Sustainable Agro-Biomass Market for Urban Heating Using Centralized District Heating System

The importance of biomass for energy production is included in the government program “Energy Policy of Poland until 2040”. Biomass is one of the most stable renewable energy sources (RES), and its resources are the largest of all alternative energy sources. The paper presents and discusses the most important conditions that are related to the possible usage of the biomass that is obtained from agriculture for heating purposes. The aim of the study is to assess the energy potential of a selected agro-biomass and to indicate its effective use for the production of district heat. The research uses: in-depth statistical data analysis (data were obtained from the Central Statistical Office and the Agency for Restructuring and Modernization of Agriculture), inference methods, shortand long-term forecasting, the minimum spanning tree (MST) algorithm, and methods of singleand multi-criteria optimization for the selection of the most advantageous variant. The research covers three different scenarios: optimistic, moderate, and pessimistic. The results of the study present: technical and energy potential of biomass, taking into account its type, energy properties, and places of its collection; optimum location of intermediate storage and processing sites; and the optimum storage frequency of its transport to energy companies.


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 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.

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]. 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 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: P z -grain yield, I s/z -ratio of straw yield to grain yield, I s/a -ratio of straw yield to land area, I ns -indicator of grain surplus, A-land area dedicated to grain production, Z s -straw surplus, E s -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: ∑ e∈E w(e) → min (4) 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.

New Opportunities for the Biomass Market in System Heating in Poland-First Stage
The ratio of hard coal in the production of system heat is still significant. In the last 18 years, its share has decreased by only about 10%, from the level of about 80% in 2002 to 70% today ( Figure 2).
The diversification of fuels that have been applied for the purposes of for heat production is slightly greater among heat sources that generate heat in CHP processes. In this group of heat sources, the use of coal-based fuels is also dominant, but about 30 percent is represented by other fuels, including 11 percent of RES sources, 9.8 percent of natural gas, and 6 percent of heating oil [33].

New Opportunities for the Biomass Market in System Heating in Poland-First Stage
The ratio of hard coal in the production of system heat is still significant. In the last 18 years, its share has decreased by only about 10%, from the level of about 80% in 2002 to 70% today ( Figure 2). Up to the present, the ratio of gaseous fuels increased by 5.8 percent and the ratio of renewable energy sources increased by 6.6 percent [6][7][8][9][10][11][12][13][14][15][16]33,50].
The diversification of fuels that have been applied for the purposes of for heat production is slightly greater among heat sources that generate heat in CHP processes. In this group of heat sources, the use of coal-based fuels is also dominant, but about 30 percent is represented by other fuels, including 11 percent of RES sources, 9.8 percent of natural gas, and 6 percent of heating oil [33].
Among the renewable energy sources, biomass takes the position of the most important fuel that is applied in heating. Its ratio among the available renewable energy sources in 2020 was 97.5 percent. In addition to biomass, the ratio of biogas has been registered since 2016 at the level of 0.4%, and other renewable energy sources at the level of 2.1 percent [6][7][8][9][10][11][12][13][14][15][16]33,50].
The ratio of biomass use in heating systems has been growing steadily since 2002. This level has now increased to over 9.9 percent compared to the level that was recorded at 2.5 percent in 2020, and it can be clearly noted that in the last two years there has been an increase in the dynamics of this growth ( Figure 3). Among the renewable energy sources, biomass takes the position of the most important fuel that is applied in heating. Its ratio among the available renewable energy sources in 2020 was 97.5 percent. In addition to biomass, the ratio of biogas has been registered since 2016 at the level of 0.4%, and other renewable energy sources at the level of 2.1 percent [6][7][8][9][10][11][12][13][14][15][16]33,50].
The ratio of biomass use in heating systems has been growing steadily since 2002. This level has now increased to over 9.9 percent compared to the level that was recorded at 2.5 percent in 2020, and it can be clearly noted that in the last two years there has been an increase in the dynamics of this growth ( Figure 3).  Below, a discussion is included with regard to the demanded increase in the ratio of renewable energy sources by 1.1% per year until 2030 and how that could affect the variations in the demand of biomass on the local market as a consequence of increasing its ratio in the overall structure of fuel use in Polish heat and power plants. Straw was considered as the basic source of biomass, as the easiest source of agro-type biomass combined with waste that is derived from the production, and processing of wood in forests that can be obtained [4,42]. In both cases, the investigations include the local determinants that define the ways in which it can be achieved.

