1. Introduction
The climate and energy policy of the European Union (EU, including its long-term vision of striving for EU climate neutrality by 2050), and regulatory mechanisms that are designed to stimulate the achievement of its outcomes in the coming decades have a significant impact on the shape of Poland’s domestic energy strategy. Poland’s energy policy until 2040 (PEP2040) sets out the framework for the energy transformation in Poland. It contains strategic decisions that are related to the selection of technologies that are designed for developing a low-emission energy system. PEP2040 contributes to the implementation of the Paris Agreement that was concluded in December 2015 at the 21 Conference of the Parties to the United Nations Framework Convention on Climate Change (COP21), taking into account the need to conduct the transformation in a fair and solidary manner.
PEP2040 contains a description of the state and conditions of the energy sector, and points to three pillars on which the eight specific objectives have been based, along with the measures that are necessary to implement them and outlines the strategic projects. The article refers to the second pillar of good air quality and the seventh goal of decommissioning from fossil fuels by investing in the transformation of the heating system sector, which should use local energy sources such as agro-biomass [
1].
In accordance with the objectives of the Poland’s domestic and EU policy, a significant increase in the ratio of renewable energy sources in heating is assumed in the years 2021–2030 [
2,
3,
4].
In Poland, the ratio of ineffective heating systems is still about at a level of around 90 percent [
5]. The high proportion of ineffective systems applies mainly to the so-called district heating in poviats. Small towns with a population of up to 20,000 have heating systems with a capacity of 10–20 MWt and only 14 percent meet the standard of effective heating system. The situation is not much better in towns up to 100,000 residents. Here, less than 30 percent of the systems are classified as effective. In large cities, over 500,000 inhabitants, this level reaches 100 percent [
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17].
The potential to attain or preserve the status of an effective system requires heating companies to undertake a variety of activities that are mainly associated with investment related to the areas of modernization, repowering or expansion of existing energy sources, and more often also the construction of completely new dissipated heat sources using new technologies and fuels [
17].
The main direction of changes in the heating sector were also outlined in the National Energy and Climate Plan that was developed for the period 2021–2030 [
4]. It stipulates, among other things, that the configuration of the size of heating sources should take into account an increase in the energy efficiency of 23%, while ensuring the ratio of the final energy that is derived from RES at the level of 21–23%. This will translate in the coming years to a significant reduction in the power that is ordered in heating systems, which can only be balanced by connecting new customers. Another important recommendation indicates that in heating and cooling systems, the ratio of renewable energy sources in heat generation should increase by 11% by the year 2030 in comparison with the year 2020 [
4].
The generation of electricity in Poland is based on hard coal and lignite, which in-creases greenhouse gas emissions. Actions should be taken to promote renewable energy sources, whose supply is unlimited. As coal accounted for 77% of all energy carrier use in Poland in 2019, it is argued that the ratio of renewable energy sources in the energy balance in Poland is insufficient [
3]. The consumption of biomass fuel for heat production in 2019 was in total 9.23%, and in cogeneration it was almost 11% [
18].
The region’s economic development is relative to the access to energy, and conventional resources such as coal, natural gas and oil are no longer sufficient to meet the growing demand of the economy. Moreover, non-renewable energy sources contribute to climate change, which makes it necessary to look for alternative possibilities, including solutions in renewable energy (RES) [
19,
20].
The ratio of energy that is derived from biomass, solar energy, energy from water, wind, and geothermal sources in the structure of energy production in Poland has been increasing in recent years. RES accounted for 16.0% of the total energy production in Poland in 2019 [
21,
22,
23,
24]. This situation imposes the greater use of renewable energy with the purpose of increasing the proportion of RES in energy production.
In recent years, more and more attention has been paid to identifying the sources of by-products from agricultural production. Theoretically, agricultural biomass has the greatest potential for renewable energy purposes [
25]. Straw forms the most common of the resources that are classified as this type of biomass. However, it should be remembered that some of the straw resources are applied in agriculture, mainly as the supply of soil with carbon and for feeding animals and litter in rearing animals [
26]. In general, Poland offers a considerable potential for plant biomass, and it is equal to 305.8 thousand TJ per year [
19], that could be utilized for heating purposes without any risk to food production, the use of hay, as well as the need for: litter, straw for litter, straw for tillage [
26].
