Efficiency of an aquatubular boiler from the burning of four cultivars of sugar cane

The growing demand for energy from renewable sources increasingly seeks to implement efficient energy production systems. Thus, the objective of this work is to determine the thermal efficiency of an aquatubular boiler that will burn the bagasse from four sugarcane cultivars: SP 80-1816, RB72-454, SP80-3280 and SP81-3250. This efficiency will be determined through the calculation methods: PCI lower calorific value, PCS higher calorific value and direct method. These cultivars were planted in the south-central region of Brazil where the largest sugar cane producers in the country are located. The results obtained show the importance of the energy analysis that each cultivar provides for energy cogeneration, as well as the benefits that will directly influence its production chain for controlled management. Among the benefits of controlled management are: maximizing processes and optimizing the energy use of each cultivar. The optimum efficiency of the boiler in energy production in relation to steam production depends on the intrinsic variables of each cultivar, such as bagasse and moisture content. When calculating the boiler efficiency, Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 2 the SP 80-1816 variety proved to be more advantageous in relation to the others, considering the same characteristics of the production process, planting region, harvest time and the same type of boiler used. Still related to the study, the cultivar SP 80-1816 requires a smaller amount of bagasse in the boiler feed to produce heat, which results in greater energy production considering the same amount of bagasse of the studied varieties.


Introduction
The biomass of sugarcane bagasse occupies a prominent place among the renewable energy sources in Brazil. It has a high energy potential for electricity production through cogeneration, since sugarcane processing generates energy material and has a vast area for cultivation in Brazilian soil. In addition to the cultivation area, other points to be considered in the productivity gain of the sugar-alcohol sector that brings favorable results in electricity production is the diversity of sugarcane cultivars that can be planted, the characteristics of the arable soil, the topography of the land, the improvement of planted species and the improvement of management according to each region in which sugarcane is cultivated.  Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 4 from the processing of sugarcane used in the production of bioelectricity represented only 15% of the total potential. In this sense, if all sugarcane biomass after processing were used, bioelectricity in Brazil would have a technical potential to reach 146,000 GWh, considering the process of energy production by cogeneration in the plants. In this sense, in Brazil, the bioelectricity of origin of sugarcane has growth potential, which can exceed 50% by 2027, changing the value produced from 21,500 GWh recorded in 2018 to 33,200 GWh by the year considered (ÚNICA, 2019). Even with the growth of more than 11,700 GWh, we would still be using only 17% of the technical potential of this source, (ÚNICA, 2019).
Among the sugarcane plantation areas located in the southern central region of Brazil, the varieties analyzed in this study were planted in the municipality of Taquarituba in the State of São Paulo. The varieties of sugarcane together with their respective energy characteristics used in the calculations for the determination of boiler efficiency were SP 80-1816, RB 72-454, SP 80-3280 and SP 81-3250. For analysis, 10 stems per variety were collected in 4 random points of the plot and they were subjected to the desponte at the height of the apical yolk and the defoliation (Lima, 2009).
The four sugarcane cultivars used in this study are due to the fact that they are one of the most planted varieties in the State of São Paulo, which is the largest producer of sugarcane in Brazil. The determination of efficiency is an important factor to control the production of steam and, consequently, its use in turbines for the production of electricity in sugar and alcohol plants.
After harvest, sugarcane goes through industrial processing in mills for the removal of the broth that is used in the production of sugar and alcohol. The material resulting from this processing is bagasse, it is burned in the boilers producing necessary steam in the industrial production process and mostly in the production of electricity. The objective of this work is to identify which cultivar presents the best energy indicators to be selected as the primary source of energy for the boiler. Other points to be analyzed are: to determine, among the cultivars studied, which of them presents the best flow of sugarcane bagasse to be inserted in the combustion chamber, determine the value of the energy provided by each sugarcane cultivar and calculate the thermal efficiency of an industrial boiler of the aquatubular type when burning the sugarcane bagasse of the four cultivars (SP 80-1816 , RB72-454, SP80-3280 and SP81-3250). Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 5

