BTL: a solution to increase energy efficiency in the Brazilian alcohol business

1 Dr. Eduardo Falabella Souza-Aguiar Coordinator - GTL Cell CENPES - PETROBRAS Avenida Jequitibá, 950, Quadra 7, Ilha do Fundão, Rio de Janeiro, Brasil

2 Sirlei Sebastião Alves de Sousa Senior Consultant - GTL Cell FUJB - Universidade Federal do Rio de Janeiro, UFRJ Avenida Jequitibá, 950, Quadra 7, Ilha do Fundão, Rio de Janeiro, Brasil

3 Fernando Barbosa de Oliveira Process Engineer - GTL Cell CENPES - PETROBRAS Avenida Jequitibá, 950, Quadra 7, Ilha do Fundão, Rio de Janeiro, Brasil

1. Introduction

Due to 1973 oil crisis, the Brazilian government, then run by a military junta, initiated in 1975 the ProÁlcool program. The ProÁlcool or Programa Nacional do Álcool (National Alcohol Program) was nationwide program financed by the government to phase out all automobile fuels derived from fossil fuels (such as gasoline) in favor of ethanol. It began with the anhydrous alcohol to blend with the gasoline. This mixture has been used since then and is now done with 24% of alcohol and 76% gasoline [1].

The decision to produce ethanol from fermented sugarcane was based on the low cost of sugar at the time. Other sources of fermentable carbohydrates were tested such as the manioc [1]. Sugarcane is in itself an enormously efficient production unit: every ton has an energy potential that is equivalent to 1.2 barrels of petroleum. Brazil is the largest sugarcane world producer, having the lowest production costs, followed by India and Australia. On average, 55% of Brazilian sugarcane is turned into alcohol [2].

Sugarcane is grown in Brazil’s Central-South and North-Northeast regions, with two harvest periods. It is the force behind the 307 existing “energy powerhouses” in Brazil, 128 of which are in fueled by sugarcane grown on 2.35 million hectares of land. These are mills and distilleries that process biomass from sugarcane feed a complex chain: they produce sugar as foodstuff, electric energy from bagasse (sugar cane fiber) burnt in their boilers, hydrated alcohol as a vehicle fuel and anhydrous alcohol to improve gasoline energy and environmental performance [2].

2. Bagasse utilization strategies

When sugarcane is processed at a sugar/ethanol factory, the cane stalks are shredded and crushed to extract the cane juice while the fibrous outer residue, known as bagasse, is sent to the

1 boiler to provide steam and electricity for the factory. The fact that the sugarcane plant provides its own source of energy for sugar/ethanol production in the form of bagasse has long been a special feature of the industry. In the traditional approach, factories and distilleries cogenerate just enough steam and electricity to meet their on-site needs [5].

Boilers and steam generators are typically run inefficiently in order to dispose of as much bagasse produced from cane crushing as possible. Some older factories purchase oil or electricity, because their steam generating technologies and boilers are extremely inefficient. Any factory designed and constructed today should be at least efficient enough to cover its own energy needs. With the availability of advanced cogeneration technologies, these factories today can harness the on-site bagasse resource to go beyond meeting their own energy requirements and produce surplus electricity for sale to the national grid or directly to other electricity consumers [5].

More efficient steam turbines operating at higher pressures can significantly increase electricity production. A typical Condensing Extraction Steam Turbines (CEST) operate at 4.0 to 6.0 MPa and produce enough steam to supply a typical sugar/ethanol factory and export 30 to 100 kWh of electricity per ton of cane (kWh/tc) to other users or to the national grid. CEST systems represent the state-of-the-art for bagasse cogeneration in terms of mature technologies that are fully commercialized in the marketplace [5].

Gasification of biomass for use in a high-efficiency gas turbine is a more advanced approach to bagasse cogeneration. This approach is based on the marriage of two technologies: a biomass gasifier unit with a gas turbine. There are a number of possible configurations like the Biomass Integrated Gasifier-Combined Cycle (BIG-CC). These systems could produce over twice as much power per ton of cane as CEST systems. However, unlike CEST systems, BIG-CC systems are not at present commercially mature. Besides, they are expected to have significantly higher capital costs [5].

