Hydrothermal liquefaction is a process in which the macromolecules making up the biomass are hydrolyzed or degraded by means of water at average temperatures and high pressures. This process is usually carried out in temperature ranges between 280°C and 370°C and pressures that are in the range from 10 to 25 MPa. Experimentation is required to improve the surface area and morphology of the char derived.

The char obtained from HTL process has oil trapped in its pores which has high boiling range. If you directly perform BET surface area analysis for the char with degassing at 280 or so, it will show very less surface area and you will be able to observe droplets of oil on BET tube.

I will suggest heating it in absence of oxygen upto 375 - 400 C. I tried it for a char obtained using agricultural residue at HTL condition of 300 C. The adsorption capacity increased by more that 60 %.

A very general question in fact! First of all, it's important to know the properties of char. For most lignocellulosic biomasses the inorganic precipitated in the char are not remarkable in amount, while in some cases like sewage sludge, inorganics play an important role in pore blockage. Acid washing to neutralize and remove the inorganics could be a way to go. Moreover, chemical and physical activation procedures are other methods that can considerably increase surface area, pore volume and diameter, whereas an additional cost have to be paid for the chemicals and energy.