Most of the biomass can be processed in HTL because of the hydrophilic nature of biomass and the reasonable ease in forming water slurries of biomass particles for concentrations that are being able to pumped, typically 5–35% dry solids. In case of using lignocellulosic biomass, which is lower in moisture content, recovery and reuse of the water for slurry preparation is important. For high-moisture biomass, like algae, some dewatering is required prior to processing in order to lessen the processing costs of excessive water. Usually the woody biomass requires grinding prior to processing as is discussed in the subsequent section, microalgae, some strains of macro-algae and certain manures and sludge are of suitable small size for direct processing. It can be seen that the wet manure and sewage sludge feedstock have not been processed in continuous systems, although results from batch systems are promising for their application in continuous systems4. A further advantage of using hydrothermal processing for sludge and manures is the effect of sterilizing bioactive contaminants.
Algal biomass has been identified as a promising alternative as a resource for renewable fuels due to its higher photosynthetic efficiency and area specific yields. Also the development of third generation biofuels from microalgae has seen increasing research efforts over the last decade.
Since we have previously mentioned as well that microalgae could be cultivated in sea water or freshwater, and also since this article and the possibilities reviewed are keeping in view of scarcity of fresh water in India and other developing countries of this region, we are only reviewing the production possibilities of the species which are compatible with seawater i.e. Chlorella. Also it is important to mention that the production capability of microalgae by using sea water is encouraging than the conventional resources. Microalgae are microscopic organisms that can grow in fresh, brackish or salt water. The advantage of microalgae compared to terrestrial biomass is its much higher photosynthetic efficiency which results in higher growth rates and improved CO2 mitigation4. They are especially suited for continuous hydrothermal liquefaction due to the small size (100 h).
Outcomes of the work on HTL of lignocellulosic feedstocks included 11:
· Demonstrated high yield (>50% carbon efficiency) and efficient HTL process on forest residue and corn stove feedstocks requiring nothing other than size reduction (formatting).
· Within the limits of the corrosion assessment testing, the suitability of stainless steels for HTL applications was confirmed.
· During the test it was successfully developed a single-stage hydro treating protocol to produce distillate (57%) and naphtha (27%) and product fractionation and fuel properties were evaluated.
· Prepared a full pumping report assessing industrial operation9.
o Relevant vendor experience included industrial/municipal sludge, pastes, fibers, silage.
o Large scale testing would be required to establish pump ability limits for larger particle size feedstock.
· An HTL technological economic analysis was prepared and published to provide potential commercial partners with a detailed understanding of the process economics and sensitivities11.
The major benefit of processing algae/wet biomass is the potential for recycling of nutrients back to cultivation. The procedure of treating wet biomass is similar to that of treating lignocellulose with a few modifications.
The important difference lies in the method of treating the wet feedstock compared to dry feedstock is removing the pre-treatment step. Generally, pre-treatment is essential as the particle size of the micro algae is quite small compared to lignocellulose that eliminates the complexities of pumping slurries to the reactor. During the HTL process the aqueous phase generated is recirculated back to the algae cultivation procedure, CO2 that is released can be utilized in the process of photosynthesis in the growth of algae for next batch.