There are two main options to sell surplus electricity from a sugar/ethanol factory. One is to sell to local off-grid customers, such as local industries or rural electricity cooperatives, thereby providing electricity services without the costs (both actual and organizational) that accompany grid connections. The second option is to sell surplus electricity to established utilities or distributors as an independent power producer and transport the electricity over the national grid [5].

3. Brazilian experience overview

Brazil has a long time tradition in the use of renewable energy. A look at the primary energy supply shows that in 2002, 41% of it was renewable energy, being 14% hydropower and biomass 27%. Hydropower plants amount to 65 GW of the 82 GW of total installed capacity. This is a unique situation, which has the positive aspect of using renewable energy, but leaves the country exposed to the seasonal rain pattern. The shortage that occurred in 2001 made the Government decide to diversify the energy supply sources, favoring the inclusion of a reasonable share of thermal power plants and creating a market share for other renewable sources of energy such as wind power and biomass [6].

The sugar cane sector in Brazil produces and processes more than 300 million metric tons of sugar cane. More than 50% of the sucrose is used in the production of ethanol. The sugar cane bagasse provides all energy required to process the sugar cane and several mills are already generating surplus power and selling it to the utilities. This surplus power generation of the

2 sugar/ethanol mills could be highly increased by the use of more efficient energy conversion systems, such as biomass gasification integrated with gas turbines and recovery of part of the sugar cane trash currently burned or wasted today, so as to supplement the bagasse as fuel. Both BIG-CC and trash recovery are emerging technologies that need development and demonstration in order to reach the market [6]. Under normal conditions, Brazil annually produces and processes a quarter of the 1300 million tons grown in more than 100 countries worldwide. The Brazilian sugar cane sector gross annual income of US$ 10 billion represents around 2% of the Gross National Product [6].

Cane production and processing are highly energy intensive activities that require, under Brazilian conditions, for each ton of cane 190 MJ in agricultural area (in the form of fossil fuels, fertilizers and other chemicals) and 1970 MJ in industry (in the form of chemicals and bagasse), the latter providing nearly 100% of the industry’s energy requirement. A life cycle analysis for ethanol production has indicated, however, that for each unit of fossil energy input to the agro industrial system, follow approximately nine units of renewable energy output (ethanol and surplus bagasse) to be used outside the system [6].

This situation has a huge potential for improvement if we bear in mind that ethanol represents only one third of the energy available in cane; the other two thirds represented by fiber in the cane stalks (bagasse) and in cane leaves (trash) is almost totally used in the process in the following away [6]:

• 93% of the bagasse is used as fuel in cane processing, in a very inefficient way. • 85% of the trash is burned prior to cane harvesting to reduce the cost of this operation; the other 15% is harvested unburned but the trash is left on the ground to decay. In both cases the net result is that the carbon in the fiber returns to the atmosphere in the form of CO 2.

This fact indicates that with some effort and investment this potentially available fuel (cane fiber) can be saved and used to generate electric power for the grid. Three things are required to accomplish this [6].

• Improve process energy efficiency to generate more bagasse surplus. • Harvest unburned cane and recover a reasonable fraction of the total trash. • Use an efficient technology to generate power.

4. Biomass-to-liquid: a new era in Brazilian alcohol business?

4.1 The Fischer-Tropsch Process

The synthesis of hydrocarbons from CO hydrogenation over transition metal catalysts was discovered in 1902 when Sabatier and Sanderens produced CH 4 from H 2 and CO mixtures passed over Ni, Fe and Co catalysts. In 1923, Fischer and Tropsch reported the use of alkalized Fe catalysts to produce liquid hydrocarbons rich in oxygenated compounds – termed the Synthol process. Succeeding these initial discoveries, considerable effort went into developing catalysts for this process. In 1936, Fischer and Pilcher developed the medium pressure (10-15 bar) Fischer-Tropsch synthesis – FTS – process. Following this development, alkalized Fe catalysts were implemented into the medium pressure FTS process. Collectively, the process of converting CO and H 2 mixtures to liquid hydrocarbons over a transition metal catalyst has become know as the Fischer-Tropsch synthesis [7].