Methodology
This work was developed using empirical equations as a basic tool. The efficiencies analyzed in this article were estimated by authors considering their experience in processing studies of sugar production plants. This type of research can be considered quantitative, since the results are analyzed in comparative terms between the cultivars, and qualitative, since the energy efficiencies were determined by cultivar. The work was carried out based on the data determined in the laboratory by the aforementioned authors, but the knowledge of the variables necessary to obtain the results was very important for the use of mathematical equations. Therefore, it can be considered both industrial analysis and laboratory work. The basic criterion for analyzing the results was a comparison between the characteristics of the cultivars and the results found. The steps carried out in the study, the determination of the sugarcane bagasse feed, the useful energy and the boiler efficiency will be presented in the next sections.

Availability of electricity
The availability of electricity is a fundamental characteristic for the industrial   Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.

Biomass characterization
Biomass processing in the industry generates agricultural and industrial waste. The harvesting of sugarcane in a mechanized way, that is, with machines gives rise to agricultural residue called straw, which includes straw, green leaves, grinding wheels, roots and weeds that grow within the sugarcane field and "industrial waste includes bagasse, molasses, vinasse, filter cake and ash (Silva et al., 2014). The biomass composition of the sugar-alcohol industry is basically composed of six chemical elements in its organic phase: carbon, hydrogen, nitrogen, sulfur, chlorine and oxygen. In the inorganic phase, however, ten other elements are found, such as silicon, aluminum, iron, calcium, magnesium, sodium, potassium, sulfur, phosphorus and titanium (Jenkins et al., 1998).
Bagasse originated after sugarcane grinding is a by-product of high energy value used in the sugar-alcohol industry as an insum in steam production. Its composition varies according to the type of cultivar and can contain in its constitution 45 to 55% of water, 40 to 53% of fiber, 2 to 5% of solids and 1% of dissolved ash ( Van der Poel et al., 1998). It is of paramount importance to confirm the characteristics of sugarcane bagasse, because these parameters are necessary for the manufacture of auxiliary equipment for cogeneration, such as: bagasse feeding systems in the boiler, pneumatic conveyor and bagasse dryer. Regardless of the sugarcane broth extraction process, when a bagasse sample is checked, two distinct groups are observed. According to Meirelles (1984), the first group is fibers (larger particles) and the second of the medulla or powder (smaller particles).

Calorific power
According to Cortez at al., (2008), the calorific value of any energy source is the amount of energy released in the form of heat during the complete combustion of the fuel mass unit, being measured in kJ/kg or in lime/kg. Associated with this theme, González (2015) states that the calorific value varies with the amount of moisture present in biomass and is differentiated into two types: the superior calorific value (PCS) and lower calorific value (PCI). PCS is the energy released in the form of heat in combustion when the water vapor generated during combustion is condensed. It is the sum of the energy released in the form of heat and the energy spent on the evaporation of water formed during oxidation. The PCI is only the energy released in the form of heat, and in the boiler, one of the products resulting from the combustion of sugarcane bagasse is steam.
The value of the superior calorific value (PCS) is determined by the use of an adiabatic calorimetric pump, using the standard ASTM techniques -Standard Method for the Gross Calorific Value of Solid Fuel (Cortez at al., 2008). Considering the entire cogeneration system, after burning sugarcane bagasse the temperature of exhaust gases in most cases is higher than the condensation temperature of the gases, which makes the PCI more applicable in boiler efficiency calculations. The calculations performed in this study are based on pcs and PCI of cultivars SP 80-1816, RB72-454, SP 80-3280 and SP81-32.50. The samples were collected coincidentally during sugarcane harvest in October 2007 (Lima, 2009 Source: Authors (2020).
After performing the calculations, the results found were shown in Table 1, in which it Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 8 can be seen that the SP 80-1816 variety, even with the lowest straw volume value in m³ / t, presented the highest PCS and PCI values, since the percentage of moisture did not show significant difference between the varieties.
A more complete analysis of the chemical composition of the bagasse sample is extremely important for the determination of the PCI in order to establish the relationships between the elemental composition of the bagasse and the calorific value of a fuel. The Russian scientist Mendeliev (1987) proposed an expression for the calculation of PCI in kJ/kg from the elemental composition of the fuel as evidenced by expression 1 (Cortez at al., 2008).
(1) Still on the calorific value, equation 2 can be used to convert the PCS from dry base to work base, and to convert the superior calorific value of the dry base into lower calorific value on a work basis, equation 3 is used (Cortez at al, 2008). (2) (3)