Biomass can also undergo thermal treatment processes such as gasification and pyrolysis to produce gaseous fuel and carbonate [
27,
28]. The gaseous product in the form of syngas can be used to supply district heating boilers as main fuel or auxiliary fuel [
29,
30] or even used in industrial processes, e.g., in industrial furnaces [
31,
32,
33].
Solid biomass forms the dominant RES in Poland and it accounted for 65.6% of the total production in 2019, with 55% of this total was that which was applied by producers without conversion to another energy carrier. Solid biomass is a leader in the production of heat from renewable sources in Poland (90.1% in 2019) and also accounted for 25.1% of electricity production in 2018, with only wind energy representing a larger proportion in total energy production [
1].
Actions that are designed with the purpose of the development of renewable energy sources are dedicated to reducing the emissions of the heating sector and diversifying the energy generation structure. This will lead to a reduction in the intensity of fossil fuel use and a reduction in the state’s dependence on fuel imports, which in the long term will improve energy security [
1].
Many EU countries are located in high latitudes and at high altitudes, which means they experience long, cold winters and face considerable requirements in terms of heating. This has resulted in the priority development of biomass as a source of renewable heating in households, industry, and district heating networks. The ratio of heat supply that is derived from renewable heat sources has significantly increased in the EU countries, i.e., from 10% in 2004 to 20% in 2017. In total, biomass heating accounts for over 10% of the total final energy consumption in the EU-14 countries. Latvia depends on biomass-derived heating to the greatest extent (33.21%), followed by Finland, Sweden, Estonia, Denmark, and Lithuania (all with over 20% ratio of biomass in heat production), followed by Croatia, Austria, and Romania (over 15%). Meanwhile, the percentage of energy that is derived from biomass is at a level of around 11% in Portugal, Slovenia, Bulgaria, the Czech Republic, and Hungary. Due to differences in geographic location, weather conditions, natural resources, policies, technologies, approaches to the use of environment, and sustainable development objectives of EU countries, supply and demand and levels of development of biomass heating systems vary considerably [
34].
There are significant differences in the percentage in biomass use for energy applications in EU countries, which can be divided into five categories of comprehensive zones, which are a key development category (Finland, Sweden, Denmark, Estonia, Lithuania, and Latvia); a resource priority category (Austria, Croatia, Bulgaria, Romania, and Slovenia); a policy-oriented category (Germany, Italy, Portugal, and Cyprus); a category with good potential (Czech Republic, Hungary, Greece, Spain, Poland, France, and Slovakia); and a poorly-developed category (United Kingdom, Netherlands, Belgium, Ireland, Luxembourg, and Malta [
35].
In Spain, research is being carried out by the application of a model that is designed for the assessment of sustainable biomass heating along with a methodology for assessing district heating [
36], and a methodology for the potential of biomass heating systems in rural areas [
37].
The demand for biomass raw materials was assessed in order to expand the district heating system in urban areas. For the purposes of the assessment, a simulation model with an annual planning horizon and daily time steps was developed for the existing heating plant with a capacity of 6 MW. A total of three types of raw materials were assessed: wood, pellets and briquettes, and a mixture [
38]. Another insight in this area deals with the impact on the environment of district heating that is supplied from biomass in Italy [
39].
However, to this date there has been a scarcity of detailed studies in the literature that focus on comprehensive methodology that is applicable to the research or analysis of the energy potential of biomass in individual municipalities, taking into account the specific local conditions [
19,
40,
41,
42,
43]. Although such research is important for the diversification of local energy sources, which ultimately affects the country’s energy balance, the number of in-depth scientific studies on this subject is small [
44,
45]. Therefore, it is justified to take up this topic, as this forecast may affect investment decisions regarding energy generation, and, consequently, increase the ratio of renewable energy sources in the local generation structure [
46,
47].
This paper aims to identify the energy potential of solid biomass in one of the Polish regions of the Opolskie Voivodeship.