3 Two main characteristics of FTS are the unavoidable production of a wide range of hydrocarbon products and the liberation of a large amount of heat from the highly exothermic synthesis reactions. Consequently, reactor design and process development has focused heavily on heat removal and temperature control. The focus of catalyst development is on improved catalyst lifetimes, activity and selectivity. Single pass FTS always produces a wide range of olefins, paraffins and oxygenated products such as alcohols, aldehydes, acids and ketones with water as a byproduct. Product distributions are influenced by temperature, feed gas composition (H 2/CO), pressure, catalyst type and catalyst composition. Product selectivity can also be improved using multiple step processes to upgrade the FTS products [7].

4.2 Technology Description

There are four main steps to producing FT products: syngas generation, gas purification, FT synthesis and product upgrading. The figure 1 depicts a generic process flow diagram. When using natural gas as the feedstock, many authors have recommended autothermal reforming or autothermal reforming in combination with steam reforming as the best option for syngas generation. This is primarily attributed to the resulting H 2/CO ratio and the fact that there is a more favorable economy of scale for air separation units than for tubular reactors (steam methane reforming – SMR) [7].

If the feedstock is biomass, the conversion of biomass to an H 2 and CO containing feed gas suitable for FT synthesis takes place through gasification but, in this case, a Pre-treatment prior to gasification is required and generally consists of screening, size reduction, magnetic separation, ‘wet’ storage, drying and ‘dry’ storage. Gasification can take place at different pressures, either directly or indirectly heated (lower temperatures) and with oxygen or air. Direct heating occurs by partial oxidation of the feedstock; while indirect heating occurs through a heat exchange mechanism [8]. A wide variety of biomass resources can be used as feedstock; however, in this study, bagasse from the Brazilian ethanol production is assumed to generate FT- diesel.

If the feedstock is coal, the syngas is produced via high temperature gasification in the presence of oxygen and steam. Depending on the types and quantities of FT products desired, either low (200-240 oC) or high temperature (300-350 oC) synthesis is used with either an iron or o cobalt catalyst. FTS temperatures are usually kept below 400 C to minimize CH 4 production. Generally, cobalt catalysts are only used at low temperatures. This is because at higher temperatures, a significant amount of methane is produced. Low temperatures yield high molecular mass linear waxes while high temperatures produce gasoline and low molecular weight olefins. If maximizing the gasoline product fraction, it is the best to use an iron catalyst at a high temperature in a fixed fluid bed reactor. If maximizing the diesel product fraction, a slurry or bed fixed reactor with a cobalt catalyst are the best choice. The FT reactors are operated at pressures ranging from 10-40 bar [7].

Upgrading usually means a combination of hydrotreating, hydrocracking and hydroisomerization in addition to product separation.

4 Low T FTS

Slurry (Co) or Tubular (Fe) Waxes (>20) Coal, Reactor Natural Gas or Biomass

Hydrocracking Gas Cleanup Gasifier Clean syngas & Conditioning H2 and CO Diesel

Air or Oxygen, Steam CFB or FFB (Fe) Olefins Reactor (C 3 – C 11 ) •Particulate Removal •Wet Scrubbing High T FTS •Catalytic Tar Conversion Oligomerization •Sulfur Scrubbing Isomerization •Water Gas Shift, etc. Hydrogenation

Gasoline

Figure 1 – General Fischer-Tropsch Flow Diagram [7]

4.3 Fischer-Tropsch Products Performance

Compared to conventional fuels, FT fuels contain no sulfur and low aromatics. These properties, along with a high cetane number, result in superior combustion characteristics. Tests performed on heavy duty trucks showed decreases in vehicle emissions of HC, CO, NO x and PM when using a FT fuel. FT diesel has been tested in a variety of light- and heavy-duty vehicles and engines. The paper by Alleman and McCornick [16] summarizes FT diesel fuel property and emission information found in the literature. Overall, FT diesel showed a reduction in regulated as well as some unregulated emissions compared to conventional diesel [7].

