Production of Multi-Purpose Bio-Solid Feedstock for Oxidation in Cement Calciner using Microalgae

Abstract

The concentration of carbon dioxide (CO2) in the atmosphere is rising and its impact (e.g. climate change) is established among the scientific community. Fuel switching in commercial sectors (e.g. cement plants) is one method to reduce fossil CO2 emissions. Microalgae cultivation provides an opportunity to reduce CO2 via photosynthesis and to produce biomass crop for fuel-switching. Industrial scale production of environmentally sound fuel using microalgae systems is still a challenge due to the small cell size and the low concentration in the growth culture. This study investigates a dewatering approach consisting of electrocoagulation, gravity sedimentation, sand filtration, briquetting and passive drying to harvest the algal bio-solids and to produce a multi-purpose feedstock for use in cement plants. The microalgae suspension in solution is first destabilized via electrocoagulation by means of charge neutralization and micro-bubble flotation. The highest destabilization achieved at 15 Amp/m2 using aluminum and iron anodes, with 60 min of operational time, were 90% and 48% respectively. The variation of solution pH and stirring speed had no noticeable effects on the overall performance. Further densification of algal bio-solids took place in a sedimentation vessel in which 92% of the process water was removed, allowing for recycling, and the water content was concomitantly reduced from 99.88% to 99.24%. The algal bio-solids obtained after a combined treatment of electrocoagulation and sedimentation were characterized using ICP-OES, TGA, and bomb calorimeter. The metal hydroxides (i.e. aluminum and iron) contribute in the reduction of raw materials used in cement making. Limestone micro-aggregates were employed as filter medium to reduce the water content of pre-concentrated algal solution from 99.24% to 93.45%, removing 84% of the entrained water. Multi-purpose briquettes were prepared from algal cake, containing metal hydroxides, and a portion of the filter bed, where the water content further reduced to 65% due to the limestone blending. The water content of the briquettes was reduced to less than 15% within few days under ambient condition (i.e. passive drying). Full substitution of calciner fuel leads to 4% of CO2 reduction compared to coal, and enhances biogenic carbon dioxide (bCO2) generation which accounts for 18% of the total CO2 emissions.