2. Materials and Methods
The research that was carried out by the authors on the study of heating systems in Poland has been conducted to include various aspects of the current problems in the area. For the purposes of the article, research has been presented in a narrow sense. This research has been reduced to specific activities that are related to the study of a selected heating system with the possibility of application of biomass fuel, along with the determination of: its potential, storage, and delivery possibilities. The article aims to present unused technological and logistic processes in the studied reality. The topic that was adopted in the research will allow to define a model approach to achieve sustainable fuel economy.
The following aspects were taken into account in the research: biomass demand in a given area, biomass potential, optimization in the area of organizing transportation routes to the customer, and typing the effective location of biomass storage (logistic centers) [
48,
49]. The complete implementation of these tasks involves a number of assumptions to be taken into account resulting in, among others: the location of heating plants in relation to each other in a given region, the location of plants in relation to individual systems of transportation infrastructure and its specific subsystems, and the state of needs and possibilities of biomass application in individual poviats/communes. The environmental, social, and legal obstacles that exist in the regional environment need to be also considered in such considerations.
Research was carried out into district heating systems in Poland. Heating plants (selected according to type of fuel use, technology, and the production capacity of heating systems) were selected for the purposes of this study. A randomized survey was determined on the basis of small heating systems in the installed capacity; in the range from 10 MW to 50 MW forms the most common group of heating systems existing in Poland in 2020 (the exact percentage is 44.6%) [
50]. An expert selection method (targeted selection) was used as the current study is a pilot that is designed to test a research tool for district heating systems. The sample size: 1 out of 16 voivodships, where there are 11 heating systems from the range [10; 50] MW. Additionally, the implementation of the assumptions took into account the potential of agro-biomass in a given area. The data were collected: from the Central Statistical Office, ARIMR, and interviews with farmers (Delfi method). The correctness of the selection was verified on the basis of a comparison with previous research that was carried out by these authors.
Bearing in mind the aspects that are mentioned above, the first step involved the development of an integrated program of biomass utilization in the area of communes and towns that were located in the Opole voivodeship. The overall analysis was divided into five steps (
Figure 1) [
51,
52,
53].
Stage one—analysis of technologies and fuels that were applied in heating systems in Poland combined with the selection of the investigated location. This included the analysis and selection of the location of the main sources of district heat supply and an assessment of the efficiency of equipment that was applied for the conversion of chemical energy to useful forms of energy.
Stage two—formed an introduction to the analysis and calculation of the technical potential of biomass separately for each voivodeship in Poland and selected communes in a selected area in the Opolskie voivodeship. For the purposes of this article, the assessment of the biomass potential was estimated by taking into account biomass that was represented by straw that was derived from agriculture. It was assumed that in each source of biomass, it is primarily applied for non-energy purposes, i.e., for industrial, nutritional, litter demand, etc. [
26]. The analysis was based on the published data of the Central Statistical Office from the assessment of land use and sown area as well as forest land use, that was collected in 2019. Renewable resources of waste biomass have been considered as an amount of energy that can be derived from biomass throughout the year. The analyzed data included: the sown area of specific cereal types, the livestock population by species and utility groups, and areas that were characterized in terms of soil quality. It was assumed that the efficiency of obtaining energy from biomass was 80% [
54,
55,
56]. Additionally, for the purposes of estimating the annual energy yield from straw, it was assumed that the grain/straw ratio was: 0.8 for wheat, 1.4 for rye, 0.9 for barley, 1.05 for oats, 0.95 for triticale [
54], and only 30% of produced straw forms a surplus that could be utilized and be used for energy purposes [
26] with the calorific value of straw (with a moisture content of about 20%) on average 15 GJ/Mg [
57,
58].
The process that was applied for estimating the energy potential of the volume of biomass forms an element of a multi-stage and multi-criteria analysis, which used, among others: statistical databases of the Central Statistical Office, ARiMR, agricultural census [
51], and a qualitative forecast (the Delphi method) [
18,
59]. The Delphi method applied a group of experts who were farmers from eight adjacent communes. Expert interviews and questionnaires were conducted with the farmers in the investigated area of the Opolskie Voivodeship in order to refine the statistical data. At the same time, a more accurate calculation of the technical potential of biomass was conducted. The surveys were conducted several times and the experts could not interact with each other. The expert group was limited to individuals with extensive substantive knowledge and experience in the field of harvesting and using agro-biomass in the last five years. Each of the experts substantively justified their results. Following the collection of the results and after the analysis was completed, the authors generated another draft of the survey—whose aim was to narrow down and refine the area of operation, and subsequently the survey was conducted again. This cycle was repeated several times until a unified way of describing the subject of the experts’ opinion was developed. As a consequence of the application of the Delphic method, the views of experts could be compared with the assumptions regarding the forms of using the agro-biomass in the studied area could be established.