Several life cycle assessments (LCA) have been performed on a variety of transportation fuels including FT diesel and gasoline. Most studies have examined only greenhouse gas emissions and energy consumption with the exception of General Motors. They also examined five criteria pollutants (VOCs, CO, NO x, PM 10 and SO x). These analyses have examined the emissions and energy consumption from resource extraction to end use. The results of the studies vary based on the feedstock procurement, technology conversion and vehicle assumptions. However, in general, there is not a big advantage for FT liquids from fossil fuels in terms of energy consumption and green house gas emissions. This will not be the case for biomass systems. Because of the improved combustion characteristics of the FT liquids, performing a complete LCA including criteria pollutants will mostly likely show the overall benefits of FT liquids compared to conventional transportation fuels [7].

5. Results and Discussion

From the information available in the literature, our study proposes the use of a process route, based on the Fischer-Tropsch Synthesis aiming at the best use of the sugarcane sub-products, the bagasse and the trash, while producing high quality liquid byproducts, such as diesel, naphtha, base oils and paraffin and the concomitant generation of electricity.

5 Two schemes of the process were chosen as such (See figure 2):

• Scheme l: Biomass pre-treatment section, generation of syngas through the gasification process (Atmospheric fluidized bed air blown gasifier) and adjustment of the ratio H 2/CO to be fed to the Fischer-Tropsch reactor through a shift reactor.

• Scheme 2: Biomass pre-treatment section, generation of syngas through the gasification process (Atmospheric fluidized bed air blown gasifier) and adjustment of the ratio H2/CO to be fed to the Fischer-Tropsch reactor through a previous Hydrogen Generation Unit (Steam Methane Reforming) of natural gas.

Biomass (bagasse/trash)

H /CO H2/CO Pre-Treatment 2 Ratio < 2,0 Shift Ratio = 2,0 Reactor Off-gas

OR Syngas Gasification Fischer-Tropsch Naphtha Cleaning OR Process Process Upgrading Diesel Process Lubes

H2/CO Air H2/CO Ratio = 2,0 Ratio < 2,0

OR

Hydrogen Combined Generation CEST Cycle Unit

EE EE Natural Gas

Figure 2 – Simplified Process Sketch

In the tables below, the elementary and immediate analyses of biomass can be found in weight %, adopted in this study, and the composition of the effluent stream of the Gasification Cleaning system with the respective ratio H 2/CO, which will be the charge of the Shift Reactor (scheme l) or mixed with the hydrogen-rich stream coming from the Hydrogen Generation Unit (scheme 2) for H 2/CO ratio adjustment.

6 Biomass: Bagasse Composition (% weight *) – Ultimate Analysis Hydrogen 5.8 Carbon 44.6 Nitrogen 0.6 Oxygen 44.5 Sulfur 0.1 Chlorine 0.02 Ash 4.38 Total 100.0 * Dry basis Reference: Biomass Power Generation – Sugar cane bagasse and trash

Table 1

Biomass: Pelletized Bagasse Composition (% weight *) – Proximate Analysis Fixed carbon 12.3 Volatile matter 75.7 Ash 3.3 Moisture content 8.7 Total 100.0 LHV (kcal/kg) 4166 HHV (kcal/kg) 4323 * Dry basis Reference: Biomass Power Generation – Sugar cane bagasse and trash

Table 2

Gasifier-Cleaning system effluent stream Composition (% volume) Hydrogen 9.1 Nitrogen 53.1 Carbon monoxide 11.1 Carbon dioxide 15.8 Methane 3.3 Ethane 0.5 Water 7.1 Total 100.0

H2/CO molar ratio 0.82 Reference: Biomass Power Generation – Sugar cane bagasse and trash

Table 3

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As we can see in table 3, the effluent stream of the Gasifier Cleaning system presents a ratio H2/CO below the value required by the Fischer-Tropsch Synthesis when working with a catalyst based on cobalt, aiming the production of synthetic crude of high molar mass to be later turned into high quality diesel.

