**1. Introduction**

Compared to separate production of heat and electricity from fossil fuels, combined heat and power systems (CHPs) can potentially allow for significant reductions of climate change impact [1,2]. In Europe, coupling heat and electricity generation from renewable sources is also one of the most cost-effective decarbonization strategies [3–5]. In particular, solid biomass has attracted increasing interest by policymakers and investors especially due to the high availability of local biomass from forests and wood processing industries in some regions [6]. The environmental performance of biomass-fueled CHPs depends not only on the type of technology but also on the type of biomass, its supply chain, and the environmental impact categories in focus [7,8].

Mature CHP technologies using solid biomass as fuels have often shown restricted fuel flexibility, limited electric efficiencies, and high particulate matter emissions [9]. To overcome these three limitations, a novel technology was developed during the H2020 HiEff-BioPower project [10]. This novel technology (see Figure 1) is based on a fixed-bed updraft gasifier coupled with a novel primary gas treatment zone, a novel gas cleaning unit (GCU), and a solid oxide fuel cell (SOFC). Its current technological readiness level is between 4 and 5 (based on the definition adopted by the European Commission [11]). The biomass fuel is converted into product gas in the gasifier. Syngas derived from biomass (e.g., wood chips) contains HCl, H2S, and tars [12] making it not suitable for direct utilization in fuel cells [13], which require purified gaseous fuels. Therefore, the syngas from the gasifier is first pre-treated in a primary gas treatment unit (first tar reforming step) and then purified in the GCU. The GCU is one of the key innovations of this technology. It combines the use of ceramic filter candles and sorbents [10]. Syngas cleaning is processed in five steps: primary tar reforming, high-temperature particle filtration, HCl sorption (after cooling the product gas), H2S removal by sorbents and tar reforming (after reheating). After re-heating, the product gas is then fed into the SOFC unit to generate electricity. The off-gases from the SOFC unit are then burnt in a catalytic afterburner to recover heat. Most biomass CHPs are suited for medium and large-scale plants (1–100 MWel). The HBP is available also in small size (about 200 kW of electricity output) [9]. Among biomass technologies of this size, one of the main competitors is the organic Rankine cycle (ORC) [14].

**Figure 1.** Concept of high-efficiency bio-based power (HBP) technology. GCU = Gas cleaning unit. SOFC = Solid oxide fuel cell.

At this stage of the Hieff-BioPower project, the assessment of the environmental impacts of the current design configuration can help with minimizing impacts at an early stage of the technical HBP development. In particular, the literature reports a few life cycle assessments (LCAs) of heat and power from SOFC-based CHPs and several ones of CHPs involving biomass gasification processes but no one on their combination (in October 2019, from Scopus database searching in TITLE-ABS-KEY). These studies provided the following main findings: (1) the investigated CHPs present lower impact in terms of climate change compared to conventional technologies [2,15] and (2) the biomass fuel production has the highest contribution to total life cycle impacts [16,17]. These studies also highlighted several methodological uncertainties of LCAs that can lead to significantly different results. Such uncertainties are mainly linked to the multifunctional nature of the CHPs. A CHP is a system producing two products, heat and electricity. Depending on the goal of the LCA, it may be necessary to apportion the overall impact of the system to each of the co-products. Finding the right criterion for the allocation of impacts to each co-product is generally understood as a multifunctionality problem [18]. When a multifunctionality issue is encountered, the practitioner has to properly select the functional units and allocation methods [19,20]. The selected criterion could affect the outcome of the LCA significantly and, for this reason, this selection is broadly discussed in the literature [21,22].

The environmental LCA presented in this study has a twofold aim: (1) to identify the main sources of the environmental impact of this new technology and (2) to assess its ecological competitiveness compared to the separate production of heat and electricity and one of its main competitors, i.e., Organic Rankine Cycles (ORC). Moreover, this case study is used to analytically discuss the influence of the allocation method in the LCA results for CHP plants and provide methodological recommendations for better allocation practices.

