Energetic and Economic Analysis of Spineless Cactus Biomass Production in the Brazilian Semi-arid Region

: The Brazilian semi-arid region is marked by a variable spatial-temporal rainfall distribution, concentrated over a 3 to 4 month season. Limited water availability is the main obstacle to the production of forage plants of C3 metabolism (such as corn and soybeans) and C4 metabolism (such as sugarcane), as well as livestock. To mitigate this forage supply, the spineless cactus (SC) has been cultivated in the region, producing high biomass amounts in this harsh environment. Recently, this remarkable capacity to produce biomass has drawn the attention of the renewable energy sector, supported by recent studies demonstrating the feasibility of its biomass as a raw material for bioenergy production. However, before moving to commercial scale, it is necessary to demonstrate that large-scale production has energy and economic viability for clean energy investors. Thus, the objective of this article was to analyze the energetic and economic viability of forage cactus cultivation systems in the Brazilian semi-arid region. The data used were extracted from the literature, based on forage production. For the energy evaluation, the energy balance was performed and the energy efﬁciency, energy productivity, speciﬁc energy, and net energy metrics were applied. The ﬁnancial feasibility analysis used the Net Present Value (NPV) and Internal Rate of Return (IRR). The energy balance revealed that the SC cultivation is viable for biomass commercial-scale production, with an energy efﬁciency of 3.36, an energy productivity of 0.25 kg MJ − 1 , a speciﬁc energy of 13.5 MJ kg − 1 , and an energy balance of 127,348 MJ ha − 1 . For the economic aspect, considering an attractive minimum rate of return of 8%, production also proved to be viable, in a time horizon of three years. The Net Present Value and IRR metrics were USD 2196 and the IRR was 46%, respectively. The results found are important to encourage new investments in rural properties in the semi-arid region, and cultivation in new areas proved to be an efﬁcient alternative from an energy and economic point of view, in addition to collaborating for the energy transition to sustainable sources and in the mitigation of regional environmental impacts.


Introduction
The search for clean energy has been boosting the production of biomass, bioenergy, and chemical compounds in arid and semi-arid regions. However, the management of these areas will be a major challenge for society, especially in a scenario of their expansion, due to climate change [1]. A large part of the Brazilian Northeast region has a semi-arid climate, and those that do not consider uncertainties (static or dynamic) [29]. Static and dynamic assays use average costs and benefits over time and are also used for biomass production assessments, as shown in the work of Sgroi et al. [30] for eucalyptus production, Testa et al. [31] for poplar, Styles et al. [32] for Miscanthus and short-rotation coppice willow, and Testa et al. [28] for the cultivation of giant sugarcane in Italy. The financial assessment of the spineless cactus system is complementary to the energy assessment, favoring the expansion of its production.
Based on the above, the objective of this article was to analyze the energy balance and the economic viability of the Cactus forage (Opuntia ficus-indica) biomass production system for energy purposes, in a semi-arid region, using the case of the Brazilian Northeast region. The findings of this study can better guide producers and thus offer solutions that can increase productivity efficiency, reduce costs, and maximize profits from agricultural systems.

Definition of the Planting System
A cropping system in Northeast Brazil with a medium level of efficiency has been simulated based on previous models developed by Ramos et al. [33], Lima et al. [34], and Dantas et al. [35]. The most representative characteristics were used in the simulated cropping system. The limits of the system under study, from the soil preparation process to the harvest and sale of forage cactus, were defined.