Determinants for Development of Biomass Market in Poland
The availability and price form the decisive factors for achieving an increase in the Below, a discussion is included with regard to the demanded increase in the ratio of renewable energy sources by 1.1% per year until 2030 and how that could affect the variations in the demand of biomass on the local market as a consequence of increasing its ratio in the overall structure of fuel use in Polish heat and power plants. Straw was considered as the basic source of biomass, as the easiest source of agro-type biomass combined with waste that is derived from the production, and processing of wood in forests that can be obtained [4,42]. In both cases, the investigations include the local determinants that define the ways in which it can be achieved.

Determinants for Development of Biomass Market in Poland
The availability and price form the decisive factors for achieving an increase in the application of biomass for purposes of heat and electricity production. According to the information that is provided by the President of the Energy Regulatory Office, the cost of purchasing biomass has remained stable in recent years between PLN 15 and PLN 20/GJ ( Figure 4).
Below, a discussion is included with regard to the demanded increase in the ratio of renewable energy sources by 1.1% per year until 2030 and how that could affect the variations in the demand of biomass on the local market as a consequence of increasing its ratio in the overall structure of fuel use in Polish heat and power plants. Straw was considered as the basic source of biomass, as the easiest source of agro-type biomass combined with waste that is derived from the production, and processing of wood in forests that can be obtained [4,42]. In both cases, the investigations include the local determinants that define the ways in which it can be achieved.

Determinants for Development of Biomass Market in Poland
The availability and price form the decisive factors for achieving an increase in the application of biomass for purposes of heat and electricity production. According to the information that is provided by the President of the Energy Regulatory Office, the cost of purchasing biomass has remained stable in recent years between PLN 15 and PLN 20/GJ ( Figure 4).  Such variations are mainly attributable to the lack of a support mechanism that promotes the use of biomass in the condensing power plants. Despite the fact that the purchase cost of biomass does not increase, the trend of increasing the cost of heat generation from biomass is clearly observed. Over the last 18 years, it has increased from the average level of PLN 27/GJ to the level of nearly PLN 43/GJ. The highest increase in heat generation costs took place in the years 2002-2015 and amounted to approximately 54%. Over the last five years, this increase has not exceeded 5%, which only points to an increase due to inflation [6][7][8][9][10][11][12][13][14][15][16][17]50].
The impediments for the more extensive use of biomass in heating have been and still are related to the technical limitations resulting from the selection of an adequate technology to be applied for its conversion into heat. The solutions that are available on the market mainly depend on combustion or co-combustion technologies using specific types of biomasses. Boilers with a stepped grate from various manufacturers allow the possibility of burning biomass in the form of wood chips, sawdust, shavings, and waste fine-piece wood. Solutions for agro-origin biomass are used less frequently, mainly straw and hay, and forest biomass, which is mainly waste from wood production [5,49,66,67].
The combustion of biomass in stepped grate furnaces is limited by various parameters, and one of the most important is the humidity, which is defined by leading producers as a boundary level not exceeding 55 ÷ 60%. Ensuring the required efficiency of the combustion process in such grate designs does not offer the possibility of varying the type of biomass that is used in them, which significantly reduces the safety of such installations.
The solution that largely eliminates the above technological drawbacks and seems effective for wide application in heating systems is the gasification technology, e.g., with the use of a gasification furnace.

