Absorción química de CO2 para la obtención de una corriente de metano, procedente del biogás generado en el proceso de digestión anaerobia de una depuradora de aguas residuales urbanas

Abstract

Biogas is a mixture of gases composed mainly of methane (CH4) and carbon dioxide (CO2) that is generated as a by-product in many activities related with organic waste management. It is produced by anaerobic digestion (in the absence of oxygen) or fermentation of organic matter and it has a high calorific value (21 - 25 MJ/m3). The main objective of this thesis is to obtain a biogas rich in CH4, which we have called BIOEDAR, for use it as biofuel in vehicles. To do this, CH4 must be isolated from other pollutant gases, which reduce the energy potential of biogas or damage engines, by means of purification techniques. To achieve this objective, first of all, it is necessary to apply a cleaning process to remove impurities, present in small concentrations, such as nitrogen (0 - 3%), ammonium (0 - 200 mg/m3), H2S (0 - 10,000 ppm), water vapor (5 - 10%), oxygen (0 - 1%) and siloxanes (0 - 40 mg/m3). This process consisted of chemical scrubbing using towers with filler material or scrubbers. This first stage of the research focused on the desulphurization of biogas, optimizing the operation of the three scrubbing towers: T1, acid tower (H2SO4), T2, oxidative tower (NaClO) and T3, basic tower (NaOH). It has been concluded that the exclusive use of the basic tower T3, working in a pH range of 10.5-11 was the best choice. A complete characterization of the treated biogas has been carried out, and it is concluded that the concentrations of the 40 trace compounds analyzed are below the detection limits of the analytical equipment. Therefore, at this point, the biogas contains basically only CH4 and CO2. In the second stage, the CO2 has to be removed, because although it does not cause damage to engines, it reduces the energy potential of the biogas. For this purpose, a process of chemical absorption of CO2 by means of amines at low pressures has been applied. In order to carry out the process continuously, a balance between absorption and subsequent desorption process of the amine solution must be achieved. The main considered process variables were the optimum amine dosage and the temperature of the desorption process. After a literature review about the efficiency of amines, and taking into account economic criteria, the solutions used as CO2 absorbers were monoethanolamine (MEA) and methyldiethanolamine (MDEA), both at 20% and 40%. The superiority of MEA over MDEA was evident because both MEA solutions, at 20 and 40 %, achieved a CH4 richness of approximately 96 % at the outlet of the CO2 absorption columns, compared to the MDEA solutions, which barely reach 75 %. Finally, after optimizing the process, the optimal amine flow rate is obtained, with MEA 20%, and the number of packed towers to be used. In relation to the CO2 desorption process of the amine, it is achieved with an optimal temperature of 85 °C in the preheater boiler, and using the two thermal resistances of the desorption tower that ensure temperatures above 92 °C. In regards to biofuel, for every 100 m3/h of biomethane, net 62 Nm3/h were obtained, enough to supply four vehicles with a range of 420 km each. Comparing the CO2 emission values, the project vehicles running on this BIOEDAR generated 138 g CO2/km, while a conventional diesel vehicle emits 170 g CO2/km on average. With this data, and considering the range of the vehicle under study, it can be deduced that the project vehicles running on BIOEDAR generate 14 kg CO2 less than a conventional vehicle in that range. This aspect is crucial to the approach towards a more sustainable future in which the aim is to reduce greenhouse gas emissions and commit to a renewable energy system. This thesis proves the possibility of establishing a circular economy model in a WWTP through the energy recovery of sludge, and on the other hand, the energetic use of biogas as biofuel, which is the subject of this work, promoting more efficient production and consumption systems, establishing continuous cycles and managing to reduce the consumption of raw materials, energy, waste generation and emissions.