**1. Introduction**

Forest biomass can provide additional revenue for forest managers and supply a bioenergy market to reach renewable energy targets. Using forest biomass for bioenergy has become an integrated part of forestry and a priority for all biomass utilization projects [1]. Large quantities of forest biomass are sustainably used around the world to generate heat, steam, and electricity through gasification and combustion processes [2,3]. Opposed to global bioenergy trends [4], there is little public or political support for the use of forest biomass in Australia [5]. With the lack of economic incentives, most of the non-merchantable forest harvest residues are burned in the forest or left to decompose on site. The bioenergy market represents only 4% of total energy production in Australia [6] and, of this, forest biomass is 25%, and bagasse is 29% [6,7]. Other renewables, including hydro (16%) and wind (12%) energy, have increased in the last decade [7]. Establishing a sustainable bioenergy market from biomass in Australia requires consideration of the availability of forest biomass, sustainable harvesting, the cost of the biomass supply chain (BSC) and the greenhouse gas (GHG) emissions related to bioenergy production. The BSC encompasses many technical, economic and environmental constraints associated with harvesting, handling, storage, transport and conversion facilities [2,8–10]. Satisfying both environmental and economic objectives is a significant consideration for establishing the bioenergy industry [11].

The potential of forest biomass is categorized in the literature [12] into four sequential terms according to calculations needed in the assessment of biomass for biomass energy potential. The theoretical biomass potential relates to the annual yield of forest biomass per unit of area and can be considered the upper-bound of the potential [12]. Restrictions introduced by alternative biomass uses and efficiency at a biomass collection level are included. The term is also modified in the literature [13] as biologically available biomass and includes a range of ecological and economical reductions of the initial biomass to determine what is usable. Alternatively, a New Zealand literature example [14] refers to a similar term as total recoverable residue volume. Generally, the theoretical biomass potential captures all restrictions at a stand or production level. Secondly, the available biomass potential describes the energy that technically and economically can be harvested and transported for energy purposes before conversion [12]. This includes some limitations related to harvest machinery, truck size and transport distance. The term captures measures and restrictions of biomass energy at a harvest and transport level. Next, the technological biomass potential is defined by the energy that can be produced bound to conversion technology, the capacity of the conversion facility and the efficiency [12]. The term applies to research focused on-site identification for potential power facilities when inputs around available biomass, facility capacity and technology, and maximum allowable transport distance are known. The technological biomass potential accounts for technical restrictions on the available biomass and efficiency of the technology at an energy conversion level. At last, the economical biomass potential is the part of the energy that is distributed with respect to competing energy sources [12]. The term includes the energy production cost and the capacity of the facility, or rather the profitability of the proposed investment. A whole range of cost estimates of the entire supply chain can be included to determine if the economical biomass potential is feasible at a distribution level. The term environmental biomass potential is added to the four sequential terms described in the literature [12]. The environmental biomass potential sits at the same level of analysis as the economical biomass potential but opens perspective in emissions and other environmental measures related to bioenergy production. The term includes emissions that occur during energy production and non-biogenic emissions due to the use of machinery that affects the carbon-neutrality of the bioenergy system. In similarity to its economic counterpart, it pays respect to competing for energy sources and evaluates bioenergy in comparison with a reference or fossil energy system.

Forest biomass is considered a sustainable source of energy; however, only when grown and harvested in a sustainable manner [15]. The sustainability of forest biomass production systems must consider that forest harvest residues help sustain the fertility of the site, regulate water flow and maintain plant, microbial and animal biodiversity [1,16,17]. These considerations are classified under the constraints of the theoretical biomass potential. Additionally, economic sustainability constraints such as fuel versus food, efficient energy balance, and social constraints determine what is or is not a sustainable resource. International forestry guidelines and forest certification ensure sustainable forest managemen<sup>t</sup> but need to address the specific impact of the additional harvest of forest biomass [18]. In Australia, guidelines such as the Forest Stewardship Council (FSC) and Responsible Wood, are inclusive of forest biomass harvesting to the extent of encouraging the harvest and respecting environmental values [19,20]. However, none of these guidelines provide directions on environmental, economic and social concepts of the theoretical biomass potential in Australian forests.

