Why not harvest existing invaders for bioethanol?
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- Quinn, L.D., Endres, A.B. & Voigt, T.B. Biol Invasions (2014) 16: 1559. doi:10.1007/s10530-013-0591-z
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Some ecologists and environmentalists have asked whether existing plant invaders could be used as sources of lignocellulosic ethanol, as an alternative to the introduction of potentially invasive non-native energy crops. Although the idea is tempting and could theoretically motivate the control or eradication of large invasive populations, we recognize that a number of major economic, logistic, and legal barriers currently prevent adoption of this plan. Here, we enumerate these barriers in detail, but conclude with an idealistic vision for the role of invasive biomass in the bioenergy industry.
Discuss non-native biofuel crops with enough ecologists or other environmentally minded citizens, and the question, “why don’t we just harvest existing invasive plants?” will eventually arise (Lane 2012; Zaleski 2008). Proponents will argue that the plan could motivate the large-scale eradication of an array of troubling invaders, avoid land use conversion, resolve the food-versus-fuel debate, result in millions of gallons of clean-burning ethanol, and finally free us from our addiction to fossil fuels. In addition, the harvest of existing invaders could avoid the potential for farmers to drop out of the USDA’s Conservation Reserve Program (CRP) by establishing energy crops on their CRP land (Secchi and Babcock 2007). As ecologists and environmentalists, we worry about the potential degradation of natural capital when land loses its conservation status. Since many invaded landscapes are already degraded (Henderson et al. 2006), and some municipalities prohibit disposal of invasive biomass at green waste facilities (Goldstein 2013), utilization of this biomass as a source of ethanol seems like a win–win situation. A group of authors has proposed the harvest of wetland invader Phalaris arundinacea (common reed) for bioenergy, estimating that the amount of energy generated from the complete harvest of this species in Wisconsin could power 1.25 million households and provide additional environmental benefits (Jakubowski et al. 2010). While this is an impressive figure, the authors propose it simply as a “what if” value, admitting that complete harvest of P. arundinacea across all of Wisconsin is not feasible. Young et al. (2011) also proposed the use of existing invaders as bioenergy sources, postulating that harvested biomass could replace other combustible feedstocks given current technologies and could potentially be used to generate liquid fuels (ethanol) in the future.
A number of technologies currently exist for the conversion of biomass to energy, including thermochemical pathways—combustion, pyrolysis, and gasification to produce heat and electricity—and biochemical pathways—anaerobic digestion, esterification, and fermentation to produce biodiesel and bioethanol. In this paper, we specifically consider the pathway from biomass to ethanol, not only because the US Environmental Protection Agency (EPA) mandated through its Renewable Fuel Standard (RFS) that (non-corn) cellulosic ethanol must provide an ever-increasing proportion of our liquid fuel consumption in the coming years (Federal Register 2007), but also because the worrisome non-native feedstocks that are being proposed for large-scale production [e.g. Arundo donax (giant reed)] are intended primarily for that end use. When we have heard the argument for harvest of existing invaders, the underlying message is that we could avoid planting these invasive feedstocks if we simply harvested invaders instead. If that were true, invasive biomass would have to produce equivalent quantities of ethanol through existing harvest, processing, transportation, and conversion pathways. All plants have some potential for lignocellulosic conversion to ethanol, given the appropriate processing equipment and enzymatic substrate. We do not argue that invasive plants are inherently less suited for this conversion process—indeed, two known invaders [A. donax and Pennisetum purpureum (Napier grass)] have been approved for production by the EPA (Federal Register 2013)—but existing legal, logistical, and economic issues related to the harvest of these plants for ethanol production pose what we think are insurmountable problems, at least for the foreseeable future. Perhaps our analysis is too pessimistic, but at the very least, these issues must be considered and incorporated into any plans for ethanol conversion from invasive biomass under the RFS.
Overarching conflict: conservation goals versus business interests
Putting aside for a moment the potential for regeneration of invasive plant stands following the initial harvest, let’s assume that the environmentalist’s primary goal is to permanently remove all aboveground biomass and to eradicate the invasive population. Unfortunately, this hardly satisfies the business-person’s goal of investment in a sustainable crop that offers repeated and profitable harvests year after year. Even if the invasive population is so large or so persistent as to require partitioning of harvest attempts into multiple years, ultimately the end goal (and theoretical result) would be the drastic reduction or complete removal of invasive biomass from the area, leading to the consequent elimination of a profit stream. Due to the embedded economic issues associated with biofuel conversion facilities and the unequivocal need for a predictable and steady source of feedstock supply to operate such facilities, we see intentional eradication of invaders as a substantial barrier to their subsequent use as a profitable ethanol feedstock.
