Is Use of Both Pulpwood and Logging Residues Instead of Only Logging Residues for Bioenergy Development a Viable Carbon Mitigation Strategy?
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- Dwivedi, P., Bailis, R. & Khanna, M. Bioenerg. Res. (2014) 7: 217. doi:10.1007/s12155-013-9362-z
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This study adopts an integrated life-cycle approach to assess overall carbon saving related with the utilization of wood pellets manufactured using pulpwood and logging residues for electricity generation. Carbon sequestered in wood products and wood present in landfills and avoided carbon emissions due to substitution of grid electricity with the electricity generated using wood pellets are considered part of overall carbon savings. Estimated value of overall carbon saving is compared with the overall carbon saving related to the current use of pulpwood and logging residues. The unit of analysis is a hectare of slash pine (Pinus elliottii) plantation in southern USA. All carbon flows are considered starting from forest management to the decay of wood products in landfills. Exponential decay function is used to ascertain carbon sequestered in wood products and wood present in landfills. Non-biogenic carbon emissions due to burning of wood waste at manufacturing facilities, wood pellets at a power plant, and logging residues on forestlands are also considered. Impacts of harvest age and forest management intensity on overall carbon saving are analyzed as well. The use of pulpwood for bioenergy development reduces carbon sequestered in wood products and wood present in landfills (up to 1.6 metric tons/ha) relative to a baseline when pulpwood is used for paper making and logging residues are used for manufacturing wood pellets. Avoided carbon emissions because of displacement of grid electricity from the electricity generated using wood pellets derived from pulpwood fully compensate the loss of carbon sequestered in wood products and wood present in landfills. The use of both pulpwood and logging residues for bioenergy development is beneficial from carbon perspective. Harvest age is more important in determining overall carbon saving than forest management intensity.
KeywordsBioenergy developmentCarbon sequestration in wood productsElectricity generationPine plantationsSouthern USAWood pellets
The Energy Independence and Security Act of 2007 has set a production target of 136.3 billion liters of biofuels by 2022 out of which 60.5 billion liters will be produced in the form of cellulosic biofuels . It is also projected that the peak capacity of biomass-based electricity generation will increase from 11.5 to 49.3 billion kilowatt hour between 2010 and 2035 countrywide because of incentives announced by federal and state governments . It is quite likely that the biomass obtained from forestlands will play a critical role in meeting the projected demand for biomass feedstocks towards bioenergy development nationwide .
Several studies have analyzed carbon benefits related with the use of forest biomass for bioenergy development. Dwivedi et al.  reported that the use of wood pellets for electricity generation in Florida could reduce greenhouse gas (GHG) emissions by 82 % relative to a unit of electricity produced from coal. In another study, Dwivedi et al.  found that the use of ethanol derived from slash pine (Pinus elliottii) wood could save 76 % of GHG emissions relative to gasoline over the average life-span of a small passenger car in the USA. Steele et al.  estimated that the use of bio-oil, manufactured using fast pyrolysis technology from southern pine (Pinus taeda) biomass, could reduce GHG emissions by 70 % over residual fuel oil. Katers et al.  found that the use of wood pellets derived from forest mill residues for domestic heating in Wisconsin could save about 27 % of GHG emissions relative to natural gas. A review of existing studies reveals two methodological similarities: (a) overall saving in GHG emissions due to use of bioenergy products is based on wood obtained from only one harvest cycle and (b) simultaneous impacts of a change in harvest age and forest management intensity on GHG emissions are not considered. It is important to analyze impacts of harvest age and forest management intensity over a long planning horizon as these parameters significantly determine total wood availability, and therefore availability of different bioenergy products.
A few existing studies consider utilization of logging residues for bioenergy development [8–11]. Logging residues are mostly comprised of non-merchantable portion of a harvested tree along with branches and tops. Currently, logging residues are either burned or left in open fields by forestland owners as markets for logging residues are practically absent. It is generally thought that logging residues could be used for bioenergy development to reduce GHG emissions  and increase profitability of forestland owners . Additionally, there is an implicit assumption that the use of logging residues for bioenergy development will not affect traditional forest-based industries which are dependent on sawtimber, chip-n-saw, and pulpwood for manufacturing various wood products like lumber, plywood, oriented structural board (OSB), and paper.
