Studies on dielectric properties and microwave pyrolysis of karanja seed

Non-edible seeds are potential candidates for the production of biofuels and other value added products. The specific objective of this work is to measure the dielectric properties (dielectric constant, loss factor, loss tangent and penetration depth) of karanja seed. The data on dielectric properties of karanja seed will be helpful to understand its microwave absorbing capacity and heating behaviour in the microwave pyrolysis process. In this work, the dielectric properties of karanja seed are determined at ambient temperature and in the frequency range 0.1–3.0 GHz. The loss tangent values of karanja seed obtained at the two recognized commercial frequencies of 915 MHz and 2.45 GHz are compared with those of some biomass available in the literature. Finally, the karanja seed is pyrolyzed in a domestic microwave oven at four different power inputs of 500 W, 600 W, 700 W and 800 W. The microwave pyrolysis characteristics of karanja seed is determined in terms of temperature variation inside the reactor and pyrolysis products yields at different power inputs. The chemical composition and fuel properties of the produced bio-oil are measured. The loss tangent of karanja seed at 2.45 GHz is 1.3, which is higher than lignocellulosic biomass like oil palm fibre (0.08). The results of the dielectric properties of the present work shows that karanja seed has better microwave absorption characteristics compared to lignocellulosic biomass. In the actual pyrolysis process in a domestic microwave oven at different power inputs, the heating rate is found to range between 0.661 and 1.157°C/s which is considerably higher than that achieved using electrical heating (0.16–0.33°C/s). The bio-oil yield increases as the power input is increased from 500 to 700 W. The maximum bio-oil yield of 47% is obtained at 700 W. The GC-MS analysis of bio-oil has shown the presence of larger amounts of hydrocarbons, esters, alcohols, very little amount of aromatics and zero sulphur-containing compounds. The fuel properties of bio-oil show that it is a non-acidic and safer fuel. The calorific value of bio-oil is 40% lower than diesel, and its viscosity is 8.5 times higher than diesel. The results reveal that the karanja seed could be pyrolyzed by microwave heating to produce bio-oil which can be used as fuel for diesel engines by suitably upgrading it by blending with diesel fuel.