Most would agree that the production of biofuels is important, even necessary, for our future. There are significant differences in the production of various biofuels, especially concerning the production of biocoal, which will impact the acceptance and use of biofuel in the future. This article focuses on one technology for the production of biocoal which has seen some significant development in the past few years; hydrothermal carbonization. Carbonization is a natural process through which, under pressure but at low temperatures, coal-like solid biofuels are produced from biomass. Hydrothermal carbonization (HTC) accelerates this process from millions of years to only a few hours.
Hydrothermal Carbonization Process
The HTC process was invented at the beginning of the 20th century by Friedrich Bergius, focusing on the carbonization of biomass to biocoal within a few hours using a pressure chamber at high temperatures. The modern process was “reinvented” in the last decade as the search for alternative sources of energy has intensified, and the amount of unused organic waste from urban areas and industrial processes has increased.
As opposed to processes such as torrefaction or pyrolysis, HTC uses heated water (at temperatures around 200 °C and at an elevated pressure) to “cook” the biomass and produces a slurry that can be efficiently processed into biocoal. HTC unlocks most efficiently the energetic potential in organic waste: the ideal biowaste-to-energy technology.
A wide variety of organic material can be used to “feed” the HTC process. In municipalities, waste management companies collect significant amount of organic waste material that is mostly composted, such as grass cuttings, leaves, branches and straw. Composting costs, especially in Europe and the US, are high, and refinement to biocoal with HTC is a financially attractive alternative. In industrial settings, the economic business case for the use of HTC to refine biomass is even more attractive.
If we consider the production of palm oil in Malaysia as an example, hundreds of thousands of tons per year of empty-fruit-bunches (EFBs) are incinerated or composted, and the production of biocoal for local use or for export not only uses the energy potential in the organic material but can generate income within a relatively short time period. This is true for other industries such as fruit juice, sugar cane or paper. Depending on manufacturer of the HTC-technology and plant, very damp organic wastes can be refined. Another benefit: the central focus for most HTC-technology providers is “unused” organic waste and not the use of energy crops.
Before the HTC process begins, the organic waste is crushed and sieved, and in some cases washed to remove impurities. The biomass is then fed into an HTC-cooker, which at high temperatures and pressure “cooks” the biomass, resulting in a biocoal-slurry. This slurry is then further dried to produce a biocoal-cake, which can then be further processed for firing in the form of powder, pellets or briquettes. The resulting product is a dry and solid biocoal, with similar fuel characteristics to fossil coal, and can be used in a number of different ways.
Uses of Biocoal
Biocoal can be used directly as an energy source in existing coal-fired power plants. In addition, biocoal can be processed into syngas and secondary products, such as second generation liquid BtL-fuels through gasification (Fischer-Tropsch-Synthesis). Alternatively, the HTC process can produce biochar, which can be used as a soil-nutrient. Because of developments in HTC technology, biomass has developed a reputation as an “all-rounder” in the renewable energy space.