If a production of light olefins and petrochemical naphtha is desired, (or gasoline with the addition of suitable downstream processes) so as to increase octane rating), the iron catalyst used in the Fischer-Tropsch is able to work with this stream composition with no need of adjustment in the H 2/CO ratio.

Below are the results of biomass consumption for the two schemes of the process aiming the production of high quality liquid byproducts as well as the generation of electric power with the help of a process simulator.

BTL process – Scheme 1 Biomass consumption (ton/barrel of C 5+) 1.40

Table 4

BTL process – Scheme 2 Biomass consumption Natural Gas consumption 3 (ton/barrel of C 5+) (Nm /barrel of C 5+) 0.86 121

Table 5

From the results obtained and listed in Tables 4 and 5, we can see a decrease of around 38% in the consumption of biomass when natural gas is used to supply the process with a rich stream in hydrogen, using a hydrogen generator. Obviously, in a further stage, the advantages and disadvantages of this scheme process in true practice will be studied, considering, for example, the availability of raw materials involved plus the costs.

Using entirely the syngas generated in the gasification stage, this study also estimated (table 6) the potential of electric power generation considering:

• Combined Cycle • CEST

Just to give an example, the forecast of a BTL plant construction intended for the production of 5000 bpd of liquid byproducts for the plant’s own use or even for the use of neighboring cities, will require approximately 7000 tons of biomass, according to scheme l, and 4300 tons for scheme two. Such figures consider the adjustment of the ethanol production process with current environmental demands, optimizing the industrial procedures in search of a better quality of life.

8 According to the data supplied by the literature, considering the sugar cane harvesting in the 1997/1998 season - which was 301.6 million tons - the potential of biomass (in this case only trash-dry matter) would be 42.2 million tons [7].

EE generation Process Combined cycle CEST Biomass consumption (ton/MWe) 0.75 1.05 Energetic efficiency * 25.6 15.3 *LHV reference

Table 6

It is important to point out that the Fischer-Tropsch Synthesis produces a residual gas stream that may be used to generate electric power through a combined Cycle. This allows the sugarcane bagasse, currently used with such purpose, to be directed towards plant BTL, thus increasing the production of liquid byproducts and keeping the electric power generation for the plant and/or neighborhoods. In case you wish to perfect the production of electric power via Fischer-Tropsch Synthesis, the search for a gasification system of the atmospheric/indirect type may become a more adequate solution in view of the possibility of dilution of this residual gas with nitrogen coming from the air stream which consequently decreases its heating value.

Besides, various (gas) streams in the whole process require cooling, e.g. the exhaust gas from the gas turbine and the syngas after gasification. Superheated steam can be generated at these places and expanded in a (partly) condensing steam turbine to generate electricity too. Low temperature steam can also be used: for drying and other steam demanding processes such as the shift reactor.

In specialized literature we find various possible values of electric power generation from the chemical transformation process of natural gas into liquid byproducts (Gas-to-liquid, GTL). Thus, we find that a plant with a capacity of around 9000 bpd and a 65% volume of produced diesel with a similar technology to the process schemes followed in this paper allows the export of 85 MWe a day.

Overstepping the data above in a simplified manner, a BTL industrial unit with a production capacity of 5000 bpd of liquid byproducts will be able to generate around 47 MWe, using 7000 tons of biomass per day. We must point out that this value is not fixed, once the Fischer-Tropsch Synthesis unit may operate with reduced conversions, therefore generating a greater out-flow of off-gas, aiming the utmost generation of electric power.

