Biomass and Bioenergy

Abstract

Bioenergy from biomass can replace fossil fuels in the production of heat, electricity, and liquid fuels for transport but the potential contribution is in need of further research and objective discussions. Biomass can also provide feedstock for the chemical industry to replace petroleum. Feedstock for this purpose are major plant oil crops such as oil palm (Elaeis guineensis or E. oleifera), soybean [Glycine max (L.) Merr.], rapeseed (Brassica napus L.), and sunflower (Helianthus annuus L.). In 2015, traditional biomass accounted for 9.1%, and biofuels for transportation accounted for 0.8% of the global final energy consumption. In principle, biomass could meet up to one-third of the projected global energy demand in 2050 by bringing new land under cultivation and/or increasing productivity. However, aside physically possible, socially acceptable biomass potential scenarios must be assessed. The main feedstocks for generating heat, electricity, or gaseous, liquid, and solid fuels are forestry, agricultural and livestock residues, short-rotation forest plantations, energy crops, and the organic component of municipal residues and wastes. Traditional biomass such as fuelwood, charcoal , and animal dung is source for about 99% of all bioenergy. Of minor importance is ‘modern’ biomass such as sugar, grain, and vegetable oil crops for the production of liquid biofuels . However, in the future the bulk of liquid biofuels may be produced from lignocellulosic crops cultivated on marginal, degraded, and surplus agricultural land. Dedicated lignocellulosic energy crops include perennial plants such as switchgrass (Panicum virgatum L.), Miscanthus x giganteus, sugarcane (Saccharum spp.), Agave spp., and short-rotation woody crops such as hybrid poplar (Populus spp.) and willow (Salix spp.). Compared to conventional crops such as corn (Zea mays L.), energy crops are less depending on favorable climatic and soil conditions and require fewer inputs of agrochemicals. Thus, using energy crops would reduce the direct competition for land with food production and ecosystem services, and potentially have lower net energy and greenhouse gas (GHG) effects. However, the carbon costs of dedicating land to bioenergy will exceed the benefits. For example, conversion of native ecosystems for bioenergy often results in soil organic carbon (SOC) loss. The long-term potential of energy crops depends largely on land availability, choice of crop species, improvements by biotechnology, water availability, and effects of climate change . Aside from the dedicated bioenergy plantations, other potential feedstocks are the large volumes of unused organic residues and wastes but it is unclear whether their share can be increased. However, agricultural residues are also required on site to maintain SOC stocks, soil health, and agricultural productivity, and to reduce soil erosion . The SOC sequestration may be the key component in determining the GHG reduction potential of biofuels compared to fossil fuels. Life cycle assessment (LCA) is a widely used approach to assess the GHG balance of biomass production. Removing 25 and 100% of corn residues, for example, jeopardizes agroecosystem services and causes losses of up to 3 and up to 8 Mg SOC ha⁻¹ in 0–30 cm soil depth after 10 years, respectively. In comparison, SOC accumulates in the top 30 cm under perennial grasses at rates of up to 1 Mg SOC ha⁻¹ yr⁻¹. Thus, more intense harvest for bioenergy adversely affects the SOC stock. Also, producing energy crop feedstock by converting previously uncultivated land will cause a reduction in the SOC stock. Otherwise, adding residues from forest harvest, processing, and after end use may be beneficial to the SOC stock compared to establishing woody crop plantations. Sugarcane, perennial grasses, and trees can be cultivated sustainably for bioenergy but estimates for the potential of global bioenergy plantations when environmental and agricultural constraints are taken into account vary widely. Specifically, long-term, large-scale biomass cultivation plots, in particular, of switchgrass and Miscanthus x giganteus are scanty. While biofuels and, in particular, liquid biofuels will offset only a modest share in fossil energy use over the next decade, the impacts on agriculture and food security may be drastic. This chapter begins with a section about biomass as feedstock alternative to petroleum. Then, agroecosystem land use and management types for producing traditional and energy crop feedstocks are discussed with a focus on non-woody plants. The chapter concludes with a section about the effects of agricultural biomass production systems for bioenergy and biofuel on SOC sequestration. Additional information about the potential of woody biomass from agroforestry and plantations as feedstock for bioenergy can be found elsewhere (e.g., Buchholz et al. 2016; Lorenz and Lal 2010).