Life Cycle Energy Consumption and Carbon Dioxide Emissions of Agricultural Residue Feedstock for Bioenergy

: The depletion of fossil fuels and climate change concerns are drivers for the development and expansion of bioenergy. Promoting biomass is vital to move civilization toward a low-carbon economy. To meet European Union targets, it is required to increase the use of agricultural residues (including straw) for power generation. Using agricultural residues without accounting for their energy consumed and carbon dioxide emissions distorts the energy and environmental balance, and their analysis is the purpose of this study. In this paper, a life cycle analysis method is applied. The allocation of carbon dioxide emissions and energy inputs in the crop production by allocating between a product (grain) and a byproduct (straw) is modeled. Selected crop yield and the residue-to-crop ratio impact on the above indicators are investigated. We reveal that straw formation can consume between 30% and 70% of the total energy inputs and, therefore, emits relative carbon dioxide emissions. For cereal crops, this energy can be up to 40% of the lower heating value of straw. Energy and environmental indicators of a straw return-to-ﬁeld technology and straw power generation systems are examined.


Introduction
Since the Industrial Revolution, the world economy has developed on fossil fuel consumption [1]. Our modern civilization relies heavily on fossil fuels (their annual consumption is around 82% of the primary energy consumption) [2]. Their combustion emits greenhouse gasses are causing an increase in the average temperature in the atmosphere. However, extending a prosperous civilization requires energy consumption [3].
Climate changes are worse than expected earlier [4]. In 2017, human-induced warming exceeded 1 • C above the pre-industrial level. The Paris Agreement of 2015 was aimed to achieve a balance between anthropogenic emissions and the removals of these gasses. The above agreement supports efforts to limit the temperature increase to below 1.5 • C [5]. To meet this ambition, it is necessary to reduce carbon dioxide emissions from the burning of fossil fuels using large-scale renewable energy supply systems. This direction corresponds to the UN's Sustainable Development Goal 7: "Ensure access to affordable, reliable, sustainable and modern energy for all" [6].
A primary purpose of the UN climate policy is to hold the anthropogenic global warming to below 2 • C. The power generation sector emits at least 30% of the total greenhouse gas emissions [7]. In 2018, the European Commission put forward an initiative to achieve net-zero emissions by 2050 [8]. The Commission called for studying the sustainability of They reported that the energy consumption of straw collection ranged between 1.73 and 2.52 MJ/kg [46]. Shang et al. [48] used the same methodology as Said et al. [46]. However, a product (grain) and a byproduct (straw) have different energy equivalents. This means that their formation needs different energy inputs and, therefore, the mass (yield) indicator does not provide an objective picture of the energy consumption for the production of straw and the carbon dioxide emissions.
In some research, the energy and greenhouse gas emissions analysis was based on organic feedstock transportation [49][50][51]. A number of researchers such as Migo-Sumagang et al. [52], Nguyen et al. [53], Shafie et al. [24], etc., used the following method: crop production costs are allocated between a product (grain) and a byproduct (straw) in terms of economic value. The allocation factor is applied in the crop production stage and calculated by the formula [52][53][54][55] where Yg is the yield of grain, kg/ha; Spr is the price of straw, EUR/kg; Ys is the yield of straw, kg/ha; Gpr is the price of grain, EUR/kg. Then the energy inputs and the greenhouse gas emissions in the total crop production are multiplied with the allocation factor of straw. There are three primary types of allocation: mass, energy, and economic [54]. Here, there are three methods used to find the embodied energy and greenhouse gas emissions of straw: • taking into account only the collection and transportation of straw (no cultivation stage); • the mass allocation (the proportion of straw yield to overall biomass yield); • the economic allocation (the proportion of straw costs to the overall costs of a product and a byproduct).
In our opinion, for biomass-based power generation projects, the energy and environmental indicators must be analyzed. Therefore, the allocation should be performed by the use of energy allocation. However, this type of allocation has not been sufficiently studied.
The purpose of this study was to develop a systematic and effective method for evaluating the embodied energy and the greenhouse gas emissions in straw production that can be used as a scientific basis for future strategies and policy development. The objectives of this research were: • to develop a mathematical model for finding the allocation of the total energy inputs and, therefore, the carbon dioxide emissions between grain and straw; • to determine the energy inputs into the straw formation; • to compare the mass and energy allocation indicators; • to explore the impact of energy and the carbon dioxide emissions associated with straw formation on the power generation indicators such as energy-specific costs and specific carbon dioxide emissions; • to compare energy and the carbon footprint indicators for different pathways of straw utilization: power generation and the substitution of mineral fertilizers.