Analysis of the Energy Balance of the Cropping System
To calculate the energy balance, we used the methodology proposed by Rodón and Rodríguez [36] and De Las Cuevas et al. [37]. All energy consumed in the cropping system (input energy) was aggregated and compared with the energy obtained from the post-harvest forage cactus biomass (output energy). Table 1 shows the characteristics of the cultivation system in the Brazilian Northeast region and Table 2 shows the energy equivalents of the input energy of the cropping system that were used in the study.  Source: [10,16].
The method uses Equation (1) that aggregates the activities presented in Table 1 into groups.
where E T is the total energy used; E M is the energy in materials, manufacturing, and transportation; E F is the energy in fuel; E LF is the energy in lubricants and filters; E MR is the energy in maintenance and repairs; E S is energy in seeds; E FHP is energy in fertilizers, herbicides, and pesticides; E L is energy in labor, and E E is energy in electricity. The values are given in MJ ha −1 . E M includes all energy used to build the tractor and machinery (implements), plus the energy equivalent for field use. To estimate E M , we used the method proposed by Fluck et al. [46], expressed by Equation (2).
where M T and M M are the masses in kg of the tractor and machinery; E MT and E MM are the energies per unit mass (MJ kg −1 ), and T I and A I are the useful life in hours of the tractor and agricultural implements, respectively. The mass values of the tractor and machinery were obtained from the manufacturers' catalog. Table 3 shows the values used to calculate the E M . E F was determined through the product of the fuel consumption per hour (C, L h −1 ) multiplied by the fuel equivalent in energy (E EC , MJ L −1 ) (Equation (3)).
To estimate E LF and E MR , we used the methods proposed by Fluck [46] and Rodón, Fernandes, and Oliveira et al. [49], adopting values of 5% of E C for E LF and 129% of E M for E MR . In the method proposed by Rodón and Rodríguez [36] and De Las Cuevas et al. [37], relate the variables of the energy cost of agricultural machinery MJ h −1 . For the analysis of the energy balance, the values are expressed in energy per hectare, that is, MJ ha −1 . The conversion was done through the effective field capacity of the machines, CM (h ha −1 ) (Table 4), defined by the methodology proposed by Rodón and Rodríguez [36] (Equation (4)).
where A is the width of the machine (m); S is the working speed of the machines (km h −1 ), and τ is the time utilization coefficient in the working day. The width of the machines was obtained from the manufacturer's catalog, the working speeds were obtained from the study of Balastreire [47], and the coefficient of use of time in the working day (τ) was 0.63, a value proposed by Rondón [50] and Ibañez and Rojas [51]. Table 5 shows the values converted by the resulting field capacity factor of 2.89 h ha −1 of the elements under study referring to machinery. To quantify the energy contained in cladodes, fertilizers, herbicides, and pesticides, we used the methods proposed by Vilche et al. [45], Yilmaz et al. [42], Meul et al. [32], and Ren et al. [52] (Equation (5)). The dry matter (DM) content of cactus forage varies considerably depending on the genus and species, with values between 6.1% and 17.1%. Thus, the value proposed by Valadares Filho et al. [53] was established in the research. The average weight of the cladode was considered to be 1 kg [54]. The lower calorific value for forage cactus biomass was 13.5 MJ kg −1 [10].
where MC and MF are the masses in kg ha −1 , and E ECLA and E EFER are energy equivalents in MJ kg −1 of the planted cladodes and the fertilizers used, respectively; VH and VP are volumes in L ha −1 , and E EHER and E EPES are energy equivalents in MJ L −1 of herbicides and pesticides, respectively. Table 6 shows the values used to calculate the E S and E FHP . The biomass of the planted cladode per hectare was considered assuming a planting arrangement of 1.5 m × 0.5 m × 0.4 m with double planting rows. Energy of electricity (E E ) is the cost of availability, that is, the amount charged by the energy distributor to make the electricity service available to the customer. The cultivation system considered for this work does not have direct electricity costs, therefore it is charged through the minimum energy rate. According to the Pernambuco state distributor, CELPE, the availability cost for a single-phase customer is 30 kWh. Finally, the labor energy, E MO , Equation (6), was calculated using the methodology proposed by Tabatabaeefar et al. [44] and Mittal and Singh [40].
where T man and T mec are the hours of work, and E man and E mec correspond to the equivalent energy in MJ ha −1 of manual and mechanical work, respectively. For calculation purposes, 8 h of work per day in the field were considered. The data used to calculate the energy by human labor are described in Table 7. The energy of the cactus forage that were obtained after harvest, the output energy, E harvest , in MJ ha −1 , was necessary to estimate the mass of each plant and multiply by the quantity of plants, the energy equivalent, and the percentage of dry matter (DM) (Equation (7)). According to Silva et al. [55], the O. fícus-indica plant has an average of 5.58 cladodes per plant after two years of growth. The lower calorific power (LCP) of a forage cactus plant was considered to be 13.5 MJ kg −1 .

