Empirical and chemical equilibrium modelling for prediction of biomass gasification products in bubbling fluidized beds

In this work, two approaches to predict the producer gas composition obtained by direct (air) biomass gasification in bubbling fluidized beds were developed and compared, namely empirical modelling based on reported experimental results in the literature and non-stoichiometric chemical equilibrium modelling. For this purpose, an extensive database containing a set of 19 published experimental results from the literature was compiled and a non-stoichiometric chemical equilibrium model developed. The prediction capability of the empirical and chemical equilibrium model was evaluated by comparison with experimental data obtained in an 80 kWth bubbling fluidized bed direct (air) biomass gasifier. The empirical model shows moderate accuracy in the determination of the producer gas composition (CO, H2 and CH4), whereas the chemical equilibrium clearly overestimates the concentration of H2 and CO, and underestimates the concentration of CH4, leading to subpar accuracy in the determination of typical gasification efficiency parameters. Thus, the empirical model is suited for preliminary estimates of gasification products, while black-box chemical equilibrium modelling, without experimental knowledge integration, is considered as unreliable for these gasification conditions.publishe

Fluxos verticais de CO2 em várias fases do ciclo de crescimento de uma cultura de curta rotação de choupo no Nordeste de Portugal

As culturas energéticas lenhosas de curta rotação constituem uma oportunidade para aumentar a rentabilidade de muitos ecossistemas, principalmente nas regiões onde podem representar uma mais-valia comparativamente às culturas alimentares. Estas culturas são também consideradas um meio eficaz de mitigação do efeito estufa, devido à sua contribuição para a redução do uso de combustíveis fósseis e ao seu impacto positivo na transferência do carbono da atmosfera para o sistema solo/vegetação (Schulze et al. 2000). Com o objetivo de avaliar e otimizar a contribuição deste tipo de culturas de gestão intensiva no sequestro de carbono, procedeu-se à medição de fluxos verticais de CO2 num sistema de biomassa lenhosa para produção de energia, instalado no nordeste transmontano, em particular numa plantação de choupo híbrido

Co-combustion of residual forest biomass and sludge in a pilot-scale bubbling fluidized bed

In this work, the co-combustion of residual forest biomass from eucalyptus and its blend with different amounts of primary and secondary sludge from the pulp and paper industry was studied in a pilot-scale bubbling fluidized bed reactor. The main objective was the determination of sludge addition influence on the overall process and on the composition of the exhaust gases, with emphasis on chlorine emissions, namely present in the solid phase (fly ashes) and in the gaseous phase (hydrogen chloride), and nitrogen oxides emissions. The co-combustion process of residual forest biomass with primary sludge (up to 5% in mass) and secondary sludge (up to 10% in mass) was successfully demonstrated as a valid energy valorization option. Except specific cases, no significant emissions increase of nitrogen oxides, carbon monoxide or hydrogen chloride were found with the addition of sludge. In fact, hydrogen chloride emissions decreased, potentially due to an increase in the chlorine retention in ashes caused by the high inorganic content present in the sludge. This high inorganic content can also lead to a significant increase in ash production during the combustion process. Thus, consequently, without proper maintenance, significant ash accumulation along the combustion system may occur, which can decrease the process efficiency and cause equipment damage.publishe

Emissions from residential pellet combustion of an invasive acacia species

Currently, different types of raw materials are under investigation to fulfil the demand for pellet-based renewable energy. The aim of this study was to experimentally quantify and characterise the gaseous and particulate matter (PM10) emissions from the combustion of a pelletised invasive species growing in the Portuguese coastal areas. The combustion of acacia pellets in a stove used for domestic heating led to a noticeable production of environmentally relevant contaminants, such as carbon monoxide (CO, 2468 ± 485 mg MJ−1), sulphur dioxide (SO2, 222 ± 115 mg MJ−1) and nitrogen oxides (NOx, 478 ± 87 mg MJ−1). Besides gaseous pollutant emissions, substantial particle emissions (118 ± 14 mg MJ−1) were also generated. Particles consisted mostly of inorganic matter, mainly alkaline metals, sulphur and chlorine. About 25%wt. of the PM10 emitted had carbonaceous nature. The chromatographically resolved organic compounds were dominated by anhydrosugars, especially levoglucosan (284 μg g−1 PM10), and several types of phenolic compounds. Retene (8.77 μg g−1 PM10) was the chief compound among polyaromatic hydrocarbons.publishe

