Contributions to the reduction of particulate matter emissions in biomass combustion processes

Abstract

The current socio-economic and environmental panel is marked by the need to curb the consumption of fossil fuels and reduce greenhouse gas emissions; so the energy transition to a renewable model is fundamental from the point of view of sustainability. More recent international policies have focused, on the one hand, on the incentive for renewable energies and, on the other, on limiting emissions of harmful substances. In this context, biomass is presented as an interesting alternative, both from a technological and economic point of view. Given its vegetable origin, its carbon content allows energy use through conventional methods, such as combustion. In addition, it offers the possibility of adapting traditional technologies to environmental requirements in a simple and low-cost way, which has allowed a remarkable expansion, especially in the domestic sector. However, obtaining energy through biomass combustion carries certain drawbacks. Fundamentally, these are related to the compositional heterogeneity of the raw material, which invites reactions to take place less efficiently than in the case of other fossil fuels. Thus, apart from carbon dioxide and water vapor, during the combustion of biomass certain substances are released into the atmosphere: carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), particulate matter, etc. Most of these compounds are harmful not only to the environment but also to human health; especially in the field of respiratory, cardiovascular, or even cancer diseases. In addition, the ashes and residues generated during combustion processes favor the formation of deposits inside the facilities, which also leads to the appearance of serious problems related to line obstruction, loss of efficiency or corrosion of the materials. The urgency of remedying this problem has led to the development of a series of techniques and methods that would reduce the effects mentioned above. These strategies are generally divided into two main groups, depending on the phase of the combustion process in which they are applied. The primary measures correspond to those strategies that seek to prevent the formation of harmful substances, either by acting directly on the combustion process and improving the way reactions take place, or by improving fuel quality. Generally, these are temperature control and air-fuel injection strategies, as well as addition or removal of the most dangerous elements. On the other hand, in the event that they are not able to avoid their formation, secondary or passive measures are responsible for preventing the release of pollutants to the atmosphere. These are, for the most part, combustion gas filtration techniques. The combination of different control measures is quite widespread, especially in the industrial and power generation sectors. With regard to the domestic sector, in recent years, environmental regulations have severely limited maximum emission levels, as it has been proven that biomass is a significant source of small particles and NOx.Therefore, given the need to adapt domestic biomass combustion plants to increasingly stringent environmental standards, the present research work is proposed. Through it, the aim was to study the capacity for mitigation and control of emissions presented by three primary strategies, applied to small power systems. Specifically, the techniques investigated were: air stratification, flue gas recirculation, and fuel bed cooling. Their effects on the most relevant combustion parameters, such as temperatures, flue gas composition and particulate emission, were analyzed. With regard to the latter variable, special emphasis was placed on the description of the type of released particulate matter, both in terms of size and shape (morphology) and chemical composition. As this is an eminently experimental work, it was necessary to get a biomass combustion system that would generate the information required to carry out the relevant analysis, based on what has been said. Given the characteristics of the strategies to be studied, and the interest in controlling and recording as many variables as possible, it was decided to build an ad-hoc test bench which meets the needs of the project. Thus, the first part of the work consisted of the convenient bibliographic review, design and set up of the experimental facility on which this thesis is based. It is a fixed bed biomass combustion system, able to generate between 10 and 20 kW of thermal power, which has upper fuel supply and air stratification. Its modular distribution allows some flexibility in terms of its configuration, giving it great versatility when designing experiments and the selection of study variables. In addition, aiming to emulate the behavior of a domestic shell boiler, it has a comprehensive refrigeration system through cooling jackets and a tubular heat exchanger. In this way, the heat generated by the combustion gases can be transferred to a heat transfer fluid (water), which could be used for the production of heat or domestic hot water (...)