Economic and logistical barriers
Even if a one-off harvest was acceptable to an opportunistic investor, the issue of scale presents a problem. To be profitable, the amount of biomass arriving at a biorefinery must be reasonably large [e.g. a plant must process 1,800–9,000 MT of biomass per day, harvested throughout the course of the year from 70,000 to 350,000 hectares (Herndon et al. 2008)] to justify the investment of a biorefinery [e.g. $200 million for a commercial-scale refinery (Rosen 2012)]. A search for invasive plant removal projects reported by counties and weed management areas indicated that the typical removal area is well under 100 hectares, and is usually closer to 10 (the range in our search was 0.2–134 ha; IERCD 2010; PVWMA 2011; Romano and Henry 2012). The amount of biomass removed from these areas would vary depending on severity of the invasion and the species, but it is highly doubtful that this biomass would satisfy the demands of even a small-scale biorefinery for a single year, much less justify construction of a dedicated biorefinery.
Transportation costs and logistical issues
Due to the high costs of transport and storage (up to 30 % of the total cost of an ethanol supply chain, Hess et al. 2007), removal projects will not be cost-effective if located more than 100 km from a biorefinery (Miao et al. 2012). The 211 biorefineries operating in the US are primarily sited in the Midwest, and 90 % process only corn (Renewable Fuels Association 2013). The majority of the aforementioned “integrated” biorefineries are located in the Midwest and the south, leaving large swaths of the Mid-Atlantic, southeast, and plains and western mountain states (Fig. 1) lacking nearby facilities. Unlike dedicated bioenergy plantations, many of which will be sited in the Midwest and south to take advantage of optimal growing conditions (Wright 1994), invasive plants are found in every state and climate, and in diffuse areas. The likelihood that an invasive plant removal project would be within a reasonable distance of an existing feedstock-appropriate biorefinery is low, and, as previously mentioned, without a constant biomass stream (e.g. >15 truck deliveries per hour; Miao et al. 2012), it is not economically justifiable to build one solely to process harvested invasive biomass. Therefore, removed invasive biomass will have to be transported long distances at commensurate cost to the investor. A mobile biorefinery concept (Venere 2010) has been proposed, but it is not clear whether or when this concept will take off.
Location and scale issues also introduce complications relating to property rights, since invasive populations rarely respect landowner boundaries. This not only matters to the group coordinating eradication—acquiring permission from multiple landowners to conduct invasive species removal can be prohibitively difficult—but also matters to the investor who may find himself in the position of paying multiple landowners for their “crop”. Uncooperative landowners could perpetuate refugia for invasive plants that could reinvade adjacent properties after eradication (Urgenson et al. 2013), and “holdout” landowners could negotiate higher biomass sale prices, driving up costs for the investor (Miceli and Sirmans 2007).
Labor and processing costs
Whereas small portions of invasive populations can be tackled by weekend “weed warrior” volunteer groups armed with hand tools, large scale eradication—the only potentially profitable approach given biorefinery demands—would likely require large groups of paid workers and/or the rental or purchase of large harvesting equipment, which can be difficult to move on uneven terrain or wet soils (Jakubowski et al. 2010). Typically, farmers take on the cost of employees or equipment as an offset to profits made in the sale of their crop. In harvesting invasive populations growing across contiguous properties, it is not clear who would cover the costs of equipment or labor.
Unlike agricultural monocultures, invasive populations often co-exist with non-target species, which could be unintentionally harvested and intermixed with invasive biomass. Along with ecological impacts discussed below, this unintentional harvest could result in substantial contamination of the feedstock with material that may not be suitable for the refinery in question (e.g. if woody species are mixed with herbaceous material). It has been noted that more uniform feedstocks are preferred or required by biorefineries and carry the potential for greater profit (US Department of Energy 2011). Processing to remove extraneous material prior to shipment would require investments of labor and, possibly, building or rental of large processing facilities.
To prevent accidental dispersal, biomass would need to be well contained at every point along the transport route, or processed into pellets (Young et al. 2011) prior to transportation. Biomass densification, including pelletization, is preferred by the industry to improve handling and storage and to reduce transport costs (Clarke et al. 2011; Miao et al. 2012), though there is some disagreement about the economic benefits of pellets for conversion to ethanol (Krishnakumar and Ileleji 2010). Pelletization involves drying, size reduction through chipping or shredding, and compaction and extrusion of small, energy-dense biomass pellets. On-site pelletization of invasive biomass would require the use of portable biomass pellet mills, which are available from a number of industrial sources, but require expenditures of thousands of dollars.