Culturas lenhosas de curta rotação e produção energética no Nordeste de Portugal

A biomassa lenhosa é habitualmente utilizada com fins energéticos em Trás-os-Montes, geralmente na forma de lenhas de carvalho negral e freixo, sendo a exploração de carvalhais e comercialização de lenhas provavelmente a actividade florestal mais relevante no Nordeste, apesar do seu verdadeiro significado económico ser ainda desconhecido. De acordo com a Matriz Energética do Nordeste, as lenhas representam 27% da energia primária utilizada no Distrito de Bragança [1]. A Estratégia Nacional para a Energia ENE2020 (Resolução do Conselho de Ministros N.º 29/2010, de 15 de Abril) preconiza o aumento do contributo da biomassa para a produção energética nacional a partir de um conjunto de medidas diversas. Uma dessas medidas diz respeito ao desenvolvimento de culturas dedicadas de rápido crescimento para as quais se estabelece a meta de 30% para o abastecimento de centrais termoeléctricas a biomassa até 2020 (Resolução do Conselho de Ministros n.º 81/2010 de 3 de Novembro). As culturas dedicadas de biomassa lenhosa em Portugal são, contudo, muito raras e de âmbito experimental, não existindo ainda o necessário conhecimento para a sua expansão a curto prazo, nomeadamente no que diz respeito às técnicas de instalação, manutenção e colheita da cultura, ao material vegetal a utilizar e à gestão da água, entre outros aspectos. Perante as actuais lacunas e a necessidade urgente de produzir conhecimento sobre estes sistemas, foi submetido em 2005 o projecto “Biomassa lenhosa para produção de energia: desenvolvimento de sistemas sustentáveis de fornecimento de bens e serviços de produção, regulação e conservação”, aprovado para financiamento pela FCT e em funcionamento desde o final de 2007. O projecto foi concebido com o propósito de desenvolver tecnologia para a produção intensiva e uso sustentável de biomassa lenhosa para produção de energia com benefícios ambientais e sócio-económicos em termos de regulação de carbono, conservação da água, do solo e da biodiversidade e criação de riqueza. Pretendeu-se também avaliar o potencial da região de Trás-os-Montes para a produção de biomassa para energia e para o sequestro de carbono de carbono em sistemas florestais. O projecto incluía seis tarefas: (1) sistemas de produção de biomassa, (2) efeitos ambientais das culturas lenhosas de curta rotação (CLCR), (3) ciclo do carbono, (4) conteúdo energético de combustíveis lenhosos, (5) potencial regional de produção de biomassa e de energia e (6) análise do ciclo de vida e sustentabilidade. Nesta comunicação serão apresentados os principais resultados do projecto relacionados com a produção de biomassa em culturas dedicadas bem como a nossa perspectiva relativamente ao papel que estas culturas podem vir a desempenhar na região, num quadro de aproveitamento da biomassa como recurso energético. O ensaio de produção de biomassa foi instalado em Bragança, numa área de 4ha, com Populus x euroamericana clone I-214; Salix alba x Salix fragilis; Salix. L. clone Terra Nova, Populus nigra e Fraxinus angustifolia em densidades de 10 000 e 15 000 estacas ha-1, no caso do freixo e choupo, e 15 000 e 20 000 estacas ha-1, para o salgueiro. A plantação, em que foram utilizadas estacas de 20-25 cm de comprimento, seguiu um esquema de linhas duplas distanciadas de 0,75 m e alternadas à distância de 1,5m. Os resultados da instalação foram insatisfatórios na medida em que, após o primeiro período de crescimento, a mortalidade foi muito elevada, com excepção do choupo I-214. As parcelas que sobreviveram apresentaram ao fim do segundo período de crescimento valores de biomassa modestos, apesar de se tratar do primeiro ciclo de crescimento após a instalação e apenas estarem avaliados os 2 primeiros anos de crescimento. A instalação revelou-se ainda um investimento muito avultado, particularmente no que diz respeito aos custos com o material vegetal e a mão-de-obra. Uma parte muito relevante deste trabalho consistiu na descrição quantitativa de processos relacionados com o ciclo do carbono com o intuito de avaliar o papel destas culturas no sequestro de dióxido de carbono atmosférico. O trabalho realizado com choupo descreveu a variação diária e sazonal dos fluxos de carbono entre as plantas/solo e a atmosfera, o que contribui para o conhecimento do comportamento das plantas e das culturas a estas escalas temporais, avaliar a fixação de carbono ao longo do ciclo de crescimento e ainda definir práticas de gestão das culturas, nomeadamente rega, que possam aumentar o seu crescimento e a taxa de fixação de carbono nesta região. Outros resultados incluem a caracterização da biomassa das espécies produzidas em culturas dedicadas e das espécies utilizadas tradicionalmente para lenha (carvalho negral, freixo e azinheira) em termos de composição e parâmetros energéticos. Foram ainda realizadas análises regionais da disponibilidade de áreas potenciais para a instalação de culturas de rápido crescimento bem como da oferta de energia da biomassa o que, conjuntamente com os restantes resultados obtidos, permite definir ideias gerais para a produção e utilização de biomassa para energia. Existem, em absoluto, condições no Distrito de Bragança para a instalação de culturas dedicadas de biomassa para energia em mais de 90 000ha. O Distrito produz actualmente biomassa lenhosa em sistemas florestais convencionais para gerar energia suficiente para sustentar o sector doméstico e partes de outros sectores. O incremento da oferta de biomassa lenhosa para energia a partir de culturas dedicadas poderá contribuir muito significativamente para a sustentabilidade energética da região. No entanto, a inexistência actual de conhecimentos técnicos de instalação, gestão e colheita e o elevado custo de instalação colocam incertezas relativamente à atractividade destas culturas e à sua viabilidade económica a curto/médio prazo o que condiciona o seu efectivo papel na matriz energética da região num futuro próximo

