A small and low-cost wastewater treatment system with biogas production was developed in a campus of São Paulo State University, campus of Guaratinguetá. This system is able to receive and to treat part of the wastewater produced in this campus, diminishing costs. Previously, the overall volume of this residue has piped by a state-owned company.
Beyond treating wastewater, an attention concerning the water produced during this treatment (reclaimed water) and the produced biogas must taking into account. This type of water (which contains nutrients for the plants) could be used in the gardens. The biogas could be used for energy purposes in own campus.
The utilization of biogas as a feedstock for biohydrogen production has be widely researched due to its low environmental impact and due to its availability, since this fuel could be produced utilizing various feedstocks through various technologies and processes. In this case, due to the small amount of produced biogas and hence, small amount of hydrogen, its use as energy carrier is imperative.
Recently, a project was approved under support by FAPESP, a state-owned company responsible by financial support of projects performed in the Brazilian state of São Paulo. Firstly, a theoretical analysis was developed to evaluate the content of hydrogen to be produced through steam reforming. The choice of this technology of hydrogen production is due to the large experience devoted by Energy Systems Optimization Group, maintained in this campus. This group has developed recently technologies and process of hydrogen production utilizing inclusively other sources such as natural gas and ethanol. Another motive to choose this technology is the ease of project and development and its low cost of installation. Other measurements such as exergy efficiencies and irreversibilities have contributed to determine the steam reforming process if compared with other processes such as partial oxidation and autothermal reforming. However, due to the high amount of carbon dioxide (CO2) in the produced biogas, and due to the high cost necessary to extract it, a steam reforming process associated with dry reforming (where the own CO2 is utilized to react with methane gas (CH4), the principal constituent of biogas) is suggested. Both reforming processes are accepted and no more additional process is necessary since the volume of CO2 contained into the biogas is smaller if compared with CH4. The volume of constituents was 54% CH4 and 40% CO2 before H2S purification, and 61.8% CH4 and 34.4% CO2 after purification. Some traces of O2 and N2 were encountered in the biogas in both cases however these small amounts are negligible and no problems during hydrogen production are foreseen. The amount of encountered H2S was 2.8% before purification and 1.2% after purification process. This process is performed through low-cost devices which utilize small pieces of iron extracted from machining processes.
In this work a physical-chemical, exergetic and economic were developed as mean to determine the more optimized conditions for hydrogen production. The steam reforming and dry reforming are endothermic processes. Due to this, high temperatures of hydrogen production are foreseen and hence a major amount of fuel as heat source is necessary. For a biogas production of 9 Nm3 per day (that is the capacity of production of biogas in the wastewater treatment system), the reforming process in the best conditions could produce about 1,3 kg H2 per day, which could generate about 15-20 kWh electricity if a PEMFC (Proton Exchange Membrane Fuel Cell) is considered. A performance of 100% for hydrogen production process was adopted, and following some recent works, the electric efficiency of a PEMFC could attain 40-50%.
The suggested thermodynamic conditions detected in physical-chemical and exergetic analysis was 600°C and 1 atm, where lower irreversibilities, high exergetic and rational efficiencies and high conversion efficiency low could be encountered. The performance of hydrogen production increases as soon as temperature of reaction also increases, however, at temperatures grater than 600°C, the additional hydrogen to be produced is low if compared with high amount of energy necessary to the process. In contrary to temperature, as soon as pressure during hydrogen production increases, the performance of reaction decreases.
Connected with both analyses, an economic analysis was developed to evaluate its viability. Some findings were considered such the possibility to utilize electricity and the own biogas as heat source to perform the reactions, the annual period of utilization of developed hydrogen production system of 5000 to 7000 hours, maximum interest rate of 20% per annum (considering the Brazilian conditions), and maximum payback of 20 years. The lowest cost detected of hydrogen produced was 0.27126 and 0.22323 US$/kWh whether electricity and biogas as heat sources are utilized, respectively. In the last case, an additional cost associated with biogas was not considered. This cost should be added in some cases due to the utilization of equipments (such as H2S purification system) to maintain biogas production only although this cost is low and do not become the process prohibitive.
The introduction of emission trading schemes such as carbon credits have become possible the introduction of environmentally-friendly technologies, in special for technologies which allow the capture of greenhouse gases such as biogas. The evaluation of the produced biogas volume and the amount of greenhouse gases emitted by hydrogen production process allow to determine the value of allowance, which could be utilized to the infra-structure, diminishing the costs of produced hydrogen and hence, becoming this technology more-competitive. With cost of allowances attaining US$ 40 per ton of CO2-equivalent, the installation of a hydrogen production system with no-additional funds is possible.