Book Description

"Switching to low carbon fuel (LCF) can help the cement industry reduce carbon dioxide (CO2) emissions effectively. This is because LCF generates energy that can lower the coal consumption in the kiln. Biomass fuel originating from perennial grass crops and woody materials is LCF that can be converted to clean energy (e.g. heat or steam) via combustion, which is an independent process from cement production. Meanwhile, the residual ash from the biomass combustion may be recycled by blending with cement if this ash possesses a high pozzolanic activity, which can reduce the demand for cement and accordingly lessen the CO2 emissions associated with cement production. Hence, the overall objectives of this thesis were to 1) investigate the combustion characteristics of switchgrass (Panicum vigratum L.) compared to hardwood; 2) characterize the physiochemical properties of switchgrass combustion ash, and assess the strength in mortars made from blended cement containing up to 20% switchgrass combustion ash content; 3) and optimize the combustion conditions to achieve a high energy conversion and ash pozzolanic value simultaneously.Switchgrass and hardwood contained 17.5 and 17.7 MJ·kg-1 of energy value, which was appropriate for energy generation. Using a laboratory-scale experimental platform that studied the air supply effect on combustion performance, the greatest energy conversion efficiency and combustion completeness rate were obtained with an excess air of 20% for switchgrass and 30% for hardwood (4 mm particle size). Kinetic analysis also confirmed that increasing the oxygen availability resulted in a superior energy conversion. Switchgrass ash had lower fouling and slagging tendencies than hardwood, and could be a better biomass fuel for a commercial-scale boiler.Switchgrass combusted at 411 °C generated 5% ash by mass. After grinding for 30 s, ground ash had a porous structure with 65.0 μm of mean particle size and 41.2 m2·g-1 of BET surface area. Ground ash contained 67.2% of SiO2 and its structure was 72.2% amorphous crystal. This ash was a good pozzolan in blended cement, and its pozzolanic activity was improved by adding chemical accelerators (5% Na2SO4 and 5% CaCl2·2H2O were equally effective). Blended cement with 10% ash and either 5% Na2SO4 or 5% CaCl2·2H2O had similar material strength and expansion resistance as conventional cement. Furthermore, considering the effect of switchgrass combustion temperature (350, 450, 550 or 650 °C) and retention time (1 or 4 h), the combustion conditions of 550 °C for 4 h were recommended for concurrently optimizing the ash pozzolanic activity (114%) and energy output (4.21 kJ·g-1) from switchgrass.In conclusion, it was technically feasible to offset energy demands in cement production by relying on switchgrass or hardwood combustion in parallel to coal burning. Recycling 10% of the switchgrass ash generated under optimal combustion conditions (550 °C for 4 h) in blended cement not only offset the disturbance in the cementitious composition due to ash substitution, but also contributed to greater strength microstructural compound (C-S-H) formation, thus improving the concrete strength by 14.2%. If these findings can be extrapolated directly to an average-size cement production plant (3.06 GJ coal /1000 kg cement product), the life-cycle CO2 emission would decrease by 1.77% when 5% of the coal burned is replaced by switchgrass energy with ash recycling." --