However, evidence suggests that the production of wood pellets from pulpwood is increasing in the USA . It is likely that the present trend of utilizing pulpwood for bioenergy development will continue in the foreseeable future driven by rising demand for different bioenergy products at regional, national, and global levels . Utilizing pulpwood for bioenergy development could affect long-term carbon sequestered in wood products and wood present in landfills . Existing studies which quantify carbon benefits of forest biomass-based bioenergy development ignore this critical aspect as they mostly focus on carbon emissions avoided due to displacement of energy products derived from fossil fuels [4–7]. A few studies which do consider carbon sequestered in wood products and wood present in landfills do not consider impact of multiple harvest cycles on the dynamics of carbon sequestered in wood products and wood present in landfills [16, 17]. Some studies that do consider multiple harvest cycles only report carbon sequestered in wood products and wood present in landfills for the first 100 to 200 years [18, 19]. Furthermore, no study, to the best of our knowledge, has yet analyzed the impact of diversion of pulpwood for bioenergy development, instead for paper production, on carbon sequestered in wood products and wood present in landfills. An understanding about this alternate usage is critical as carbon stored in wood products and wood present in landfills is a major percentage of total carbon sequestered by the forestry sector. For example, the forestry sector sequestered about 159 million metric tons of carbon in 2005, of which 36.5 % (58 million metric tons) was sequestered in wood products and wood present in landfills .
Details of selected cases. First and second cases are considered as two separate baselines
Left on the ground
Burn on the ground
Parameters used for modeling carbon flow. Non-biogenic carbon emissions due to burning of logging residues on forestlands contain N2O and CH4 (100-year global warming potential). We used same value for ascertaining quantities of non-biogenic GHG emission released at the time of burning green wood (wood waste and logging residues) due to lack of sufficient information. All values of carbon emissions can be converted into CO2e by multiplying 44/12
Carbon present in harvested timber products (by mass) (percent)—25
Parameters used for plantation management
Carbon emissions with intensive forest management before age 12 (metric tons/hectare)—0.66 
Carbon emissions with intensive forest management after age 12 (metric tons/hectare)—1.3 
Carbon emissions with non-intensive forest management (metric tons/hectare)—0.60 
Non-biogenic carbon emissions due to burning of logging residues on forestlands (gram/kilogram)—143.12 
Half-life of logging residues (years)—5.7 
Parameters used for converting timber products into wood products
Conversion efficiency of lumber production from sawtimber (percent)—64.5 
Conversion efficiency of OSB production from chip-n-saw (percent)—64.5 
Conversion efficiency of paper production from pulpwood (percent)—58 
Conversion efficiency of wood pellet production (percent)—80 
Carbon emissions related with finished lumber production (gram/kilogram)—25.83 
Carbon emissions related with OSB production (gram/kilogram)—115 
Carbon emissions related with paper production (gram/kilogram)—369 
Carbon emissions related with wood pellet production (gram/kilogram)—42.46 
Non-biogenic carbon emissions due to wood waste burning in controlled conditions (gram/kilogram)—9.38 
Parameters used for modeling carbon flow during the use phase of wood products
Half-life of lumber (years)—100 
Half-life of OSB (years)—30 
Half-life of paper (years)—2.6 
Recycling rate of lumber (%)—25 
Recycling rate of OSB (%)—25 
Recycling rate of paper (%)—50 
Carbon intensity of a unit of grid electricity (kilogram/kilowatt hour)—0.212 
Conversion efficiency of a 100 MW power plant (percent)—31.6 
Calorific value of wood pellet (megajoule/kilogram)—18.5
Parameters used for modeling carbon flow in the landfill
Lumber to landfill (percent)—76 
OSB to landfill (percent)—67 
Paper to landfill (percent)—34 
Lumber changing to non-degradable portion in landfill (percent)—77
OSB changing to non-degradable portion in landfill (percent)—77 
Paper changing to non-degradable portion in landfill (percent)—44 
Lumber changing to degradable portion in landfill (percent)—23 
OSB changing to degradable portion in landfill (percent)—23 
Paper changing to degradable portion in landfill (percent)—56 
Half-life of degradable portion in landfill (years)—14 
The following steps were used to ascertain quantity of carbon sequestered in wood products and wood present in landfills under the case LEFT-LR. For the first harvest cycle at the very first year of simulation period, we allocated carbon emissions from plantation management to timber products based on their mass percentage. We did not allocate any plantation management-related carbon emissions to logging residues under LEFT-LR and BURN-LR cases. We ascertained carbon emissions related with manufacturing of different wood products—lumber, OSB, and paper (Table 2). We assumed that wood waste produced while manufacturing different wood products was burned within the manufacturing facility itself. We estimated non-biogenic carbon emissions due to burning of wood waste at manufacturing facilities (Table 2). We subtracted carbon emissions from plantation management, manufacturing of wood product, and burning of wood waste from the carbon sequestered in different wood products to estimate net carbon present in different wood products before entering the use phase.
We adjusted the above procedure for each case. For the case BURN-LR, we did not consider carbon sequestered in logging residues at all. We first estimated carbon emissions related with the burning of logging residues by multiplying the quantity of available logging residues and a suitable emission factor (Table 2). We divided total carbon emissions with the harvest age to ascertain average annual carbon emissions. Then, we calculated cumulative average carbon emissions for every year present in the simulation period. Finally, we subtracted cumulative average carbon emissions related with the burning of logging residues from the carbon sequestered in wood products and wood present in landfills to estimate overall carbon savings for all years present within the simulation period.
