SUBCRITICAL HYDROTHERMAL CONVERSION (SHC) PROCESS SUPPORTED BY MICROWAVES

Abstract

Ryszard Parosa PROMIS-TECH, Poland Subcritical Hydrothermal Conversion (SHC) process can be used for utilization of wet biomass with recovery of energetic gases and diesel-like fluids. Process is carried out in high pressure (over 200 Barr) and wet biomass is heated up to 350°C. In our research we have apply microwave system to heat wet biomass in high pressure vessel – with control of the most important process parameters like temperature, pressure, microwave power and reflection coefficient. In the first stage process was tested with use of small metallic vessel connected to microwave line (rectangular waveguide) by special ceramic window. Microwave generator 2.45 GHz with controlled output power in range of 200 W to 3 kW was applied. Sewage sludges with moisture content up to 60% were heated by microwaves to temperature from 250°C to 350°C with pressure ca 300 Barr. After heating by 15 - 20 minutes inside of vessel fluid like oil was obtained with solid particles in form of carbon black. Fluid after separation of water contains hydrocarbons – with  biodiesel structure. The similar process with conventional heating by metallic walls of vessel needs much longer time – about 40 minutes, and higher energy consumption.In the next stage a semi-industrial system was designed with process reactor in form of ceramic tube located inside of metallic cavity with two single mode microwave radiators connected to microwave generators of 2.45 GHz and with output power 3 kW (CW) each.Heating process is provided in stop/flow system: biomass inside of high pressure cavity is heated by microwaves to adjusted temperature (280 -350°C) and product is transported by heat exchanger to separator. At the same time next portion of biomass is injected to the cavity. During the process reflected microwave power was recorded. When treated material was in very high pressure, close to subcritical state, specific fluctuations of reflected power level ware observed.  Water in subcritical condition lose polar structure and dielectric constants change, but in the hydrothermal conversion process another chemical compounds are formed (for example diesel like hydrocarbons) and treated material still effectively absorb microwaves. In the described system ca 60 – 80 kg of wet biomass (with 50 – 55% of water) was treated and ca 8 – 15 kg of diesel – like fluid was obtained. Basing on the preliminary results a new industrial scale system was designed and actually is under construction. In industrial scale installation a microwave heating system of 60 kW, 915 MHz is planned, with two radiators installed along ceramic tube.  Estimated efficiency of the system is in range of 200 to 400 kg of biomass treated in hour.