Diesel Displacement in the Sugarcane Ethanol Life-Cycle: A Comparative Analysis of Different Integrated Systems

Diesel displacement in the sugarcane ethanol life-cycle: a comparative analysis of different integrated systems

Simone P. Souzaa,b,
*
, Joaquim E. A. Seabraa,b

a Faculdade de Engenharia Mecânica, UNICAMP. Rua Mendeleyev 200, Cidade Universitária “Zeferino Vaz”, Campinas,SP, Brazil. Postal code 13083-860.

b Brazilian Bioethanol Science and Technology Laboratory (CTBE) – CNPEM/ABTLuS – Rua Giuseppe Máximo Scolfaro 10.000, Polo II de Alta Tecnologia, P.O. Box 6170. Campinas, SP, Brazil. Postal code 13083-970.

1. Introduction

Integrated crop systems have been proposed as an alternative to enhance the efficiency and to improve the interaction among bioenergy, food and chemicals production (Cherubini, 2010). They can be able to produce such products from different raw materials (Taylor, 2008) and, additionally, may reduce the commitment of land for bioenergy production and provide diversification and optimization of agricultural systems (Cavalett et al., 2011; Souza and Seabra, 2013).

The biodiesel and the sucroenergetic sectors in Brazil are already important model of biorefinery due to the diversity of products and the opportunities to use the residues as fertilizer, energy source, irrigation, and others. However, there is still potential for improvement and other integration alternatives for these sectors (Bonomi et al., 2012; Lombardi et al., 2009; Oliverio et al., 2007; Ometto et al., 2007; Pereira et al., 2014). The integration in the crop and industrial systems can improve the environmental aspects over the life-cycle, such as reducing the GHG emissions and the fossil energy use.

In previous works (Souza and Seabra, 2013; Souza et al., 2013, 2012) we assessed the gains of the integrations between palm-sugarcane (PSIS), soybean-sugarcane (SSIS) and algae-sugarcane (ASIS) in terms of GHG emissions and fossil energy use. In this work, we aim to demonstrate a comparative analysis among these systems and to indicate the advantage and disadvantage of each integrated system, focusing the diesel displacement in the sugarcane ethanol life-cycle.

2. The sugarcane sector and its potential for integrated systems

The diesel consumption in the sugarcane sector is responsible for about 4% of all diesel consumption in Brazil, without accounting for the diesel used in the transport of ethanol and inputs (fertilizers, pesticides, fossil diesel). Since the focus of this work is replacing the fossil diesel by a first generation biofuel, the proposed systems aims to produce biodiesel from palm, soybean or algae integrated to the sugarcane sector.

Soybean is a well-established culture in Brazil and relevant in the Brazilian agribusiness as a result of a suitable development of agronomic, industrial and logistic aspects over decades. For these reasons, soybean has been leading as the main feedstock for biodiesel production, comprising about 80% of the biodiesel sources. Other feedstocks include beef tallow, cottonseed, waste frying oil, swine and chicken fat, palm oil, peanut and sunflower (ANP, 2013).

Palm is the main feedstock used for oil production worldwide; its main application is food and chemical industries. Over ten years, the palm oil has remained as the lowest price in the international market (MPOB, 2012). There are great expectations that the palm oil will be an important raw material for the world, especially in developing countries (OECD, 2013). Although palm is a typical culture from wetter areas, promising results have been achieved in the Brazilian Cerrado under irrigated condition. The water consumption is relatively small and occurs only during the dry season (Embrapa, 2011). The integration of this crop to the sugarcane sector can enable the use of vinasse (a liquid effluent from the destilary) for irrigation. Also, the mechanization in the sugarcane harvesting has restricted the plantation in areas with a slope exceeding 12%. Palm can be planted in these areas because its harvesting does not require machines.

As for the algae-sugarcane integration, the ethanol distillation unit is an excellent source of pure CO2, which is generated during the sugarcane juice fermentation, along with substantial quantities from the combined heat and power system. Carbon dioxide, light, water and nutrients are essential inputs for algae production. Due to a favorable climate, Brazil has the potential to become a major algae producer (Franz et al., 2012).

