Mahbod Shafei, an expert on biomass conversion from Iran, shared with ennomotive some insights about the hydrothermal carbonization process of biomass materials and the applications of its byproducts.
Hydrothermal Carbonization (HTC)
Biomass, as a non-fossil-based, feedstock has applications in many industries such as renewable energies, chemicals, agricultural and pharmaceutical products. Several articles on biomass thermo-chemical conversion process cited that direct combustion or gasification is not a satisfying option owing to properties of biomass, such as high moisture and oxygen content.
There are several methods for biomass pre-treatment to reduce moisture, such as torrefaction. This process produces more carbonaceous material to be used in thermo-chemical processes. These methods have limitations because of the high moisture content of some biomass, such as fresh compost, organic waste, leaves, industrial waste, and finally, sewage sludge from municipal sewage treatment. On the other hand, the amount of human waste (untreated excrement and fecal sludge) from wastewater treatment processes is continuously increasing not only through the sanitation in developing countries but also through wastewater treatment in developed countries.
Hydrothermal carbonization of biomass (HTC) is a promising method for pre-treatment of high moisture content biomass with high dewatering properties. The produced material from the HTC process, hydro char, is like brown coal with its elemental composition and calorific value. Here is a technical description of the HTC and the application of its byproducts in new technologies.
Hydrothermal Carbonization (HTC) process
The process was explored by Friedrich Bergius in 1913. Technically, hydrothermal carbonization (HTC) is a chemical process that converts the wet biomass with less than 12 hours of the operation into a material comparable to brown coal called hydro char. Among thermal methods, HTC offers significant benefits for biomass conversion, including less energy for the drying process, high conversion efficiency, and relatively low operating temperature. The HTC process operates with high moisture biomass suspended in a pressure vessel under a temperature of 180C to 250C and resulting water vapor between 15 to 20 bars. HTC is an exothermic process. The experiments show that the energy consumption required maintaining the reactor temperature during the carbonization process is significantly lower owing to the heat emitted from the exothermic processes within the reactor.
Several investigations show approximately 75%-80% of carbon input found in the solid phase, about 15%-20% dissolved in the liquid phase, and the remaining 5% converts to gas, mainly carbon dioxide . The liquid phase highly consists of organic components, which are easily degradable. HTC is a promising technology for the conversion of organic waste with high water contents and might become of importance also for CO2 sequestration. The differentiation of the mass balance concerning the dry and wet biomass indicates that carbon recovery of the process is around 90%. Also, comparing the energy content of the input and output materials show that 60%-90% of the calorific values are available in hydro char.
According to several studies, various parameters influence the yields of the HTC process in which the temperature has a significant effect. Consequently, pressure and residence time are the function of temperature to optimize the hydro char of the HTC process. In the table, the effect of parameters categorized for several feedstocks is shown. Results show that with a temperature around 230 C and proper residence time, the biomass carbon yield increased, and H/C and O/C ratio decreased in all cases in the hydro char .
|Solid yield db %||C %||H %||O %|
|HTC 230 °C, 4.5h||57||75.2||6.4||11.1|
|HTC 250 °C, 20h||46||75.2||6.4||11.1|
|HTC 230 °C, 6h||51||72.6||7.2||15.6|
|HTC 230 °C, 5 min||75||56.1||5.9||37.9|
Hydrothermal carbonization application
HTC sets up the process to a variety of nontraditional biomass such as wet animal manures, human waste, sewage sludge, municipal solid waste (MSW), and algae. The commercial product of the HTC process is hydro char with brown coal qualities.
- Human waste/sewage sludge:
Statistics show that around 10 million tons on a dry weight basis of sewage sludge exist in Europe per year. In some plants HTC process on sewage sludge done today. The hydro char produced through the process shows that the conversion raises the energy density of the biomass, carbon content, and declined oxygen content. Also, with technical temperature control, ash content decreases significantly. Besides, the analysis of the liquid phase shows that it is biodegradable. By this means, an energetic utilization of the liquid phase in the form of CH4 is possible.
For most of the Thermochemical processes pretreatment, it is needed to destroy the fibrous nature of biomass particles such as entrained flow gasification. HTC is an extraordinary pretreatment process that raises the heating value and mechanical properties of biomass, such as grind ability. Some specific characteristics of the hydro char, such as higher ignition temperature and wide combustion range, make it appropriate for boilers of heating systems.
- Soil amendments:
Hydro char have the potential to sequester carbon in the soil, enhance water holding capacity, boost nutrient retention, and reduced greenhouse gas emissions such as CO2, CH4, and N2O.
- Catalyst production:
Catalyst production from the HTC process reported several times for hydrogen (H2) production owing to high stability and tunable surface qualities of hydro char. The experiment noted HTC of algae with the aim of H2 production, produced hydro char with the catalytic ability that not only promoted H2 generation but also surged up phenol formation in the liquid phase meantime, acid production was limited .
Efficiency and cost estimation
It is necessary to consider the development of the HTC process in the market estimation of economic feasibility. Many investigations reported HTC profitability in regards to the efficiency with parameters such as electric power demand and specific thermal energy. It is predictable that in the HTC process, higher efficiency is available with high moisture biomass.
For example, an estimation reported that the plant with an operating time of 8000 h/y and a capacity of 20000 ton/y (2500 kg/h) of biomass work efficiently at 220 C, one hour residence time with wet biomass. For this plant, specific thermal energy consumption and specific electricity consumption are around 1.17 KWh/kg hydro char and 0.16 KWh/kg hydro char, respectively, and total efficiency is 78%.
For economic estimation, the cost of installation, and capital cost for site preparation, water, and water treatment that is around 1157 kg/hr considered. The bare cost and capital cost estimated to be 3 million Euros.
For production cost, estimation is around 157 Euros per ton by considering fuel for burners, electricity, maintenance, pelletizing, and taxes. This plant produced 5500 tons of hydro char per year, and the break-even value of hydro char is 200 Euros per ton. In contrast with wood pellets price in the range of 150-200 Euros per ton, it makes hydro char competitive with wood pellets .
There are HTC plants in Europe working with various capacities, including Germany, Switzerland, Italy, and the United Kingdom.
Finally, hydrothermal carbonization can homogenize wet biomass and produce hydro char with developed fuel character. On the other hand, there is a knowledge gap in the link between biomass properties, the end-use application, and environmental impact that can compensate with more laboratory research and practical work. For example, the organic compounds dissolved in the liquid phase of HTC are the hefty energy losses from the plant. Therefore, it is essential to focus on concepts for limiting dissolved compounds and convert them into useful products such as biogas.
 Hans-Günter Ramke, Dennis Blöhse, Hans-Joachim Lehmann, Joachim Fettig. Hydrothermal Carbonization of Organic Waste. University of Applied Sciences Ostwestfalen-Lippe, Campus Hoexter An der Wilhelmshöhe 44, 37671 Hoexter
 Zeno Robbiani. Hydrothermal Carbonisation of Biowaste/ faecal sludge. Swiss Federal Institute of Technology Zurich. April 2013.
 Silvia Rom. Libra Nicole Berge, Eduardo Sabio, Kyoung Ro, Liang Li, Beatriz Ledesma, Andr és Álvarez and Sunyoung Bae. Hydrothermal Carbonization modeling, Final Properties Design and Application. A Review. University of Extremadura, Avda. Elvas, s/n, 06006 Badajoz, Spain.
 Michela Lucian, L fiori. Hydrothermal carbonization of waste biomass, process design, modeling, energy efficiency and cost analysis. Department of Civil, Environmental and Mechanical Engineering, University of Trento.