Once the qualitative forecast was carried out using the Delphi method, the technical potential was calculated taking into account the available solutions and technical equipment that was available in the region. Formulae and data were utilized to calculate the technical potential of straw. The amount of straw production depends on the area of which specific plants are cultivated as well as grain yield. Straw is used for various economic purposes, and its surplus could prove practical in heating systems. However, this surplus depends on the following factors: type of soil, size of the farm, and type of farming (number of animals, type of litter etc.). In order to estimate the straw surplus in a specific commune, it is necessary to obtain data on the existing grain production or the size of the acreage. On the basis of the formulae 1–3 given below, the energy that can be obtained from straw was determined.
where:
Pz—grain yield,
Is/z—ratio of straw yield to grain yield,
Is/a—ratio of straw yield to land area,
Ins—indicator of grain surplus,
A—land area dedicated to grain production,
Zs—straw surplus,
Es—volume of energy generation.
By the application of the above formulas and the Delphi analysis of the obtained data, the forecasts were determined, and the technical potential was determined [
18], which offered the means to proceed to the further stages [
60].
Stage three—analysis and the characterization of natural and climate conditions, as well as heating systems. The area, soil classes, number of arable lands, climate, farmlands, forests, beneficial climate and soil conditions, as well as the available results of the analysis of the area subject to nature and landscape protection in the given communes were examined.
Stage four—multi-variant analysis of looking for the optimal location for the bio-mass supply.
The distance minimization method is most common approach that is applied for solving location problems. For this purpose, graphs theory was used, which is currently one of the dominant mathematical methods that is used, among others, in format, automatics, and also in logistics. As a result of its application, it is possible to establish the shortest paths between a set of points (graph vertices) based on the given weights. For this purpose, tools such as the Kruskal algorithm, the Prim algorithm, the Dijkstra algorithm, the A algorithm, or the Floyd-Murchland algorithm are used.
Graph given as G = (
V,
E,
w(
e)) forms a simple set of vertices vi that can be connected by edges
ei in such a way that each edge has its end and beginning at the vertex points of the graph [
61]. The characteristic features of graphs include the edge weights that are expressed by the function
w(
e), which gives them attributes in the form of these weights. The edge weights can be a real number and express, among other things, edge intervals, frequency, and quantity of deliveries. Searching for the shortest route is about finding the so-called Minimum Spanning Tree (MST), i.e., one whose sum of weights is the smallest of all that are possible:
In this study, the Prima algorithm was utilized for the determination of the MST for graphs representing locations of biomass production and places of its conversion to energy applicable for utility purposes, as well as the road network that connects them. This algorithm offers the means to build an MST according to a specific procedure. The construction of the minimal spanning tree starts from any vertex of the graph, e.g.,
vi. Then, from the incident edges in vi, select the one with the lowest severity. Let it be an edge in the form {
vi, vj}. In each of the steps, the algorithm searches for the edges with the lowest weight connecting some vertex outside the set
L of edges that were considered in the previous step. The edge that is selected in a given step is added to
L and the process is repeated until a tree is obtained with a set of vertices that are equal to the set of vertices in the graph. The following notation has been adopted where: [
vi, vj, w] is the edge connecting the vertices (
vi, vj) with the weight equal to
w [
61,
62].
The article utilizes MST in two ways:
- -
A single-criterion method for minimizing the distance of biomass transportation from agricultural areas to the designated study area (eight communes + a single existing heating system), this study will not offer the possibility of the simultaneous determination of locations for more than one logistic facility. The method is simplified and additionally, an analytical-descriptive method should be used to assess the location,
- -
A multi-criteria analytical-descriptive method that compares design alternatives by the application of the simplest analytical methods. This enables the included location factors to be reduced to one criterion and the selection of the best location for biomass storage prior to its transport to the selected heating system.
