Electric energy from vinasse

ELETRIC ENERGY FROM

VINASSE

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This is a theme that frequently appears when we are discussing the design criteria for the implementation of new industrial units, especially when the client wants to maximize the sale of electricity.

The vinasse from sugarcane ethanol production is a liquid effluent with great potential for pollution. This potential is basically characterized by its organic load, evaluated by its chemical oxygen demand (COD), and by its potassium content. Potassium is an element that practically does not participate in physical chemical reactions during sugarcane processing. In this way, all the potassium found in sugarcane ends up in the vinasse, whether it is ethanol produced from juice, honey or a combination of the two. What can vary is its concentration, depending on the amount of vinasse produced per liter of ethanol, but it will all be there. When vinasse is applied to crops, its potassium content is decisive in calculating the maximum application rates per hectare per year. COD is not relevant, as long as there is no infiltration to the water table, as the organic matter disposed in the soil will be used by the sugarcane ratoon.

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When the vinasse undergoes an anaerobic digestion treatment, there can be a COD removal in the order of 80% to 90% at most. Again, potassium does not participate in the reactions, and the potassium that enters is the potassium that leaves the reactor. As the potassium content does not change and the outflow COD is still high for the disposal of the effluent in nature, this treatment cannot be made viable as a pollution control system. Therefore, the treatment is only viable, especially in Brazil, if the energy produced pays for the investment and operating costs.

The energy source is mainly methane gas, contained in biogas from the anaerobic digestion of vinasse. Anaerobic digestion is provided by bacteria that are properly selected and adapted to the effluent and the environment in the reactor. Most of the COD is converted, and the sludge resulting from the reaction (about 1.5% to 2.5% of the effluent at the inlet) should be removed from the reactor and properly disposed of as other waste. It can also be sold at a price of around R$ 60.00/t, as today the activated sludge market is a buyer.

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There are basically two categories of bacteria used in reactors. Mesophilic bacteria operate in a temperature range of 35 to 37 C, and thermophilic bacteria in a range of 55 to 57 C. Good temperature control is crucial for good process efficiency. In addition to temperature, pH control is also important. At the beginning of the process, as the pH of the vinasse is very low and the bacteria work in a basically neutral medium, it is necessary to use soda or another alkalinizer to neutralize the medium. As the reactors always work with high recirculation rates, as the process progresses, the alkalinizer consumption decreases rapidly.

To define the process, the designer must face two essential tasks. It is necessary to try to adequately characterize the effluent in question and to try to determine, with pilot tests, the rate of expected COD removal and the characteristics of the biogas produced.

The correct characterization of the effluent is essential for any project. Manufacturers such as Dedini, for example, which has already supplied numerous reactors for the brewing industry, have a large database about

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vinasse. However, for sugar mills that use sulfur in the process, knowing the hydrogen sulphide (H S) content in ₂ the resulting biogas is also essential to determine the need for treatment before its use. High content of H S in ₂ biogas may require, for example, internal combustion engines with special materials, which are much more expensive, or may cause undesirable corrosion in boiler heat exchangers.

In Brazil, vinasse has COD in the range of 20 to 25 kg/m³ (coming from fermentation with juice) and from 30 to 35 kg/m³ (fermentation of juice and honey). As they say in the industry, COD in effluent means lost product. In this case, the highest COD in honey vinasse means that some of the reducing sugars are turned into unfermentable sugars during processing for sugar production, which are industrial losses.

There are several categories of reactors for anaerobic digestion. In all cases, the engineering challenge is always to try to homogenize the medium as much as possible, in order to allow bacteria to come into contact with organic matter, their source of survival.

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Anaerobic lagoons, with a depth in the range of 4 to 6 m, can be used and must be lined with PAD-based geomembrane and also covered by the same material, for the collection of biogas. They are reactors that require large volumes, as the application rate (TA) varies from 2 to 3 kg COD/m³/day. Large volumes make it difficult to homogenize the medium and can create preferential paths, reducing the COD conversion rate. As the membranes cannot withstand internal pressures without leaks, they need exhaust systems to remove the biogas. They use mesophilic bacteria, operating at a temperature of 36 C, but even so temperature control is difficult in function of the large volumes that must be homogenized.