Energy Indicators
To determine energy indicators, methodologies were used proposed by Tabatabaie et al. [56] and Nabavi-Pelesaraei et al. [57]. The energy efficiency of the crop system was defined as the energy provided by the forage cactus at harvest time (output energy), divided by the sum of all the energy spent in the crop cycle (input energy) (Equation (8)). Overall, this indicator is used when the purpose of the process is energy generation [19].
Energy productivity was defined as the amount of cactus forage that can be obtained, in kilograms, for each MJ used in the production process (Equation (9)).

Energy Productivity =
Cactus Forage productivity(kg ha −1 ) Deposited Energy(MJ ha −1 ) Specific energy (MJ kg −1 ) was defined as the amount of energy contained in each kilogram of forage cactus after harvest (Equation (10)).

Speci f ic Energy
The net energy was defined as the difference between the output energy obtained after harvest and the total energy used in production (input energy) (Equation (11)).

Economic Analysis of the Cropping System
The production process (for one hectare of planted area) considered the price of purchase of cladodes for planting and manure (organic fertilizer), and the human and mechanized labor. The purchase and depreciation of machinery were not considered, since the machinery would be leased [34,35].
The financial viability analysis was carried out through discounted profits and expenses using the cash flow method [58]. The Net Present Value of investment (NPV) and the Internal Rate of Return (IRR) for the production of forage cactus were measured. The temporal plane was only the first rotation of the forage cactus, considered to be three years.
The NPV transfers to the initial period of analysis all the future cash flow of the study, with a discount rate (r), rhe r, of 8% [35,58]. The purpose of the NPV is to identify whether the investment made has a return greater than its cost [59]. When the result shows a positive NPV, the project is viable, while a negative NPV indicates a return below the minimum assumed rate and an economic unviability. The NPV was obtained according to Equation (12).
where I is the initial investment; t is the time (in years); n is the total project time (3 years); r is the discounted rate (8.0%), and FC is the cash flow per period. The project is accepted if the Internal Rate of Return (IRR) is greater than the r and rejected otherwise. This indicator guarantees that the investor obtains, at least, the required rate of return and was calculated using Equation (13).
where I is the Initial investment; t is time (in years); n is total project time (3 years), and IRR is Internal Rate of Return (8.0%).
In order to apply the NPV and IRR financial instruments, it was necessary to determine the prices of all necessary items in the cultivation system, from the planting stage to the harvest. The data used were extracted from the literature, following the common cultivation conditions in the Brazilian Northeast semi-arid region [34,35,60]. Table 8 shows the prices of the items used in the investment phase.
Investments in planting cladodes, animal manure, herbicides, labor for cleaning the land, planting, fertilizing and applying pesticides, tractor plowing, furrowing, and subsoiling were considered as initial investments, that is, were carried out in period 0 of the project's cash flow. The investment in harvesting labor was considered to be paid in the second period of the cash flow. Finally, the investment in electricity was divided into two equal parts between period 1 and 2. To calculate the income, we assumed the mass of each plant at the time of harvest, the spacing of plants in the field and the market price of the cladodes. Equations (14) and (15) were used to calculate the final sales weight (W final ) and revenue, respectively. The number of cladodes per plant was 5.58 and the mass of each cladode 1 kg [54,55].
Sales revenue = W f inal (kg) × Cladode unit (R$) (15) After calculating the values of the final cactus mass and the revenue from its sales, the cash flow for the period between the first investment made and the receipt of the value referring to the commercialized cladodes was constructed ( Figure 1). After calculating the values of the final cactus mass and the revenue from its sales, the cash flow for the period between the first investment made and the receipt of the value referring to the commercialized cladodes was constructed ( Figure 1).