3. Method

In the proposed model, sugarcane and the oils (palm, soybean or algae) are processed in a combined ethanol-biodiesel plant, which uses only bagasse as fuel. In the palm-sugarcane model, fibers and shells from the palm bunches processing are also used as fuel. The biomass burning provides the utilities for the biodiesel and ethanol plants. In the SSIS, the oil is provided by the soybean grown in the sugarcane reforming areas, including the direct oil from the grain and the additional oil acquired from the sale of soybean meal. Ethanol, electricity and glycerin are the output products of the SSIS system. In the ASIS system, carbon dioxide is captured from the sugarcane juice fermentation process to feed the algae production in a photobioreactor which also receives sunlight (solar prism) and fertilizers. Algae flows to a mixotrophic reactor fed with organic carbon source and fertilizers. We chose the microalgae species, N. salina because of its high lipid content and high biomass productivity (SAT, 2013). Glycerin is used as carbon source because it is available as a co-product of biodiesel production onsite and because N. salina has demonstrated a better carbon assimilation using this substrate (Sforza, 2012). The output products in the ASIS are meal, glycerin, ethanol and electricity. As for the PSIS, the empty fruit bunch and the POME (industrial effluent) are used as organic fertilizer. The PSIS system has kernel cake, kernel oil, glycerin, electricity and ethanol as output products. For all integrated system, ethanol from the sugarcane distillery is used as transesterification reagent.

The data was collected from sugarcane mills, sugarcane and soybean suppliers, palm sector, algae company and biodiesel plants. The data are from the states of São Paulo, Mato Grosso and Goiás. As for palm, the data is from Para state.

We applied an attributional life-cycle assessment technique (ISO 14040, 2006; ISO 14044, 2006) and choose 1 MJ of biofuel as the functional unit. Note that the goal is to compare the ethanol production (1 MJ of ethanol) between the traditional (TSES) and the integrated systems: soybean-sugarcane (SSIS), algae-sugarcane (ASIS) and palm-sugarcane (PSIS). As further study we will include a sensitivity analysis and Monte-Carlo simulation.

4. Results

The palm-sugarcane and algae-sugarcane integrated systems shows better results for both fossil energy use and GHG emissions. The energy balance (output energy per unit of fossil energy input) is 18:1 for both PSIS and ASIS. As for TSES and SSIS, the energy balances are 9:1 and 11:1, respectively (Figure 1). The great difference among the systems is due the diesel displacement. Because of the high oil yield of the palm and algae, these integrated systems are able to displace 100% of the diesel used in the agricultural sectors. While SSIS reduces around 15% the fossil energy use in the ethanol life-cycle, both PSIS and ASIS reduce over 50% (Figure 2).

Figure 1. Comparative fossil energy use for different systems integrated to the sugarcane sector

Figure 2. Life cycle fossil energy use of the traditional system and the integrated systems

Fertilizers and residues are the most impacting parameter in the life-cycle GHG emissions. For this reason, replacing diesel by biodiesel does not reduce the CO2e emission burdens as much as the fossil energy use. Even so, the emissions reduction is about 20% when palm and algae are integrated to the system. As for SSIS, the GHG savings is negligible (Figure 3).

Figure 3. Ethanol life cycle GHG emissions of the traditional system and the sugarcane-soybean integrated system.

5. Conclusion

The proposal was evaluating and comparing the GHG balance and fossil energy use when the sectors of palm, algae or soybean biodiesel are integrated into the sugarcane sector. The main goal of integrating biodiesel production is to reduce the fossil energy in the ethanol production. In our models, such reduction would be achieved by using biodiesel in the agricultural stages.

Integrating the sugarcane sector to the palm and algae sectors shows better results for fossil energy use and GHG emissions. This indicates that the traditional ethanol sugarcane system is able to improve the life-cycle by replacing the diesel consumption. Also, the current biodiesel scenario using soybean as feedstock is not the best option as diesel substitute in terms of reducing fossil energy and GHG emissions. Algae biodiesel is not a current realistic option since technologies are not yet economically feasible and technical issues are still under development. So far, palm seems to be the most interesting choice to integrate to the sugarcane sector. However, the best alternative depends on the local conditions and the economic feasibility.

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