Biomass Potential in POLAND-Stage Two
The increase in the ratio of renewable energy sources in the country's energy balance increases energy security, reduces the degree of dependence on the import of energy carriers, and offers savings in fossil resources. Poland has large resources of biomass that can be applied for heating energy production purposes [56].
The greater use of renewable energy sources will also enable greater diversification of heat supplies from various sources, as it promotes regional development and creates conditions for the development of district heating based on local sources. According to experts, the use of biomass as the main fuel in a heating system is indeed a solution that positively affects the natural environment [49].
Straw is an easily accessible source of biomass from agricultural production that can be applied for heating purposes [25,49,64,68]. After the re-use of some straw resources in agriculture, its surplus can be considered as waste and used for the production of "green energy" [49]. The total straw resources can be estimated on the basis of statistics on cereal production (Eurostat) and knowledge of the proportion between the main and secondary yields [49].
The territory of Poland is separated into regions by means of an administrative division. Poland is divided into 16 voivodships, 314 poviats, and 2477 communes forming the smallest administrative units. Among them there are 302 urban communes, 642 mixed urban-rural communes, and 1533 rural communes. In a significant part of rural areas there are potential sources of solid biomass that can play a special role in the local heating systems (Figure 4) [3,19,68,69].
The results of the biomass potential calculations are presented graphically in Figure  577 km 2 , we can see that the potential in the Opolskie Voivodeship, which is the smallest voivodeship (9 412 km 2 ) in Poland, accounts for 3% of the area. On the other hand, the biomass potential is 4.7055 PJ/year, which constitutes a supply source that covers 5% in the scale of the whole country. The estimation of this potential indicates the possibility of using straw in heating plants to a larger extent than it occurs at present. The ratio of using the energy potential of biomass in the production of system heat, compared to the possibilities for most voivodeships, is significantly below 50%. The level is close to 50 percent, and it is achieved only in Mazowieckie (50.9%) and Małopolskie (49.5%) voivodships. The only voivodship in Poland which is not capable of achieving the required level of participation using local biomass resources is Śląskie Voivodeship; it has a deficit of 15.3 percent. In this voivodeship, the fulfillment of the requirements will have to be carried out either with the use of imported biomass or with the use of another form of renewable energy source.  The estimation of this potential indicates the possibility of using straw in heating plants to a larger extent than it occurs at present. The ratio of using the energy potential of biomass in the production of system heat, compared to the possibilities for most voivodeships, is significantly below 50%. The level is close to 50 percent, and it is achieved only in Mazowieckie (50.9%) and Małopolskie (49.5%) voivodships. The only voivodship in Poland which is not capable of achieving the required level of participation using local biomass resources isŚląskie Voivodeship; it has a deficit of 15.3 percent. In this voivodeship, the fulfillment of the requirements will have to be carried out either with the use of imported biomass or with the use of another form of renewable energy source.
Taking into account the above-described conditions in terms of the requirements and the possibility of a greater application of local biomass resources for the production of system heat, we should forecast an increase in its importance in the years to come. It seems that biomass can constitute a significant supplement to the heat and electricity generation market in addition to such new sources as heat from thermal waste treatment installations or sources using other alternative fuels. However, the increase in the ratio of biomass in heat production will be determined by properly organized supply chain logistics on the local market, a result of which both farmers and energy producers will be able to jointly organize the market for this fuel and reap mutual benefits [25,48,[70][71][72].

