**1. Introduction**

In order to maintain 2 ◦C as the maximum increase in the global average temperature, the levels of atmospheric concentrations must be kept below 450 ppm of CO2eq during the 21st century [1]. Therefore, worldwide emissions of CO2 have to be drastically reduced in the coming decades, inducing deep changes in the energy systems [2]. This scenario requires that emissions from electricity generation should be negative by the end of the century, with fast progress in energy efficiency and promotion of low-carbon technologies. In this context, carbon capture and storage (CCS) is crucial because it represents a process by which large amounts of carbon dioxide can be captured and stored for the long term [1].

The CCS technology involves four main steps: conditioning processes to separate CO2 into a pure stream, carbon capture itself, its compression and, finally, storage for long term periods [1]. In the case of CCS applied to power units, significant losses in efficiency are expected; for instance, the Intergovernmental Panel on Climate Change (IPCC) indicates a 9% net reduction in efficiency for coal-fired power plants (pulverized) and 7% for combined cycle gas-fired power plants [1].

Post-combustion technology consists in the removal of CO2 from the exhaust gases. Capture by absorption is recognized as the reference technology [3,4] and considered mature for power plants [5]. The removal from flue gases uses a solvent, generally amines, to absorb CO2 molecules, being CO2 then released by heating or drastic pressure reductions [6]. Flue gases need to be cooled before getting in contact with the solvent: the temperatures must be between 40 and 60 ◦C at the entrance of absorption columns [7]. Costs and energy requirements—also called energy penalties—from a CCS unit using absorption capture depends mainly on the solvent properties. It is estimated that the heating for solvent regeneration is responsible for over 25% of the energy penalty when compression is included [7].

Combining bio-energy with carbon capture and storage (BECCS) offers the prospect of energy supply with net negative emissions and is clearly an important approach to reach the target of 2 ◦C. BECCS combines production of fuels and electricity from renewable biomass with carbon capture and storage of the CO2 emitted when biomass is converted [2]. As the CO2 is removed from the atmosphere during the growth of the raw material, life-cycle absolute emissions of BECSS could be negative [8]. In this sense, BECCS technology applied in sugarcane mills would be fundamental, contributing with very low greenhouse gas (GHG) emissions in both the transport sector (with avoided emissions due to the displacement of fossil gasoline) and in electricity generation [9].

The production of ethanol (via the fermentation of sugars) releases a pure stream of CO2, which means there is no penalty for its separation in the CCS process. This is the most obvious option to capture CO2 in sugarcane mills, and it is estimated that, considering the ethanol production figures of 28.5 million m3 in Brazil, it would be possible to reduce CO2 emissions by 27.7 million tonnes per year [9]. Carbon capture in sugarcane mills could at least double—or triple—with the adoption of CCS technologies in cogeneration systems in which residual biomass is burned—usually bagasse, and more recently, bagasse combined with straw.

This work focuses on assessing the technical and economic impacts of BECCS in a typical Brazilian sugarcane mill. First, a hypothetical sugarcane mill was selected to perform the evaluation that considers advanced steam cogeneration systems. A literature review was conducted to select the CCS technology and obtain representative data to model the integration of the CCS unit with the cogeneration plant. The feasibility analysis is based on typical costs (i.e., investments, operation and maintenance costs) and efficiencies, and the final assessment is based on the costs of CO2 avoided emissions.

#### **2. Materials and Methods**

#### *2.1. Cogeneration Plant*

A typical Brazilian sugarcane mill, but rather representative among the more than three hundred existing mills, was considered for this study, with a 4 Mt/y (million tonnes of sugarcane crushed per year) milling capacity. The cogeneration unit would be fully integrated to the mill to supply electric power and steam to the industrial process, and also maximizing surplus electricity. The steam demand for both sugar and ethanol production was assumed equivalent to 340 kg of steam (at 2.5 bar and 137 ◦C) per tonne of sugarcane; this is the minimum consumption of steam currently considered economically viable [10]. The power plant would operate along the whole year (with 90% capacity factor), being as a cogeneration unit during the harvest season and as a single power plant during off-season. Biomass would be stored to assure the operation during off-season and this is already a common practice for mills that generate electricity throughout the year; in general, mills have area available for this, and the costs are not prohibitive. The cogeneration technology is the one known as condensing-extraction steam-turbine (CEST), with live steam at the highest possible pressure and temperature (120 bar/535 ◦C is the state-of-art in Brazilian sugarcane mills, according to [9]).

