Filling a Need: Forest Plantations for Bioenergy in the Southern US
The growing number of renewable energy projects in the Southern U.S. utilizing woody biomass will require the development of short-rotation bioenergy plantations.
The U.S. forest products industry is already the nation's largest producer of renewable energy and the southern region is no exception. For many decades, the forest products industry has been utilizing, harvesting and manufacturing residues in boilers and kilns for on-site energy usage as well as selling excess energy into the grid. This trend is increasing in light of the recent and expected future volatility in other energy sources such as coal and natural gas.
Against this backdrop are a number of published studies on logging residuals available for bioenergy. What is increasingly obvious is that the amount of truly available logging residues will be nowhere near enough to supply the current and announced bioenergy processors in the Southern U.S. This indicates that appropriate technology for short-rotation bioenergy plantations must be rapidly developed to fill this growing need.
Forest Plantation Concept-Tried and Tested
Forest plantations have been sustainably grown in many parts of the world. While exact records do not exist, it is commonly understood that the Japanese have been planting forests since the 10th century. Forest yields continue to increase on these sustainably managed acres.
In the Southern U.S., the history of forest products and forest plantations is long and successful. It is projected that more than one-half of the wood harvested for processing will be obtained from planted forests in this region. This could not have been possible without the utilization of outreach, education and research from land grant universities, the U.S.
Forest Service, state forestry services, private and state forestry associations and the participation of literally millions of private landowners as well as large timber land companies.
The typical forest plantation today in the Southern U.S. is planted to loblolly pine (Pinus taeda L.) on average at 600 seedlings per acre, on a 25-year rotation with a thinning at the age of 15 years. The thinning typically removes trees for pulpwood while the final harvest is for saw timber and pulpwood. This management scheme has been derived to fill the wood needs of current processors in pulp and lumber. In addition, plantation acres have been established for other conifer or hardwood species in the Southern U.S.
This work is supported by highly trained forestry and logging professionals and land grant universities, amongst others, that yearly yield more than 100 masters and doctoral students.
Novel Idea, But Not New
Bioenergy forest plantations have been practiced in the Southern U.S. since the oil embargoes in the 1970s. In countries such as Brazil and South Africa, eucalyptus plantations have been managed for bioenergy production for decades. What makes the current Southern U.S. situation novel is the short timetable given to develop existing genetic improvement programs and their required silvicultural systems for widespread early adaptation. A forest bioenergy plantation can take 18 months to eight years to reach financial maturity, and the sooner it is planted the sooner it will be ready for commercial harvest.
The tried and tested forest plantation concept in the Southern U.S. produces conifer wood because it is the backbone of the forest products industry for both pulp and lumber. The bioenergy plantation, however, will be more complex and many questions need to be addressed. Does the bioenergy stream allow bark, branches, leaves and wood or is only wood preferred? Does the bioenergy stream need higher lignin content typical for certain species and tree ages? What will be the usage of the ash in co-generation or single-source biomass combustion? How will the development of enzymes change the tree species or rotation age? What are the logistics of harvesting and transporting feedstock cost effectively?
At this time, a number of landowners, research institutions and government entities are researching forest bioenergy plantation management schemes. Early phase testing is also underway on feedstock suitability for wood pellets, cellulosic ethanol and combustion. These research efforts are rapidly expanding due to both research funding and private company research interests.
The forest bioenergy plantation will have more trees per acre, possibly 1,000 to 2,000, and shorter rotations. In fact, for hardwood species that re-sprout (coppice) after harvest, the rotation lengths can be 18 to 36 months. New harvesting systems are being developed for this smaller material and most of these have an on-site chipping or grinding capacity so that the delivered feedstock is ready to be processed directly into bioenergy.
One type of forest plantation takes both traditional and bioenergy concepts to a nonconventional system. This utilizes one row of wide spacing high-value genetics for saw timber (lumber) while the adjoining row is tightly spaced for bioenergy. This system will work with loblolly pine with a bioenergy harvest at six to eight years and a saw timber final harvest at 18 to 22 years, allowing farmers and forest landowners to increase the possibility of positive cash flow in the first years, capturing new markets for bioenergy and retaining existing markets for saw timber.
The authors believe that the region's seedling nursery capacity, genetic improvement programs and land management technologies (silviculture) are robust and more than adequate for developing highly productive forest bioenergy plantations. Two areas are in need of continued investment. First, those organizations involved in bioenergy processing need to share their findings on what is and will be the desired biomass processing characteristics, such as whether softwood or hardwood is preferred as well as the most suitable species for biofuels. Sharing this with the landowner is crucial to their forest management decisions. Second, additional effort is needed to conduct financial modeling of the various forest bioenergy plantation systems with regard to species, trees per acre, rotation lengths and harvesting systems.
North Carolina State University in partnership with state and federal institutions, private companies and other universities is actively working to identify the most promising biomass and the most profitable pathways for biofuel production. The NCSU Wood and Paper Science Department is performing complete analysis of the supply chain with strong technical basis in process design while also accurately measuring the financial impact. From current research, the major features identified for ideal biomass for biofuel are:
›Maximum delivered cost of $62 per bone dry tone of biomass
›Carbohydrate content of 65 percent to 70 percent on dry mass basis (mostly true for chemical pathways for biofuel production)
›A crop that may be harvested and supplied year-round (instead of three-to-five-month harvesting windows for switchgrass and sweet sorghum, requiring further logistics and storage)
Fast-growing, short-rotation forest plantations can fulfill these requirements and be used for bioenergy including but not limited to electricity generation, wood pellets and biofuels.
The increasing scale of forestry biomass for bioenergy will only be possible with developments in forest bioenergy plantations as there will be insufficient feedstock from logging residuals for all announced and planned facilities. Existing technologies can be utilized to rapidly establish forest bioenergy plantations and research is underway to expand these possibilities. Bioenergy processors and forest plantation managers must continue to interact to ensure that woody feedstock demand does not exceed supply.
Ronalds Gonzalez is a PhD student at North Carolina State University in Raleigh working on cellulosic ethanol from various feedstocks, Dr. Jeff Wright is an adjunct professor at North Carolina State University and Dr. Daniel Saloni is an assistant professor at North Carolina State University working on supply chain and life-cycle analysis of woody biomass and biofuels. Reach them at firstname.lastname@example.org, email@example.com and firstname.lastname@example.org.