Investigation of the Biomass Potential on the Selected Example of Opolskie Voivodeship
In recent years, the greatest diversification of fuels that were applied for heat production was observed in the following voivodeships: Kujawsko-Pomorskie 25%, Podlaskie 25%, and Pomorskie 7%, with a significant ratio of renewable sources, mainly biomass. On the other hand, the group of voivodeships that do not apply biomass include: Lubuskie, Mazowieckie, Lubelskie, Opolskie, and Dolnośląskie, despite the fact that these regions have a high biomass potential ( Figure 6) [17]. Figure 6 presents the energy potential of biomass in Poland in each of the provinces. The total area of crops is presented, including separately: agricultural crops and forests. The level of the demand for system heating for each voivodeship was also marked. However, the most important point in the graph is the calculation of the energy potential from the biomass that can be used. The voivodeships where the heat demand can be fully covered by biomass are: Opolskie, Lubuskie, Podkarpackie, Zachodniopomorskie, Warmińsko-Mazurskie, and Lubelskie. These sets of data should be considered as the first step to the use of biomass regionally, and even poviat and commune due to the diversity and variability of biomass. Lubelskie. These sets of data should be considered as the first step to the use of biomass regionally, and even poviat and commune due to the diversity and variability of biomass. One of the 16 voivodeships in Poland (Opolskie Voivodeship) which currently does not use biomass in heat production, was selected for the study. When a comparison was carried out of the results of GUS data and on the basis of the analysis of existing research [55], the calculations demonstrate that in the Opolskie Voivodeship the biomass potential is 16 306.6 TJ/year. [55]. However, base on the amount of straw that can be managed according to [60] it is confirmed that the amount of straw to be used for heating purposes in the Opolskie Voivodeship is in the range [2873; 50 624] tonnes per year, the largest amounts of which are found in the southern part of the voivodship (Figure 7) [17]. One of the 16 voivodeships in Poland (Opolskie Voivodeship) which currently does not use biomass in heat production, was selected for the study. When a comparison was carried out of the results of GUS data and on the basis of the analysis of existing research [55], the calculations demonstrate that in the Opolskie Voivodeship the biomass potential is 16,306.6 TJ/year. [55]. However, base on the amount of straw that can be managed accord- ing to [60] it is confirmed that the amount of straw to be used for heating purposes in the Opolskie Voivodeship is in the range [2873; 50,624] tonnes per year, the largest amounts of which are found in the southern part of the voivodship (Figure 7) [17]. One of the 16 voivodeships in Poland (Opolskie Voivodeship) which currently does not use biomass in heat production, was selected for the study. When a comparison was carried out of the results of GUS data and on the basis of the analysis of existing research [55], the calculations demonstrate that in the Opolskie Voivodeship the biomass potential is 16,306.6 TJ/year. [55]. However, base on the amount of straw that can be managed according to [60] it is confirmed that the amount of straw to be used for heating purposes in the Opolskie Voivodeship is in the range [2873; 50,624] tonnes per year, the largest amounts of which are found in the southern part of the voivodship (Figure 7) [17].

Characteristic of Selected Area of Investigations-Stage Three
When an analysis is carried out with regard to the characteristics of the area of the Opolskie Voivodeship, the following input data were collected: agro-industrial region with an area of 9412 km 2 , agricultural land-60%, and forests-27%. The highest value of the straw development potential in the entire voivodship lies in the southern part of the voivodship.

Characteristic of Selected Area of Investigations-Stage Three
When an analysis is carried out with regard to the characteristics of the area of the Opolskie Voivodeship, the following input data were collected: agro-industrial region with an area of 9412 km 2 , agricultural land-60%, and forests-27%. The highest value of the straw development potential in the entire voivodship lies in the southern part of the voivodship.
When we investigate the location of the basic sources of system heat supply in the Opolskie Voivodeship, the following data can be derived: in 11 out of all 12 poviats, there are 13 system sources above 10 MW (Table 1). System sources are usually located in larger poviats towns, and their fuel is, among other things, fine coal, natural gas, heating oil, as well as other liquid fuels [73]. When we consider the potential of finding a place for the storage and delivery of biomass, an optimal location should be identified, i.e., a radius of up to 20 km from each district heating source, which will offer the measure to decide the system that will offer the Energies 2022, 15, 4268 13 of 23 possibility of using biomass resources (Figure 8.). In the Brzeg, Nysa, and Opolski poviats, there are two heating systems each and we can note that the possibility of using biomass may be limited. Considering this situation and taking into account the existing potential, it is necessary to consider how the supply districts can be moved so as to be able to use the remaining biomass from other poviats, where the reserves remain unused.
strzelecki Strzelce Opolskie 30.00 When we consider the potential of finding a place for the storage and del biomass, an optimal location should be identified, i.e., a radius of up to 20 km fro district heating source, which will offer the measure to decide the system that w the possibility of using biomass resources (Figure 8.). In the Brzeg, Nysa, and poviats, there are two heating systems each and we can note that the possibility o biomass may be limited. Considering this situation and taking into account the potential, it is necessary to consider how the supply districts can be moved so as to to use the remaining biomass from other poviats, where the reserves remain unus We can note that the energy potential of straw is different in each poviat means that it is necessary to consider and decide for which poviat the analysis We can note that the energy potential of straw is different in each poviat, which means that it is necessary to consider and decide for which poviat the analysis will be carried out. We should take into account the fact that the calculation of the potential for a given poviat or commune does not always need to be considered. However, the area of the district heating system should be verified because, in some poviats as already mentioned, there is more than one system. The article focuses on the area of the Opole poviat, where there is the largest heating system with a capacity of 180 MW and the second from the end of Ozimek 14.00 MW. The delivery range coincides, but it should also be noted that a smaller heating system will require a smaller amount of biomass.
The technical potential of biomass was estimated in selected communes of the Opolskie Voivodeship (potential suppliers of biomass from the immediate vicinity of the Ozimek heat plant). The calculated potential was based on data from ARiMR, GUS, Agricultural Census, as well as in-depth interviews ( Table 2.).