The CEST technology is very common in modern sugarcane mills. Bagasse used to be the only fuel but, recently, a blend of bagasse and straw has been used [11] due to the growing straw availability at the mill site as a consequence of mechanized harvesting. Table 1 presents a summary of the main characteristics

of the reference mill operating with CEST system and burning biomass—bagasse and straw—as fuel. Bagasse availability is defined by the fiber content of the sugarcane plant (14%), i.e., 280 kg of bagasse with 50% moisture per tonne of cane. As for straw its availability at the mill was considered 50% in relation to the total amount available at the field, resulting 161 kg per tonne of cane (with 13% moisture).


**Table 1.** Characteristics of the reference mill and the power plant.

The CEST system was modelled in a non-commercial software able to simulate its integration with sugarcane mills and to estimate electricity generation [12]. The current experience with straw use as fuel has shown that problems like slagging, fouling and surface corrosion are common when the straw share is above 15–20% in the fuel blend (mass basis). These problems are due to biomass and their ash compositions, which have much more chlorine, CaO and K2O in the case of straw compared to bagasse [13]. As it is predicted that in the considered case straw would represent about one third of the fuel input, the hypothesis is that the problems mentioned would be solved in the future. Biomass consumption—bagasse and straw—would be distributed along the year to assure fuel supply according to system's requirement.

The emissions of CO2 from combustion were estimated considering full combustion with 30% excess air [14], and carbon content 48.6% in the dry fuel for both bagasse and straw [15]. For estimating CO2 from fermentation, it was assumed a sugar mill with a medium to large annexed distillery (i.e., 50% of the sugarcane would be used to ethanol production). A typical Brazilian mill with such a capacity (4 Mt/y) produces both ethanol and sugar with some flexibility (in general, each output varies between 40% and 60%; basis is the sugarcane input) [16]. Therefore, CO2 from fermentation was calculated for an ethanol production of 86.3 L per tonne of sugarcane [17] and a CO2 production of 0.96 kg per kg of ethanol [9], resulting in an emission index 0.78 kg of CO2 per liter of ethanol (calculated for an ethanol density of 0.809 kg/L).

#### *2.2. CCS Unit*

Carbon dioxide both from fermentation and from biomass combustion were considered to be processed in a CCS unit. CO2 from the combustion passes through the complete CCS process (referred to as absorption, regeneration and compression), while CO2 from fermentation only passes through the compression steps. These two streams are combined in the transportation and storage stages. It was assumed a post-combustion technology based on capture with monoethanolamine (MEA).

#### 2.2.1. MEA Technology

Capture process by absorption is the reference technology and MEA is the incumbent solvent used. Absorption characteristics of the solvent determine energy penalties and impact the economic feasibility

of capturing. In this study, parameters of the capture process based on MEA technology were taken from [7]. Solvent regeneration was considered using steam extracted at the medium-pressure stage of the steam turbine, that coincidentally is the same pressure required by the industrial process (i.e., 2.5 bar, 137 ◦C). The three levels of heat requirement for solvent regeneration are related with the different stages of technology development: 4.4 GJ/t CO2 (1998 kg of steam per tonne of CO2); 2.6 GJ/t CO2 (1180 kg of steam per tonne of CO2); and 1.6 GJ/t CO2 (726 kg of steam per tonne of CO2). It was assumed 90% as the maximum possible capture rate in the CCS unit. Absorption and regeneration take place in the unit hereafter referred to as CCS. Power requirement for flow gases treatment (at the CCS unit) was estimated at 25.84 kW per unit of exhaust gas flow, in kg/s [18]. This figure includes electricity requirement for pumps and blowers, capture pre-treatment pumping, cooling water pumping and blower duties and, finally, solvent pumping duties.
