Development of Solid Biomass Production in Poland, Especially Pellet, in the Context of the World’s and the European Union’s Climate and Energy Policies

: The aim of this research was to present the changes in biomass production, especially pellets in Poland, in the context of world’s and European Union’s (EU) climate and energy policy, compared to other renewable energy sources. We also analyzed the law concerning the biomass production in the EU. Finally, we have elaborated the prognosis of the pellet production on the world scale. We have used different methods to achieve the goals, among which the most important are the Generalized Autoregressive Conditional Heteroscedasticity (GARCH model) and prognosis. We also compared the results of pellet production in different countries in the European Union. The results were presented in tabular and graphic form. We have received the data from Eurostat and the Main Statistical Ofﬁce (MSO) in Poland. Our research proves the increase of biomass and pellet production on the global scale. Moreover, global wood pellet production increased by 972% in the years 2005– 2018. We can conclude that this increase was the result of increasing demand for renewable energy sources. The ﬁrst research hypothesis assumed that the changes in the European Union Policy have impacted the increase of biomass production in the world. Our prognosis conﬁrmed the second hypothesis that the development of pellets will increase as the result of increasing global demand. The use of more renewable energy sources is necessary to decrease the degradation of the environment.


Introduction
In 1997, the European Council and the European Parliament adopted the "White Paper for a Community Strategy and Action Plan". It is worth mentioning that the share of renewable energy sources (RES) in gross internal energy consumption in the EU in 1997 was 6%. Then, the EU proposed an integrated Energy and Climate Change package in 2007. Moreover, in 2009, the EU established the Renewable Energy Directive (RED) promoting renewable energy sources. As a result, the EU established a goal of a reduction of 20% of In general, the range of instruments for the environmental direction of European policy has significantly expanded in recent years [31].
The aim of the paper was to present the changes in solid biomass utilization in Poland in the context of world and European environmental protection policies, especially pellets. To achieve this goal we needed to answer following questions: 1.
What is the renewable energy mix in Poland, the EU, and the world? 2.
What is the state of law concerning the renewable energy sources? 3.
What is the prognosis of pellet production in Poland and the world?
From the literature and methods we have withdrawn the following hypotheses: Hypothesis 1 (H1). The changes in European Policy have impacted the increase of biomass production.
Hypothesis 2 (H2). The development of pellets will increase as the result of increasing global demand.
The rest of the paper is organized as follows. We present a Literature Review pointing out the renewable energy regulations including the United Nations Framework Convention on Climate Change, the Kyoto Protocol, and the Paris Agreement. Then, we present Materials and Methods, and Research Results with Discussion. The final part contains Conclusions.

United Nations Framework Convention on Climate Change (UNFCCC)
The problem of climate change was first included in the political agenda of the international community in the mid-1980s. Scientists from different countries drew their first conclusions about the relationship between climate change and human activity. It was ratified by 195 countries [32]. The problem of climate change is global. The concern about the environment forced national governments to undertake actions to reduce the emission of greenhouse gases [33]. Climate change is becoming a priority issue [34]. Only policy intervention can reduce the crises that can be created by climate pollution [35].
The Convention was opened for signature on 4 June 1992 at the United Nations Conference on Environment and Development held in Rio de Janeiro, and came into force on 21 March 1994. Currently, more than 190 countries are parties to the Convention. It was the first attempt to cap carbon emissions [36]. This includes all industrialized countries, all countries with economies in transition, and most developing countries [37].

The Kyoto Protocol
The further process of developing an international agreement on climate change refers to December 1997, when the Kyoto Protocol was adopted by delegates from more than 160 countries. The main provisions of the Kyoto Protocol included the determination of the permissible amount of greenhouse gas emissions in 2008-2012 for all industrialized countries participating in this agreement.
Over recent years, 55 million tons of gas emissions were sold in deals. In 2005, the European Union introduced the system of trade in quotas. After exiting the Kyoto Protocol, Russia, which generates 17% of gross greenhouse gas emissions, has become one of the largest suppliers of emission permits [38].
Since different countries had different degrees of interest in implementing the Kyoto Protocol, its ratification faced serious obstacles. In March 2001, the United States announced its decision not to participate in the Kyoto Protocol, and its ratification was threatened. The US position, in particular, was voiced at the UN conference on climate change in Buenos Aires in 2004, where the head of the US delegation, Harlan Watson, stated: "The Kyoto Protocol is not a scientific document, but a political one, so we are not going to participate" [39,40].

