Valorization of Willow Lignin Recovered in a Pilot-scale Biorefinery Based on Hot Water Extraction

摘要

The United States consumed a record 101 quadrillion British thermal units (Btu) worth of energy in the year 2018. Fossil fuels contributed to 80% of this energy. Primary sources of energy under focus in the US have undergone several changes over the course of history. These include wood in the 18th and 19th centuries, later coal and petroleum, followed by renewable sources, such as nuclear energy in the 20th century and more recently, modern renewable sources, such as hydropower, biomass, wind and solar in the 21st century. Modern renewable sources contributed to ～11.4% of the total energy consumption in the year 2018[1]. With energy research becoming increasingly focused on the renewable resources, coal consumption has fallen to the lowest since year 1970[1]. Biomass has been the largest renewable energy source for the last several years, making up around 44% of the total renewable energy and ～5% of the total energy consumed in the US in the year 2018[2]. It is estimated that ～331 million Mg of biomass is used annually in the US for energy production with an additional 750 million to 1 billion Mg of biomass being potentially available, while still meeting food, feed, and fiber demands[3]. The worldwide interest in biomass as an energy source has largely been stimulated by the heightened threat of climate change caused by emission of greenhouse gases. Latest agricultural growth has enabled biomass surplus, while the technological growth has enabled higher biomass to energy conversion efficiency[4]. Several types and species of biomass have been suggested as potential energy crops. An ideal energy crop is expected to have following characteristics: high yield of dry biomass per hectare, low cost and energy input to produce, low nutrient requirements, and low amounts of contaminants within the biomass[4]. Short rotation woody plants such as willow (Salix spp.) and herbaceous plants such as perennial grass miscanthus (Miscanthus spp.) fit well into these criteria. Previous experiments in our research group have focused on utilization of miscanthus biomass[5], while current experiments focus on willow. Willow, a short rotation coppice has been increasingly researched as a dedicated energy crop in the US over the past several years, e.g. as a part of the Salix Consortium's Willow Biomass project, under which willow biomass has been planted in the Northeastern and Midwestern regions of the US[6]. Commercial willow biomass has been planted over 500 hectares in the Northeast region alone[3]. In the NY State, the average yield of Salix clones is 13.6 odt/ha/year, with minimal chemical and energy input (net energy ratio of 55)[6]. In addition to the high yields that can be sustained in 3-4 years rotation, willow has a favorable chemical composition and energy content (average 8340 Btu/dry pound of three-year old willow stems)[7]. It also offers an easy propagation from dormant hardwood cuttings, wide genetic base for selective breeding, and an ability to sprout back after multiple harvests. Therefore, it is suitable to select for specific characteristics and to sustain[8]. In addition to energy provision, willow biomass is expected to provide economical benefits such as creation of new markets and jobs in rural areas. Also, it allows an array of environmental benefits, such as enhanced landscape diversity and wildlife habitat, increased erosion resistance due to its perennial nature and extensive fine root systems, carbon sequestration, significant reduction in greenhouse gas, NO_x, SO_2 and particulate emissions during energy production and land remediation[3][6][7][8]. Willow, however, is a lignocellulosic biomass (typical composition: extractives 2-10%; lignin 20-26%; glucose 35-42%; xylose 20-25%; mannose, galactose, arabinose, glucuronic acid 3-6%; acetic acid 3-4%)[9] and hence, presents recalcitrance toward the chemical and/or mechanical treatments employed to harness the energy and the chemicals within the biomass.