Estudios de alternativas de valorización energética para el aprovechamiento de residuos agrícolas procedentes de la agricultura intensiva bajo plásticoAplicación en una planta de producción de clínker

Abstract

Nowadays, there is a great social concern regarding the growing accumulation of the wastes derived from intensive greenhouse agriculture in the environment, particularly in those areas where an intensive agricultural activity is concentrated. The inadequate management of these wastes causes the generation of unmanageable environmental problems using the traditional methods of recovery such as composting, animal feed or landfill. In this framework, it is crucial to encourage the search for innovative solutions that might be able to achieve an adequate and sustainable use of these residues. The heterogeneity of the wastes and the seasonality associated with their production make difficult the progress of these activities and discourage the development of solutions towards its use to the energy recovery. This dissertation aims at promoting the use of wastes derived from intensive agriculture of greenhouse as a source of thermal energy to be used in the industry, emphasising the quality of the fuels derived from the processing of this type of waste. In addition, this work evaluates the benefits of the use of these wastes in a clinker production plant. Firstly, the characterization of wastes derived from intensive agriculture of greenhouse were done in order to evaluate the potential energy associated with their recovery. In particular, it was analysed the wastes generated in the region of Almería (Spain), where around 2 million of tonnes of this kind of wastes per year are produced. The results obtained from the above-mentioned waste characterization were extrapolated to the national level. In this sense, an evaluation methodology was developed to analyse the data provided by a waste treatment plant located at the same region. Different samples of wastes were characterized in the laboratory facilities. The results extracted from these analyses highlighted the need of a waste pre-treatment before its use as a source of thermal energy for the industry. Secondly, three conversion alternatives were assessed to transform the greenhouse wastes in suitable fuels to be used in a clinker production plant, covering the needs indicated above. Among all the available options, three different alternatives were studied to improve the chemical and the energy properties from the original gross wastes. These technologies are named the direct combustion, the thermal gasification and the anaerobic digestion technology. The analysis of the co-combustion process of wastes derived from intensive agriculture of greenhouse was carried out at laboratory scale. To meet this target, a portfolio of pre-treatment operations was evaluated in order to reduce both the chlorine and the moisture content. These results allow to establish an optimal pre-treatment system to reduce the chlorine content up to 50% and to increase the lower calorific value up to 8% compared with its original value. The thermal gasification technology was assessed through a series of laboratory tests carried out in a pilot gasifier combined with a simulation model based on the characteristics of these particular wastes. On the one hand, the laboratory tests showed that the gasification of greenhouse wastes might generate a high-quality gaseous product - lower calorific value more than 5 MJ per Nm3 of syngas- that can be compared with the retrieved from the orujillo gasification. However, waste processing is difficult due to the amount of ash content. On the other hand, the simulation model allows to establish that the industrial gasifier should be operated at 852 °C and a ratio air per stoichiometric air next to 0.39 to ensure the best quality of the fuel gas to industrial scale, The anaerobic digestion of the greenhouse wastes was analysed using theoretical models reported from the literature. The use of this conversion technology provides a fuel gas with a methane potential of 0.56 Nm3 of methane per kilogram of wastes supplied, and a lower calorific value of 17.2 MJ per Nm3 of biogas. The energy content of the biogas generated is higher than those obtained from other conventional residues such as alperujo, sludge from sewage or slurry. The co-digestion of a blend formed by 90% of greenhouse wastes combined with 10% of the meat industry residues allows to increase up to 22%the quality of the biogas produced. The energy associated to this fuel gas might achieve a lower calorific value close to 21 MJ per Nm3 of biogas with a chlorine content less than 5.0 mg per Nm3 of biogas. Finally, the technical and economic feasibility of the energy recovery of each waste derived fuel generated was evaluated in a clinker production plant located at Almería (Spain). To do this, it was considered the addition of the maximum amount of alternative fuel authorized into the process instead of part of the fossil fuel currently used. The impact of the alternative fuel added into the process was also evaluated on the operation of the clinker production plant. For this purpose, an analysis tools based on a Decision Tree Model was developed. This tool allows to predict the risks associated to the use of each fuel generated in terms of operational issues such as incrustation problems and frequent blockages. In addition, the analysis tool evaluated the interest the substitution of a certain amount of fossil fuel by these new fuels. In this case, the input of 65,000 tonnes per year of alternative fuel derived from waste might achieve the substitution of up to 51% of the thermal energy provided by petroleum coke. This recovery energy provides both savings of up to 2.9 € per tonne of clinker produced that represents 2.1 M€ per year. The analysis tool predicts that this addition improves the current operation and decreases both the frequency of the appearance of incrustation and the generation of system blockages. However, an enrichment of chlorine content in the process is also highly probably. For this reason, it is recommended the installation of a bypass located at the highest concentration of chlorine in the gas likely might be found. In addition, the incorporation of a new fuel should affect the specific thermal energy consumption of the process, expressed in GJ per tonne of clinker produced. The influence of the novel fuel addition was determined using a Multivariate Linear Regression Model, which allows to identify the most relevant operational parameters on the specific thermal energy consumption. The input of 65,000 tonnes per year of each fuel derived from waste could increase the specific energy consumption up to 15% compared with the current indicator value. For this reason, it is recommended to carry out the monitoring and control of the flame during the incorporation of these new fuels to the clinker kiln. Although the cost associated with the different conversion technologies evaluated in this work should be analysed more carefully, the results obtained in this dissertation show that there is a sustainable and friendly solution for minimizing the environmental impact associated to the accumulation of wastes from intensive agriculture of greenhouse. Moreover, this solution could provide an added value non-existent so far on this kind of waste. The use of these wastes as an alternative fuel for a cement plant should report an important economic benefit for the cement industry and substantially improve the operation performance of the cement plant production in terms of a reduction of the production losses derived from the increase of incrustation or system blockages.