Biomass fuels have variable chemical and physical properties which differ significantly from that of coal. This places limits on the amount of coal that can be substituted with biomass in conventional coal boilers. In order to encourage the use of biomass fuels, the reliability, efficieny and economics of boilers should be at least maintained. This is in addition to maintaining the emissions requirements and being able to fulfill flexibility demands.
Torrefaction for upgrading the fuel properties of biomass is a novel technology. Torrefaction involves heating of biomass in the absence of oxygen to a temperature of 200°C to 300°C. Torrefied biomass is reported to have better storage properties, improved grindability, higher energy densities and more homogenized properties.
Quantifying the grindability of thermally treated biomass is necessary for predicting its performance in coal milling systems. While there are improvements in grindability brought about by torrefaction, at this time the question of whether the improvements are sufficient enough for torrefied biomass to be fractionated in typical coal mills is still unanswered. For utilities that have tested torrefied biomass, the practice is to perform large scale mill trials which are expensive and time consuming. This work evaluates the grindability of densified torrefied biomass in industrial coal mills with the aim to develop and validate a lab procedure for predicting the grindability in such mills. Tests performed range from lab-scale to industrial-scale validation tests, utilizing various fractionation devices and mechanisms. The work also examines grinding performance in a typical biomass mill such as the hammer mill in order to quantify the benefit gained by torrefaction.
Torrefied biomass particles have a different shape and morphology compared to non-treated biomass particles. Their particle shape descriptors (aspect ratios and sphericities), inner surface areas, and total pore volumes are different compared to non-torrefied biomass. The volatility of biomass also decreases as a result of torrefaction. Particle size and volatility significantly impact the structure of the volatile flame in the near burner region. The near burner volatile flame zone is important for flame stability as well as the formation and primary control of NOx. The experimental work which involved de-volatilization and combustion tests examined various biomass fuels including torrefied biomass in co-firing configurations with coal as well as mono-firing test cases. The de-volatilization tests provide information on the gas phase composition, mass release and nitrogen distribution in volatiles and char. The combustion tests were performed in a 20kW reactor, a 500kW test rig while large scale co-firing trials were done in an industrial utility boiler.
The milling results show that the HGI method is flawed when applied for the grindability of biomass materials. Three adaptations were applied to the standard method to extend it to thermally treated biomass materials. These adaptations relate to the amount of sample for the determination, the particle size criteria for defining grindability and the generation of the grindability calibration curve. The adapted approach, termed thermally treated biomass grindability index, TTBGI was then substantially tested and validated for a range of torrefied biomass materials. The method showed good repeatability within ±2TTBGI units, analogous to requirements of the HGI characterization method. In a typical biomass hammer mill, the advantage of torrefaction can be seen and quantified in terms of lower milling energy consumption and the fineness of product output for the torrefied biomass.
De-volatilization tests show mass release to be higher for non-torrefied wood compared to torrefied wood (with both being much higher than coal). For torrefied and non-torrefied biomass, more than 90% of the fuel nitrogen is released with the volatile matter. The retention of fuel nitrogen in char is for both fuels below 6% (much lower than that of bituminous coal that was about 75%). Compared to coals, biomass combustion is dominated by homogeneous gas phase reactions. Primary NOX control techniques such as air-staging are more active in the volatile gas phase homogeneous reduction of fuel nitrogen intermediates, making higher volatile fuels potentially ideal for NOX reduction by this method. In addition to the fuel nitrogen content, NOX formation is also influenced by flame structure and flame structure by fuel volatility, particle size and burner aerodynamics. As larger biomass particles heat-up slower, particle breakthrough of the initial volatile combustion zones may occur leading to further volatile release (with accompanying release of nitrogen intermediates). Depending on local stoichiometries, the volatile-nitrogen intermediates will be oxidized to NOX or reduced to N2. Optimization of swirling intensities as well as particle size distribution will ensure effective utilization of the volatile flame zone for implementing primary NOX control techniques like air-staging. Torrefaction in this regard is positive for biomass as the degree of fineness is improved due to better grindability. Burner and firing concepts have to be adapted in order to optimize the primary NOX reduction potential, especially in the near burner region.