#### **2. Materials and Methods**

#### *2.1. Goal and Scope Definition*

The LCA has been conducted according to ISO 14040:2006 and ISO 14044:2006 [18,23]. The intended audience of this LCA consists of technology developers, researchers involved in the field of bioenergy and LCA practitioners. An attributional LCA (ALCA) approach is followed since the goal of this study is to identify the activities within HBP causing the highest contribution to the environmental impacts, and not the consequences of changes in these activities [21,24].

Two technologies are considered for environmental comparison: (1) a combination of the electricity mix (EMIX) from the German national grid plus heat provided from a natural gas boiler (NG) and (2) biomass-based organic Rankine cycle (ORC) CHP.

As the ORC CHP has a different heat to electricity ratio compared to the HBP, the definition of two functional units was preferred to the definition of a single functional unit with a fixed heat/electricity ratio. Hence, two functional units were defined as follows: 1 kWh of electricity or 1 MJ of heat.

The HBP technology finds one of the main strengths in its fuel flexibility [10] since it can operate with various biomass feedstocks in the forms of chips or pellets. To explore the effect of different feedstocks on the environmental impacts of the HBP CHP technology, this study explored the use of three different types of biomass fuels: wood chips, wood pellets, and *Miscanthus* pellets. The operation with wood chips was considered as the baseline scenario (WC), while the operation with wood pellets (WP) and the operation with *Miscanthus* pellets (MP) as alternative scenarios. The baseline scenario with wood chips was also used for comparison with the competing technologies, i.e., the ORC technology (fueled with wood chips as well) and the combination of grid electricity plus natural gas boiler. Additionally, this last competing option was also compared to the WP and MP alternative scenarios.

Figure 2 shows the process diagram of the HBP product system. The system boundaries follow a cradle-to-gate approach. As shown in Figure 2, all the life cycle stages from the extraction of the raw materials to the final dismantling and waste treatment are included. The final distribution and consumption of the products, i.e., heat and electricity, are not included in the LCA. After the power plant is dismantled and parts are recycled, the use of the recycled materials is outside of the system boundaries. Biomass transport stages from the forest to the processing plant and from the processing plant to the HBP plant were included in the study. The transportation of plant components (e.g., the gasifier) from the production site to the power plant location and the construction activities of the plant were not included in the analysis. The exclusion of these activities was based on their expected minor contribution to the total environmental impacts, as also found in similar studies, for example [16].

**Figure 2.** Flowchart of the HBP product system, including system boundaries (dashed lines).

The temporal scope of the study is placed in the near future (the next 5–10 years) when the HBP technology should be commercialized. The HBP is assumed to be installed in Germany, being the country with the maximum potential sales for the HBP technology in Europe [25]. Nevertheless, some components for the HBP (e.g., the gasifier) might also be manufactured outside Germany (in other EU countries).

Seven mid-point impact categories were selected and the adopted impact assessment models for each impact category were selected following the ILCD recommendations [26] (see Table 1). Climate change (CC) and depletion of mineral, fossil, and renewable resources (MFRD) were chosen because they are considered top priorities in the current societal and political challenges [27]. Particulate matter (PM) and photochemical ozone formation (POF) are selected because of their relevance to the energy sector [28]. Acidification (AC), Terrestrial eutrophication (TE), and Water resource depletion (WRD) were selected because of their relevance for agricultural systems, and therefore for biomass production [29].

To assess the robustness of the results, two sensitivity analyses were conducted. As anticipated in the introduction, a comprehensive sensitivity analysis was performed on the allocation choices to explore their influence in the outcome of the LCA (and as recommended by ISO [18]). The second sensitivity analysis was performed to explore parameters that are potentially sensitive for the results and that might environmentally improve or make less attractive the technology in the future.


**Table 1.** Selected impact categories and models.
