Thermochemical conversion of non-woody biomass: upgrading cotton gin waste into solid fuel

Thermochemical conversion of non-woody biomass: upgrading cotton gin waste into solid fuel

Non-woody biomass is a common waste material found in agriculture. Despite its abundance, the waste is not widely utilised due to unfavorable physical properties (bulkiness, irregular size and varied composition) and low energy content.

The aim of this research is to study the solid fuel properties of a non-woody biomass in order to improve their qualities. Cotton gin waste (CGW), a source of non-woody biomass from the processing of cotton, was selected. Methods of densification and blending of biochar were proposed and evaluated for transforming CGW into pellets in order to create a fuel with high density and energy content, as well as uniform physical properties. The development of CGW pellets was achieved by using a small scale pellet mill. CGW was blended with 5 to 20 percent weights of biochar. The developed CGW pellets were accordingly defined as CGW100, CGW95, CGW90, CGW85 and CGW80 pellets, implying the weight percentages of CGW as much as 100%, 95%, 90%, 85% and 80% in pellets, respectively.

It has been found that pelleting the CGW increases the bulk density from 112 kg/m3 to 600 kg/m3. The biochar blends upgraded the heating values of CGW pellets from 14 MJ/kg of CGW100 to 18 MJ/kg of CGW80. In the process of stabilisation, the blended pellets slightly shrank, while the pure CGW pellet marginally expanded. In contrast to the pellet durability, the hardness was significantly influenced by the biochar addition. The biochar in the pellets diminished the rancid smell of raw CGW.

It has also been found that CGW95 and CGW90 behaviours in the thermogravimetric (TGA) combustion were almost identical with CGW100 combustion. In addition, CGW95 pellets had the highest conversion rate and resulted in the least residual ash. On the contrary, CGW85 and CGW80 pellets were slow in conversion and burn out at closer to the biochar ignition temperature. From the examination of ash content and activation of energies, all the blended pellets show a synergism in co-combustion.

Similar to combustion, the TGA pyrolysis using inert gas also resulted in a slightly higher conversion for CGW95. Other biochar blended pellets show a lower and more linear conversion as a function of biochar content.

A CFD model has been developed using ANSYS Fluent 17.2 software. The approaches are the discrete phase and non-premix combustion models. The model shows an accurate prediction of the gasifier temperature and resulting gas composition. The simulation also predicts that CGW95 will have a higher CO yield than CGW90. The gasification of CGW95 pellets with air to fuel ratio of 1.3 v/w results in a gas composition of CO, CO2, H2 and CH4 gas of 19.8%, 11.6%, 14.2% and 0.2%, v/v respectively. The estimated gas heating values are in the range of 3.9-5.1 MJ/m3.

It has been found that 30% energy produced from CGW pellet gasification is sufficient to cover the energy need for pellet production. The costs of energy in the ginning house can be reduced by 20-40% from the use of produced gas. The GHG emission is also lowered. Overall, it can be concluded that upgrading the non-woody biomass into pellets and applying it in a co-gasification could potentially provide an effective alternative fuel source to achieve agricultural energy self-sufficiency and off-grid operation.