Boiler
The use of heat has been part of human life since the beginning and this energy is not restricted only to the direct use of fire. Industrially, this thermal energy is obtained through boilers resulting in steam being widely used in various industrial sectors such as sugar and alcohol factories. The availability of water near industrial parks is relatively high and, as steam has a specific high heat, it is used for heating or mechanical activation purposes. In addition, due to its easy obtaining employing boilers and the low cost lead their use on a large scale in industrial processes and facilities that require steam energy for its operation.
The boiler is a heat exchanger that works at a pressure higher than atmospheric pressure, produces steam from the thermal energy extracted from a primary fuel, which can be gaseous, liquid or solid. Thus, "heat transfer is widely used in many applications in heat exchangers, chemical process, gas and oil production, air condition, automotive or food industries" (Vahidifar & Kahrom, 2015), and there are two types: flamotubular and aquatubular boilers, which may receive different classifications for their employability and ( ) Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4  As shown in Figure 3, the principle of operation of these generators is the exchange of heat with water, so that the combustion gases circulate inside the pipes and the water around them is heated (Botelho, 2011).
An example of using this model to generate steam for small capacities, is through vertical configuration. The steam generated in this equipment is saturated or supersaturated steam type and can have a production of 160 to 50 ton/h of steam and pressures that can go from 10 to 18 bar, producing from 112 to 34,000 kW. The boilers designed in the horizontal configuration limit the flow of steam to 13 ton/h and pressures of up to 14 kgf/cm² (Botelho, 2011).
In these boiler models, after burning the fuel, the combustion gases pass through pipes heating the water that surrounds them. When the oven is built outside this type of boiler, the fuel to be used in burning is low calorific value, such as rice husk, coffee and peanuts, straw, sawdust, black liquor and heavy oils. Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 This type of boiler is more difficult to build and, in the scope of its operation, generates a large amount of steam, reaching 750 ton/h. But usually the amount of steam is between 15 and 150 ton/h, with high operating pressure reaching a level between 90 kgf/cm² to 100 kgf/cm² (Botelho, 2011).
One of the operational principles of aquatubular boiler is the circulation of water through the pipe. And most of these boilers work with natural circulation caused by the difference in density when the water is heated until it reaches the water-steam mixture. The density difference causes the downward displacement of the water in the direction of the lower tube, while on the other side there is an upward flow from the mixture of water and steam to the upper tube. This cyclical movement is facilitated by the continuous supply of water, steam outlet and hot gas flow.
In boilers that work with high pressure, the density difference decreases, just as the water-steam mixture decreases greatly and natural circulation becomes slow. To maintain water circulation, a pump is installed to maintain sufficiently higher steam demand than natural circulation. This measure provides that the aquatubular boilers meet the needs of the application, such as high steam consumption, high steam pressure or overheated steam (Santini & Telhado, 2015).
The macroprocess of steam production in an aquatubular boiler begins with the supply Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 11 of low temperature water that is pumped to the economizer which has the function of heating the boiler feed water, performing the first heat exchange to increase the temperature. This allows the mixing of heated water with the steam generation system. Which is a necessary process to avoid thermal shocks and possible temperature fluctuations. Continuing the cycle, the water from the steam pipe passes into the lower tube through the pipes at low temperatures. After reaching the lower tube, the heated water passes through the hottest tubes, with the partial transformation of the water into saturated steam, following a biphasic flow until it reaches the steam nozzle. The design presented Figure 5 shows a simplified scheme of water circulation in an aquatubular boiler with vertical tubes. The advantages of this boiler model are: the possibility of working with pressures between 50 and 165 bar, the rapid production of steam, the fast start-up and ease of adapting to different types of fuels. Although it has the disadvantages of having large dimensions, of presenting sensitivity to sudden variations of load, high demand for quality of the feed water due to the high operational pressure, high installation cost and complexity in the assembly (Botelho, 2011).

