Author: Xu, Linji
Title: Ammonium transport and reduction in deionization cell integrated anaerobic treatment and solid oxide fuel cells as a net energy producer
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Sewage -- Purification
Waste products as fuel
Department: Department of Civil and Environmental Engineering
Pages: xxvi, 244 pages : color illustrations
Language: English
Abstract: Since human entered industrial era, our behaviors have accelerated energy scarcity and environmental pollution, which is threatening our unique living planet. To decelerate fuel consumption and prevent our earth from further deterioration, we are thinking of ways to save the traditional fossil fuels and search for sustainable energy. Biomass, as one type of new energy, has been investigated for some decades. Most of the biomass comes from organic wastes, especially from wastewater treatment plants. It is widely known that wastewater treatment plants represent a portion of the broader nexus between energy and water, which has raised the worldwide attention. Logically, energy capture from wastewater streams is considered to be a new and noteworthy approach for eliminating environmental pollution and offsetting the traditional fuel consumption. Such as anaerobic treatment (AT) combined thermal engines has been employed to convert the organic wastes to useful electricity. Yet, an enormous amount of recoverable energy like ammonia (NH₃) or hydrogen (H₂) has not been well considered up to date. Toward this, this thesis reported a hybrid system of three techniques including anaerobic digestion (AD), electrodeionization (EDI), and solid oxide fuel cells (SOFCs), named AD/Landfill-EDI-SOFCs. This system was used to separate and remove different kinds of ammonium wastewater. When AD/Landfill-EDI-SOFCs was used to capture energy from anaerobic digestate and landfill leachate, the experimental results displayed excellent performances of inorganic ion removal and energy conversion, which signified that this system would be a potential perspective of environment protection. Specifically, under the optimal conditions (3.0 V applied voltage and 7.5 mm internal electrode distance, IED), EDI section removed 70-95% NH₄⁺ ion from 0.025-0.5 M synthetic NH₄⁺-N wastewater. Regarding energy benefits of EDI, the energy balance Ratio (EBR) indicated that concentrated NH₄⁺-N sewage (> 0.25 M) was its appropriate range according to collected fuels (NH₃ and H₂) and consumed electricity. In terms of electricity generation of SOFCs, synthetic gases (NH₃-H₂ and CH₄-CO₂) and the real biogas from the lab-scale reactor can be used as its fuels for electricity generation because it gained 900-1380 mW cm⁻² peak power densities at 750 °C operating temperature. To assess the net energy benefit, the comparison of energy recovery between the existing process (landfill, ammonia stripping, and combined heat and power, Landfill-AS-CHP) and the proposed system (Landfill-EDI-SOFCs) was carried out via a municipal landfill site in Hong Kong as an example. The results demonstrated that EBR rose from 1.11 (Landfill-AS-CHP) to 1.75 (Landfill-EDI-SOFCs). The removal rate of inorganic ions reached 80% from the raw landfill leachate, and the removal rate of ammonium ion attached 99%. Hence, AD-EDI-SOFCs hybrid system was thought to be feasible for the upgrades of anaerobic processes for energy potential extraction from wastewater streams.
According to energy footprint of the AD-EDI-SOFCs system, whereas the process of NH₄⁺ separation including the solution, ion exchange membrane, and electrochemical reactions consumed about 56% recovered energy by EDI. This would be owing to the weak conductivity of the ammonium electrolyte. To understand the reason for such a high level of energy consumption in this process on the earth, studies thus went to the mechanisms of ion separation and electrode behaviors. The effect of the supporting electrolyte on NH₄⁺ separation was initiated through experiments and theoretical calculations. The results signified that the concentrated Na₂SO₄ electrolyte (0.125-0.75 M) contributed an increase in current density from 1.6 mA cm⁻² at 0.125 M to 40 mA cm⁻² at 0.75 M. The concentration polarization between two sides of ion exchange membrane was described. The difference in the magnitude of concentration polarization: NH4+ concentration decreased to almost 0 M in the left-side boundary layer but increased to 0.65 M in the right-side boundary layer in 0.125-0.75 M. Also, the increase in Na2SO4 concentration contributed to increases in the limiting current density (LCD, from 14 mA cm⁻² to 20 mA cm⁻²), the total thickness of boundary layer (from 370 μm to 430 μm), and the potential (from 0.2 V to 2.0 V) at the corresponding concentration. As a result, the resistance of the bulk solution was dominant for the diluted Na₂SO₄ electrolyte (< 0.5 M) while the resistance of the boundary layer and water splitting became dominant for the concentrated Na₂SO₄ electrolyte (> 0.75 M). Interestingly, NH₄⁺ reduction (NH+₄(aq) + e- -> 0.5H₂(g) + NH₃(g)) was found at the EDI cathode during the ion separation. This reaction brought a positive impact on increasing the H₂ yield. Experiments demonstrated an increase in reduction performance of NH₄⁺ when the Na₂SO₄ concentration increased at low Na₂SO₄ concentration, but this trend was reversed when the concentration was higher. So, in order to dig its fundamentals, the supporting electrolyte influencing on NH4+ reduction was studied via molecular dynamics (MD) simulation followed by experimental tests using 0-1.5 M Na₂SO₄ solution. The results indicated that the concentration of Na₂SO₄ supporting electrolyte significantly executed adverse effects on the reduction and migration of NH₄⁺ ion in the electric double layer (EDL). In detail, NH₄⁺ ion displayed a stronger competition capability than Na+ ion at the diluted Na₂SO₄ solution (< 0.25 M). The competitive absorption of Na+ formed a thick layer blocking NH₄⁺ approaching and electron transfer in the EDL at the concentrated Na₂SO₄ electrolyte (>0.5 M). Concerning NH₄⁺ recovery, Na₂SO₄ concentration thus should be not more than fed concentration (0.25 M). An unreported result that the migration rate of NH₄⁺ (2.43 × 10⁻⁹ m² s⁻¹) was faster than that of Na+ (1.54 × 10⁻⁹ m² s⁻¹) occurred at over-concentrated Na₂SO₄ was found, which was thought to be related to hydrogen bond. The mechanisms disclosed the co-ion competition molecularly and allowed the manipulation of EDI capacity optimization. In short, the hybrid system (AD-EDI-SOFCs) proposed in thesis performed great potentials for energy recovery from wastewater, particularly suitable for the upgrades of processes used at landfill sites and concentrated NH4+-N wastewater treatment. The supporting electrolyte was indispensable from the results of this study. Nevertheless, the over-concentrated supporting electrolyte caused apparent concentration polarization and the fierce co-ion competition, which caused the intensive energy consumption and the decay of NH₄⁺ reduction. Therefore, the concentration of Na₂SO₄ solution should be controlled within a specific range (0.25 M< cNa2SO4< 0.5 M) as considering migration and reduction processes together.
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