Heterogeneous cracking of wood pyrolysis tars over fresh wood char surfaces

Abstract

Char-induced conversion of newly formed wood pyrolysis tars was measured for independent variations in temperature and tar-char space time. Tar vapors were generated by controlled devolatilization of shallow (~2 cm deep) packed beds of 45-250-µ particles of sweet gum hardwood.The tar vapors were extensively diluted in a helium carrier gas and then rapidly conveyed to an adjacent reactor for controlled thermal treatment with (or without), 0.2-12 cm deep beds of fresh char from pyrolysis of the same type of wood. Heterogeneous conversion is defined as the net tar loss from exposure to the wood char (i.e. after subtraction from the total tar lost during thermal treatment the amount of tar destroyed by vapor-phase cracking upstream and downstream of the char bed). Heterogeneous conversion was significant but essentially constant at 14 ± 7 wt % of tar (to 2 standard deviations), for temperatures from 400 to 600 °C and space times from 2.5 to 100 ms. In contrast, vapor-phase tar conversion upstream of the char bed ranged from 0% at 400 °C to 30% (wt % of tar) at 600 °C. The implication is that an otherwise thermally stable fraction of newly formed wood pyrolysis tars is very reactive in the presence of wood char. Literature data and the present results, including measurements of char yields from devolatilization of wood beds of different depths, imply that this fraction contains oxygen, is more aromatic than the whole tar, and may amount to as much as 35 wt % of the total quantity of tar released at the surface of pyrolyzing wood. Lignin is indicated as a major source, but not the only source, of this char-reactive tar fraction. Carbon dioxide and CO are definite products of this char-induced tar conversion, and additional char (coke) is a highly probable product. to this work as part of MIT undergraduate research programs. We also thank Professor H.-M. Chang and his colleagues, Department of Wood and Paper Science, North Carolina State University, who specially prepared samples of sweet gum hardwood, milled wood lignin, and other wood constituents for our biomass research program.Registry No. C02, CO,

Removal of Residual Char Fines from Pyrolysis Vapors by Hot Gas Filtration

“…Ash present during biomass pyrolysis has been reported to polymerize pyrolysis intermediates to produce more char [10]. A similar effect has also been reported for the char itself [11,12] when biomass pyrolysis vapors are passed over it. Thus, pyrolysis vapors may be catalytically reacting with the ash present on the surface of the filter and also with the char in the filter cake.…”

Effect of Biomass Size and Aspect Ratio on Intra-Particle Tar Decomposition during Wood Cylinder Pyrolysis

“…At the outset of each run, each wood cylinder ( ρ = 400 kg/m 3 ) was dried at a temperature of 383 K for at least 10 min. Previously, our research showed that the intra-particle tar decomposition rate during a heating rate of 30 Ks -1 was the highest in our experimental conditions (8) . Thus, the pyrolysis process was performed at heating rates of 30 K/s in an atmosphere of argon flowing at 0.8 l/min.…”

“…S t2 = ε bk g2 ρ t1 (8) Intra-particle tar and gas transport velocity (U, V) are estimated using Darcy's law. Many researchers have used Darcy's law to calculate intra-particle flow field because bulk flow is mainly driven by pressure gradients caused by an increasing in volatile matter (14,15,16) .…”

“…It is concluded that syngas derived from biomass can be purified efficiently from tar using a nickel catalyst; however, a nickel catalyst increases the investment costs of the process (7) . Meanwhile, some researchers have studied the potential of using biomass char for tar decomposition (8)(9)(10)(11) . There have noticed that char provides good catalytic activity for tar removal.…”

Thermal decomposition and gasification of biomass pyrolysis gases using a hot bed of waste derived pyrolysis char

“…The decrease in BET surface area of the chars after the char-tar reaction could be attributed to the deposition of high molecular weight tar compounds on the char surface, followed by condensation and polymerisation to form coke. Boroson et al (1989) also reported a decrease in the surface area of chars due to deposition of tar onto the surface of the char during the cracking of wood pyrolysis tars over a hot bed of biomass char. A similar observation was reported by Zhao et al (2015).…”