Multivariate Analysis of Looking for the Optimal Location for Biomass Delivery-Stage Four
The fourth key stage involves the designation of a multi-variant MST that is related to the selection of a storage location as well as the delivery to the selected heating system. The available data demonstrate that it is necessary to find the optimal place for storing and transporting biomass from its supply sources in the area of three communes. The resulting details show that for the research it is necessary to obtain biomass from eight communes in three different poviats: Opole, Olesno, and Strzelce. A total of seven communes are adjacent to the eighth one with the heating system, Ozimek Commune (O). The remaining municipalities are: Turawa (T), Chrząstowice (CH), Zębowice (Z), Dobrodzień (D), Kolonowskie (K), Strzelce Opolskie (SO), and Izbicko (I). As we can see in the Table 2 are a potential candidate for finding the optimal place for the investment-the construction of a biomass storage location center. The points on the map (Figure 9) correspond to 18 localities in eight communes from which biomass will be delivered to the heating system with a variable frequency. The course and results of the optimization analysis that was performed using the Prima algorithm that was described above were used for the eight selected municipalities. The conducted analysis is aimed at finding the shortest route connecting the places of biomass supply to individual customers in the given localities. The localities have been connected by national, provincial, district, and communal roads. The next step is to simplify the graph so that it shows only vertices, i.e., places and edges, i.e., roads. The above-mentioned towns were selected on the basis of the map characteristics, divided into agricultural, forest, water, and nature protected areas.

Single-Criterion Method of Minimizing the Road Distance
For the graph that was developed in this manner, an optimization analysis was carried out, determining the shortest network of biomass delivery routes from individual towns to the place of its technical use ( Table 3).
The The smallest sum of weights for the connections of the indicated places is 128 km, which is shown in Table 3. It also includes weight indications for individual sections with the shortest (bold and shaded values) and the number of recommended routes from a given peak (locality). Daniec (DA), Zębowice (Z), Radawie (R), Ligota Dobrzeńska (LD), Rzędowice (RZ), Szmerowice (SZM), Staniszcze Wielkie (SW), Szymiszów (SZY), Rozmierka (RO), Błotnica Strzelecka (BS), Ligota Czamborowa (LCZ), and Otmice (O). The localities have been connected by national, provincial, district, and communal roads. The next step is to simplify the graph so that it shows only vertices, i.e., places and edges, i.e., roads. The above-mentioned towns were selected on the basis of the map characteristics, divided into agricultural, forest, water, and nature protected areas.

Single-Criterion Method of Minimizing the Road Distance
For the graph that was developed in this manner, an optimization analysis was carried out, determining the shortest network of biomass delivery routes from individual towns to the place of its technical use (Table 3.). Table 3. Weights wi for specific distances-single-criterion method of minimizing the road distance.    Figure 9 contains an outline with the course of the algorithm that was applied in search of the minimum distance between points in municipalities, and Table 3 shows the distances between the individual localities. For the selected localities in the study area, constituting the vertices of the graphs, the minimum spanning tree and the corresponding sums of weights were found-the shortest distances connecting these locations.

