Greenhouse gas emissions of local wood pellet heat from northeastern US forests
Introduction
Wood pellet heat is a new and growing heating alternative in the US and has been proposed as a climate-beneficial energy source to replace fossil-fuels. However, little work has been done to assess this claim. The opportunity for switching to wood pellet heat is particularly great for the Northern Forest region of northern Maine, New Hampshire, Vermont and New York which is home to more than 2 million people who live in rural communities, larger towns, and small cities surrounded by the largest intact forest in the eastern US (1). Around 42% of all energy consumed is for space heating [1] and the predominance is derived from fossil-fuels [2]. New York and the five New England states comprise 88% of the entire US consumption of home heating oil [3], which is a distillate fuel similar to diesel fuel. Though natural gas is used widely for heat throughout the northeastern US, the northern states of Maine, New Hampshire, Vermont, and the northern portion of New York still rely on home heating oil as a heat source (62%, 45%, 43%, and 50% of homes respectively; [4]. Propane and electricity account for the majority of the balance of heating fuel sources in the region.
Use of wood for heat is variable throughout the region, ranging from 17% of homes in Vermont to 8% in New Hampshire and northern New York [4]. Though the use of wood pellets is increasing, cord wood represents almost 82% of wood use for heat in the five-state New England region [5]. Wood pellet heating systems are up to 15% more efficient than non-catalytic cord wood stoves [6] and prices per Gigajoule of energy for pellets are competitive or better than split wood. For instance, pellet fuel for home heating was 12% less expensive than split wood for the same energy generation in Maine as of December 2016 [7].
GHG emissions from residential energy consumption in the New England states are responsible for 18% of the total GHG emissions for the region [8]. The widespread use of home heating oil contributes disproportionately to these emissions because of the low efficiency of heat conversion and high GHG emissions rates per thermal unit relative to other fossil-fuels [9]. In 2014, about 14.8 billion liters of heating oil were sold to residential consumers in New York and the five New England states [3]. An estimate for the broader northeastern US (Maine to Pennsylvania), suggests enough wood is economically available to replace 16% of the liquid fossil-fuels (i.e., home heating oil) used in the residential heating sector [10]. Though some are encouraging movement towards technology such as air-source heat pumps to meet heating needs and GHG reduction goals [11], others are advocating the conversion to modern wood heat systems such as wood pellet stoves and boilers that rely on locally-derived fuel (i.e., wood) and can support forest-based economies hit hard by recent solid wood and pulp and paper mill closures [12]. While forest-based bioenergy can be renewable if harvest does not exceed growth, these systems can also provide GHG benefits compared to fossil-fuel alternatives under specific conditions [13], [14].
A declining marketplace for low-grade wood in the Northern Forest region creates a sense of urgency for local forest sector economies to replace these markets or face the further loss of jobs and logging infrastructure that have been essential elements of the economy. In Maine alone, paper mill and biomass electric facility closures since 2014 have resulted in the loss of more than 3.6 million green metric tonnes (MT) of the low-grade wood market for landowners and loggers [15]. Wood pellet manufacturing represents one growing aspect of the forest sector that could be developed to replace a portion of the lost low-grade marketplace. In early 2015, ten pellet manufacturing facilities were in operation within the Northern Forest region, though low oil prices and the warm winter of 2015–2016 forced many to curtail operations or temporarily shut down [16]. To create incentives for converting to wood pellet heating systems, states such as New Hampshire are offering rebates to homeowners of 40% of the installed cost of qualifying new residential bulk-fed, wood-pellet central heating boilers or furnaces [17]. Similar programs exist in Maine, Vermont, and New York [18]. One underlying assumption of these incentive programs, since funding typically comes from the Regional Greenhouse Gas Initiative carbon auction proceeds, is that the conversion to modern wood heat systems results in GHG reductions.
Much of the research conducted to date to study the potential GHG impacts of switching from fossil-fuel derived energy to woody biomass energy has focused on the electricity sector and has not addressed comprehensively the thermal uses of wood [14]. Greenhouse gas emissions implications are often expressed in terms of the carbon “payback period”, which is the time required by the forest sequester an equivalent amount of carbon dioxide from woody biomass energy combustion. Modeling has shown that payback periods for electricity uses can be long (e.g., 45–75 years) when harvest rates must be increased to meet the demand of a new wood-consuming facility [13]. But models also show the payback period can be relatively short, especially when the new market creates incentives for landowners to plant trees in previously un-forested areas [19]. When modern thermal uses of wood were evaluated, carbon payback times were generally shorter than when wood is used for electricity [13], [14]. Greater efficiency of wood for thermal uses compared to electricity as an end use is the key factor in this difference.
To date, only one study we are aware of has looked at the atmospheric implications of switching from fossil-fuel heat sources to wood heat in the northeastern US [13]. This study was focused on one state (Massachusetts) and only looked at wood chips used for industrial thermal and combined heat-and-power outputs and did not evaluate wood pellet systems.
The goal of the study presented below was to explore the GHG impacts of locally sourced, produced, and consumed wood pellets (referred to hereafter as “pellets”) for heating applications including both the biogenic and fossil-fuel carbon cycle. The approach included a rigorous LCA framework that considered a range of plausible forest market scenarios to capture an uncertain future.
We focus on a case study area in Maine and discuss the relevance to the broader region through an analysis of survey data from pellet manufacturing facilities throughout Maine, New Hampshire, Vermont, and northern New York.
Section snippets
Study area
One representative softwood dominated wood supply area in Maine was chosen to evaluate the impacts of adding a pellet manufacturing facility to the forest landscape. The wood supply areas each were defined by an 83 km (50 mile) radius centered on an existing wood pellet manufacturing facility. We queried USDA Forest Service Forest Inventory and Assessment (FIA) data to categorize the current acreage within the radius based on forest cover type, tree diameter size class, and stand density. FIA
Climate impacts by economic scenario and feedstock choice
Net GHG emissions were mostly influenced by economic scenario and feedstock source when comparing wood pellet heating scenarios that displace a home heating oil baseline (Table 2). Considering the uncertainty associated with forward modeling of ecosystems paired with economic scenarios (see 4.2), results in Table 2 need to be interpreted cautiously and on a high level. We categorized the results as: 1) Climate Negative; 2) Climate Neutral; or 3) Climate Beneficial. Climate Negative indicates
Overall impact and GHG emissions metrics
Mixing pulpwood trees with sawmill residue to make wood pellets for residential heat produces climate benefits up to a certain point (around 75% pulpwood). This remains true particularly if harvest rates do not exceed current rates. Pellets made from sawmill residues alone show the strongest overall climate benefits in all scenarios as a residential heating source compared to other existing heating alternatives. We observed wide variability in feedstock inputs for existing pellet facilities in
Conclusions
An industry-average feedstock mix consisting of equal parts of sawmill residues and pulpwood-quality wood appears to generate pellet heat in the northeastern US that is at least climate-neutral compared to fossil-fuel heating alternatives when harvest levels are not changed as a result of wood pellet demand and contribute to energy independence in rural areas. The recent loss of pulp and paper and biomass electric facilities creates a surplus of harvested low-grade wood that could be used to
Acknowledgements
This work was supported by funding from the Northern Forest Center, USDA Rural Development, and in-kind support by Spatial Informatics Group, LLC. Partial funding was provided by the New Hampshire Agricultural Experiment Station. We are grateful for input received on scientific context by two anonymous reviewers as well as Chris Hennigar and on regional context by Maura Adams, Joe Short, and Kelly Short.
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