 290 \({}^{\circ}\)C \\ Intermediate pyrolysis & 400–600 \({}^{\circ}\)C & 1–5 min & Bio-oil & Yes, if up to 565 \({}^{\circ}\)C \\ Fast/flash pyrolysis & 600–1000 \({}^{\circ}\)C & 1–2 s & Syngas & No \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Operating conditions for pyrolysis processes. Regarding methane-containing mixtures (e.g., natural gas, biogas, and biomethane), pyrolysis methodologies can be considered as ways to produce hydrogen with zero CO\({}_{2}\) emissions, without the need to use systems for carbon dioxide capture [134]. Actually, these processes allow for straightforward coupling with an HTF and the TES used in CSP and can lead to a direct separation of carbon and H\({}_{2}\), where the former can be stored in a solid state and, if valuable products are obtained, placed on the market. This way, the usual \"carbon sequestration\" route can be replaced by \"carbon vaporization\" methods [135; 136]. Among the different possible approaches, a very innovative and interesting one is to perform the pyrolysis reaction directly in a molten bath made of liquid metals or MSs (typically chlorides) [137]. This is so to avoid the employment and possible poisoning of catalysts, and the carbonaceous products can be readily recovered from above the surface of the melt [138]. In this context, the C-zero consortium realized a TRL 6 plant utilizing the KCl/MnCl\({}_{2}\) mixture at 1000 \({}^{\circ}\)C and 1 bar, obtaining graphite-like carbon as a co-product [139].