MST
A study was performed by the application of the single-criterion method of minimizing the distance of biomass supply from agricultural areas to the designated study area (eight communes + one existing heating system). On this basis, we can conclude that the transfer would take place from three different directions to the heating system. This will not enable the simultaneous designation of locations for more than one facility for biomass storage. Any simplification of this method leads to the need to carry out an additional assessment of the location.

Multi-Criteria Method of Minimizing Road Distances
Due to the considerable diversity of the existing technical potential of biomass, from 3 [GWh/year] to 34 [GWh/year], the analyzed area was separated into smaller parts that were identified in terms of technical potential. Hence, four values of MST were calculated. The investigated area was divided into four smaller areas according to the amount of biomass potential in a given commune (Tables 4-7     The conducted analysis was aimed at establishing the shortest routes within the existing potential. In the area of groups connecting areas with the same amount of biomass potential, i.e.,: range [3-7 GWh/a], 11 [GWh/a]; 15 [GWh/a]; 34 [GWh/a] was determined for the Ozimek heating system (OZ). Figure 10 illustrates four minimal spanning trees with the respective potentials. The commune of Strzelce Opolskie has the potential with the highest volume of biomass that is potentially generated in it and it amounts to 34 GWh/a-marked in green color. The potential of 15 GWh/a is represented by the area of the Turawa commune-marked in blue for MST. As we can see from the results that are presented in Figure 10, MST designated two direct transportation routes to the heating system in Ozimek (OZ). Another MST for a potential of 11 GWh/a-marked in purple in the Dobrzeń commune. The smallest amount of potential occurs in five communes, i.e., [3-7 GWh/a]. These are the following communes: Zębowice, Chrząstowice, Izbicko, Kolonowskie, and Ozimek. MST is marked in red in the figure. Such classification makes the planning of the amount of transportation from given areas easier for the purposes of the biomass supply chain.

Optimization of Supply Chain of Biomass in the Selected Region in Poland-Stage Five
In the consideration of the multi-faceted characteristics of the localization issue that formed the criterion guiding the approach that was applied in the selection of the location of the facilities in the transport and logistics network, the final step involves the multi-site consideration of the possibility of selecting the simultaneous existence of several warehouses. On the basis of the fourth stage, single and multi-criteria analysis offers the means to select potential locations of biomass storage. The study based on a single criterion demonstrates that warehouses can be built in four municipalities, i.e., in four identified localities: LD, LT, DA, and RO ( Figure 9). These localities are located at the following distances from the heating system in OZ: OZ-LD; 18 km, OZ-LT; 13 km, OZ-DA; 11 km, and OZ-RO; 19 km, respectively ( Figure 10, Table 8.), and the total number of kilometers is 62. If we take into account the multi-object analysis, we can see that the number of selected warehouses is greater (as there are six warehouses in such a system). The localities are located at the following distances from the heating system, respectively: OZ-SZY 23 KM; OZ-RO; 19 km, OZ-LD; 18 km, OZ-Z; 17 km, OZ-DE; 12 km, and OZ-LCZ; 18 km. There two warehouses that have been selected in the commune of Strzelce Opolskie, as this area has the greatest biomass potential ( Figure 10, Table 8). In some municipalities, biomass harvesting straight from the field was proposed instead of a warehouse.

