Impact of post-torrefaction process on biochar formation from wood pellets and self-heating phenomena for production safety

Highlights

• Self-heating phenomena are investigated using three different scenarios.

• Three scenarios under different post-torrefaction processes are taken into account.

• Self-heating occurs when oxidative post-torrefaction with keeping temperature is operated.

• Oxidative post-torrefaction without keeping temperature intensifies fuel’s calorific value.

• XPS analysis reveals the depletion of the CO functional group in biomass after torrefaction.

Abstract

The advancement of torrefaction in the industry raises safety issues lately. This work aims to investigate the impact of post-torrefaction upon biochar formation and the self-heating phenomena of torrefied wood pellets. Three operating scenarios are taken into account. Specifically, dried wood pellets are first torrefied in nitrogen at various temperatures (200–300 °C) followed by immediate exposure to N₂ or air with/without keeping temperature, namely, the post-torrefaction, to examine the self-heating behavior. Meanwhile, the influences of torrefaction temperature, duration, and self-heating on biochar’s characteristics are analyzed. It is found that self-heating with the highest temperature rise of 61 °C is triggered when the biochars are in an oxidative post-torrefaction process with keeping temperature. Alternatively, oxidative post-torrefaction without keeping temperature can efficiently intensify the pellets’ calorific value up to 42%. The X-ray photoelectron spectroscopy analysis reveals the depletion of the CO functional group. It is concluded that uncontrolled self-heating in large scale production will lead to combustion and make biochar unsafe. Alternatively, if heat accumulation inside the biochar piles can be prevented, the oxidative environment that causes self-heating can open a lot of research opportunities that can help in the advancement of torrefaction technologies for biomass as an alternative source of energy.

Introduction

Currently, the majority of our energy consumption is coming from fossil fuels [1]. Coal plays an important role in fossil fuels and is more efficient than biomass. In addition to combustion, coal gasification is another economical choice to produce synthesis gas [2]. On the other hand, Chen et al. [3] discussed that considering the depletion of resources, there were alternative fuels that could be a potential substitute for coal. Biomass energy or bioenergy is considered as one of the promising resources that can match the characteristics of fossil fuels for GHG emission reduction. As emphasized by Tillman et al. [4], the most carbon-efficient use of biomass materials is by co-firing with coal.

Torrefaction is one of the pretreatment technologies to improve the properties of biomass as fuel. Torrefaction has been a topic of interest for the past years but a very limited number of plants had it on a full commercial scale [5]. Table 1 shows a summary of some torrefaction studies using different biomass materials, reactors, and operating parameters for evaluation. Torrefaction is a mild pyrolysis process that is typically used for upgrading the properties of woody (softwood and hardwood) and algal biomass. It is usually carried out at 200–300 °C without the presence of oxygen [6,7]. The prime product of torrefaction is biochar; biochar is also the by-product of the pyrolysis which has several possible applications from solid fuels to adsorbents. Chen et al. [8] found that biochar produced in a high torrefaction temperature had a comparable energy density to low ranking coals. In comparison to raw biomass, biochar has a better energy density, lower toxicity levels from combustion, higher grindability, and low water affinity [9]. These characteristics and properties made the biochar a good alternative to coal.

For the utilization of fuels such as coal and biomass, their self-heating followed by self-ignition is considered a serious problem for fuel producers and users due to safety issues. The advancement of torrefaction studies also raises safety issues. The study of Cocchi [10] showed that combustible materials were prone to oxidation and might promote self-heating and self-ignition. The self-heating of biochar can affect its properties such as energy yield, enhancement factor, and ash content. Without proper prevention, it can raise safety issues, particularly on large stockpiles, storage, and transportation. Several methods have been conducted to identify and prevent the self-heating of coal. The common tests used in industry and academia to study coal self-heating are outlined in Table 2. In the present study, a horizontal tube furnace was used to identify the onset temperature-time runway of the biochar.

According to the study of Jones et al. [11], coal and biomass share similar properties including self-heating propensity. The self-heating phenomenon usually starts at a low temperature and gradually increases with the heat released by the reaction. Cruz et al. [12] showed that at higher temperatures, the degree of self-heating was higher with enough oxygen exposure. The self-heating phenomenon occurs when there are exothermic reactions [13] like oxidation reactions. A temperature rise occurs when the heat release rate is faster than the heat dissipation rate [6,14]. The dissipation rate can be increased by purging of inert gas to overcome the heat release from the biochar. The volume of the piled material affects the self-heating propensity of biomass, depending on the dimension of the storage which affects the dissipation rate, and the biomass can be stored safely with minimal controlled storage conditions [15]. Large volume of biochar (greater than 1 ton), on the other hand, may experiences a self-heating phenomenon because of the lower dissipation rate of heat through the large stockpile boundaries. The boundary serves as a thermal blanket which hinders the heat dissipation rate. Moisture content and humidity also affect the self-heating propensity phenomenon. Beamish et al. [16] explained that self-heating could be significantly delayed by the level of moisture content because the heat released by oxidation was dissipated by the evaporation of moisture. Kiel et al. [17] had identified the self-heating phenomenon as the cause of a fire incident in the storage after the torrefaction process [18]. Hazard and risk issues are now considered in that the increase in temperature can lead to heat generation, and spontaneous ignition can lead to self-combustion, the critical state of self-heating. Evangelista et al. [6] observed that self-combustion was a known scenario for coal. With coal and biomass having some similar properties, self-combustion can also be observed in biochars. Verhoeff et al. [19] observed that in small scale experiments, self-ignition could occur.

According to the literature review, the recognition of self-heating in an industrial scale is crucial, but the literature for this topic is limited so far [20]. Past studies are limited to the intrinsic effect of self-heating on biochar’s elemental composition, and to determine the O/C and H/C ratio difference [[21], [22], [23]]. Studies for self-hating oxidation kinetics and its effect on energy yield, parameters causing the elevation of self-heating to self-ignition, and analysis of self-heating on large scale samples are still not available. For this reason, this work aims to identify the existence of the self-heating phenomenon that can be possibly observed in biochar discharging and storage stages after biomass is torrefied and exposed to air in a scaled-up setting. Potential safety hazard arises when biochar exhibits thermal runaway. This thermal runaway can lead to combustion and thereby causes a fire. This study specifically aims to compare the existence of self-heating under different scenarios and identify its effect on the higher heating value (HHV) which can be used to evaluate the quality of biochar as a fuel.