DEVELOPMENT AND CHARACTERISATION OF HYBRID FUEL BRIQUETTE FROM COAL SLACK AND TORREFIED WOODY BIOMASS
Nigeria is blessed with over 2.7 billion tonnes of coal and several hectares of forest reserves. However, millions of tonnes of coal slack and wood residues (biomass) generated from coal mining and wood processing respectively, are often discarded as wastes because of near absence of processing. These wastes have enormous potentials as energy sources for both domestic and industrial applications if well processed. Therefore, the aim of this study was to develop and characterise Hybrid Fuel Briquettes (HFB) from lean grade coal fines and torrefied woody biomass using pitch, molasses and starch as binders. The objectives were to: (i) characterise coal fines and woody biomass from Teak (Tectona grandis) and Melina (Gmelina aborea); (ii) optimise and determine the effect of torrefaction conditions on the combustion properties of the woody biomass; (iii) produce HFB from coal fines and torrefied woody biomass; and (iv) evaluate the mechanical and combustion properties of the HFB. Teak and Melina woody biomass from Benin, Edo state, and coal from Okaba mine, Kogi state were used in this study. Thermal decomposition studies were carried out on the woody biomass and Okaba coal fines in a STA7300 Thermobalance system. Each of the woody biomass samples of particle sizes 2 and 6 mm, were torrefied at temperature of 240 and 300 , for 30 and 60 min residence time and then characterised for their combustion properties. The HFB were produced from the coal fines and torrefied biomass aggregates using the organic binders in a hydraulic press at 14-55 MPa. The briquettes were cured in an inert environment at 200-300 for 60-120 min. Mechanical and combustion properties of the HFB were evaluated to determine its strength and energy contents, respectively. Phase identification and chemical composition were determined using Fourier Transform Infrared Spectrophotometry, X-ray Diffractometry and X-ray Fluorescence spectrometry. Microstructural and elemental studies were also carried out using Scanning Electron Microscopy and Electron Probe Microanalysis. The findings of the study were that: (i) the woody biomass (Teak and Melina) had Fixed carbon (FC), Volatile Matter (VM), ash, Higher Heating Value (HHV) in the range of 8.92 - 11.73%, 79.26 - 81.42%, 1.73 - 2.15%, 18.39 - 18.72 MJ/kg, respectively, while coal had 66.92% FC, 13.77% VM and HHV of 23.13 MJ/kg; (ii) optimised torrefaction conditions of the woody biomass were: particle size, 2 mm; temperature, 260 ; and residence time, 60 min, which resulted in higher FC (41-42%), lower VM (54%) and greater HHV (23 MJ/kg); (iii) the best formulation ratio for the HFB (coal and torrefied biomass) production was 97:3 using blend of 8% pitch and 7% molasses under 28 MPa compared with other formulations; and (iv) mechanical strength such as drop to fracture (200 times/2m) and cold crushing strength (11 MPa) as well as energy content (24 MJ/kg) of the HFB were higher compared with standards. The study concluded that the HFB developed has sufficient mechanical strength with desirable combustion properties and thus can be used for energy generation. Therefore, this HFB is recommended for domestic and industrial applications.