Energy Analysis
The total energy required for the production of cactus forage biomass was Eha = 54,008

Energy Analysis
The total energy required for the production of cactus forage biomass was E ha = 54,008 MJ ha −1 (Table 9); such value is higher than those reported for corn and sweet sorghum crops in Poland, which had energy expenditures of 25,517 MJ ha −1 y −1 and 22,816 MJ ha −1 a −1 , respectively [62], and sugarcane, cassava, and corn in Brazil with values of 14,370 MJ ha −1 y −1 , 9528 MJ ha −1 y −1 , and 15,634 MJ ha −1 y −1 , respectively [63]. The cladodes for planting represented the highest energy consumption (32,501 MJ ha −1 ), followed by fertilizers, herbicides, and pesticides (E FHP ), with a third of this amount (8.999 MJ ha −1 ). The lower consumption was that of lubricants and filters for the machinery (90 MJ ha −1 ).
Biomass productivity varies according to the genetic potential of plants, soil conditions, climate, and agricultural operations [21]. The energy obtained in the cactus harvest, after two years of cultivation, was 181,357 MJ ha −1 (Table 10), a value higher than other crops such as soybean (67,641 MJ ha −1 ) and corn (66,971 MJ ha −1 ) in southern Brazil [64], and lower than sweet sorghum (average of 346,725 MJ ha −1 ) in Mexico [19]. The energy efficiency was approximately 3.36, that is, the energy at the end of the process was 3.36 times greater than the energy invested (Table 11). Therefore, cultivating Cactus forage (Opuntia ficus-indica), with a moderate level of technological intensity is capable of generating a positive balance energy. According to Schroll [65], an energy efficiency of 2 is already considered reasonable in Denmark's ecological sustainability development policy. Jankowski et al. [62], in Poland, obtained energy efficiencies of rapeseed (Brassica napus subsp. Napus) and mustard production that ranged from 1.24 to 4.92.
The energy productivity was 0.25, representing a ratio of 1 /4 of the cactus forage biomass production for each MJ consumed in the crop system (Table 11). This is a smaller value when compared to sweet sorghum, which had energy productivity values ranging from 1.81 to 2.62 [19], and higher value when compared to wheat, which had values ranging from 0.018 to 0.114 [44]. For each kilogram of cactus harvested, 13.50 MJ of energy was obtained (specific energy), which represented a net energy of 127,349 MJ. Such values are greater than those reported for sweet sorghum [19].