Post-Kyoto System of International Relations
The negotiations on a new post-Kyoto agreement for the period after 2012, that began in 2007, was unprecedented for the complexity and difficulty of problems covered, and the impact on the economic development of individual countries and humanity as a whole.
In August 2009, another round of international climate talks took place in Bonn (Germany). Negotiations in Bonn were carried out within the framework of two working groups. The first group, working under the aegis of the United Nations Framework Convention on Climate Change, was preparing a new text for the post-Kyoto agreement. The second action was a discussion of the possible extension of the Kyoto Protocol for the second period and the commitments of developed countries after 2012. Negotiations in Bonn were held in the form of informal meetings, focusing on convergence of the positions of the two groups in agreeing on a single text for the future agreement [41,42].

Paris Agreement
The implementation of the 2015 Paris Agreement requires support of the development of renewable energy not only in the EU, but also worldwide. The EU is playing an important role in the decarbonization of the EU according to the Paris Agreement [43]. The EU is trying to keep the rise above two degrees of Celsius below levels [44,45]. The legal norms play a key role in energy security. The energy policy is regulated by numerous documents and laws [46].

Data Sources
We used different sources of information. First, we presented the gross energy availability from solid biofuels in the EU. Further, we have examined the share of individual renewable energy carriers in obtaining energy from renewable sources in 2005-2018 in Poland. In the research section, we have also analyzed the changes in pellet production in the world and the EU. The sources of information were data obtained from Eurostat and Statistics Poland. We also used the Global Status Report 2019 to analyze the changes in RES in the world.

Methods
To analyze the changes in pellet production, we used the Augmented Dickey-Fuller test (ADF) to determine if the pellet production is non-stationary [47].
Finally, we calculated Generalized Autoregressive Conditional Heteroscedasticity (GARCH) models that were elaborated by Engle [48] and Bollerslev [49] to analyze time series.
One of the most important applications of GARCH models is the description and forecasting of volatility, understood as a measure of uncertainty. In the case of GARCH models, such a measure of uncertainty is conditional variance or conditional standard deviation. GARCH models are the most commonly used models of variation. This is primarily because they allow the description of most of the empirical properties of the analyzed time series. In order to avoid problems related to the estimation of parameters of the ARCH model, the GARCH model is used. The GARCH model has all the advantages of the ARCH model, and also describes the variability of most time series better [50].
The models are widely used for risk management, and they can capture both volatility clustering and unconditional return distribution [51]: Energies 2021, 14, 3587 5 of 22 In the GARCH notation, the first subscript refers to the order of the y 2 terms on the right side, and the second subscript refers to the order of the σ 2 terms [44]. The GARCH is used to analyze conditional variance. The process of variance is not stationary [51]: where ψ t−1 denotes series history up to time t − 1, t(υ, 0, h t ) is a Student t distribution with υ degrees of freedom (υ > 2), and h t is the conditional variance. Given the regularity conditions, α 0 > 0, α 1 ≥ 0, β 1 ≥ 0, the required stationarity condition is α 1 + β 1 < 1.
Additionally, due to the presence of autocorrelation, the roots of the polynomial z p − ϕ 1 z p−1 − ϕ 2 z p−2 − . . . − ϕ p = 0 lie inside the unit circle. According to the GARCH models, the residual depends on the previous period's mean and variance [51]. The GARCH model can satisfactorily explain the volatility of an analyzed series [52]. The authors of the paper limited the analysis to the class of models AR (1)-GARCH (1,1) because it was dictated in previous research, as well as the fact that in most of the cases analyzed in the literature on the subject, process order selection autoregression at the AR (1) level and the GARCH process at the GARCH (1,1) level are sufficient to capture the autoregressive effect and heteroscedasticity [50,53].
We could not use the threshold GARCH (TGARCH) model because the defined conditional variance was not a linear piecewise function [54].