UASB (Up-flow Anaerobic Sludge Blanket) reactors use thermophilic bacteria, operating at a temperature of 56 C, designed to operate with a TA in the range of 8 to 10 kg COD/m³/day. It is the category of reactor that is in operation at Usina São Martinho, processing around 10% of the vinasse produced and generating the energy necessary for the drying of yeast.

Type IC (Internal Circulation) reactors use mesophilic

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bacteria, operating at a temperature of 36 C, and are designed for a TA of 25 to 30 kg COD/m³/day. The volumes are drastically reduced, mainly due to the use of the biogas generated for the homogenization of the environment. They can operate with enough internal pressure to pump the biogas to the next steps in the process. They are typically used in breweries for pollution control purposes.

To assess the energy potential of a plant of this type, it is possible to exemplify the typical situation of a plant producing only ethanol, as indicated below:

Typical ethanol/cane production: 0.090 m³/t Typical vinasse/ethanol production: 10/1 Typical vinasse COD: 20 kg/m³ Typical COD/cane production: 18 kg/t Reactor COD removal rate: 85% Biogas production: 0.35 to 0.40 nm3/kg COD removed Average concentration of methane in biogas: 60% to 80% Typical methane production: 0.26 nm3/kg COD

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Specific methane/cane production: 4.7 Nm3/t Lower calorific value of methane: 34450 kJ/Nm3 Equivalence with bagasse (PCI = 7325 kJ/kg): 4.7 kg of bagasse / Nm3 of methane Additional equivalent bagasse % cane: 2.2% Bagasse % typical cane: 26.5% Percentage increase, expressed in bagasse, . provided by methane: 8.3% Additional generation in steam cycle (68 bar / 520 C) with condensing turbine: 12.7 kW.h/tc Additional generation in Otto cycle internal combustion engines: 17.1 kW.h/tc

The methane produced can basically be burned in boilers for steam generation or in internal combustion engines.

If it is burned in boilers, we are talking about a condensation cycle, as there would be no consumption of process steam. It is a cycle with low efficiency, in the order of 30% only. With a regenerative system, with taps on the condensation turbine for pre-heating the condensate, we generate about 0.50 kW.h/t of bagasse.

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Therefore, with the use of methane in the boiler, we would be producing an additional around 12.0 kWh/t cane. Although the energy recovery efficiency is lower, the advantage is that the equipment (boiler and condensing turbine) would already be available.

If burned in internal combustion engines of good efficiency, we can achieve efficiencies of up to 38%. In electricity generation. Therefore, with the use of methane in engines, we would be producing an additional around 17.0 kW.h/t cane. These engines lose about 22% of the primary fuel energy in cooling water and oil, and about 40% in the flue gases. Thus, although there is an additional investment in engines, one can think of using the heat of these gases, which are at around 400 C, for drying yeast or for producing ice water, creating a cycle of greater efficiency than the condensation cycle.

We will obtain the equipment installation costs. Complete type IC anaerobic digesters, including the H S ₂ elimination system, cost from R$ 85.00 to R$ 95.00 per kg of COD applied per day. Internal combustion engines cost from R$1,700.00 to R$2,000 per kW installed,

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including the cooling tower system for the water circuit.

The operating costs of the boiler and condensing turbine are known. The operating costs of the engines must be well evaluated, as they are equipment that use a lot of lubricating oil and filters. The operating costs of anaerobic digesters are estimated at between BRL 0.016 to BRL 0.018 per kg of COD applied, but it is necessary to confirm with the manufacturer, as the inputs can vary greatly from one technology to another. The total cost of operating the plant with internal combustion engines is around R$ 40.00 / MW.h.

It is important to remember that, in all cases, it is necessary to discount the parasitic energy, represented by auxiliary equipment such as exhausters, pumps, cooling towers, etc. It should be in the about 10% range.

Now it's time to do the math and, depending on the sale price of electricity, check if it pays.