Type 4 waste biomass includes agricultural waste such as rice hulls, rice straw, coconut husks and nut shells. It includes naturally occurring forestry debris as well as waste from forest harvest operations. Every year during dry season entire villages and cities in the highland areas of Vietnam are enveloped in soot and smoke from the prescribed burning of forestry debris. Also, there is woody biomass from construction and demolition debris, from sawmills, from the manufacturing of wood-based products and so forth.
Land should never be set aside to grow biomass to fuel gasifiers, Wood Pellets: Green Energy or New Source of CO2 Emissions? We must always stay within the framework of waste biomass, co-cropped biomass or biomass harvested in the reclamation of land infested with invasive plants.
Three invasive plants stand out in the United States
In the southern United States, Chinese tallow covers over 185,000 hectares of land. In the western United States, salt cedar covers about 365,000 hectares of land. But the land infested by mesquite is far bigger.
Mesquite, a nitrogen-fixing plant, has become “the dominant woody plant on 38 million hectares of semiarid grasslands” in the United States. About 25% of Texas grasslands, representing over 20 million hectares, are now covered with mesquite, thanks to the spread of seed by grazing cattle. “Prosopis juliflora leaves are unpalatable for most livestock but mature pods (with or without seeds) are highly palatable” (Mesquite Pods as a Feed Resource for Livestock). Woody biomass from mesquite makes, of course, an excellent gasifier fuel. Mesquite was introduced to Puerto Rico, Hawaii, New South Wales and Queensland and it is abundant in all of these places. Instead of employing expensive technologies to eradicate mesquite, this plant can be managed and controlled to provide food, feed and fuel (Mesquite in Mexico: The Renaissance of an Ancestral Staple Crop in a Time of Climate Change).
“The invasion of alien species is now regarded as one of the major threats to biodiversity of the world (IUCN, 2000)” (Alien Invasive Plants of the Mekong: An Overview). “Worldwide 653 woody plants species have been recorded as being invasive” (An Overview of invasive woody plants in the tropics). In South East Asia “Invasive species are threatening forest habitats (Invasive Species The livelihoods threat). Properly managing invasive woody plants is very much needed in the restoration of biodiversity in many areas throughout the world.
In Vietnam, Mimosa pigra is a huge problem. This nitrogen-fixing plant, however, could provide a lot of woody biomass to fuel gasifiers. Its heating value is higher than most agricultural residue (Renewable energy from thermal gasification of a giant sensitive plant – Mimosa pigra L.).
At the same time, its leaves have a high nutritive value for goats and rabbits. Its leaves can serve as the sole feed for goats. It can reduce the production of enteric methane in goats by as much as 42% (Effect of a tannin-rich foliage (Mimosa pigra) on feed intake, digestibility, N retention and methane production in goats fed a basal diet of Muntingia calabura) and Mimosa pigra for growing goats in the Mekong Delta of Vietnam). It makes an excellent feed for rabbits (Effect of Feeding Head Lettuce, Water Spinach, Ruzi grass or Mimosa pigra on Feed Intake, Digestibility and Growth in Rabbits). Since Mimosa pigra has a lot of thorns, it can be fed to cattle only if leaves are flailed off its thorny stems and branches.
Mimosa pigra contains strong antioxidants: “Myricetin, quercetin and their glycoside derivatives are strong antioxidants; and elicit cytotoxic effect on human cancer cell lines among other pharmacological activities” (Two new acylated flavonol glycosides from Mimosa pigra L. leaves sub-family Mimosoideae). “It has been used as a medicinal plant for colds, fever, toothaches, eye medicine, snakebite, weak heart and diarrhea, and it has antimicrobial activity” (Uses for Mimosa pigra).