Development and characterization of new feedstock from biomass blends
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Date
2017
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University of New Brunswick
Abstract
This thesis investigates the biomass blends to develop standard quality fuel pellets. New knowledge in the synthesis of chemical, physical and mechanical properties of pellets made from the biomass blends is being reported. Pellets made from agricultural biomass are inherently known for poor quality and do not meet the pellet quality requirements. Through blending proposed in this study, the heating value, ash content, density and durability were demonstrated to meet the standard requirements.
Theoretically, most relations developed for compaction of powdery material significantly depend on diameter, thickness, porosity, density, pressure, and strength. The relevant relationships were reported in various equations such as Gurnham Model, Jonnes Model and Fell and Newton Model. The effect of particle size and blending for compaction of biomass was the greatest interest in this study. Results obtained for this study are in line with the following hypotheses:
- Blending may change in the initial and final degradation temperature of the biomass and thus chemically improves its content, gases production, thermal stability and heat capacity.
- Smaller particle size can yield higher strength than the coarser particle size of pellet made from either individual or blended biomass. Thus, pellets quality met the standard requirement of the density and durability set by Fuel Pellet Institution (PFI)
- Blending may reduce the energy of compaction while increasing the inter-particle bonding caused by the change in the chemical composition. Forestry biomass may contribute binding agents such as lignin to compact the particles of biomass blends.
- After grinding agricultural biomass, it is difficult to keep in moisture, thus compaction is hard to achieve. Blending may improve the adsorption rate of moisture within the required moisture content of pellets (8 – 12 %) that can provide strong inter-particle bonding and most importantly can sustain the combustion stability.
- Pellet strength for biomass blends may be best evaluated at significant dependants such as particles size, blend ratio and moisture content, for which an experiment is done at constant process parameters (Temperature 80 °C, pressure 159 MPa and holding time 30 seconds)
The result shows that an increase in carbon, hydrogen, volatile and char content, and a significant decrease in chloride were observed after blending. Since blending helped to improve in its chemical composition, similarly the strength, durability, and density of the blended pellets was found improved compared with individual agricultural biomasses. Corresponding to the theory, the yield stress was found inversely proportional to the particle size. The pellet durability index (PDI) of pure agricultural biomass pellets was less than 90, which is well below the PFI (Pellet Fuels Institute) standard requirement. This study revealed that using a blend ratio of 50:50 it was possible to achieve PDI of more than 95. At this ratio, pellets made from lower particle size (150 – 300 μm) exhibited higher density (950 – 1178 kg/m3 for spruce and pine; 668 – 800 kg/m3 for reed canary grass, timothy hay and switchgrass; 900 – 970 kg/m3 for biomass blends). The yield stress exhibited differences in values for individual forestry (40 MPa) and agricultural biomasses (27-48 MPa). However, after blending the values converged closest to that for forestry biomass. The ground biomasses were also evaluated for their compaction energy requirement into pellets. The compaction conditions were set as at 159 MPa load and a temperature of 80 °C. The energy required in compacting ground reed canary grass, timothy hay and switchgrass was lower (1.61, 1.97, and 1.68 kJ) than spruce (2.36 kJ) and pine (2.35 kJ). After blending, the values were around 2 kJ with the pellet quality approaching almost similar to that of pellets made from woody biomass.
Besides the particle size and blending, moisture content was also manipulated to see its effects on the strength and quality of pellets. The proposed model developed based on pine and Timothy hay can be used to predict the strength of pellets made from similar species with moisture content range of 8 – 12%, blend ratio in the range of 25 – 50%, and the particle size in the range of 300μm and below.
Besides evaluating the pellet quality, this study was further investigating the behavior of pellets made from biomass blends. The heat released from the blended biomass (6.94 – 9.26 kJ/kg) was higher than the individual agricultural biomass (4.59 – 6.78 kJ/kg) but lower than individual spruce (10.2kJ/kg) and pine (11.13kJ/kg). The findings indicate that the reactivity of the individual agricultural biomass material changed due to blending.
As for torrefaction, the chemical functional groups were recognized using TG-FTIR and TG-MS techniques, which were paired to refine the identification of gases. The FTIR analysis indicated that biomass blends had similar functional groups but their decomposition was largely at higher torrefaction temperature of 290 °C compared to switchgrass alone, which was largely at lower torrefaction temperature of 230 °C. The MS quantified the degradation of combustible gases: CH4, C2H4, CO and O2 from individual biomass in the range of 20-30% at torrefaction temperature of 200 °C and 230 °C. At the increased torrefaction temperature of 260 °C and 290 °C the degradation of the gases was more than 30%. The blended biomass degraded into gases in the range of 28-30 %(v/v) at torrefaction temperature higher than 290 °C. All gaseous products evolved from the torrefaction of agricultural and forestry biomasses were almost similar in characteristics, but varied in their proportions. The composition of product gas generated from torrefied biomass depends on the types of biomass blended and the temperature. In conclusion, blending can help increase the energy capacity, improve the pellet quality, increase in pellet fuel production, and improve the combustion performance. The pellet fuel industry can successfully implement the biomass blending approach as this research has demonstrated achievement of positive effects in pellet fuel quality.