Discussions and Conclusions
This study assesses the impact of diversion of pulpwood for bioenergy development instead for paper manufacturing on overall carbon savings. We estimated carbon dynamics for two additional cases and compared our original results with them to obtain a better understanding of overall carbon savings. For estimating overall carbon savings, we included carbon sequestered in wood products and wood present in landfills along with avoided carbon emissions due to the use of logging residues (with and without pulpwood) as a feedstock for manufacturing wood pellets, which were then used to generate electricity. We also assessed impact of changes in forest management intensity, harvest age, and simulation period on overall carbon savings. This study clearly indicates that use of pulpwood and logging residues for bioenergy development helps in significant amount of carbon savings under realistic baselines depending upon the length of simulation period.
We acknowledge that validation of our results on carbon sequestered in wood products and wood present in landfills using real data is not possible considering the peculiarity of the problem analyzed. However, we have followed standard guidelines to account for carbon sequestered in wood products and wood present in landfills . Moreover, reported carbon sequestered values are well within ranges reported by other similar studies over first 100 or 200 years of simulation [18, 19]. Additionally, trajectory of carbon sequestered in wood products and wood present in landfills reported is very similar to the existing work [18, 19].
Intensive forest management sequesters more carbon in wood products and wood present in landfills relative to non-intensive forest management.
Leaving logging residues will sequester highest amount of carbon in wood products and wood present in landfills relative to burning them without displacing fossil fuels.
Carbon sequestered in wood products and wood present in landfills reaches to a steady state around 500 years under multiple harvest cycles at a constant harvest age.
An increase in harvest age will not always lead to more carbon sequestration in wood products and wood present in landfills.
Harvest age and intensity of forest management play an important role in determining quantities of carbon sequestered in wood products and wood present in landfills.
Availability of large-diameter timber products is a significant determinant of carbon sequestered in wood products and wood present in landfills.
Carbon sequestered in wood products and wood present in landfills decreases when pulpwood is diverted to bioenergy development instead when it is used for paper manufacturing. However, this decrease is insignificant over time.
Avoided carbon emissions play a significant role in determining overall carbon savings especially under a long planning horizon.
Carbon credits generated due to use of wood pellets derived from pulpwood for electricity generation fully compensates the loss of carbon sequestered in wood products and wood present in landfills due to diversion of pulpwood for wood pellets instead of paper manu-facturing.
Harvest age is more important in determining overall carbon savings than forest management intensity.
Simulation period is a key determinant of overall carbon savings. However, trajectory of overall carbon savings does not change a lot with simulation period but magnitude does.
Overall carbon benefit varies significantly with respect to the selected baseline. Therefore, a caution should be exercised in selecting a baseline while promoting use of either logging residues, pulpwood, or both as a feedstock for bioenergy development, in general or electricity generation, in particular.
We have not considered any carbon sequestered in above- or belowground in this study. Aboveground carbon shows a cycling trend with respect to various silvicultural practices and carbon in forest soils attains an equilibrium value in a relatively short time on reforested forestlands . This suggests that net carbon sequestered on those lands which are continuously being used for raising forest plantations might be negligible relative to carbon sequestered in timber products. However, we strongly feel that more information is needed to objectively define the role of carbon sequestered in above- or belowground while calculating carbon intensity of wood-based bioenergy products. Additionally, we have assumed that all logging residues will be collected at the time of harvest. Collection efficiency could be varied along with several other parameters used in this study to assess changes in trajectories and quantities of carbon sequestered in wood products and wood present in landfills.
We only considered major wood products for ascertaining impact of use of small-diameter timber products for electricity generation on carbon sequestered in wood products and wood present in landfills. However, there are many more wood products available in the market having different manufacturing and use characteristics. The developed model in this study can be extended to include additional wood products to get a more precise estimate of carbon sequestered in wood products and wood present in landfills. Additionally, a need exists to evaluate other forest species growing in different regions of the country for understanding the broader impact of utilizing timber products on carbon sequestered in wood products and wood present in landfills. This is particularly true as yields and wood characteristics vary based on management practices adopted by forestland owners, local weather, and soil conditions. Furthermore, we have assumed that all parameters (yields, wood product use characteristics, and GHG intensities of different wood products) will remain constant over the length of selected simulation period. This model can be further improved by including time-dependent values of these and other parameters. Finally, we have not included any transport-related carbon emissions in this study. Including transport-related emissions will further improve the model. We hope that the present study should be able to guide future research in a suitable manner. It is expected that the findings of this study will suitably guide policy deliberations over carbon benefits of utilizing logging residues and pulpwood for bioenergy development in the USA and elsewhere.
The authors appreciate the funding support provided by Yale Climate and Energy Institute and Energy Biosciences Institute, University of Illinois at Urbana-Champaign. The authors are also thankful to Drs. Jianbang Gan (TAMU), Wendell Cropper (UF), Douglas Carter (UF), Timothy Martin (UF), and Gary Peter (UF) for their helpful suggestions.