Pellet Production in the World
The global supply of biomass was 55.6 EJ in 2018. Solid biomass plays an important role and constitutes 47.6% of total biomass. The solid biomass mainly includes wood chips, wood pellets, and traditional biomass sources. The next positions in biomass mix are taken by liquid biofuels (3.98%), municipal and industrial waste (1.45%), and biogas (1.36%) ( Table 1). Such structures demonstrate the strong position of solid biomass in the energy mix [55]. A global increase of biomass was observed in 2005-2018. The biggest increase was observed in liquid biofuels (355.2%), biogas (166.70%), and industrial waste (151%). The solid biofuels recorded the smallest increase (11.20%) in the period. The growth forecasts for renewable energy investments in the next 10 years are very optimistic. It is even estimated that the renewable energy market will be one of the fastest growing energy sources in the world, and its impact will be cross-sectoral [56].
According to Rokicki [57], the most important obstacles in production of renewable energy sources are the high costs of installation. Another obstacle can be the high investment cost in production facilities [58]. The growth for renewable energy is a consequence of economic development. Forecasts expect the demand for energy will increase in accordance with the population growth, automation, and modernization. The global demand for energy will increase worldwide about 1.6% each year. Renewable energy sources are very important in satisfying future demand for energy. These energy sources are emission-free and they contribute to worldwide power with fewer emissions of greenhouse gases (GHC) [59]. The concentration of greenhouse gases in the 21st century caused a rapid rise in the global temperature. Coal-fired power stations are considered to be the source of the pollutants. Most were built two decades ago and are responsible for 80-85% of pollution [60].
Renewable energy sources contribute to sustainable energy development worldwide, which include economic, technical, political, and social factors. Economic factors are linked to regional economic development. Technical factors indicated that the development of renewable energy sources will require new technologies. The political factors are the effects of policy solutions. Social factors are focused on social resources and ecological awareness [61]. To meet the environmental protection concerns, it is necessary to invest in renewable energy sources. These investments can help reduce greenhouse gases and increase energy security [62]. The renewable energy sources can replace fossil fuels that are estimated to be depleted in less than 50 years, and are responsible for most of greenhouse gas emissions [63].
As we can see from Figure 1, global wood pellet production has increased from 5.2 MMT (million metric tons) in 2005 to 55.7 MMT (million metric tons) in 2018. The increase reached 971% in the period. Such an increase is the effect of the increasing demand for renewable energy sources in the world.
Source: Own elaboration based on Ren21 Renewables. Global Status Report 2019.
The growth forecasts for renewable energy investments in the next 10 years are very optimistic. It is even estimated that the renewable energy market will be one of the fastest growing energy sources in the world, and its impact will be cross-sectoral [56].
According to Rokicki [57], the most important obstacles in production of renewable energy sources are the high costs of installation. Another obstacle can be the high investment cost in production facilities [58].
The growth for renewable energy is a consequence of economic development. Forecasts expect the demand for energy will increase in accordance with the population growth, automation, and modernization. The global demand for energy will increase worldwide about 1.6% each year. Renewable energy sources are very important in satisfying future demand for energy. These energy sources are emission-free and they contribute to worldwide power with fewer emissions of greenhouse gases (GHC) [59]. The concentration of greenhouse gases in the 21st century caused a rapid rise in the global temperature. Coal-fired power stations are considered to be the source of the pollutants. Most were built two decades ago and are responsible for 80-85% of pollution [60].
Renewable energy sources contribute to sustainable energy development worldwide, which include economic, technical, political, and social factors. Economic factors are linked to regional economic development. Technical factors indicated that the development of renewable energy sources will require new technologies. The political factors are the effects of policy solutions. Social factors are focused on social resources and ecological awareness [61]. To meet the environmental protection concerns, it is necessary to invest in renewable energy sources. These investments can help reduce greenhouse gases and increase energy security [62]. The renewable energy sources can replace fossil fuels that are estimated to be depleted in less than 50 years, and are responsible for most of greenhouse gas emissions [63].
As we can see from Figure 1, global wood pellet production has increased from 5.

Gross Energy Availability from Solid Biofuels in the EU
The policies concerning climate and energy in the European Union encouraged the development of renewable energy sources including biomass, biogas, and others. The basic benefits include support schemes for renewable energy sources. Moreover, using RES delivered economic and environmental benefits [1]. Each country of the EU included bioenergy policy in environmental policy. Furthermore, the production of biomass is also linked to agricultural policy [64]. Biomass is an organic substance created in the process of accounting solar energy [6]. Biomass also includes processed forms, such as pellets and briquettes [65]. Poland ranks seventh in terms of gross energy availability from solid biofuels in the EU (approx. 6.2 Mtoe) ( Figure 2)

