Thermochemical Conversion of Biomass Using Solar Energy: Use of Nanoparticle-Laden Molten Salt as the Working Fluid

Solar energy can potentially be used to convert biomass into more readily usable fuel. The use of solar energy in such a process improves the overall conversion efficiency of the system significantly by eliminating combustion of a portion of biomass needed to heat the rest of it to a temperature where pyrolysis occurs. The present study models the thermochemical conversion process during pyrolysis of biomass matter into product gases. Concentrated solar radiation is used as the source of heating of the biomass. The biomass is indirectly heated by a mixture of molten salts (Na₂CO₃ and K₂CO₃) and nanoparticles (copper), which acts as the absorbing medium and in turn heats the biomass matter (cellulose). A two-stage heat transfer and chemical reaction analysis is carried out in order to simulate the simplified operating conditions of a solar-powered gasifier. The temperature of the molten salt at the exit of the reactor is held fixed at 1000 K (727°C). The calculations are carried out at different values of solar concentration factor ranging from 10 to 60. The results show that the temperature of the molten salt mixture at the exit of the solar collector increases with an increase in the solar concentration factor. Moreover the temperature inside the biomass reactor is a function of the concentration factor as well and largely the determining factor of the rate of biomass conversion into product gases. At the highest concentration factor (Cf = 60), the model predicts that the reactor is able to convert 1.1 tons of biomass into product gases each hour using 900 kW of solar radiation at an overall efficiency of 8%. The main finding of this study is that under similar operating conditions a solar collector using a direct absorption fluid (mixture of nanoparticles and molten salt) would require significantly less concentration factor (an order of magnitude reduction) than a conventional solar collector. A conventional solar collector is defined as one where the solar radiation heats up a solid surface (such as tube walls) which in turn heats up the working fluid (molten salt). Such a reduction in concentration factor would translate into lower concentrator area, and consequently lower initial capital cost.