Experimental effective parameters on a single-chamber microbial fuel cell with a ring structure using chocolate industry effluent

Dissertation for Master's degree

Chemical Engineering

Abstract:

Microbial fuel cell biotechnology is a new science, in which microorganisms, as cheap catalysts, convert the chemical energy contained in organic and inorganic compounds into electrical energy. they do Optimizing parameters affecting battery performance is one of the first laboratory steps towards the development of this technology on a practical scale. The production power is one of the noticeable characteristics of the performance of the microbial fuel cell, which is influenced by several factors such as electrode spacing, input substrate concentration, conductivity and acidity conditions of the anolyte solution, the type and type of electrodes used, operational parameters such as: temperature, pH, and so on. is In this research, the following goals were followed in order to simultaneously treat wastewater and produce power and electricity using a single-compartment microbial fuel cell. First, the stainless steel mesh with graphite coating as the anode provided a porous surface for the proper growth and connection of the biofilm. Second, by using the anode in the helical geometry, the surface of the anode electrode increased and the time for the substrate to reach the microorganisms decreased. Wastewater of chocolate industry, which contains hard degradable and stable compounds such as solvents, detergents and oils, was used as a substrate. In this research, two annular single chamber microbial fuel cells with a volume of 90 cm3, with the same configuration and the only difference in the electrode distance, were tested discontinuously. The maximum voltage in the open circuit state and the power density for the first system with an electrode distance of 1.3 cm were obtained as 742 mV and 7.98 W/m3, respectively. In order to optimize the electrode distance, the second fuel cell was built and started at three electrode distances of 1, 0.7 and 0.4 cm, and its performance in these three distances was investigated and compared with the results obtained from the first fuel cell. The maximum voltage in the open circuit state at the optimal electrode distance of 0.7 cm was 856 mV. The experiments were repeated to obtain the maximum current and power density. The maximum power density at the optimal distance was 22.898 W/m3. The performance of the microbial fuel cell as an electricity generator is shown based on the polarization behavior and the cell potential. In the next stages of the experiments, the input substrate concentration, chemical oxygen demand, turbidity and operational parameters such as temperature, pH and their effect on the behavior of the system at the optimal electrode distance were analyzed. A significant decrease in turbidity and chemical oxygen demand was observed after 96 hours, 79.66% and 91.2%, respectively. By reducing the chemical oxygen demand of the wastewater from 1400 to 700 mg/liter, the duration of the reduction phase decreased and the output current decreased from 3.77 to 2.76 mA. Also, by examining the effect of pH on system performance, the maximum current was obtained for pH between 7 and 8. Examining the effect of temperature also indicated the negative performance of the system at a temperature of more than 35 degrees Celsius. Finally, the scanning electron microscope images of stainless steel mesh with graphite coating before and after the biofilm formation showed the proper adhesion of bacteria on the electrode surface.

Foreword

The increase in global energy consumption and the issue of global warming have made the use of new and renewable energies inevitable. Microbial fuel cells[1] are particularly attractive for reasons such as cheap raw materials and relatively high efficiency. In this chapter, first things are stated about the challenges of energy and renewable energies, and then microbial fuel cell technology is proposed as a solution to deal with these challenges. At the end, the important applications of microbial fuel cells are presented.

1-1       Population increase and energy need

Currently, the population of the earth is more than 6 billion people, which is estimated to reach more than 9.4 billion people in 2050 [1]. In the past years, fossil fuels contributed to the development of the industry of advanced countries and their economic growth. It is predicted that in the years 2015 to 2025, the demand for more production will empty many oil tanks and reserves [2].. Based on the predictions and taking into account the population growth and economic growth, they have estimated the need for energy in 2050 at 41 terawatts [2]. This prediction is based on current energy consumption. Considering this trend, according to a reasonable forecast, the expected energy for the year 2050 is 27 terawatts and for the year 2100, 43 terawatts [1].

1-2     Fossil fuels and current challenges

The use of fossil fuels, especially oil and gas, has gained a lot of momentum in recent years. Fossil fuels have caused the industrial and economic growth of countries, but it is clear that they cannot support the global economy indefinitely. The consumption of such fuels, since it leads to their direct combustion, has brought many problems to humanity, it should be noted that more than 20% of the required energy is produced in the form of electricity in power plants. Considering that the efficiency of power plants is about 33%, so the energy used to produce such electricity is three times the amount of production. The most important problem that will endanger the future of humans is the problem of global warming, which is caused by the presence of greenhouse gases, and these gases themselves are obtained from the direct combustion of fossil fuels. In addition, the combustion of fossil fuels leads to environmental pollution such as air pollution, acid rain and its negative effects on fields, forests, pastures, surface water and historical buildings, etc. Another problem that threatens the world community due to the increasing use of these fuels is the energy crisis, the consequences of this crisis will be much more unfortunate, and it will no longer be an environmental issue, but it will lead to political, social and economic problems. When America faced its first oil crisis in the 1970s, it sought to find solutions to overcome this problem. Among these solutions are discovering new oil reserves, increasing the efficiency of oil extraction from existing sources or using other fossil fuels such as tar sands[3].

Another solution is to use nuclear energy, but it also has its own limitations. Limited global uranium reserves, problems related to environmental issues and human health caused by the extraction of uranium from mines and the lack of sufficient safety and finding a long-term solution for nuclear waste storage are among these limitations.

Solar energy is a long-term solution, but it all depends on how this energy is used. The sun does not shine every day and not all its rays are the same everywhere. So solar panels can help meet the electricity requirements of the day. However, they cannot be useful as a source of energy supply during the day and night without efficient methods of energy storage.

Together, all these factors have led scientists to look for suitable alternatives for energy supply, so renewable energies have been considered as one of the ways to reduce this crisis. Many efforts have been made to create other methods of generating electrical energy. New methods of producing electrical energy from renewable sources without net emission of carbon dioxide are of great interest [3].

1-3    Renewable energies

Renewable energies are basically compatible with nature, do not pollute, and because they are renewable, there is no end to them. Among other features of these resources, we can mention their dispersion and expansion in the whole world, easy technology and low price. Renewable energies are categorized as follows [4].