Ukrainian power plants annually generate more than 150 billion kWh. Approximately 37.8% of electricity is produced from fossil fuels (coal and natural gas) [1
]. In 2018, Ukrainian thermal power plants consumed 26.22 million tons of coal [2
]. To reduce greenhouse gas (GHG) emissions and improve energy security, renewable power generation needs to be developed. Ukraine currently operates 10 biomass and 33 biogas power plants [1
Since 2011, Ukraine has been a member of the Energy Community [3
]. Therefore, the country is obliged to implement European Union directives concerning renewable energy, including for biomass utilization. Despite the fact that Ukraine’s electricity consumption is decreasing, renewable power generation is almost stable (Figure 1
). Its share of the country’s power generation ranged from 5.49% in 2009 to 9.54% in 2018.
Large scale biofuel production from edible crops (corn, rapeseed, soybean, sugar beet) may have a negative impact on food supply and its prices, soil erosion, and biodiversity [7
]. Indeed, agricultural crop residue may be an environmentally and food friendly source of renewable energy [8
]. European Union policy is currently aimed at reducing the use of crops and increasing the use of agricultural residue as feedstock for biofuel production [9
]. Therefore, the availability of biomass for energy and material production is an important question for the development of energy policy. There are a number of studies discussing the agricultural crop residue energy potential available in the world, including at separate country and regional levels. Bioenergy is in the spotlight for researchers.
Most earlier studies were focused on energy potential, conversion technologies, or existing problems (environmental, financial, and political) of bioenergy [10
]. Gao et al. studied the spatial distribution and total biomass potential for energy production in China [12
]. Bentsen et al. provided estimates of agricultural residue production on a global scale. Its geographic variation in 227 countries was revealed [13
]. Ren et al. reviewed wheat straw utilization issues on a global scale and a national level (China). They analyzed utilization technologies and their commercialization, benefits, and barriers. Their study revealed that straw utilization technologies are currently becoming profitable and straw utilization results in fossil fuels saving and reducing hazardous emissions, especially greenhouse gases [14
]. Some studies assessed the bioenergy potential at local levels [15
]. The utilization of biomass residue for power generation was studied in some countries. The availability of biomass and its potential disadvantages were revealed [16
]. In addition to agricultural waste, agrifood industry waste was the subject of research for the production of energy resources [18
There are a number of studies devoted to the analysis of power generation from solid biomass in Ukraine. State-of-the-art of electricity production and choosing steam turbines for combustion based biomass combined heat and power (CHP) plants were carried out by Geletukha et al. [20
]. However, a combustion based technology is only one of the main existing technologies. Fedorchenko [21
] and Melnychenko [22
] used cluster analysis to evaluate the energy potential in the regions of Ukraine. In addition, Fedorchenko [21
] focused on energy crops (corn and rapeseed) for biofuel production and Melnychenko [22
] analyzed field-based crop residues. However, a number of the other crops (soybean, rapeseed, etc.) were left out along with any discussion on process-based crop residues and the residue removal rate. Moreover, the aforementioned studies only focused on a single region of the country. Zvorykin et al. [23
] studied the status and future developments of electricity generation in Ukraine. The development of power generation and existing barriers were studied too [24
]. To evaluate the efficiency of alternative energy resources, mathematical models have been widely used [25
All in all, the previous studies did not examine the potential for power generation from crop residue in Ukraine in a systematic manner. Specifically, the following limitations apply to the earlier research: only extant major power generation technology was considered (i.e., direct biomass combustion); availability of major crop residues, including field-based resources, was ignored (e.g., sunflower stalk and soybean straw); also, process-based resources were neglected (e.g., sunflower husk). Therefore, further comprehensive analysis is needed to estimate the available crop residues (based on current agricultural practice) and power generation technologies. In order to assess the potential of agricultural crop residue for power generation, this study focuses on: (i) the quantity of agricultural crop residue; (ii) their geographical locations; (iii) analysis of technologies for biomass energy transformation into electricity; (iv) potential power generation.
2. Literature Review on Biomass Energy
There are four main pathways for crop residue utilization (Figure 2
). Straw burning is still a common practice in some developing countries. A proportion of crop residue is stored at landfills. An alternative is to return crop residue to fields as a fertilizer and to prevent soil erosion. Straw is used for livestock bedding and housing too [14
Agricultural crop residue can be converted into electricity. Thermochemical, methanation, and fermentation processes can be used to obtain fuels [27
]. The thermochemical process is the most common. The fuels can be used in boilers (combustion based crop residue power plants), internal combustion engines (ICE), gas turbine engines, and fuel cells. Biomass co-firing with fossil fuels is an alternative to direct biomass burning. It has a number of advantages over direct biomass power plants: minimal investment costs and higher efficiency [16
] (Figure 3
Crop biomass can have a high mineral content. That is why its combustion results in combustion problems [3
]. To improve the quality of crop residue, different technologies are used, including gasification, pyrolysis, and liquefaction.
Being able to run at full load without unplanned outages and governmental subsidies would indicate the economic viability of straw utilization projects [30
]. The above technologies have different efficiency and maturity levels.
2.1. Combustion-Based Biomass Generation Plants
The electric efficiency of straw-based power and CHP plants depend on their capacity (Figure 4
] and ranges from 18% to 32%. This technology is widespread.