Gross Energy Availability from Solid Biofuels in the EU
The policies concerning climate and energy in the European Union encouraged the development of renewable energy sources including biomass, biogas, and others. The basic benefits include support schemes for renewable energy sources. Moreover, using RES delivered economic and environmental benefits [1]. Each country of the EU included bioenergy policy in environmental policy. Furthermore, the production of biomass is also linked to agricultural policy [64].
Biomass is an organic substance created in the process of accounting solar energy [6]. Biomass also includes processed forms, such as pellets and briquettes [65]. Poland ranks seventh in terms of gross energy availability from solid biofuels in the EU (approx. 6.2 Mtoe) ( Figure 2) In 2017, the production of basic energy from renewable sources in the EU-28 amounted 226.5 million tons of oil equivalent (toe). Compared to 2004, this is an increase of 66% in the amount of renewable energy produced in the EU-28, which corresponds to an average annual increase of 4.7% [66].
Among renewable energy sources in the EU-28, the most important source is wood and other solid biofuels, which accounted for 42% of primary renewable energy production in 2017. Wind energy is on second place in terms of share of the renewable energy mix (13.8%), followed by hydropower (11.4%). Biogas accounts for 7.4% of the share in the production of energy from renewable sources, liquid biofuels were 7.4%, and solar energy was 6.4%. The thermal energy of the environment (captured by heat pumps) was 5%, geothermal energy was 3%, and the share of renewable waste increased to 4.4% [67].
In recent decades, the development of renewable energy sources (RES) has become one of the main goals of the energy policy of the EU countries, including Poland, where biomass receives special attention [64].
The decreasing energy sources of fossil raw materials and the need to reduce greenhouse gas emissions, especially CO2, indicate the need to use RES [68].
The independence of the economy from fossil fuels and the reduction of greenhouse gas emissions are the main reasons for introducing renewable energy sources. Self-combustion of coal causes a significant deterioration of air quality through the emission of toxic substances such as SO2, NO2 and CO2 [69].
Combating climate change and the desire to reduce CO2 emissions and other harmful gases have made the EU's policy of counteracting the increase in pollutant emissions a priority task. As a result, a number of EU industry policies were subordinated and a common agricultural policy was launched, which is responsible for the functioning of the agricultural sector to support renewable energy. It has been contractually assumed that renewable fuels for the purposes of the climate package have zero emission [70]. In 2017, the production of basic energy from renewable sources in the EU-28 amounted 226.5 million tons of oil equivalent (toe). Compared to 2004, this is an increase of 66% in the amount of renewable energy produced in the EU-28, which corresponds to an average annual increase of 4.7% [66].
Among renewable energy sources in the EU-28, the most important source is wood and other solid biofuels, which accounted for 42% of primary renewable energy production in 2017. Wind energy is on second place in terms of share of the renewable energy mix (13.8%), followed by hydropower (11.4%). Biogas accounts for 7.4% of the share in the production of energy from renewable sources, liquid biofuels were 7.4%, and solar energy was 6.4%. The thermal energy of the environment (captured by heat pumps) was 5%, geothermal energy was 3%, and the share of renewable waste increased to 4.4% [67].
In recent decades, the development of renewable energy sources (RES) has become one of the main goals of the energy policy of the EU countries, including Poland, where biomass receives special attention [64].
The decreasing energy sources of fossil raw materials and the need to reduce greenhouse gas emissions, especially CO 2 , indicate the need to use RES [68].
The independence of the economy from fossil fuels and the reduction of greenhouse gas emissions are the main reasons for introducing renewable energy sources. Selfcombustion of coal causes a significant deterioration of air quality through the emission of toxic substances such as SO 2 , NO 2 and CO 2 [69].
Combating climate change and the desire to reduce CO 2 emissions and other harmful gases have made the EU's policy of counteracting the increase in pollutant emissions a priority task. As a result, a number of EU industry policies were subordinated and a common agricultural policy was launched, which is responsible for the functioning of the agricultural sector to support renewable energy. It has been contractually assumed that renewable fuels for the purposes of the climate package have zero emission [70].
Biomass requires considerably less investment than other types of renewable energy to produce a unit of energy. In economically developed countries, there is an over-production of food products, thanks to which, it is possible to use a part of agricultural land for the production of biomass for non-food purposes. Biomass is a solid fuel that can be stored compared to wind or solar energy. Creation of a new direction of agricultural production creates new jobs in agriculture and its surroundings, and increases agricultural income which stimulates the development of local industry and rural areas [71].
Renewable energy sources have lower emission of hazardous substances. The modern concept of bioeconomy includes the production of biofuels and other renewable energy sources [72]. Solid biomass is mainly used for heat and electricity, whereas biofuels can be used in transport.
Solid biofuels account for 44.6% of all RES consumed in the European Union and are the main source of clean energy [73,74]. Solid biomass is a very important stock for the production of heat and electricity ( Table 2). Total gross heat production (ktoe) produced from solid biomass achieved the biggest increase (151.2%), the next highest was gross electricity production (100.6%) and gross inland consumption (49.3%). The smallest increase was achieved by primary energy production (43%). In 2016, domestic primary energy production amounted to a total of 2,804,263 TJ (terajoules), of which renewable energy from solid biomass amounted to 265,780 TJ, giving a total of 265 petajoules (PJ) [75].
Reflecting the availability and current use of biomass compared to others, as carriers of renewable energy, gives us a picture of the advantage of this raw material over other sources of renewable energy in Poland.

