# Influence of Hydrothermal Carbonization on Catalytic Fast Pyrolysis of Agricultural Biomass
## 1 Introduction  
Extensive effort is ongoing to fulfil the challenging climate goals which have been set by the agreement signed in Paris. Biomass is an upcoming renewable energy source, which has fulfilled around 19% of heating demand, as well as 3% of electricity demand in 2019 in the EU [1]. Due to these reasons, valorization of biomass has been intensively investigated recently, considering its tremendous potential [2; 3; 4]. Pyrolysis is a thermal process in which decomposition takes place in the absence of oxygen [5; 6], thus converting biomass to a mixture of solid, liquid, and gaseous compounds [7; 8; 9; 10] (unlike gasification, where partial oxidation takes place [11; 12; 13]). Solid products can be used as solid fuel or applied to soil in the form of biochar [14]. Gaseous products can be used on-site to supply the process heat. Condensable products can also be used on-site for energy needs. However, it is also possible to condense those compoundsand obtain pyrolysis oil, which can be considered an excellent energy carrier due to its relatively high energy density. A wide range of different processes comes under the common name of pyrolysis. Torrefaction is also known as mild pyrolysis [15; 16; 17]. Slow pyrolysis aims at the production of biochar, pyrolysis oil, and gases for various purposes, e.g., the production of H\({}_{2}\)[18; 19; 20]. Fast pyrolysis maximizes the yield of bio-oil, which is also known as pyrolysis oil. It employs high heating rates of 100 \({}^{\circ}\)C/s with temperatures between 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C and a residence time of 1 s [21; 22; 23]. The University of Twente (Thermal Engineering group) in the Netherlands has developed PyRos, which is a fast pyrolysis installation capable of processing approx. 30 kg/h of biomass. This reactor was patented with number WO0134725. PyRos is a cyclonic shaped type of reactor, and biomass is injected into the cyclone along with preheated inert material (sand) and gases. Centrifugal forces help in separating the solids from generated vapors and gases. Solids start sliding down the reactor walls. The biomass has direct contact with hot inert sand, which helps to achieve high heat transfer of the order of 500 up to 700 W/m\({}^{2}\)K. It also helps in a non-problematic in situ introduction of the catalyst. Produced chars find application in a fluidized bed combustor, generating the heat necessary for the process. CaO has been used as a catalyst in many different applications, such as oxidation of unburned pollutants in the flue gas [24], Fisher Tropsch synthesis [25], gasification [26], production of syngas from pyrolysis [27], and fast pyrolysis [22; 28; 29; 30]. HTC is considered a thermal vaporization process, typically performed at 200 to 260 \({}^{\circ}\)C in subcritical water at high pressure [31; 32]. At temperatures of 200 up to 280 \({}^{\circ}\)C, the ionic constant of water increases significantly. Hence, water starts acting as a non-polar solvent [33; 34]. The process generates a multitude of concurring reactions, yielding multiple types of products, especially with different types of biomass [31; 32]. The first stage of the HTC process is hydrolysis, followed by dehydration as well as decarboxylation [35]. Dehydration results in decreasing the number of hydroxyl groups [31]. Decarboxylation promotes the formation of CO\({}_{2}\)[36; 37]. A decrease in the amount of OH groups also results in a lower O/C ratio. Decarboxylation results in decreasing the amount of COOH and C=O groups, also slightly decreasing the O/C ratio of the solid product [31; 38; 39]. This is followed by further polymerization and aromatization [31; 35]. The ability to decrease the O/C ratio is considered beneficial when vaporization is performed, from the point of view of subsequent pyrolysis [40; 41; 42]. Due to these factors, HTC is considered one of a viable means of valorizing wet biomass before pyrolysis [43; 44; 45].