Optimization of Supply Chain of Biomass in the Selected Region in Poland-Stage Five
In the consideration of the multi-faceted characteristics of the localization issue that formed the criterion guiding the approach that was applied in the selection of the location of the facilities in the transport and logistics network, the final step involves the multisite consideration of the possibility of selecting the simultaneous existence of several warehouses. On the basis of the fourth stage, single and multi-criteria analysis offers the means to select potential locations of biomass storage. The study based on a single criterion demonstrates that warehouses can be built in four municipalities, i.e., in four identified localities: LD, LT, DA, and RO ( Figure 9). These localities are located at the following distances from the heating system in OZ: OZ-LD; 18 km, OZ-LT; 13 km, OZ-DA; 11 km, and OZ-RO; 19 km, respectively ( Figure 10, Table 8.), and the total number of kilometers is 62. If we take into account the multi-object analysis, we can see that the number of selected warehouses is greater (as there are six warehouses in such a system). The localities are located at the following distances from the heating system, respectively: OZ-SZY 23 KM; OZ-RO; 19 km, OZ-LD; 18 km, OZ-Z; 17 km, OZ-DE; 12 km, and OZ-LCZ; 18 km. There two warehouses that have been selected in the commune of Strzelce Opolskie, as this area has the greatest biomass potential ( Figure 10, Table 8). In some municipalities, biomass harvesting straight from the field was proposed instead of a warehouse. On the basis of on the multi-criteria analytical and descriptive method, the design alternatives can be compared and the transport scenarios can be determined, with particular emphasis on the profitability analysis [74]. The alternatives were identified as: optimistic, moderate, and pessimistic and determine the accurate fuel demand schedule and the transportation costs. However, the process of the logistics infrastructure design requires the development of additional design methods, with particular emphasis on mathematical modeling methods supporting design decisions.
The present study includes a description of the supply chain in which a group of agricultural enterprises and a heating plant in Ozimek can perform joint activities that are necessary to meet the demand for biomass in the entire flow chain. It is also necessary to consider among the approaches that can be taken into account: (1) deriving raw materials and their storage in a selected location, followed by delivery to the specific heating system generation facility; (2) deriving the raw material that is to be collected directly from croplands, followed by the delivery to the specific heating system generation facility; (3) a system that combines (1) and (2). The pessimistic alternative is represented by alternative (2) due to the large dispersion of biomass producers throughout the investigated area. The moderate alternative involves a decision to build four warehouses followed by a single-criterion analysis in combination with direct harvesting from designated places in croplands as defined in alternatives (1) and (2). The optimistic variant is a combination of a single-and multi-criteria analysis together with the selection of three common warehouses (Table 8) RO, DE, and LCZ and emergency delivery of biomass to farmlands from the locations that were identified previously, that are easily accessible for cars that supply fuel to the heating system.

Discussion
The increase in the ratio of renewable energy sources in the country's energy balance offers an increase in the energy security, decreases of dependence on the import of energy carriers, and results in savings in terms of fossil fuel resource use. One of the basic sources of renewable energy in our country is biomass. Poland has considerable resources of this raw material that has a potential for energy use in heating [56].
The greater use of renewable energy sources may serve as a tool leading to an increase in the diversification of heat supplies, as it promotes local development and creates conditions for the development of heating that are based on local resources [75,76]. According to experts, the use of biomass as the main fuel in heating systems offers a solution that has a positive impact on the natural environment [49].
Actions that are designed with the aim of the development of renewable energy sources are dedicated to reduction of the emissions of the energy sector and THE diversification of the structure of energy generation capacity, leading 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 run, will improve energy security [77]. In accordance with the objectives of the domestic and EU policy that are specified for the period between 2021 and 2030, a significant increase in the ratio of renewable energy sources use in heating systems is assumed. This ratio needs to be characterized by an increase of at least 1.1 percent annually year after year in the period, and the total increase in 2030 will then assume a minimum level of 11% [4,25,26].
When we consider the differences that are occurring in the use of heating based on biomass in EU countries, the cases involving countries with considerable potential, including Poland, the Czech Republic, Hungary, Greece, Spain, France, and Slovakia, could be applied to expand the scope of the research or to transfer the presented results and calculate analogue forecasts in space and time [35].

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 agrobiomass 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:

1.
Analysis of the technologies and fuels that are applied in the heating systems, 2.
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, 3.
Analysis of the characteristics of the studied area (including: natural and climatic, heating systems, arable land surfaces, and arable lands), 4.
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, 5.
The transport scenarios were determined, taking into account optimistic, moderate, and pessimistic alternative solutions.
Throughout the study, heuristic, single-multi-criteria, multi-object, and analyticaldescriptive 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.