Gross Energy Availability from Solid Biofuels in Poland
Poland has enormous potential, consisting of forest biomass, agricultural products, extensively used grassland, and special purpose crops as willow and poplar. On the other hand, the main source of biomass is forestry and the wood industry, as well as agriculturepartly derived from by-products and waste [74]. The technical potential of biomass in Poland is estimated at about 900 PJ/year [76].
Due to favorable climatic and soil conditions, Poland is obtaining energy from renewable sources of approx. 70% from biomass [77]. This is much more than the EU average for obtaining energy from biomass as a single carrier from renewable energy sources. Solid biofuels dominate other energy carriers, including the most popular wind energy, overtaking it by almost four times.
In order for biomass to be used in the process of conversion to secondary energy carriers, it is necessary to incur low material and energy expenditure for its acquisition. It can be used to produce heat, transport fuels, or electricity. The use of this raw material is increasing rapidly [76].
Poland's task is to meet the environmental protection requirements resulting from international obligations and agreements. This contributes to the increase in the amount of en-Energies 2021, 14, 3587 9 of 22 ergy produced from renewable sources, including from biomass. The production of energy from biomass undoubtedly contributes to the reduction of environmental pollution [69].
Directive 2009/28/EC of the European Parliament and of the Council of April 23, 2009, on the promotion of the use of energy from renewable sources, obliges European Union countries, including Poland, to share renewable energy in the final gross energy consumption at the level of 15%. This is lower than the overall EU level (20%) [78]. In Poland, the share of energy from renewable resources in the overall energy balance is still too low, which is a problem [69]. The diversity of biomass and the possibilities of its processing make it ideal for the production of thermal energy, especially on the local market, as part of distributed and cogenerated energy [79].
The use of renewable energy sources in Poland is slowly becoming economically viable. Renewable energy sources in the country could play an important role in the economy, and biomass, especially from agriculture, should play a significant role in this, while contributing to the development of rural areas and agriculture [79].
Solid biomass production is strongly linked to rural areas. Poland is a country with great possibilities for the development of biomass because 93% of the total area is rural. The arable land share is 65% of the country, which shows the possibilities. Moreover, lower quality land including 34% of class V and VI can be used for energy crop production [80].
Finally, we have analyzed the renewable energy carriers from renewable energy sources in Poland (Table 3). As we can see, solid biofuels have the biggest share in Poland in the years 2005-2019. However, the share of solid biofuels decreased 26%, which raises a question about how much of the market will stay in solid biofuels. In the same period, the share of wind energy increased almost 4700%, fluid biofuels 385% and biogas 250%. Water energy decreased its share by −43%. The data in Table 3 are very optimistic for Poland because problems with energy exists. For example, most of the buildings in rural areas are old and have not been modernized, as well as having poor thermal installation. That is why the usage of biomass for heating rural homes is important [81]. The share of renewable energy sources in the energy balance will increase in Poland as result of requirements and development of the sector [82].
The production of heat and electricity from solid biomass in Poland is presented in Table 4. As we can see, the solid biomass production in Poland increased in the years 2005-2019. The highest increase was observed in total gross heat production (628.3%), gross electricity production (339.1%), primary energy production (149%) and gross inland consumption (158.3%). Solid biomass is the most important source of renewable energy, not only in Poland but also in Europe and the world. Poland has good conditions for the production of solid biomass. It has 2 million hectares of less productive land that can be useful in biomass production. Poland is a major source of solid biomass because the farmland area per person is 0.41 ha., whereas in EU-15 the area is 0.19 ha. Biomass is mainly used for heat production in Poland [12]. It can be used for pellet and briquet production, too. The storage and transport of pellets and briquets have many advantages mainly because of saving time, as well as surface storage and lower costs of transport [83].
Forests play an important role as the pellet source in the process of conversion of woody biomass into conventional. Forests also play an important role in the process of carbon dioxide absorption from the atmosphere. The value of forests can be estimated in its role in culture, tradition, and aesthetic and spiritual values [84]. The proper utilization of forests can improve the sustainable management of this natural resource and also sustainable development of society [85].
However, the development of renewable energy sources meets many obstacles in Poland. The main problem is imports of cheaper biomass from markets outside the EU. Pellets from biomass from Russia, Ukraine, and Belarus are cheaper compared to the Polish production. Moreover, strict requirements of biomass combustion factories including average and long-term contracts are also factors hindering the development of biomass in Poland [86].
The production of biomass in Poland is particularly important in the context of fossil fuel usage. Poland is the second largest EU coal consumer after Germany [87]. Poland, as a country, is an important producer of hard coal (70.7 million tons in 2016) and importer (8.3 million in the same period) [88]. Most of the electricity is produced in Poland from hard coal (92%) and heat (about 89%) [89].
One main product of biomass is pellets. They can be produced both from wood and straw. The data in Figure 3 show that wood pellet production increased from 160 thousand metric tons (TMT) to 1300 (TMT) in 2005-2019. The increase of more than 800% shows the possibility of potential production in Poland. One main product of biomass is pellets. They can be produced both from wood and straw. The data in Figure 3 show that wood pellet production increased from 160 thousand metric tons (TMT) to 1300 (TMT) in 2005-2019. The increase of more than 800% shows the possibility of potential production in Poland.  Table 5 shows the descriptive statistics of global pellet production. As we can see, the coefficient of variation reached a high level of 89.06%. The average global wood pellet production was 18.59 million metric tons.

