Optimisation of thermal treatment of invasive alien plants (IAPs) for char production for use in combustion applications

[摘要] ENGLISH SUMMARY: Due to the popular worldwide demand for need to use cleaner fuels, lignocellulosic-derived char is gaining importance as a possible component in co-firing with coal. In order to avoid deforestation of indigenous forests in Zambia for char production, possibilities of using alternative feedstocks from invasive alien plants (IAPs) were investigated.In the present study, torrefaction and slow pyrolysis were used for char production from IAPs for energy applications. Both processes were optimised individually at milligram-scale in a thermogravimetric analyser (TGA) for char yield and higher heating value (HHV), through manipulation of the temperature, heating rate and holding time. Two IAPs, namely Lantana camara (LC) and Mimosa pigra (MP), from Zambia were used as feedstock materials. The feedstock particle size distribution (PSD) used was from 425 to 600 μm. The optimisation results for torrefaction and slow pyrolysis showed that temperature majorly influenced char yield and HHV. In case of torrefaction, operating at temperatures ≤ 300 ˚C, heating rate and hold time also influenced char HHV, while neither parameters had a statistically-significant influence on char yield and HHV during slow pyrolysis.During torrefaction at 300 ˚C, LC recorded a higher char yield of 43 wt.%, and a corresponding HHV of 27.0 MJ kg-1, mainly due to increased hemicelluloses content, compared with MP that had a char yield of 52 wt.% with HHV of 24.4 MJ kg-1. In case of slow pyrolysis, MP recorded the highest char HHV of 31.0 MJ kg-1 at 580 ˚C, due to increased lignin, in comparison with LC that had a highest char HHV of 30.0 MJ kg-1 at 525 ˚C. Based on optimised conditions from milligram-scale, LC and MP samples of PSD from 850 to 2800 μm were used for char production at gram-scale in a bench-scale reactor. Scaling-up promoted secondary char formation due to mass and heat transfer limitations in larger particles and increased sample size, thereby increasing char yields for both biomasses. Char yields were increased by 4 and 2 wt.% for MP and LC, respectively, due to scale-up. The highest HHVs at bench-scale were 30.8 MJ kg-1 (614 ˚C) and 31.6 MJ kg-1 (698 ˚C) for LC and MP, respectively.For the purposes of coal substitution and co-firing, a combustion study was conducted in a TGA reactor using LC and MP chars (torrefied and pyrolysed) from gram-scale of PSD from 850 to 2800 μm. LC and MP chars were blended with three South African coals between 5 to 90 wt.% (biomass char). The combustion characteristic results showed that LC chars were more reactive than MP chars, with significantly lower combustibility temperatures than the coals. During co-combustion, the combustion indices for blends < 30% were similar to those of the individual coals, showing that partial coal substitution could be done without significant modifications to existing equipment. There was better combustion performance through increased combustion indices for blends > 60%, though probably with a likelihood of modifications to existing reactors that were initially designed for coal combustion, as the conversion was much faster.In summary, this study has shown that LC and MP IAPs could be valorised through torrefaction and slow pyrolysis to produce char for direct energy applications and co-firing with coal. LC samples torrefied at 300 ˚C were found to be equivalent to high volatile bituminous C coal, while pyrolysed chars for LC and MP were equivalent to high volatile bituminous B coal. To confirm the practicality of co-firing possibilities, it is recommended that scale-up studies to pilot-scale be conducted in order to assess overall energy efficiency, techno-economics, operating conditions of industrial reactors and a life cycle assessment.

[发布机构] Stellenbosch University