Methodology, data and assumption
The analysis performed is based on the result of the calculations performed to determine the efficiency of an aquatubular boiler, using the data of biomass burning of Research, Society and Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 12 sugarcane. The PCS value of the four sugarcane cultivars SP 80-1816, RB72-454, SP80-3280 and SP81-3250 were obtained as follows: the samples of the cultivars were ground, dried, sieved in the ABNT 70 sieve according to the ABNT -NBR 8633 standard, after this process they were pressed to the form of tablets with a mass of approximately 1 g. Later they were left in a greenhouse with a temperature of 105 ºC resulting in a dry sample. After this stage, an ALEMMAR KL-5 calorimetric pump was used to burn the sample and obtain the superior calorific power, the process of obtaining was based on the calorimeter instruction manual and this one was adapted according to the ABNT-NBR 8633 standard (Lima, 2009).
After the determination of the PCS of the cultivars, doat (1977) was used to determine the PCI, this method takes into account the amount of hydrogen present in the sample, the heat absorbed for the vaporization of the water contained in the sample and the proportion of water that was formed during combustion (Lima, 2009).
"Producing economical heating and cooling systems with high performances is a constant concern in the industry (Touzani et al., 2019)", and thus, the calculation of the yield of these equipments becomes quite necessary. To calculate the efficiency of the boiler are used parameters similar to those of a real boiler. The steam production of the boiler used as a model is 300,000 kg/h at a temperature of 520ºC with a working pressure of 6.7 Mpa. The gas resulting from combustion has a temperature of 160ºC and the estimated yield of the PCI is 90%. The feeding of sugarcane bagasse in the combustion chamber is carried out by conveyors and the consumption of bagasse necessary to maintain the operational characteristics of the boiler is determined by dividing the total heat transferred by the fuel, which in the studied model is equal to 991,858.2 MJ/h. This value is determined by the WET bagasse PCI (Sampaio, 2015).
The consumption of sugarcane bagasse in the boiler is determined by the relationship between the total heat transferred by the fuel (991,858.2 MJ/h) and the lowest calorific value of the bagasse of each variety studied (Sampaio, 2015 Development, v. 9, n. 11, e5469119859, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9859 13 Another calculation to be performed to determine the efficiency of the boiler is through the inputs and outputs method, also called gross efficiency, for which expression 6 is used.

(6)
After the burning of the bagasse in the boiler furnace, the combustion products will be at high temperature and yielding part of their energy to the evaporative surfaces in the heater and superheater, the sum of these energies represents the useful heat of the boiler. Useful heat is the energy transferred to the working substance, considering the following data: energy consumed for the evaporation of the feed water and the overheating of the steam to the required pressure and temperature conditions and the energy contained in continuous extraction waters (Cortez et al., 2008). The calculation of useful heat (Qu) is obtained by equation 7.
The available heat determined per unit of fuel mass (kg) is calculated using equation 8.
Using the calculation parameters and to meet normative aspects, the boiler efficiency calculation for the four cultivars used was performed based on the recommendations manual published by the Institute of Technological Research (IPT) (Camargo, 1990). Where it is recommended that parameters such as PCI and PCS be used to quantify the thermal efficiency of the boiler, so that the final results calculated by different methods are interpreted and compared with the requirements of the boiler. In addition to the PCI and PCS values, other values used in the efficiency calculations are the data from the boiler that served as a model for the work, are the following: steam temperature 520 ºC, flue gases 160 ºC, working pressure 6.7 MPa and energy from the steam produced in the boiler of 991,858.2 MJ / h. The data in Table 2 are reference values that are commonly used in efficiency calculations thermal power of boilers, for example, mass of dry air entering the combustion chamber, specific average heat of the fuel. The boiler efficiency calculation was performed considering the main characteristics of the studied cultivars. In this sense, the result found is directly linked, among other factors, to the rate of supply of sugarcane bagasse for burning in the boiler and the energy provided by each cultivar studied from the combustion of bagasse.