2.2. Gasification- and Pyrolysis-Based Biomass Power Generation Plants
The atmospheric oxidation and pyrolysis gasification technologies are mature for household supply. However, a number of technological problems and high investment costs hinder their commercial development [33
]. Their electric efficiency ranges from 10% to 33% [33
]. Their energy conversion factor ranges from 0.5 to 0.8 [34
]. According to experts’ recommendations, gasification based crop residue technologies are preferable if their electrical power production is less than 2 MW [38
]. Therefore, this technology is not widespread and still in the early commercial stages. Pyrolysis technology for power generation is currently not at a mature commercial stage [39
The Polohy oil extraction plant PJSC (Zaporizhya, Ukraine) has experience in utilizing residue (sunflower husk) gasification technology. However, this technology proved to be economically infeasible. Therefore, the plant abandoned this technology and currently uses direct burning of biomass.
2.3. Biogas-Based Power Generation Plants
Internal combustion engine (ICE) generators are more efficient than steam turbine generators. However, ICE generator need liquid or gaseous fuels (biogas, syngas, etc.), whereas steam turbine power plants can use all types of fuels. ICE generators’ electrical efficiency reaches 50% [40
], whereas combustion based biomass steam turbine power plants have an efficiency of under 35% [31
]. Therefore, it is appropriate to study biogas production. Straw may be used as substrate for biogas production. Cereal straw is considered to be a promising feedstock for biogas production in the European Union [27
]. The methane yield ranges from 242 to 324 m3
]. Sukhesh, M. J., and Rao, P. V. summarized the following information: a yield of 125 to 380 m3
/t for wheat straw, and a yield of–165 to 383 m3
/t for corn stalk [44
]. Straw has a high methane yield. However, straw is rarely used for biogas production due to the fact that it requires pre-treatment.
The energy conversion factor for biogas production varies from 0.38 to 0.88, while direct burning (conversion into heat) provides the opportunity to reach an efficiency of up to 0.9.
There have been demonstration projects in some countries such as Germany, China, and France [45
]. Since 2005, the Chinese government has supported straw biogas plants [46
]. Some of these demonstration projects are presented in Table 1
To convert biogas into electricity gas turbine engines, fuel cells, spark-ignition engines, and dual-fuel engines can be used. Internal combustion engines are currently widely used to convert biogas into electricity. In fact, in the European Union, biogas plants use spark-ignition engines (50%), and dual-fuel internal combustion engines (50%). Fuel cells and gas turbine engines are rarely used. The electrical efficiency of ICE is in a range between 34% and 48.7% (the capacity ranges from 100 kW to kW) [40
There are technologies to enhance the above indicator. These are turbo-compound, electric turbo-compound, thermoelectric generator, and organic Rankine cycle (ORC) [42
]. However, ORC is the most promising technology [52
]. It can increase the obtainable power by 5% to 19% [40
]. This means that the total potential power generation efficiency ranges from 12.9% to 50.2%.
In 2015, the total biogas production in Ukraine exceeded 600 TJ [53
]. Installed electric capacity of biogas plants is increasing. The electrical production capacity rose by up to 70 MW [54
]. Domestic and foreign practices related to biogas technology stimulate Ukrainian businesses to employ it further. Finish and German investors plan to build straw biogas plants in the Khmelnytsky and Vinnitsa regions [55
2.4. Ethanol Based Crop Residue Power Generation Plants
Crop residue including straw can be converted into ethanol. Commercial projects have been realized in some countries such as Italy, Brazil, the USA, etc. [56
]. Some examples of these plants are presented in Table 2
For straw, the ethanol energy conversion factor ranges from 0.24 to 0.32. Ethanol can be used for power generation in ICE or fuel cells. The efficiency of ICE can reach 40%. The same value for fuel cells is up to 50% [33
]. Due to relatively high production costs, it is fuel cells can be used for vehicle fuel, but are unprofitable for power generation.
Electricity consumption is distributed unevenly in Ukraine. The Dnipropetrovsk, Donetsk, and Kyiv regions consume 44% of total national electricity production. This poses certain challenges for development of the renewable energy sector in Ukraine.
Ukraine has abundant agricultural crop residue energy resources. Our results showed that the country annually generates up to 128.47 million tons of agricultural crop residues. Indeed, 48.66 million tons of residues may be used for energy production. Corn and sunflower stalks represent 65% of the total crop residues. In 2018, their energy potential was 774.46 PJ, which could be used for power generation.
Using cluster analysis, we identified five groups of regions. The energy potential of Vinnitsa and Poltava regions is 133.8 PJ, or 17.2% of the total national energy potential. They are preferable for setting up crop residue power plants. Regions with high electricity consumption do not have enough crop residue potential to meet their own requirements for electricity.
Power generation potential of crop residues is up to 108 billion kWh. This value is 67.8% of current national power generation. Thirteen regions have enough crop residues to satisfy their own needs in electricity. Combustion based biomass power plants can generate from 38 to 68 billion kWh power per annum. To increase electricity generation, methanation technology should be developed.
The results provide a scientific foundation for the development of biomass electricity policy in Ukraine. They may be helpful for creating autonomous power generation zones. The results can support policymakers and investors in their decisions. The advantages of biomass power plants are the following: the reduction of GHG emissions; job creation; strengthening the energy security of Ukraine; the utilization of crop residues; the reduction of fossil fuel consumption; increasing farmers’ income. Meanwhile, there are a number of risks [76
] such as significant harvest fluctuations, which may negatively affect the reliability of feedstock supply for electricity generation; increasing biomass feedstock prices; logistics problems. Finding possible solutions is the goal for further research.