The overall cost of the BSC includes the economics related to harvest, collecting, transport and conversion of forest biomass. The cost of an economically sustainable BSC is heavily influenced by operating costs and the need to maintain a supply of forest biomass [21,22]. The economics related to harvesting technology and collection methods [23] as well as biomass handling and storage are critical

to ensure the reliability of supply [8,23]. The biggest cost contribution comes from transport which is determined by the quality and moisture content of the biomass and the mode of transport [24]. Forest biomass densities are generally low (400–900 kg/m3) and moisture contents high (>50%), which results in transportation contributing 20–40% of the BSC cost [8,22,24–26]. Many of these cost measures determine the available biomass potential. Processing costs for converting forest biomass to bioenergy are determined by the technology used, the capacity of the plant, and the consistency/quality of supply [22]. In general, biochemical and thermochemical techniques are the most suitable for the conversion of forest biomass [27,28]. Processing into liquid or gaseous fuels can be done by biochemical conversion while combustion, gasification, and pyrolysis can be used to produce fuels, heat and electricity [11,27–30]. However possible, the forest biomass to the biofuel supply chain is still at a pre-commercial level in terms of technology [8]. These characteristics, together with the large, complex equipment required, and often the need for different transportation modes, create complex economic and logistic issues and result in losses of the technological biomass potential [8,22,31,32]. Several measures are in place to define the technological biomass potential. The input of energy, or primary energy that is already delimited by theoretical and available biomass potential constraints throughout the supply chain, and the type of technology and conversion efficiency, define how much useful energy we can ge<sup>t</sup> from the primary energy source. Different types of energy products are then sent to customers through the grid, networks or channels of distributors, wholesalers and retailers as net energy. In order to substitute for fossil fuel, it is important to check the energy balance of the proposed bioenergy system [33,34]. The net energy ratio, for example, is an indicative measure to ensure the system does not use more energy than it creates [33,35]. The ratio is defined by the produced energy/consumed energy ratio that equals the primary energy at the gate. The ratio is a supportive measure of the technological biomass potential and provides additional insight into the profitability of the system and thus the economical biomass potential.

The economic potential of the forest biomass supply is a function of the biomass availability and the profit during sequential steps of the value chain [8]. These measures aim to determine if the electricity production cost of a bioenergy facility is lower than the conventional power facility. Factors like internal return rate and net present value define profitability measures of the economical biomass potential and are indicators that allow us to accept or reject a proposed investment [12]. Several cost parameters should be considered to compare bioenergy and fossil energy systems. These include equipment and capital cost, the construction cost of power line and grid connection cost, stumpage cost for forest biomass, supply chain cost and additional maintenance and administrative cost [8]. Each of these costs contributes to the energy production cost or cash outflow in the net present value of the investment. The impact of the value chain can be extensive, and there is an unavoidable degree of uncertainty in the supply of forest biomass that makes the estimation of cost and profit hard to predict. Optimization and simulation models are tools that can provide further insight into the economical biomass potential, where the inclusion of a reference fossil fuel scenario should be considered [8].

Using forest biomass for bioenergy reduces GHG emissions compared to the use of fossil fuels and thus mitigates climate change. Biomass from a sustainably managed forest can be considered as a carbon-neutral energy source [36–38] since the carbon emitted during the energy conversion process is fixed relatively quickly during subsequent photosynthesis and tree-growth [37,39]. Life cycle assessment (LCA) of sustainably-sourced forest biomass for energy shows a period of climate warming impacts as a result of the delay for the CO2 to be captured by new tree growth [36,40–42]. Evaluating environmental impact in an LCA must be comprehensive [22] and include all non-biogenic carbon emissions from the consumption of fossil fuels for production, transport, harvesting, collection, and pre-processing [43]. Other factors such as other atmospheric pollutants (e.g., methane) and the e ffects of direct and indirect land-use change a ffect the value chain and are important to the result of the LCA [11,43]. Land-use changes, due to replacing crops with intensive forest plantations, can increase GHG emission [11,44] but a change from crops that demand high fertilizer and pesticide inputs to a forest that produces biomass for bioenergy can reduce GHG emissions [45–47]. Including emissions

as a measure of the environmental biomass potential in balance with energy cost as a measure of the economic biomass potential in value chain optimization is becoming increasingly important for the sustainable utilization of forest biomass [22].

This case study review aims to identify gaps and approaches used to assess the potential of forest biomass for bioenergy generation in Australia and is structured around the hierarchical nature of biomass potential as defined in [12]. Evaluating Australian studies on forest biomass for bioenergy in Australia in their methods used to determine the theoretical, available, technological, economical and environmental biomass potential. In the next section, we explain the scope and methods that define the extent of the literature review. The following five sections review how research achieved the respective levels of detail in forest biomass potential. We discuss distinctive features and limitations in such a way as to make recommendations with regard to measures of forest biomass potential.
