Biogás para cocción a partir de la digestión anaerobia de pastos en zonas rurales de clima tropical

Resumen

Energy for cooking is the most essential energy service for peasants in countries with tropical climates such as Colombia where a large number of families can only access to firewood as energy source. An alternative energy source is the production of biogas, with an estimated consumption of 0.68 m3·d-1 per dwelling unit. The lowest cost and most universal biomass that exists is grass, natural or cultivated, while Anaerobic Digestion (AD) can be a very attractive technology to generate renewable energy due to the versatility and potential uses of the generated biogas, to the continuous optimization of the process and to the possibilities of its application at a domestic level with low-cost reactors. The use of grass or forage as substrate for AD is especially interesting in areas with high agricultural productivity, where the main challenges consist of producing grass in a sustainable way at the lowest cost and having the appropriate technology to produce biogas from grass at an acceptable investment and operation cost. The objective of this research was to study an alternative proposal for the production of cooking energy for rural areas with a tropical climate, from the biogas resulting from AD from pastures. In a first evaluation stage, assays of specific methane yield (mLCH4·g-1SV), methane yield per hectare (m3CH4·ha-1) and agronomic yield (tMS·ha-1·ha-1·y-1), were conducted with three types of grass widely distributed in the equatorial zone of the American continent (King Grass, Elephant Grass and Star Grass), cultivated in the Andean zone of Valle del Cauca This screening showed that, among the grasses with the highest yield under these conditions, Pennisetum purpureum species are the most suitable in Valle del Cauca (Colombia) due to its dry tropical climate conditions. Furthermore, King Grass was the one with the highest methane yield per hectare and most feasible for harvesting and chopping due to its morphology. The next stage at pilot scale evaluation, allowed to conclude that AD of pastures in domestic digesters type Discontinuous Dry Bio-digestion System (SDBS) of a single phase, without heating or temperature control, is viable with and without recirculation of leachate at a mean digestion temperature of 24°C, using King Grass as substrate, being the option without recirculation of leachate the most sustainable. Complementary tests of Bio Methane Potential (BMP) allowed to detect a s ignificant increas e in AD yield with harves ting ages lower than 60 days and to conclude that the “m achine chopped” method of size reduction (1-10 cm) was the most efficient pretreatment. Two options for the production of cooking energy were designed and evaluated economically and environmentally from the biogas resulting from the AD of grasses: one of them with organic cultivation (BOFG) and the other with complementary mineral fertilization (BMFG). Although the investment cost of the biogas cooking alternative was high for the target population, its annual operating costs was lower than those of current sources: firewood and LPG, which indicates that it is an economically viable alternative provided there is external support for initial investment. The evaluation of the environmental benefits / impacts of the technological proposal using the Life Cycle Analysis (LCA) technique, showed good environmental sustainability in the emissions of polluting substances, compared to firewood and LPG. Among the type of fertilization, the BOFG presented lower impacts than the BMFG in most categories. As a general conclusion, it can be said that the viability of producing biogas from AD of pastures cultivated in situ using domestic SDBS has been demonstrated, optimizing the cultivation and digestion processes. Therefore, this technological