Storage of bulk biomass materials is essential along the feedstock supply chain of bioenergy production. The self-heating accompanying biomass storage has hazardous consequences. With the increasing biomass utilization for bioenergy production, the risk and economic and environmental concerns about biomass storage have been attracting extensive research attention aimed at understanding the processes of heat generation, evaluating the propensity of a material to self-heating and spontaneous ignition, describing and predicting the self-heating process and consequences in biomass storage, and developing strategies and technologies for storage management. The advances in the understanding and description of heat generation processes including water-associated physical processes, microbiological degradation processes, and chemical oxidative processes during the self-heating in biomass storage are reviewed, with the focus on understanding the processes and their underlying mechanisms, reaction kinetics, and heats of reaction through experimental studies and description of heat generation and self-heating processes through kinetic analyses and modeling. This review highlights the need to improve the mechanistic model description of the self-heating processes and associated material changes and product formation, which demand a better understanding and description of microbial degradation and chemical oxidation through experimental study.

The moisture sorption isotherm (MSI) is very useful for biomass drying and storage. In the present work, measured MSI data of herbaceous and agricultural biomass (HAB) were collected from the literature. The adsorption or desorption isotherms at the same temperatures (15, 20, 25, 30, 35, and 40 °C for adsorption and 20, 25, 30, and 35 °C for desorption) were compared, separately, to demonstrate the similarity, evaluate the difference in the MSI among HABs, and explore the feasibility of applying the similarity for MSI prediction. It was observed that the MSIs of HABs at the same temperature are similar, with most measurements falling within a range of ±20% from an average given by a Guggenheim, Anderson, and de Boer (GAB) model. The chemical composition and distinct sorption behaviors of the biomass components were identified as the major factors determining the similarity and defining the variation in the MSIs of HABs. Factors associated with the measurement, including the temperature of drying for sample preparation and uncertainty related to sample heterogeneity, may cause the variation of the MSI comparable to the range of ±20%. Considering the similarities among HABs, using the average to estimate the MSI of an unknown HAB has a relative error of < ±20%. Estimating the MSI based on the chemical composition and isotherms of the major components (cellulose, lignin, and hemicelluloses as well as the extractives) can achieve the same accuracy. Both may be acceptable for practical applications. Additionally, it was found from the comparison that HABs, on average, have evidently higher MSIs than wood species mainly as a result of their higher contents of hemicellulose and the extractives.

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This work presents a comprehensive study on the effects of pyrolysis parameters (pyrolysis temperature, residence time, and heating rate) on the distribution of pyrolysis products of Miscanthus. Py-GC/MS (Pyrolysis-gas chromatography/mass) was conducted to identify building blocks of value-added chemical from Miscanthus. The results showed that the main pyrolysis products of Miscanthus were ketone, aldehyde, phenol, heterocycles, and aromatic compounds. The representative compounds of ketone and aldehyde compounds produced at different pyrolysis temperatures changed obviously, while the representative compounds of phenolic, heterocyclic, and aromatic compounds had no obvious change. Large-scale pyrolysis of Miscanthus had begun at 400°C, and the relative content of pyrolysis products from Miscanthus reached the maximum of 98.34% at 700°C. The relative peak area ratio of phenol and aromatic compounds reached the maximum and minimum at the residence time of 5 and 10 s, while the relative peak area ratio of ketone compounds showed the opposite trend. The relative peak area ratio of aldehyde compounds was higher under shorter or longer residence time. For heterocyclic compounds, the relative peak area ratio reached the maximum of 27.0% at residence time of 10 s. The faster or slower heating rate was beneficial to the production of aldehyde and phenol compounds. The relative peak area ratio of ketone compounds reached the maximum at 10,000°C/s, 70°C/s, and 10°C/s, and the relative peak area ratio tendency of heterocyclic compounds was similar to ketone. For aromatic compounds, the overall fluctuations were large, and the relative peak area ratio was the highest at the heating rate of 100°C/s.

A mathematical model was developed to predict the self-heating and self-ignition processes of relatively dry biomass during storage, considering in detail the effects of moisture exchange behaviour, low-temperature oxidation reaction and associated heat and mass transfer. Basket heating tests on fir pellets and powder at temperatures of 180–200 °C were conducted to observe the heating process and determine the kinetics of low-temperature chemical oxidation for model validation. As a result, it was demonstrated that the developed model could reasonably represent the self-heating and spontaneous combustion processes of biomass storage. Furthermore, the numerical study and model sensitivity analysis revealed that reasonably describing the low-temperature oxidation and associated heat and mass transfer process with reliable estimations of kinetic and thermophysical parameters of the biomass material is necessary for predicting the self-ignition, considering the effect of water exchange behaviour is essential to predict the self-heating process even for relatively dry biomass, such as pellets, with the moisture content up to 15–20%.