Fast Pyrolysis of Tropical Biomass Species and Influence of Water Pretreatment on Product Distributions

Abstract

The fast pyrolysis behaviour of pretreated banagrass was examined at four temperatures (between 400 and 600 C) and four residence times (between ~1.2 and 12 s). The pretreatment used water washing/leaching to reduce the inorganic content of the banagrass. Yields of bio-oil, permanent gases and char were determined at each reaction condition and compared to previously published results from untreated banagrass. Comparing the bio-oil yields from the untreated and pretreated banagrass shows that the yields were greater from the pretreated banagrass by 4 to 11 wt% (absolute) at all reaction conditions. The effect of pretreatment (i.e. reducing the amount of ash, and alkali and alkali earth metals) on pyrolysis products is: 1) to increase the dry bio-oil yield, 2) to decrease the amount of undetected material, 3) to produce a slight increase in CO yield or no change, 4) to slightly decrease CO2 yield or no change, and 5) to produce a more stable bio-oil (less aging). Char yield and total gas yield were unaffected by feedstock pretreatment. Four other tropical biomass species were also pyrolyzed under one condition (450°C and 1.4 s residence time) for comparison to the banagrass results. The samples include two hardwoods: leucaena and eucalyptus, and two grasses: sugarcane bagasse and energy-cane. A sample of pretreated energy-cane was also pyrolyzed. Of the materials tested, the best feedstocks for fast pyrolysis were sugarcane bagasse, pretreated energy cane and eucalyptus based on the yields of 'dry bio-oil', CO and CO2. On the same basis, the least productive feedstocks are untreated banagrass followed by pretreated banagrass and leucaena.

Fig 1. Schematic diagram of the variable-freeboard pyrolysis reactor.

Numbers 1 through 6 show the locations of the thermocouple measurements in the multi-point temperature probes (SS—stainless steel).

Fig 2. Ternary plot of the compositional analysis of biomass samples examined in this study—using the approach proposed by Vassilev et al. [23].

The data from Table 2 was normalized to 100% before plotting the points.

Fig 3. Ternary plot of the ash forming elements in the biomass samples—using the approach proposed by Vassilev et al. [23].

The data from Table A in S3 File was normalized to 100% before plotting the points (wt% of the ash), and no data was available for MnO.

Fig 4. Ternary plot showing position areas of 86 biomass samples and 38 solid fossil fuels in the chemical classification system of the inorganic matter in biomass, wt.% [23].

[Reproduced with permission from Vassilev et al., Fuel 94 (2012) 1–33].

Fig 5. Pyrolysis bio-oil yields (dry bio-oil, daf feedstock) from banagrass as a function of temperature and residence time (bed position, BP).

Left side shows untreated banagrass and right side pretreated banagrass.

Fig 6. Permanent gas yields (daf feedstock) from banagrass as a function of temperature and residence time (bed position, BP).

Left side shows untreated banagrass and right side pretreated banagrass.

Fig 7. Elemental analysis (C, H, N and O by difference) results for the dried bio-oils from pyrolysis of pretreated banagrass as a function of temperature and vapor residence time.

Results are presented as wt% of the bio-oil. The standard deviation of the C and O results is < 2.0 wt% (absolute) and for H < 0.5 wt% and N < 0.3 wt% (absolute).

Fig 8. Elemental analysis (C, H, N and O by difference) results for the dried bio-oils from pyrolysis of pretreated banagrass as a function of temperature and vapor residence time.

Results are presented as wt% of the element in the Feedstock (daf). The standard deviation for the C and O results is ≤ ±3.0 wt%, for H ≤ ±5.0 wt% and for N ~±10 wt% (absolute).

Fig 9. Permanent gas data (CO, CO₂, CH₄ and H₂) from the pyrolysis of banagrass S3 as a function of temperature and vapor residence time, presented as wt% of the daf feedstock (BP, bed position).

The standard deviation for the CO values is ≤ ±1.5 wt% (absolute), for CO₂ ≤ ±0.5 wt%, for CH₄ ≤ ±0.2 wt% and for H₂ ≤ ±0.05 wt%.