Combustion characteristics of micron-size hydrochar particles

In comparison with biomass and coal, hydrochar has relatively smaller amounts of volatile matter, ash, and moisture, which makes hydrochar an attractive energy source. In the present study, the combustion characteristics of single micron-size carbon-rich hydrochar particles were compared to those of coal and arbutus bark (the wood feedstock from which hydrochar is derived). A Holthuis burner was used to combust the particles in the products of a premixed flat flame. For each type of the studied particles, five oxygen mole fractions of the premixed flame combustion products (12% to 38%) were examined while the adiabatic flame temperature was fixed at 1850 K. Simultaneous shadowgraphy and broadband luminosity experiments were conducted to study the combustion behavior of the particles and measure the ignition delay and combustion time. Arbutus bark particles ignited heterogeneously at high oxygen mole fractions, while hydrochar and coal particles exhibited an overlapping two-stage combustion process. At low oxygen concentrations, the arbutus bark particles burnt following a sequential two-stage process, while hydrochar and coal demonstrated temporal overlap between the stages. Coal particles burned with a dark sooty flame, while arbutus bark and hydrochar particles featured a transparent flame. Hydrothermal carbonization of arbutus bark created pores on the hydrochar particles which are hypothesized to cause fragmentation, rocketing, and fast ignition of the particles. This treatment increased the energy density of hydrochar, increasing its combustion time, yet hydrochar burnt faster than coal. The ignition and combustion behavior of hydrochar were closer to coal, highlighting hydrochar's enhanced combustion characteristics and viability as an improved fuel source.