Abstract
In this paper, a hydrothermal carbonization (HTC) process is designed and modeled on the basis of experimental data previously obtained for two representative organic waste materials: off-specification compost and grape marc. The process accounts for all the steps and equipment necessary to convert raw moist biomass into dry and pelletized hydrochar. By means of mass and thermal balances and based on common equations specific to the various equipment, thermal energy and power consumption were calculated at variable process conditions: HTC reactor temperature T: 180, 220, 250 °C; reaction time θ: 1, 3, 8 h. When operating the HTC plant with grape marc (65% moisture content) at optimized process conditions (T = 220 °C; θ = 1 h; dry biomass to water ratio = 0.19), thermal energy and power consumption were equal to 1170 kWh and 160 kWh per ton of hydrochar produced, respectively. Correspondingly, plant efficiency was 78%. In addition, the techno-economical aspects of the HTC process were analyzed in detail, considering both investment and production costs. The production cost of pelletized hydrochar and its break-even point were determined to be 157 €/ton and 200 €/ton, respectively. Such values make the use of hydrochar as a CO2 neutral biofuel attractive.
Highlights
Hydrothermal carbonization (HTC) is an induced coalification process that converts raw biomass into a coal-like product, called hydrochar, characterized by high carbon content and high calorific value
The process was simulated at various process conditions (T = 180, 220, 250 ◦ C; θ = 1, 3, 8 h; dry biomass to water ratio (DB/W) in the range 0.07–0.19)
Plant efficiency was found to be highly dependent on initial feedstock moisture content and DB/W: an increase in DB/W from 0.07 to 0.19 largely reduced energy consumption
Summary
Hydrothermal carbonization (HTC) is an induced coalification process that converts raw biomass into a coal-like product, called hydrochar, characterized by high carbon content and high calorific value. This type of thermo-chemical conversion, referred to as wet pyrolysis (or wet torrefaction), can be applied to a variety of non-traditional sources such as the organic fraction of municipal solid waste, wet agricultural residues, sewage sludge, algae and aquaculture residues [1,2,3,4]. Hydrochar was proposed as a feedstock for supercritical water gasification in a scenario where small HTC plants, placed in the production location of the Energies 2017, 10, 211; doi:10.3390/en10020211 www.mdpi.com/journal/energies
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