O panel socioeconómico e medioambiental actual está marcado pola necesitade de frear o consumo de combustibles fósiles e reducir as emisións contribuíntes ao efecto invernadoiro; polo que a transición enerxética cara un modelo renovable resulta fundamental dende o punto de vista da sustentabilidade. As políticas internacionais máis recentes puxeron o foco, por un lado, no incentivo ás enerxías renovables e, por outro, na limitación das emisións de substancias nocivas. Neste contexto, a biomasa preséntase coma unha alternativa interesante, tanto dende o punto de vista tecnolóxico coma económico. Dada a súa orixe vexetal, o seu contido en carbono permite o aproveitamento enerxético mediante métodos convencionais, como a combustión. Ademais, brinda a posibilidade de adaptar as tecnoloxías tradicionais aos requirimentos ambientais dun xeito sinxelo e a baixo coste, o que permitiu unha expansión notable, especialmente no sector doméstico. Non obstante, a obtención de enerxía a través da combustión de biomasa carrexa certos inconvenientes. Fundamentalmente, estes relaciónanse coa heteroxeneidade composicional da materia prima, que invita a que as reaccións teñan lugar de xeito menos eficiente que no caso doutros combustibles fósiles. Así, a parte de dióxido de carbono e vapor de auga, durante a combustión de biomasa libéranse á atmosfera substancias coma son o monóxido de carbono (CO), óxidos de nitróxeno (NOx), óxidos de xofre (SOx), ou materia particulada, entre outros. A meirande parte destes compostos resultan nocivos tanto para o medio ambiente, coma para a saúde humana; especialmente no campo das enfermidades respiratorias, cardiovasculares ou, incluso, o cancro. Ademais, as cinzas e os residuos xerados durante os procesos de combustión favorecen a formación de depósitos no interior das instalacións, o que tamén conleva á aparición de problemas graves de obstrución de liñas, perda de eficiencia ou corrosión dos materiais. A vontade de remediar esta problemática propiciou o desenvolvemento dunha serie de técnicas e métodos que permitirían reducir os efectos anteriormente mencionados. Estas estratexias divídense, xeralmente, en dous grupos principais, en función da fase do proceso de combustión na que se apliquen. As medidas primarias correspóndense con aquelas estratexias que buscan evitar a formación das substancias nocivas, ben sexa actuando directamente sobre o proceso de combustión e mellorando o xeito no que as reaccións teñen lugar, ou a través da mellora da calidade do combustible. Xeralmente, trátase de estratexias de control de temperatura e inxección de airecombustible, así como de adición ou eliminación dos elementos máis perigosos. Por outra banda, no caso de non ser capaces de impedir a súa formación, as medidas secundarias ou pasivas son as encargadas de evitar a liberación dos contaminantes ao exterior. Trátase, na meirande parte, de técnicas de filtrado dos gases de combustión. A combinación de diferentes medidas de control está bastante estendida, especialmente no sector industrial e de xeración de enerxía. No tocante ao sector doméstico, nos últimos anos as regulacións ambientais acoutaron fortemente os niveis máximos de emisións, xa que se comprobou que a biomasa se trataba dunha fonte notable de partículas de pequeno tamaño e NOx. Por tanto, ante a necesidade de adaptar as instalacións domésticas de combustión de biomasa aos estándares ambientais cada vez máis estritos, proponse o presente traballo de investigación. A través do mesmo pretendeuse estudar a capacidade de mitigación e control de emisións presentada por tres estratexias primarias, aplicadas a sistemas de pequena potencia. En concreto, as técnicas investigadas foron: a estratificación de aire, a recirculación de gases de combustión e a refrixeración do leito de combustible. Analizáronse os seus efectos sobre os parámetros máis relevantes da combustión, como son as temperaturas, a composición dos fumes e a emisión de partículas. Con respecto a esta última variable, fíxose especial fincapé na descrición do tipo de materia particulada emitida á atmosfera, tanto a nivel de tamaño e forma (morfoloxía) coma de composición química. Por tratarse dun traballo iminentemente experimental, foi preciso contar cun sistema de combustión de biomasa que permitise xerar a información requirida para realizar a análise pertinente, en base ao comentado. Dadas as características das estratexias a estudar e o interese en controlar e rexistrar a maior cantidade posible de variables, optouse pola construción dun banco de ensaios ad-hoc que permitise satisfacer as necesidades do proxecto. Deste modo, a primeira parte do traballo consistiu na revisión bibliográfica oportuna, no deseño e na posta en funcionamento da instalación experimental sobre a que se fundamenta esta tese (...)