Bio-coal for the sustainable industry : A scientific approach to optimizing production, storage, and usages

• Bio-coal produced from biomass is a promising material to replace fossil coal in order to achieve net-zero greenhouse gas emission from the industrial sector. Bio-coal with quality comparable to that of fossil coal can be produced by high-temperature pyrolysis at ≥500 ºC, but the production efficiency is relatively low due to low bio-coal yield at high pyrolysis temperatures. This trade-off suffers the economic feasibility of bio-coal production. The overall objective of this doctoral thesis is to develop a pyrolysis process that can produce bio-coal for fossil coal replacement in the industrial sector, while maintaining a high process efficiency.   To increase bio-coal yield and process efficiency, secondary char formation during the pyrolysis of thick biomass, for example, woodchips, is the primary method considered in this work. Secondary char formation can be promoted by increasing volatile concentration during pyrolysis and/or extending residence time of volatiles inside the pore structure of wood particles. This study investigated how to increase secondary char formation using bio-oil recycling and CO2 purging. Bio-oil recycling increased bio-coal yield by not only increasing the reactants, but also through the synergetic effect between bio-oil and woodchips upon physical contact. Using CO2 as a purging gas reduced mass diffusion of volatiles inside the pore structure of woodchips, producing extra bio-coal. In addition, the effect of these techniques can be maximized by ensuring good contact between the volatiles and the solid surface using thick particles and slow heating. In parallel, a numerical model of pyrolysis in a rotary kiln reactor was developed to increase the understanding of parameter implementation in pyrolysis reactors. Two important parameters were studied: rotation speed and feeding rate. Rotation speed controlled the solid residence time, while the feeding rate influenced the heat capacity of holdup materials and product distribution.   Bio-coal is prone to self-heating and usually causes spontaneous ignition during production, storage, and transportation, which can lead to losses in the production and health of workers. In this study, self-heating at low temperatures was investigated by using numerical simulations describing the changes in local properties inside different bio-coal containers such as closed metal containers and woven plastic bags. The kinetic parameters of bio-coal were measured and implemented in the model. It was observed that the bio-coal temperature slowly increased from the initial temperature due to the heat released during O2 chemisorption. Thermal runaway occurred in some storage conditions, even at intial bio-coal temperatures of ca. 155 ºC. The simulation results suggest that self-heating can be mitigated by using small and wide particle distribution, limited storage volume, and low ambient temperature. This study also provides the criteria for estimating the cooling demands in bio-coal production processes.   Bio-coal properties are the main challenges for utilizing it as a substitute for fossil coal. Although the elemental composition and heating value of the bio-coal produced in this study are equivalent to those of fossil coal, the reactivity of bio-coal is relatively high. To replace fossil coal in existing industrial processes, bio-coal reactivity is preferred to be similar to that of fossil coal to avoid major process modifications. This thesis has concluded that pyrolysis temperature, heating rate, and biomass feedstock are the major parameters influencing the gasification rate under chemical reaction limitation. It was found that potassium in biomasses increased bio-coal reactivity even at low gasification temperatures such as 800 ºC, while calcium did not play a significant role at temperatures below 1600 ºC. Furthermore, bio-coal reactivity increased only slightly by promoting secondary char formation using the proposed methods. These findings suggest that we can achieve high bio-coal yield, both mass and energy, while maintaining similar fuel properties through pyrolysis with bio-oil recycling and CO2 purging.   In the most industrially relevant applications, the gasification rate is dominated by diffusion mass transfer. Therefore, it is necessary to reflect gasification behavior of bio-coal under these circumstances. At the particle scale, where intraparticle diffusion controls the overall reaction rate, bio-coal particle size was nearly constant until high conversion. This implies that particle size changes should be considered only at high conversion. Meanwhile, large particles exhibit low gasification rate at the particle scale following the Thiele modulus. The contrary result appears at the packed bed scale, where both intraparticle and interparticle diffusions play roles. Large particles increased the gasification rate in packed beds because of the large bed channel size, high void fraction, and low tortuosity. This observation led to an opportunity to minimize the apparent gasification rate in a packed bed by using polydisperse particles, which have a wide particle size distribution. Large particles maximize the intraparticle diffusivity of CO2, while small particles fill the gaps between large particles, thus increasing interparticle diffusivity, which reduces apparent reactivity. This outcome was confirmed experimentally.   By combining the knowledge obtained in this doctoral thesis, an efficient pyrolysis process is proposed to produce bio-coal for a sustainable industry.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Energiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Energy Engineering (hsv//eng)

Nyckelord

Energiteknik

Energy Engineering

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