Materials and Methods
A systematic literature review was conducted in this study to gather required data from scientific sources. A number of relevant criteria were applied to analyze the information collected.

Embodied Energy
Energy requirements in agricultural production were divided into three groups: direct and indirect (two subgroups) ( Figure 1). A direct energy flow included the energy of fuels Appl. Sci. 2021, 11, 2009 4 of 17 and the electricity that are used by machines. An indirect energy flow consisted of two subgroups. The first was the energy used in the production processes of fertilizers, herbicides, and other chemicals. The second subgroup was the energy used in the manufacturing processes of farm machinery. The direct energy is determined as: where B i is the fuel consumption of the i th type, kg/ha; EEf i is the energy equivalent (embodied energy) of the i th fuel, MJ/kg; W is the electricity consumption, kWh/ha; n is the number of fuels; EEe is the energy equivalent (embodied energy) of electricity, MJ/kWh. The indirect energy of the first subgroup is equal to: where M j is the mass consumption of j th chemical, kg/ha; EE j is the energy equivalent of j th chemical, MJ/kg; m is the number of chemicals. The manufacturing energy requirements for farm machinery depend on a number of factors such as a certain machinery's mass, use lifetime, etc., and can be computed by a formula: where EEM j is the embodied energy of k th machinery, MJ/kg; MM k is the mass of k th machinery, kg; ULT k is the utilization lifetime of k th machinery, h; OFC k is the operation field capacity of k th machinery, ha/h; l is the number of machinery. The total energy inputs are: The embodied energy (or energy equivalents) of main fuels, chemicals, and machinery are presented in Table 1. They are used to determine the indirect energy inputs. The embodied energy of electricity can be presented in different units such as MJ/MJ or MJ/kWh. Consumers usually measure electricity in kWh. However, some sources of information express embodied energy in MJ/MJ. To simplify subsequent calculations, in this study, we converted MJ/MJ to MJ/kWh. For this conversion, we assumed that 1 kWh = 3.6 MJ.

Energy Output Flow
Energy output flow (EOF) of crop production is the energy generated by a product and a byproduct. For cereal crops, EOF is calculated as the sum of the calorific values of grains and straw: where LHVg is the specific calorific value of grain, MJ/kg; LHVs is the lower heating value of straw, MJ/kg. The yield of straw is calculated based on the grain yield and a residue-to-crop ratio: where RCR is the residue-to-crop ratio of a certain crop.

Carbon Dioxide Emissions
Carbon dioxide emissions occur due to the combustion of fuels, well-to-tank emissions (extraction of raw material, refinery, transportation, etc.), the emissions associated with electricity generation (from fuel production to electricity generation, distribution, and transportation), and the production of machinery, fertilizers, and chemicals ( Figure 2). of straw, MJ/kg.
The yield of straw is calculated based on the grain yield and a residue-to-crop ratio: where RCR is the residue-to-crop ratio of a certain crop.

Carbon Dioxide Emissions
Carbon dioxide emissions occur due to the combustion of fuels, well-to-tank emissions (extraction of raw material, refinery, transportation, etc.), the emissions associated with electricity generation (from fuel production to electricity generation, distribution, and transportation), and the production of machinery, fertilizers, and chemicals ( Figure 2). The carbon emissions caused by fuels are formed by the following components: the fuel consumption per hectare, the carbon content in the vehicle fuel, and the well-to-tank (WTT) emissions of a certain fuel. For standard diesel fuel, WTT-based carbon dioxide emissions vary from 6.7 to 24 g CO2/MJ [67]. This corresponds to the following range: from 0.284 to 1.020 kg CO2/kg. Thereby, well-to-wheel (WTW) emissions caused by the use of diesel fuel is equal to: , kg/ha, where CCi is the carbon content in i th type of fuel, kg/kg; WTTi is the well-to-tank carbon dioxide emissions of i th type of fuel, kg CO2/kg. The carbon dioxide emissions from electricity generation are determined by the following formula [68,69]: where EFe is the emission factor, kg CO2/kWh.  The carbon emissions caused by fuels are formed by the following components: the fuel consumption per hectare, the carbon content in the vehicle fuel, and the well-to-tank (WTT) emissions of a certain fuel. For standard diesel fuel, WTT-based carbon dioxide emissions vary from 6.7 to 24 g CO 2 /MJ [67]. This corresponds to the following range: from 0.284 to 1.020 kg CO 2 /kg. Thereby, well-to-wheel (WTW) emissions caused by the use of diesel fuel is equal to: where CC i is the carbon content in i th type of fuel, kg/kg; WTT i is the well-to-tank carbon dioxide emissions of i th type of fuel, kg CO 2 /kg. The carbon dioxide emissions from electricity generation are determined by the following formula [68,69]: where EFe is the emission factor, kg CO 2 /kWh. During the chemical (fertilizer, pesticides, etc.) production process, carbon dioxide is emitted. Its quantity is computed by: where MF i is the consumption of i th chemical, kg/ha; CDE i is the carbon dioxide emission during the production process of i th chemical, kg CO 2 /kg. The manufacturing and assembly of agricultural machinery result in carbon dioxide emissions. Their specific values are equal to: where CDM k is the specific carbon dioxide emission during the manufacturing and assembly of the k th machinery, kg CO 2 /kg. The total carbon dioxide emissions are:

Allocation of Energy Inputs between Grains and Straw
The input energy creates the harvest, including grains and straw. Therefore, the energy must be allocated between grain and straw formation. We suggest carrying out this division, considering the energy value of each part of the harvest obtained. The total energy output is formed by the product (grain) and a by-product (straw): where EGout is the energy content of grain, MJ/ha; ESout is the energy content of straw, MJ/ha. The straw formation requires the following amount of energy: The share of energy, which is used for the straw formation, is computed as After the transformation of Equation (15), we obtained the following expression: or: A residue-to-crop ratio is not constant, and it depends on the yield [70,71]: where a and b are the parameters. Scarlat et al. [71] suggested an exponential relationship between the residue-to-crop ratio and the crop yield. Parameters a and b were estimated on the basis of data published by researchers [71][72][73].
If we use Equation (18), then the share of energy is:

Energy Input into Straw Formation
The data on the RCRs and the energy parameters per crop type are presented in Table 2 [70,71,74,75]. In this study, the residue-to-crop ratios and the shares of energy used for the straw formation are expressed in percent. The results of our calculations are shown in Figure 3. The shares of energy used for straw formation range from 24% to 70% of the total energy inputs. The SR indicators for the actual data of wheat production reported in some articles are presented in Figure 4 [76][77][78][79][80]. The increase in yield results in the decrease in the SR. The impact of crop yield on the SR was studied using Equation (18). RCR functions for wheat, barley, and corn were taken from the results found by Bentsen et al. [70]. Our calculations showed that the share of energy used for straw formation has a significant dependence on the yield and type of crop ( Figure 5). Corn has a higher value than other cereal crops. The energy consumption for straw formation and the energy consumption for straw collection were compared. Nguyen et al. [45] reported that the energy consumption for straw collection ranged from 0.37 to 0.588 MJ per kg of straw. Ou et al. [50] reported about 0.4256 MJ per kg of straw. However, if the energy consumption for straw formation is taken into account, the embodied energy ranges from 1.1 to 6.2 MJ/kg. Therefore, the energy consumption for straw formation may be up to 10-fold the energy consumption for straw collection, and it should be taken into account in an energy analysis of straw use.

Comparison of Mass and Energy Allocation Indicators
The mass allocation indicator is calculated by the following formula: This expression resembles Equation (17), which is used to determine the energy allocation indicator. The only difference is that the energy allocation indicator takes into account lower heating values of grain and straw. Based on Figure 6, it is apparent that the mass allocation indicator can be considered as a special case of the energy allocation indicator. We took the derivative of Equation (17) with respect to the lower heating value of the grain-to-straw ratio and found the rate of change: The results of our calculations are presented in Figure 7. The shape of the graph (Figure 7) depicts that the increase in the lower heating value of the grain-to-straw ratio results in a decrease in the rate of change of the energy allocation indicator.

Power Generation: Energy Specific Costs and Carbon Dioxide Emissions Associated with Straw Formation
The carbon dioxide emissions of straw-fired power generation can be expressed as: η e · LHVs · EFs, kg CO 2 /kWh, where η e is the efficiency of power generation; EFs is the carbon dioxide emission factor associated with straw formation, kg CO 2 /kg. And the saving of carbon dioxide emissions compared with a fossil fuel power plant can be expressed as: η e · LHVs · EFs, kg CO 2 /kWh, where EFcf is the carbon dioxide emissions factor (fossil fuel), kg CO 2 /kWh. The carbon dioxide emissions factor associated with straw formation was calculated for conventional wheat production technology used in Ukraine. The following initial data were used: the yield of grain-4110 kg/ha; the yield of straw-5138 kg/ha; the total carbon dioxide emissions-1747.9 kg CO 2 /ha; the share of energy that is used for straw formation-0.5. Our calculations showed that the carbon dioxide emissions factor associated with straw formation is around 0.17 kg CO 2 /kg. It results in 0.146 kg carbon dioxide emissions per kWh for a straw-fired power plant.
Energy specific costs (ESC) are the energy-input-to-electricity-production ratio: where ∆H is the energy used for straw formation, MJ/kg; Ws is the quantity of electricity generated by one kilogram of straw, kWh/kg.