Descriptive Statistics of Wood Pellet Production in the World and EU
Skewedness and kurtosis measure the asymmetry of the data [51,52]. However, kurtosis is useful in thin tails. For example, in the normal distribution, skewedness describes the extent to which the analyzed distribution is different from the normal distribution. The skewedness was positive, whereas kurtosis was negative.   Table 5 shows the descriptive statistics of global pellet production. As we can see, the coefficient of variation reached a high level of 89.06%. The average global wood pellet production was 18.59 million metric tons. Skewedness and kurtosis measure the asymmetry of the data [51,52]. However, kurtosis is useful in thin tails. For example, in the normal distribution, skewedness describes the extent to which the analyzed distribution is different from the normal distribution. The skewedness was positive, whereas kurtosis was negative.

Descriptive Statistics of Wood Pellet Production in the World and EU
Europe is a very important producer of renewable energy. However, the production of renewable energy sources are diversified in Europe. Iceland (70.551%), Norway (64.717%) and Sweden (48.446%) achieve the largest average share of renewable energy sources ( Table 6). The Eurostat data [4] show that the smallest average share of renewable energy sources was achieved in Malta (3.1353%), Luxemburg (3.8492%) and Netherlands (4.7304%).
Increasing climate change, which goes hand in hand with the increase in population, contributes to air pollution through the use of conventional fuels. This causes a continuous increase in energy demand. The steps taken by the EU offer a chance to improve the situation, as well as the possibility of dealing with research issues and taking appropriate steps to prevent future actions of states in its harmful contribution to the devastation of our planet. The coefficient of variation informs us about the changes that took place in renewable energy sources in European countries. As we can see, the largest changes were found in Malta (99%), Luxemburg (56.97%) and Ireland (45.68%).
The highest share in renewable energy sources has solid biomass. It is mainly produced from straw, wood and other ingredients. It can be used in the combustion or as a source to produce different products such as pellet. Table 7 presents the descriptive statistics of wood pellet production in the EU. Germany (2.0871 Mtoe), Sweden (1.4286) and Poland (730.00 Mtoe) achieved the highest average production of pellet in the European Union. The coefficient of variation was the highest in Latvia (172.80%), France (93.43%) and Austria (79.71%).
A normally distributed variable can be described by skewedness and kurtosis near zero [90,91]. Skewedness was negative for Germany, Sweden, France, Austria, Estonia, and Spain, which means that the mean of negatively skewed data was less than the median. Kurtosis was negative in all countries. Our results depend only on stationarity and the existence of some moments as it was achieved by other authors [51].
Europe is also an important pellet producer. Table 7 shows the main pellet producers in the EU. Poland is in seventh place as the producer of pellet with the amount of 950 million metric tons. The biggest producers in 2017 were: Germany (2.250 million metric tons), Latvia (1.55 million metric tons) and Sweden (1.42 million metric tons). The average pellet production in the EU increased 50.5% in 2011-2017. The biggest increases of pellet production were observed in Estonia (189.5%), Spain (129.2%) and France (127.3%). Only Portugal recorded a decrease in pellet production in 2011-2017 (−48.7%).

