Forest Trees - Bioenergy Crop Plants - page 381

15.5 other Forest sPecIes BIoenerGy use
and PotentIal WorldWIde
In addition to Eucalyptus , Pinus , and Populus species, many other trees have promise for bioenergy
production in various parts of the world. Salix species covered in Section 3, Chapter 4 of this topic
are well suited to wet temperate climates. Numerous others have been proposed for use in different
climatic zones (Little 1981; NAS 1977, 1979, 1980, 1982; NRC 1983a, 1983b, 1983c, 1984).
Humid Tropics— Acacia auriculiformis, A. mangium, Albizia falcataria, Bursera simaruba,
Calliandra calothyrsus, Casuarina equisetifolia, Coccoloba uvifera, Derris indica, Gliricidia
sepium, Gmelina arborea, Guazuma ulmifolia, Hibiscus tiliaceus, Leucaena leucocephala,
Maesopsis eminii, mangrove genera, Mimosa scabrella, Muntingia calabura, Psidium guajava,
Sesbania bispinosa, S. grandiflora, Syzygium cumini, Terminalia catappa, Trema spp.
Tropical Highlands— Acacia decurrens, A. mearnsii, Ailanthus altissima, Alnus acuminata,
A. nepalensis, A. rubra, Gleditsia triacanthos, Grevillea robusta, Inga vera, Melaleuca quinque-
nervia, Melia azedarach, Robinia pseudoacacia, Sapium sebiferum
Arid and Semiarid Regions— Acacia brachystachya, A. cambagei, A. cyclops, A. nilotica,
A. saligna, A. senegal, A. seyal, A. tortilis, Adhatoda vasica, Ailanthus excelsa, Albizia lebbek,
Anogeissus latifolia, Azadirachta indica, Balanites aegyptiaca, Cajanus cajan, Cassia siamea,
Colophospermum mopane, Combretum micranthum, Conocarpus lancifolius, Dalbergia sissoo,
Emblica officinalis, Haloxylon aphyllum, H. persicum, Parkinsonia aculeata, Pithecellobium
dulce, Propsopis alba, P. chilensis, P. cineraria, P. juliflora, P. pallida, P. tamarugo, Sesbania
sesban, Tamarix aphylla, Tarchonanthus camphoratus, Zizyphus mauritiana, and Z. spina-christi
Many of these are multipurpose trees that have more than bioenergy uses. They tend to have
many favorable characteristics such as wide adaptability, easy establishment, low maintenance
requirements, tolerance of difficult environments, nitrogen-fixing ability, rapid growth, coppicing,
and high energy content. Those that are multistemmed, poorly formed, and short-lived may be best
for family and small-scale use. Most are suitable for plantations to meet larger-scale bioenergy
needs. Because these species are aggressive, grow rapidly, and often seed early and prolifically,
they are potentially invasive and should be used carefully. Native species should always be strongly
considered for bioenergy plantations.
15.6 Pretreatment oF Forest BIomass
Biochemical conversion of lignocellulosic biomass through enzymatic saccharification and fermen-
tation is a major pathway for liquid fuel production from biomass (USDE 2005; NSF 2007). In this
approach, biomass cellulose is converted to glucose through microbial or enzymatic actions. The
glucose is then fermented into alcohols, such as ethanol. Residues resulting from microbial and
enzymatic actions contain mainly lignin and can be converted to energy thermochemically. Whereas
starch functions as an energy storage material in plants, wood functions as a structural material.
As a result, woody biomass has natural resistance—often called “recalcitrance”—to microbial and
enzymatic deconstruction (Himmel et al. 2007). Forest (woody) biomass differs from agriculture
biomass physically, structurally, and chemically. Specifically, forest biomass is physically large and
structurally very strong. Its density is higher than agricultural biomass. It has higher lignin, higher
cellulose, and lower hemicellulose contents than most agricultural biomass. As a result, forest bio-
mass has much greater recalcitrance than does agricultural biomass. On the other hand, the high
density and high lignin and cellulose content increase energy content and reduce transportation cost
for forest biomass, which is favorable for advanced energy production.
Recalcitrance of lignocellulosic biomass is a major barrier to economical development of bio-based
fuels and products through the biochemical pathway. This is especially true for forest biomass because
it has greater recalcitrance than does agriculture biomass. The technical approach to overcome this
recalcitrance has been pretreatment to make cellulose more accessible to hydrolytic enzymes for