One kilogram of straw generates:
Ws = η e · LHVs 3.6 , kWh/kg. (25) Then, the required energy specific costs are: Energy specific costs were calculated for the following conditions: • The efficiency ranges from 10% to 40%.

•
The relative energy used for straw formation (100 ∆H/LHVs, %) has two meanings: 0% (energy for straw formation is not taken into account) and 40% (maximum possible value for biomass direct-fired power plants).

Power Generation or Biofertilizer: Energy and Carbon Dioxide Footprint Comparison
There are some forms of straw use, such as straw-to-energy and straw return-to-field. Liu et al. [51] studied the benefits of straw return-to-field. Straw retained in arable lands can increase organic nitrogen, phosphates, and potassium, which enhance crop yield [81][82][83]. Researchers found a number of disadvantages of straw return-to-field technology, such as nitrous emissions, methane emissions, and the possible reduction in crop yield (in the case of high levels of straw retained) [84][85][86][87][88].
In this study, we evaluated the environmental and energy indicators of straw use as biofertilizers compared with power generation.
Crop residues can be used for power generation and use as fertilizers. The use of agricultural residues as a substitute for mineral fertilizers can reduce the energy consumption and carbon dioxide emissions of agricultural practice. That is why the comparison of different pathways for crop residues is of scientific and practical interest.
The use of crop residues as fertilizers saves the following amount of energy: where NG i is the content of the i th nutrient component in a crop residue, %; EEq i is the energy equivalent of the i th nutrient component, MJ/kg. An organic fertilizer produced from crop residues can reduce carbon dioxide emissions caused by the production process of mineral fertilizers as such: where CDFP i is the carbon dioxide emissions caused by the production of the i th nutrient component in a mineral fertilizer, kg CO 2 /kg. The carbon dioxide emissions and energy used by the production process of mineral fertilizers are presented in Table 3 [62][63][64]. As an example, corn stover was considered. Corn stover has the following composition of nutrient components: nitrogen-5.95; phosphorus-0.56; potassium-7.91 g/kg [89]. In our study, we used the data reported by Kim et al. [90,91]. The energy used for straw formation is around 8.81% of the total energy potential of straw ( Figure 9). The embodied energy of mineral fertilizers substituted by straw is at least four-fold compared with the energy used for straw formation. Therefore, the use of straw for energy generation is more energy-efficient. Carbon dioxide emissions were analyzed too. The total carbon dioxide emissions of corn production are around 1895 kg CO 2 /ha. The energy associated with straw formation is 1048 kg CO 2 /ha of straw. Therefore, WTT is equal to 0.112 kg CO 2 /kg. In our calculation, the following emission factor (power generation) was assumed: average-0.578; minimum-0.146; maximum-1.593 kg CO 2 /kWh [90]. Our study revealed that the use of straw for power generation saves more carbon dioxide emissions compared with the straw return-tofield technology ( Figure 10). Energy saving was analyzed too. The straw return-to-field technology can reduce primary energy consumption in the range of 3.28 to 7.65 MJ/kg. Straw for power generation can produce electricity in the range of 2.55 (the electricity efficiency is 20%) to 5.11 MJ/kg (the electricity efficiency is 40%) (Figure 11).

Conclusions
Agricultural residues including straw play a significant role in renewable energy supply systems. The use of agricultural residues can mitigate climate changes. They can also complement unstable renewable power technologies such as solar and wind in electricity generation systems. This study of energy and the environmental evaluation of straw as a fuel for generating power resulted in the following findings: The energy consumption of straw production accounts for 30-70% of the total energy inputs. This value depends on the yield and species of crop.
The use of straw for power generation has better energy and environmental indicators compared with the return-to-field technology (biofertilizer production).
Energy allocation provides the opportunity to account for the effects of the agricultural byproduct (straw) production on the energy and ecological indicators of power generation.
The authority should take into account the environmental protection and energy efficiency indicators when designing straw use policies and strategies. From the perspective of carbon dioxide emissions mitigation, straw-to-electricity is superior to the straw returnto-field strategy.
This study provides a framework for the energy allocation of energy inputs and, therefore, carbon dioxide emissions between a product and a byproduct.
There are some limitations in the present study. Bioelectricity is generated by strawfired power plants; however, there are different environmental-friendly and energyeffective technologies for straw use, such as the production of biogas, bioethanol, construction materials, etc. In addition, there are problems associated with the high cost of transporting and storing straw, as well as the use of advanced technologies for power generation, including biomass gasification). They can be the topics of future research explorations.