Evaluation of Steam-Exploded Wood Pellets Storage and Handling Safety in a Coal-Designed Power Plant

Abstract

Solid biofuels are currently used in increasing volumes to replace fossil equivalents. Next to the traditional wood-derived fuels, the advancement of biomass upgrading technologies like steam explosion (SE), followed by pelleting, further broadens the variety of materials on the market. This thermochemical and mechanical upgrading improves greatly the general properties of such fuels (homogeneity, energy density, water resistance, biological decomposition, grindability, pneumatic transport, etc.) and at the same time offers the possibility for co-production of green chemicals/biorefinery processing. However, these alternative SE-material pellets have different properties from the fossil counterparts with regard to dust formation, dielectric, ignitability, and explosive characteristics, hence posing an increased risk of fire and explosion in handling, transport, and storage of the biosolid fuels. These divergent properties need to be properly addressed before their large implementation in the industry. To shed more light on the variability of such material properties, several batches of SE pellets varying in bulk density and subjected to different handling and storage conditions were evaluated. This was done using a number of standardized equipment and following industrial standard measurement methodologies to compare steam-exploded material with standard white wood pellets and reference coals. The main body of the work focused on the explosivity of dust generated in the handling of the pellets, studied using a Hartmann tube, coupled with a high-speed camera to evaluate the sensitivity [minimum ignition energy (MIE) and minimum explosive concentration (MEC)] and the severity maximum flame front velocity (FFVmax) of explosions of the generated SE-pellet dust. Thermogravimetric analyses (TGA) was used to measure and compare the ignitability of dust layers of SE-pellet dust vs coal and as a function of co-mixture. Finally, the SE-pellet self-heating behavior was addressed using the standard “basket method” and compared with that of a commercial coal sample. The results show that the SE pellets produce very low levels of dust even after extensive weathering, hence limiting the explosion potential. The MIE and MEC of SE-pellet dusts exhibited similar values compared to those of the parent raw biomass material; however, the FFVmax revealed lower values when compared to that of the parent raw biomass. Also, the self-heating propensity is very low as the pellets are basically immune to biological degradation, while the spontaneous ignition temperature is high and in the range of nonhazardous bulk materials.

Review on Self-Heating of Biomass Materials: Understanding and Description

“…Biochar is the solid product from biomass pyrolysis, as well as gasification, which can be used as a fuel; in particular, biochars from mild thermal treatments, including those by torrefaction (mild pyrolysis) and steam explosion, have been used as upgraded feedstock for bioenergy production . The thermal decomposition and carbonization processes of biochar production result in changes in material properties that influence the risk of self-heating and spontaneous combustion of the biochar as compared to the parent biomass. ,− It therefore has prompted extensive studies to evaluate the propensity of the spontaneous ignition − and off-gassing , of biochars and explore the underlying mechanisms. ,, …”

Energetic and Exergetic Performances of a Retrofitted, Large-Scale, Biomass-Fired CHP Coupled to a Steam-Explosion Biomass Upgrading Plant, a Biorefinery Process and a High-Temperature Heat Network

“…This increases the energy density of the pellets. It also improves their durability, their grindability, and their pneumatic transport properties [13][14][15]. Steam-exploded pellets are also hydrophobic [14,15].…”

“…Three heat exchangers (9,10,11) transfer heat from the steam extracted from the three tapping points (outlet HP, outlet IP and LP turbines) to an intermediate closed loop. This intermediate loop in turns delivers heat to internal heat clients (12) and to an external heat network (13).…”

Uncertainty Quantification for Thermodynamic Simulations with High-Dimensional Input Spaces Using Sparse Polynomial Chaos Expansion: Retrofit of a Large Thermal Power Plant

“…This results in a lower investment cost to retrofit the power plant. Compared with white pellets, the other advantages of steam-exploded pellets are the higher lower heating value (LHV) and the fact that they are hydrophobic [38][39][40]. On the other hand, there is the higher price of the steam-exploded pellets.…”