Stationarity of Solid Biomass and Pellet Production in the World and Poland
In order to analyze changes in total gross heat production and pellet production, the Dickey-Fuller test was carried out, which examines the stationarity of time series [92]. The results presented in Table 8 show that the p value is quite large, which does not justify rejecting the hypothesis H0 which states that the series is non-stationary due to the presence of the unit root. The values of the mean, variance and autocorrelation function change over time, which proves that the time series are not stationary. On the other hand, the H1 time series hypothesis is stationary, i.e., static and dynamic properties remain unchanged at any time shift, and therefore, the mean value and variance of the time series elements are constant [93]. Finally, we conducted the ADF test with intercept, and with intercept and trend (Table 9). We took the differences and did the ADF test once again and achieved the stationary variables. Autocorrelation of the first-order suggests that the variables were stationary. The ADF test proved the existence of unit root.
The empirical evidence suggests that there is no serial correlation for the total gross heat production from solid biomass because the α1 < β1, which suggests that the conditional variance is independent from previous observations (Table 10).
According to our prognosis, the wood pellet production in the world will slightly decrease (Table 11). This can be the effect of looking for cheaper and cleaner sources of renewable energy. Table 9. ADF test for total gross heat production from solid biomass and pellet with differences.    Table 12 presents maximum-likelihood estimates for the Generalized Autoregressive Conditional Heteroscedasticity (GARCH) model.
The empirical evidence suggests that there is no serial correlation for the analyzed series. The conditional t distribution is distinctly fatter-tailed than the normal. For global wood pellet, the α1 < β1 which suggests that the conditional variance is independent from previous time series.
Our final step was to prepare a prognosis of global wood pellet production. Our prognosis suggests that the global wood pellet production will increase almost 81% (Table 13). Even though other renewable energy sources will increase their share in the energy mix, the pellets, which are organic, increase also.
Poland is an important producer of wood pellets in Europe. Polish pellet production reached 1.3 million metric tons. Most were used by residential consumers, 25% by commercial or industrial entities, and 13% by commercial stakeholders to produce energy for heat for sale [94].   Table 14 presents the Generalized Autoregressive Conditional Heteroscedasticity (GARCH) model of wood pellet production in Poland. The empirical evidence suggests that there is no serial correlation for the analyzed series. The conditional t distribution is distinctly fatter-tailed than the normal. For global wood pellet, the α1 < β1, which suggests that the conditional variance is independent from previous observations.  Table 15 shows the prognosis of wood pellet production in Poland. As we can see, the production of wood pellets could increase as the result of changing policy, increased demand for renewable energy sources, and the development of the wood processing industry. Poland, an important producer of pellets in the EU, is responsible for a reduction of emissions of greenhouse gases into the atmosphere. The projection of pellet development will undoubtedly decrease the gas emissions that are responsible for global warming which creates losses for the environment and the economy as a whole [95].
Poland is achieving more energy from renewable energy sources each year because the EU imposed a duty to adjust emissions in all countries. Poland's aim was to achieve 15% of renewable energy sources of gross energy consumption in 2020. Poland has a good climate and soil conditions for plant cultivation for energy purposes. Many lands can be devoted to cereals, rapeseed, straw, and forest. The forest cover in Poland is over 30% and it is a basic source for timber waste. Basket willow and other energy plants can be harvested in Poland for energy [96].

Discussion
The development of renewable energy sources depends on global demand from clear energy. Its development is also regulated by law.
The European Union established the necessary reduction of GHG gases (greenhouse gas) emission reduction by 80-95% by 2050. Such an ambitious goal can be achieved by using renewable energy sources which could be increased between 55% and 75% in gross final energy consumption in 2050. Moreover, changing to a competitive green economy requires constant changes in law concerning climate, environmental pollution levels, and renewable energy sources [1].
However, to achieve these ambitious goals, the EU must make significant investments. In some EU countries, the investment levels are insufficient and actions should be taken to avoid shrinking the green economy [97]. Such an investment should amount to 60-70 billion euros to achieve the planned development of a green economy, which is very important for humans [98].
This green economy based on biomass instead of fossil fuels will generate constant changes in agricultural, economic, and social systems. This economy can also be called a bio-based economy "or a knowledge-based bio-economy". This approach generates many benefits for sustainable development, and perspectives for people and the environment [29]. This economy is described as a low-carbon and resource-efficient strategy that will improve energy security and human well-being [99]. According to this concept, the use of raw materials, innovation development, and biotechnology in different sectors is necessary [100].
The European Union enabled each member state to decide its own ways to increase renewable energy use. This bottom-up approach gives independent choice for each member state to elaborate its way to meet the goal, and the European Commission is monitoring the activities. The production of biomass plays an important role in policies of member states [97].
Biomass has an important role in EU strategies that restrict energy security and climate change reduction. It can contribute to the decrease of carbon dioxide concentrations in the atmosphere. Supporting plant production enables carbon dioxide storage in vegetation and substitution of fossil fuels [101].
Sustainable agricultural production and biomass utilization require intensive, productive technologies to enable high yield achievements from plants. The increasing demand for energy and rational use of energy sources are of particular interest in the European Union. The changing of replacement of traditional fossil fuels are inhibited by high costs of new technologies and low pace of legislations [11].
Moreover, the increase of biomass can be achieved by using land that is not valuable for agricultural production. Willow, miscanthus, and other plants deliver solid biofuels and do not require good land. The further utilization of existing resources and intensification of current production can also be the source of the development of bioenergy production [102]. This can create a competition between good quality lands that can be used for food production and for renewable energy crops. Another reason that can create the market for renewable energy sources are production costs. Lower costs of biomass compared to fossil fuels can stimulate the development of renewable energy sources [103].
The development of bioenergy requires an evaluation of the bioenergy potential. This should be done not only globally but also on a regional scale. Each EU country has different potential for different renewable energy sources and the strategy should be adjusted to existing resources [104].
The usage of biomass has advantages and disadvantages. The most important advantages include: the safety of transport compared to gas, a low balance of carbon dioxide, the possibility of using low quality lands for energy purposes and the possibility to create jobs in rural areas. The most important disadvantage is the low energy value of biomass, and some parts of biomass are only accessible seasonally [105]. The other barriers to the development of biomass include: small biomass markets, poor subsidies for crops devoted to energy purposes, or the lack for emission control systems for energy installation [106].
In many countries, the development of alternative energy is stimulated, including a partial transition to biofuels and the deployment of capacity for its production. The consequences of this step are ambiguous as intensive production of biofuels might significantly raise the price of food, deplete the soil, and increase water scarcity. Already, the production of biofuels is called as a cause of rising grain prices.
However, all these steps are not sufficient to keep the Earth's temperature at a relatively safe level [107].
Solid biomass is a major source of renewable energy mainly used for heating in Poland and Europe. However, its share in renewable energy sources is decreasing. The future of solid biomass in Poland and the European Union will depend on various factors such as wood and straw access. It is worth mentioning that the possibility of using straw in biomass production and using it in heating depends on agriculture development and its utilization in animal production. Some regions of Poland suffer from the lack of straw because of intensive cow production [12].
The solid biomass can help the countries to achieve planned goals. In China, nonfossil energy in primary energy consumption should be increased from 15% to 20% in the 2020-2030 [108]. In the EU-28, the achievement is of 40% mitigation in domestic GHG emissions over the period 2021-2030. Moreover, the contribution of renewable energy sources in total energy consumption should reach about 32% by 2030. Finally, the target of about 32.5% increase in energy efficiency in 2030 at EU level should be reached [109]. Global actions should be taken to secure a likely chance of meeting the 2 • C target in the future. A 2030 target of 67% below 1990 for the EU-28, a 2025 target of 54% below 2005 for the USA or a 2030 target of 32% below 2010 for China should help to solve the problem of achieving a fully decarbonized world [110,111].

