Biochemical and thermochemical conversions of short rotation coppice poplar for carbohydrate and fuel production in lignocellulosic biorefineries

Abstract

The economic success of a potential biorefinery is directly related to the use of economic feasible biomass. Due to high cost, it is unlikely that a wood-based biorefinery would use only whitewood material from conventional forestry plantation as feedstock. Instead, short rotation coppice can be a potential feasible substitute because of its low cost. Yet, the efficacy of conversion using short rotation coppice, a more heterogeneous feedstock, has not been investigated. This work studied the influence of using 2-year-old poplar coppice on the overall sugar yield via biochemical conversion and the bio-oil production via thermochemical conversion. By harvesting all aboveground parts of 2-year-old poplar coppice, the biomass was comprised of four different fractions – leaves, bark, branches, and whitewood chips. From this, three samples from coppice plantation – whole tree coppice (WTC), noleaf coppice (NLC), and leaf coppice (LC) were prepared and studied. A sample of mature wood from forestry plantation – whitewood forestry (WWF) was used as standard for reference. In biochemical conversion, all samples were processed via steam pretreatment and enzymatic hydrolysis for sugar production. In thermochemical conversion, samples were fast pyrolyzed in a fluidized bed reactor and the final bio-oil yield and composition were measured. Results show that converting the 2-year-old poplar coppice is promising in biorefinery. Leaf removal is essential for biochemical conversion, as it improved the sugar yield by 150 kg/tonne and the sugar recovery by 40%. The NLC achieved over 350 kg/tonne overall sugar yield and up to 70% sugar recovery. Given these results, we evaluated the economics of converting 2-year-old poplar coppice in biochemical conversion. For thermochemical conversion, leaf removal did not impact the bio-oil yield, as both WTC and NLC achieved similar bio-oil yield of 55%. Energy recovery rate was used to evaluate the energy efficiency of thermochemical conversion. By lowering the energy recovery rate by 2.8%, leaf removal showed only small impact on the energy efficiency of fast pyrolysis. Further investigation was conducted to assess the biochemical conversion performance by using two leafless poplar coppice hybrids from two plantation sites. Processed in the same condition, poplar coppice showed different overall sugar yields between low-productive hybrid and high-productive hybrid for both sites. Correspondingly, the difference illustrates a remarkable fluctuation in the potential product yield, leading to a pronounced variability in economics for feedstock user. Economic modelling showed that the biorefinery will achieve a 7% to 12% revenue increase by using the low-productive hybrid compared to high-productive hybrid. In contrast, the feedstock productivity dramatically influences the economics of the feedstock grower. Growing the high-productive hybrid means either 11% - 25% reduction in land use or considerable extra revenue from the surplus feedstock. A new business model integrating both plantation and biorefinery could help to fully accommodate the needs of both sides and enhance the overall efficiency in feedstock production and biofuel production. One reason of low biochemical conversion yield from short rotation coppice poplar is its high content of non-structural components (NSCs), which include inorganic ash and organic extractives. To study the impact of NSCs and improve the biochemical conversion yield, we preprocessed the coppice poplar by neutral and/or acidic wash. Preprocess significantly reduced ash and extractives content as much as 70% and 50%, respectively. It appears that preprocess changed the buffering capacity of the biomass via ash removal and thereby facilitate the hemicellulose solubilization and enzymatic hydrolysis. The overall sugar yield after pretreatment and hydrolysis was 18-22% higher when the biomass had been preprocessed. Meanwhile, removal of extractives during preprocessing reduced the formation of inhibitor and improved the fermentation yield. The ethanol yield was 36-50% higher for the preprocessed biomass during fermentation of liquid fraction. The economic assessment shows that introduction of one preprocessing unit (acidic-neutral wash) in the biorefinery could increase $43.1 MM/year in gross revenue and greatly benefit the economics.