Fundamentally, hydrothermal liquefaction is pyrolysis, and therefore degradation and polymerization occur. Simple reaction scheme is shown in Fig. 4.6.2. At first step, biomass can be degraded into water-dissolved materials. Then the water-dissolved materials are

polymerized to form oil. When reaction is prolonged, the formed oil is polymerized into char.

4.6.3 Product oil from hydrothermal liquefaction

Properties of obtained oil by liquefaction are shown in Table 4.6.1. The reaction was conducted without reducing gas, such as hydrogen and carbon monoxide, and with alkali catalyst for wood and without catalyst for sewage sludge. The obtained oil has oxygen content of around 20 wt%, and therefore, its higher heating value is lower than that (around 42 MJ/kg) of heavy oil from petroleum. In addition, its viscosity is very high.

The obtained oil from wood has much amount of acid fraction, and it can corrode and polymerize during storage.

On the other hand, the obtained oil from sewage sludge has nitrogen originated from protein, and NOx treatment is needed at its combustion.

As chemical fraction, pyridine derivative, pyrazine derivative, and amid compounds are detected. If they can be separate, they might be used as chemicals.