6. Conclusions

Various studies are being done worldwide by professionals in the areas of research, development and engineering to perfect the generation of electric power and more recently the production of liquid byproducts with or without generation of electric power via Fischer-Tropsch Synthesis. All start from the most diversified charges generators of biomass, aiming the insertion of this kind of raw material in the world’s energy sources, thus decreasing the dependence of non renewable sources.

9 Currently, most efforts are concentrated on the development of the gasification processes adequate for each type of biomass. In the case of Brazil, some studies have already demonstrated the viability of the use of bagasse and trash from the sugarcane processing in the ethanol fuel producing mills as input of such processes.

Another important area is the development of synthesis gas cleaning originating from gasification because of the level of contamination in this stream, not making its use viable in processes of power generation or in Fischer-Tropsch Synthesis because of the inactivation of the cobalt based catalysts.

In short, the GTL process is not only linked to the use of natural gas as a raw material charge but also to any source of carbon that may generate a synthesis gas in the adequate ratios for the Fischer-Tropsch process adopted (H 2/CO between 0,5 and 1,2 for iron based catalysts and 2,0 for cobalt based catalysts). Then, why don’t we begin to understand the GTL process as “Synthesis gas for liquid byproducts”, once that we will thus be preparing for a not so distant future when the knowledge of this technology will be extremely important to face the upcoming changes in the world sources of power with the gradual substitution of non-renewable fossil sources for renewable ones?

Brazil has a long time tradition in the use of renewable energy. A look at the primary energy supply shows that in 2002, 41% was renewable energy: hydropower contributing with 14% and biomass with 27%. The hydropower plants amount to 65 GW of the 82 GW of total installed capacity.

In short, this study aims at making available a new and promising process route for the production of high quality liquid byproducts concomitant with ethanol production from sugarcane and its sub-products generators of biomass. This will contribute to meeting the future need for “green” fuels and the decrease of pollution caused by the low energy efficiency of the ethanol production process.

References

[1] Wikipedia, the free encyclopedia – Ethanol fuel in Brazil - http://en.wikipedia.org

[2] São Paulo Sugarcane Agro Industry Union – UNICA – http://www.unica.com.br

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10 [7] Spath, P.L. and Dayton, D.C. - Preliminary Screening: Technical and Economic Assessment of Synthesis Gas to Fuels and Chemicals with Emphasis on the Potential for Biomass-Derived Syngas, National Renewable Energy Laboratory, Colorado, 2003.

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[9] Choi, G.N., Kramer, S.J., Tam, S.S., Fox, J.M., Carr, N.L. and Wilson, G.R. – Design/Economics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co- Production, 1997.

[10] Schulz, H. - Short history and present trends of FT synthesis - Applied Catalysis A: General 1999; 186.

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[12] Espinoza, R.L. and Steynberg, A.P. - Low-temperature Fischer-Tropsch synthesis from a Sasol perspective - Applied Catalysis A: General 1999; 186.

[13] Faaij, A., Van Ree, R., Waldheim, L., Olsson, E., Oudhuis, A., Van Wijk, A., Daey Ouwens, C. and Turkenburg, W. - Gasification of biomass wastes and residues for electricity production - Biomass and Bioenergy, 1997; 12(6).

[14] Cerqueira, H.S. and Sousa-Aguiar, E.F. - X-to-Liquids - Take Your Pick: x=gas, coal, biomass - Energy Tribune, October, 2006.

[15] Tijmensen, M. - The production of Fischer-Tropsch liquids and power through biomass gasification - Utrecht, The Netherlands: Utrecht University/Science Technology and Society, 2000.

[16] Alleman, T.L. and McCormick, R.L. - Fischer-Tropsch Diesel Fuels - Properties and Exhaust Emissions: A Literature Review - SAE World Congress 2003, CI Engine Combustion Processes & Performance with Alternative Fuels (SP-1737), Detroit, Michigan.

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