Stage five—determination of transport scenarios with particular emphasis on the profitability analysis of the following alternatives: optimistic, moderate, and pessimistic.
In the literature, a lot of space is devoted to the methods of locating storage facilities in the logistics network. Taking into account the multifaceted nature of the localization issue as the criteria for dividing the methods of locating objects in the transport and logistics network, the following methods were applied [
63,
64,
65]:
- -
heuristic ones, that is ones that already account for the quality of a solution;
- -
single- as well as multi-criteria ones (derived from stage four of this analysis);
- -
multi-object ones, which take into account the possibility of simultaneous location of several warehouses and building a hierarchy in the logistics network of supply;
- -
analytical and descriptive ones.
5. Conclusions
The potential that is estimated in this study demonstrates that there is a possibility of using straw in heating systems on a larger scale than it occurs at present. Taking into account the conditions that are described in terms of the requirements and the possibility of a more extensive use of local biomass resources for the production of energy for district heating systems, its role can be forecast to increase in the years to come. Biomass seems to play the roles of a significant supplement to the heat and electricity generation market, in addition to new sources such as heat from thermal waste treatment installations as well as sources using other alternative fuels. However, logistics that are properly organized on the local market will determine an increase in the ratio of biomass use in heat production, which will allow farmers to organize the market for this fuel together with local heating plants and consequently, mutual benefits can be gained.
The article included a description of the conditions that are related to the use of agro-biomass for heating purposes. The study demonstrated that the examined rural area can soon be crucial for deriving solid biomass, and individual communes can become sustainable areas with simultaneous diversification of the currently applied energy resources. Besides, the analysis indicates favorable conditions for its application for the generation of district heating locally.
An integrated agro-biomass management program was developed in the area of municipalities/cities for a selected research sample taking into account the district heating systems in Poland. The analysis reported here was divided into five stages:
Analysis of the technologies and fuels that are applied in the heating systems,
Calculation of the biomass potential in all the voivodeships in Poland using data from the last five years (data from the Central Statistical Office and ARiMR) by the application of the Delphi Method for the selected research area,
Analysis of the characteristics of the studied area (including: natural and climatic, heating systems, arable land surfaces, and arable lands),
Multivariate analysis that was applied in search for the optimal location for biomass delivery. The Prima algorithm was utilized to calculate the MST. MST was applied by the application of single-criteria methods and multi-criteria in decision-making processes. The Prima algorithm serves for establishing the shortest routes to connect the selected locations in the investigated area. On the basis of these results, we can conclude that the proposed concept of the biomass transportation model within the selected area can be developed with further analysis concerning: the elaboration of biomass supply schedules in “just in time” system, building warehouses (related to their costs and specifications), considering alternatives to be taken into account in the analysis of the number of warehouses needed for the capacity of the investigated heating system,
The transport scenarios were determined, taking into account optimistic, moderate, and pessimistic alternative solutions.
Throughout the study, heuristic, single-multi-criteria, multi-object, and analytical-descriptive methods were used. In the next step, it would be necessary to consider the limitations concerning, among others: straw transport as well as the dry effect which may reduce the biomass potential in the future. Therefore, an integrated system for the diversification of the type of biomass (forest, agro) should be developed for the purpose of economic efficiency of the use of various renewable energy sources.
The research results demonstrate the possibility of transferring further research towards extending it to other renewable energy sources and developing a logistic system of integrated environmental management for heating systems in Poland in the direction of, among others, variations in the type of fuels, supplies, and technology changes.
The logistic system of biomass supply is designed individually for specific heating plants. In the multi-criteria analysis, the regionalization approach plays the decisive role for the economy, as its successful implementation can ensure the raw material supply for future biomass utilization purposes. The result of the MST analysis takes the form of a map with a list of optimal storage regions for the indicated locations of the heating system that apply biomass. Therefore, the actual locations based on the studies of the local diversity of biomass resources in the unit will lead to the selection of even more beneficial biomass storage locations, i.e., those that are in the immediate vicinity of the selected heating system. The presented analysis is one of many possible scenarios, locations of storage, and the transportation of biomass to the final destination, which takes the form of a heating system.