Results and Discussions
To determine the boiler efficiency, process variables such as humidity, lower calorific value and consumption of sugarcane bagasse in the boiler should be checked. Knowing the proportion of moisture in the sugarcane bagasse assists in determining its combustion capacity as well as in storage, handling and transport. Not being able to be quantified, the lower calorific value is found by subtracting from the higher calorific value the water condensing energy that is produced during the combustion of bagasse and the moisture present in the fuel before burning.  The consumption of bagasse for burning in the boiler is determined through the relationship between the energy of the steam in the boiler and the lower calorific value, the results obtained after the calculations are described in Table 3 Tables 3 and 4, it is possible to determine the boiler performance which was used as a model for the analysis of energy efficiency based on the characteristics of the studied cultivars. This is determined by the direct method using three parameters: useful energy for the energy supplied, higher calorific value of the basic PCS, lower base caloric PCI, whose numerical values are shown in Table 5. From the results shown in Table 4, it is possible to observe that the boiler efficiency calculated by the PCI method was higher. The efficiency determined by the PCS and the ratio between the useful energy and the energy supplied did not show any significant difference in the results.

Conclusion
One of the items needed to determine the thermal efficiency of the boiler is the amount of sugarcane bagasse that must be introduced into your combustion chamber over time. This value was determined by the relationship between the steam flow produced in the boiler and the PCI of each cultivar. The lowest bagasse feed flow in the boiler was found to cultivate And with the use of cultivar SP81-3250, the value obtained is 97.11%. This value is justified because the direct method was used in the calculation, disregarding the losses that occur in the system. The values found using the PCS and useful energy / supplied energy methods were smaller and closer because in these methods the values used in the calculations take into account the real requirements of the boiler operation. Thus, it demonstrates the importance of determining the thermal efficiency of a boiler when burning sugarcane bagasse for different cane cultivars, because each cultivar will provide a different amount of energy for a given boiler model. Another point to be noted in this regard is that with the determination of the energy quantities of each cultivar, there will always be an improvement in cogeneration in the plants that use sugarcane as a raw material.
Of the cultivars analyzed, the best choice for using sugarcane bagasse as a primary source of energy is the cultivar SP80-1816 because it has less bagasse consumption in the boiler and more useful energy available for steam generation. We indicated that it could be appropriate to determine the efficiency of each cultivar considering the losses that occur in the steam production process in different boilers. This can enable the identification of the efficiency provided by the cultivars in different equipment, considering the point of work of each one. That is, what would be the best cultivar to be used to produce bagasse in an industrial unit. We also claim that another important topic to be developed is the calculation of the thermal energy that steam can supply to a turbine and produce mechanical energy for the purpose of generating electrical energy when driving an electric generator.
As a suggestion for future work, we recommend determining the efficiency of each cultivar considering the losses that occur in the steam production process in different boilers.
This can enable the identification of the efficiency provided by the cultivars in different equipment, considering the point of work of each one, that is, which will be the best cultivar to be used in an industrial unit. We argue that another item also as an important topic to be developed in future works is the calculation of the thermal energy that the steam will supply to a steam turbine for the purpose of producing electric energy.