Economic Analysis
The mass harvested was 139,500 kg and the revenue from its sale was USD 5987. The results indicated economic viability for cultivating cactus forage in Northeast Brazil. The IRR of 46.1% was 5.76 times higher than the r, indicating a highly viable investment that can attract attention to this culture. The NPV was USD 2196, which represents a profitable investment for the producer (Table 12). The highest investment was the purchase of cladodes for planting and, even if the price undergoes a considerable adjustment, the project is economically viable. The profitability found in this work was lower than in similar studies that evaluated the economic viability of cactus cultivation systems. Dantas et al. [35] found an NPV of USD 22,723) and an IRR of 88%, considering an r of 8%, but considering higher biomass production (300 t ha −1 y −1 ) and a 10-year time interval. However, both studies were carried out under an irrigated system, which has high costs of production and also a much higher biomass productivity per hectare than the rainfed system we evaluated.
One of the advantages of the cactus system is the positive return in small, planted areas (1 ha), different from that observed for other crops, such as sugarcane, which is only economically viable in areas above 51 ha [66]. Naturally, larger cactus areas would have higher returns, given that the profitability of the system should increase, due to lower logistical costs [13,67].
The price of the cladodes for planting was the largest cost associated with the crop system (Table 7). Due to the large extension of the semi-arid region in Brazil, this price tends to be one of the variables with the greatest variability; thus, different scenarios were simulated for different prices and their influence on the associated NPV and IRR (Table 13). It is important to mention that Cactus forage is cultivated in all states with the existence of this climate condition in Brazil.
Scenarios were also simulated for different cactus cladodes sales values. The high costs with the establishment of crops is a specificity of this and other crops with vegetative reproduction, such as sugarcane, when compared to crops such as corn, sorghum, and millet. The costs of planting sorghum in India were estimated at USD 1218, while that of millet at USD 1133. In all scenarios, the business showed economic viability. Keeping the other variables with constant costs and revenues, the investment becomes unfeasible from the purchase price of cladodes of USD 0.13 and the sale of cactus below USD 0.02.
Another extremely important variable is associated with the average growth of the cactus plant. Silva et al. [55] suggested an average value of 5.58 cladodes in each plant, every two years. Table 14 presents different growth scenarios. For the other variables, the sensitivity analysis was carried out considering a variation of 20%, up and down ( Figure 2). The harvest and planting variables had the greatest impacts on NPV and IRR. It is important to emphasize that technological optimizations can result in lower costs and, consequently, increased economic returns.
Plowing, subsoiling and furrowing involved expenditure on renting machines, the sensitivity analysis showed that these agricultural practices do not represent a high cost, nor can they make the business unfeasible, as long as they have low fluctuations. In the Northeast Brazil, it is common to have large local variability, which makes this assessment important. Renting the machines is advantageous in the short term, considering that depreciation costs can be neglected. Kaneko et al. [68] evaluated the outsourcing of these processes for sugarcane production, in São Paulo state, Brazil, and identified IRR lower than discount rate and negative NPV. sensitivity analysis showed that these agricultural practices do not represent a high cost, nor can they make the business unfeasible, as long as they have low fluctuations. In the Northeast Brazil, it is common to have large local variability, which makes this assessment important. Renting the machines is advantageous in the short term, considering that depreciation costs can be neglected. Kaneko et al. [68] evaluated the outsourcing of these processes for sugarcane production, in São Paulo state, Brazil, and identified IRR lower than discount rate and negative NPV.

Conclusions
In general, the spineless cactus cultivation system in the semi-arid region of Brazil had energetic and economic viability. Large variations in the purchase and sale values of the cactus biomass could turn the production system unfeasible, but variations of up to 20% in the prices of other inputs and processes would not make it unfeasible.
These results, although initial, stimulate the sustainable production of spineless cactus in semi-arid areas and guide investors. In several locations around the world, there are indications that the production of cactus for the purpose of biogas production is possible and viable. The main limitation was the availability of data on spineless cactus production in a semi-arid region. Subsequent work needs to assess not only the production of biomass, but also incorporate the transport and processing of spineless cactus in the conditions of the Brazilian Northeast, and may also include risks in financial analyses.

Conclusions
In general, the spineless cactus cultivation system in the semi-arid region of Brazil had energetic and economic viability. Large variations in the purchase and sale values of the cactus biomass could turn the production system unfeasible, but variations of up to 20% in the prices of other inputs and processes would not make it unfeasible.
These results, although initial, stimulate the sustainable production of spineless cactus in semi-arid areas and guide investors. In several locations around the world, there are indications that the production of cactus for the purpose of biogas production is possible and viable. The main limitation was the availability of data on spineless cactus production in a semi-arid region. Subsequent work needs to assess not only the production of biomass, but also incorporate the transport and processing of spineless cactus in the conditions of the Brazilian Northeast, and may also include risks in financial analyses.