Conclusions
The usage of energy from renewable sources is increasing worldwide. Biomass is actually the most important source of renewable energy. Pellets can be a good example of solid biomass. The international market for biomass and pellets is increasing. The European Union is an important producer and consumer of biomass [105]. The further development of the biomass market requires a proper climate and energy policy.
In general, environmental policy remains the most important activity of the European Union. In this area, an extensive system of environmental legislation has been formed, the norms of which are successfully implemented in practice. Each member state should adjust its own policy to the general requirements of the EU [38].
Our research demonstrates the development of solid biomass production in the world, EU, and Poland. Such an increase can demonstrate the increasing demand for renewable energy sources created by environmental policy.
Poland is an important biomass and pellet producer in the EU. It has large potential for solid biomass production. However, the further development of solid biomass depends not only on agriculture and forestry, but also the pace of other renewable energy source developments [84]. However, about 28% is distributed on the domestic market. It can be the result of a small investment in burning installations, as well as instability of prices and supply [112]. Most of the pellet and biomass are achieved in Poland from forest waste. However, biomass can be obtained from perennial energy crops, such as Salix coppice (Salix spp.), Sida hermaphrodita (Virginia mallow) and Miscanthus giganteus, which can be cultivated on poorer quality agricultural land [113]. The market for pellets is increasing because the EU is importing about one third of the pellets used [114]. The largest producer of pellets in the EU is Germany, and the largest consumers are the United Kingdom and Italy [115].
Our prognosis demonstrates that wood pellet production will increase in the world and Poland. Such a result can be achieved by the support of renewable energy in European policy. Climate change and environmental protection will play an important role in world policy.
Our research confirms both hypotheses positively. Environmental policy is becoming stricter and requires an increase of global awareness by society. Our common environment is in danger and the policy should protect it.
The analysis of the wood pellet production in the EU proved that kurtosis was negative in all countries. Skewedness was negative for Germany, Sweden, France, Austria, Estonia, and Spain, which means that the mean of negatively skewed data was less than the median [116,117].
The ADF test proved that the analyzed variables were not stationary. That is why we took the differences and did the unit root test, achieving the stationary variables.
Moreover, the GARCH model confirmed that there is no serial correlation for the analyzed series. The conditional variance is independent from previous observations. Based on the obtained results, it can be concluded that modeling the series of pellet production by means of models is entirely sufficient AR (1)-GARCH (1,1) with conditional normal distribution [118][119][120].