Given the overwhelmingly negative impacts of invasive species on landscapes (Henderson et al. 2006), we wholeheartedly believe that it is appropriate to remove them and revegetate with native species. However, it has been pointed out that control of invasive species can result in unintended negative consequences (Kettenring and Adams 2011; Myers et al. 2000). For example, removal of a dominant invasive plant can create a “weed-shaped hole” to be occupied through reinvasion of the same or different invasive species (Buckley et al. 2007) through release of bound resources (e.g. sudden increase in canopy gaps) and/or removal of direct suppressive interspecific interactions (Kettenring and Adams 2011; Thompson et al. 2001). Also, invasive plants have become the preferred habitat for native fauna in some rare cases [e.g. the endangered southwestern willow flycatcher in invasive saltcedar (Tamarix spp.)], and sudden removal of such invasive biomass could have direct detrimental effects on these species (Zavaleta et al. 2001). Also, as pointed out above, the use of heavy harvesting equipment in patchy invasive populations could cause collateral damage to resident vegetation (Zarnetske et al. 2010), limiting capacity for natural regeneration. The potential for negative consequences of large-scale eradication underscores the necessity for careful restoration following invasive plant harvest.
Because many invasive plants can establish from small vegetative fragments, harvest crews and heavy equipment operators would need to be extremely careful to avoid leaving behind material that could re-invade the area (Boland 2008). And unless pelletized or otherwise densified on site, transport of invasive biomass may become a vector for escape of viable propagules along the route or at the loading or unloading sites. Many invaders can establish from small vegetative fragments—for example, A. donax can readily sprout from stem or rhizome fragments with a single node (Boose and Holt 1999)—and we know that vehicles are a major dispersal vector for many invasive plants (von der Lippe and Kowarik 2007, 2008). In addition, disturbed roadsides and other transportation routes are highly susceptible to invasion, and invasive plants that establish along roadsides are often able to migrate along highways for long distances, sometimes spreading into adjacent undisturbed areas (Christen and Matlack 2009; von der Lippe and Kowarik 2007).
Restoration plans would need to be targeted specifically to the ecosystem in question, potentially correcting any ecosystem effects of the invader (e.g. soil salt accumulation in sites invaded by saltcedar or other halophytic species; Zavaleta et al. 2001) prior to revegetation with native species. Costs associated with large-scale restoration could be extremely high (Gerla et al. 2012), and include not only the native plant material, but also the time and expertise of a restoration ecologist during the planning and monitoring stages, and the labor of a large planting crew. Any revenue from the sale of the biomass after transportation to the biorefinery could be depleted after accounting for the cost of restoration.
As an example, the cost of controlling and revegetating areas invaded by A. donax has been estimated at $25,000 acre−1 (0.4 ha) (Giessow et al. 2011). Yield for A. donax in dedicated biomass plantations has been estimated at 16 t ac−1, with a target price of $30–$50 dt−1 of biomass delivered to the biorefinery (Leightley 2012). The sale of this biomass, at approximately $800 acre−1, certainly does not justify the cost of its removal, from a purely profit-driven standpoint.
The US Plant Protection Act provides for a noxious weed list that operates at the federal level to ban the import, transport, and sale of over 100 plant taxa into and within the country. Furthermore, the 50 US states, through their respective noxious weed laws, collectively prohibit an additional 620 taxa (Quinn et al. 2013). While these statutes are intended to prevent the introduction and establishment of live plants or propagules, many states prohibit the sale or distribution of noxious weeds in any form. Therefore, the harvest, transport, and sale of declared noxious weed biomass within or across states may be technically illegal. However, most invasive plants occurring outside of cultivated landscapes are not regulated by noxious weeds laws (McCubbins et al. 2013; Quinn et al. 2013), so there is likely to be a wide selection of invaders not subject to noxious weed regulations in any given state. Still, these regulations must be considered and possibly changed before proceeding with the removal, transport, and sale of invasive biomass.
The Energy Independence and Security Act established the federal RFS. Under the RFS, the EPA assesses whether a proposed biofuel will qualify as a renewable fuel based upon an energy balance equation with respect to greenhouse gas (GHG) reductions. In a June 2013 EPA rule qualifying A. donax and P. purpureum as feedstocks under the RFS for production of cellulosic ethanol, the agency will require the fuel producer to demonstrate no significant likelihood of the plants spreading beyond the intended area of cultivation (Federal Register 2013). As such, plants harvested from outside the intended area of cultivation (that is, escapes) would not qualify as a renewable fuel under the RFS. Based on this rule, one could assume that the EPA is unlikely to approve under the RFS other potentially invasive feedstocks unless cultivated with confinement procedures such that the plant is unlikely to spread. Moreover, the agency has not signaled a willingness to qualify a harvested invasive plant under the RFS, perhaps due to the perverse incentive for unscrupulous actors to introduce or otherwise maintain known invaders for their profit potential (Lambertucci and Speziale 2011; Nunez et al. 2012).
An optimist’s vision
Despite the challenges discussed above, it is theoretically possible that invasive plant biomass could be sustainably harvested as a contributing, but probably not primary, feedstock for the lignocellulosic ethanol industry. Assuming invasive biomass will continue to be generated through ongoing control programs, we can envision optimistic models for the productive use of this biomass.
One could imagine a consortium of landowners that organized to sell the invasive biomass on its collective land, potentially generating enough biomass to satisfy demands of a small local biorefinery for a time. Such landowners would have a vested interest in improving the quality of their land through invasive species removal and restoration and may be willing to pay the up-front costs to fulfill this interest. Noxious weeds laws would further motivate this action in states like Montana that require eradication on private land. If noxious weeds laws are improved to better represent invasive species in natural areas (McCubbins et al. 2013; Quinn et al. 2013), a greater number of landowners may be required to eradicate. Recouping some of the costs of eradication through sale of biomass would be attractive to many landowners—especially if accompanied by a potential government incentive payment. However, as mentioned above, noxious weeds laws currently prohibit the transport and sale of listed species. Statutory exemptions would be required for the transport and sale of noxious weeds for bioenergy, and would ideally include requirements to prevent propagule escape throughout the harvest and transportation processes. Furthermore, despite EPA’s recent rule signaling its reluctance to add to the invasive species problem, it is possible that the agency could establish a generic RFS qualification for invasive biomass, provided it is not intentionally planted and is harvested as part of a certified invasive species restoration effort. Bioenergy sustainability certification groups are beginning to form, and could potentially provide third-party verification for these efforts (Endres 2013).
A complementary model would involve creation of a coordinating body that could help to synchronize the efforts of multiple regional control or eradication projects, and direct the collective biomass to storage and/or processing centers prior to shipment to the nearest feedstock-appropriate biorefinery. These coordinating bodies could also help to source personnel and equipment for large-scale eradication and restoration projects. Of course, securing adequate funding for these coordinating entities is a serious concern in this era of federal sequestration and general budget austerity by states/localities. Nonetheless, given the high cost invasive species impose on the economy (Pimentel et al. 2005), the relatively minor expenditures required to establish and operate coordinating entities offers an excellent return on taxpayer investment.
Alternate forms of energy
We have focused our discussion on the use of invasive biomass for conversion to ethanol, but this does not preclude the conversion of invasive biomass to other forms of energy. A few small-scale projects have tested the feasibility of conversion of invasive biomass to alternative sources of energy. One of these found that Tamarix spp. biomass could be efficiently gasified, or converted to a combustible gas to create electrical power (Nielsen et al. 2011). However, a similar study using Tamarix spp. and Elaeagnus angustifolia stated that the cost of generating liquid fuel from these two invaders through gasification would be greater than that of natural gas or coal, and therefore would be unmarketable (Rindos et al. 2012). However, the fuel properties of these materials were similar to other woody species and therefore the potential for combustion was strong (Rindos et al. 2012). In fact, we see combustion as the most viable option at present for conversion of invasive plants to energy sources. While the need for processing (drying, size reduction, densification) (van Loo and Koppejan 2008) may present some barriers, invasive biomass could drop into the existing supply of biomass being co-fired with coal in the huge network of electrical power plants across the country (Baxter 2005), and this is currently being done in some municipalities (Goldstein 2013).
Perhaps as the biomass-to-ethanol industry matures over the next half century, technical innovation may reduce the currently insurmountable logistic and economic concerns associated with utilizing existing invasive feedstocks for viable sources of liquid fuel. In the meantime, however, this concept of invasives-to-energy currently thrown about with unbounded optimism warrants careful scrutiny grounded in a life-cycle analysis of the bioenergy supply chain.
We acknowledge and thank James S.N. McCubbins and Elise C. Scott for their intellectual contributions toward this concept, and members of the Sustainable Bioenergy Network for stimulating discussions on the topic. This work was funded by the Energy Biosciences Institute.