Sustainable woody biomass production systems: a novel solution for energy, agriculture and the environment in Portugal

Bioenergy is a major current issue within the agriculture and energy sectors and the society in general. The environmental and social concerns raised by first generation biofuel crops increased the interest on woody biomass. Biomass production for direct conversion to heat and/or electricity is common in northern European countries, the USA, and, more recently, southern Europe, mainly based on short rotation woody crops (SRWC). These crops are renewable sources of energy, CO2 neutral, able to preserve biodiversity and water, and to contribute to socio-economic development. In Portugal, there is an increasing demand of renewable and sustainable sources of energy. Portugal also presents conditions to expand biomass production and use for energy, namely through SRWCs. There is, however, a lack of experience in this field. With the purpose of studying short rotation woody crops dedicated to energy production, we have launched in Bragança, Portugal, a research project aiming to develop technology for the sustainable production, processing, and use of woody biomass for energy. The goal is to analyse environmental and socio-economic aspects of these crops considering carbon cycle, conservation of water, soils, and biodiversity, as well as creation of wealth locally. It also aims to evaluate the potential of northeastern Portugal for biomass production and carbon sequestration. In this presentation we address the rational behind the project, the methodologies followed, and the results obtained during the first year of the study. We also present our perspectives on the role of woody biomass production at several scales and for several scenarios.Fundação para a Ciência e Tecnologia. (PTDC/AGR‐CFL/64500/2006

CO2 fluxes in short rotation coppices for energy production

Short rotation coppices (SRC), which usually consist of densely planted and high-yielding willow and poplar clones, can provide a sustainable source of energy. Furthermore, the potential of these systems as carbon sinks, recognized in the international debate on the global carbon balance and climate change impacts, may also make them a good option for many countries to receive ‘carbon credits’ to offset the effects of greenhouse gases emissions. As part of a research project and with the purpose of evaluating and optimizing the contribution of this type of intensively managed crop in carbon sequestration, CO2 vertical fluxes have been measured in a SRC stand installed in Bragança, Portugal. Chamber based-techniques were designed to measure both CO2 soil effluxes and exchange rates between the crop canopy and the atmospheric boundary layer. Soil fluxes are measured at various locations by means of an open top dynamic chamber. Biosphere/atmosphere CO2 exchange rates are evaluated by using a large ventilated open top chamber. CO2 analyses are performed by using an ADC 2250 differential infrared gas analyzer coupled to an automatic sampling and acquisition system. Measurements are taken every 10 seconds and averaged over half hour intervals. These measurements will be continued throughout several seasons, with the purpose of investigating daily and seasonal patterns of net ecosystem CO2 exchange rate. To support our analysis, other relevant environmental variables as meteorological, soil temperature and soil humidity have also been monitored. The results obtained so far are still scarce, but very encouraging since they show the ability of the system to perform reliable and continuous measurements of CO2 fluxes. This presentation will focus on the methodologies and on the evaluation of the magnitude of these production systems as carbon pools, in order to contribute to a better description of the current carbon fluxes pools

Local-scale fluxes of carbon dioxide in a poplar short rotation coppice

Short rotation crops can provide biomass fuel for a wide range of users in co-firing plants and to replace other conventional energy resources used for heating and/or power generation. These biomass systems have also a considerable potential for carbon sequestration, which may constitute a good option to offset greenhouse gas emissions. With the purpose of evaluating the contribution of these intensively managed crop systems to carbon sequestration, we measured daily and seasonal patterns of net exchange rates of CO2 and soil effluxes in a poplar short rotation plot installed in the vicinity of the city of Bragança, Portugal. Open top chamber based-techniques were designed to measure both CO2 soil effluxes and exchange rates between the crop canopy and the atmosphere. Chambers inflow and outflow CO2 concentrations were measured by means of an ADC 2250 differential infrared gas analyzer coupled to an automatic sampling and acquisition system. This study started in the summer of 2010 and the data collected so far covers only part of the annual growth cycle of the poplar coppice. Poplar trees were already in an advanced stage of their growth cycle and from then on they have been subjected to stress environmental conditions, especially heat and high soil moisture deficit, which were observed until the end of September. The main results showed a significant variation of CO2 fluxes over the evaluation period. Day/night variation pattern was a common characteristic, but with shapes and magnitudes clearly influenced by the physiological activity of the poplar trees. The pronounced diurnal cycle with CO2 uptake during daytime and an upward flux at night registered in late September gave gradually rise to a CO2 emission pattern in the second half of November, slightly before the falling of the leaves. These preliminary results showed the potential of this energy crops to absorb large amounts of carbon dioxide during the growing season, but in the senescent period and/or under very stressful environmental conditions these biomass systems become predominantly a source of CO2

Method for online measurement of the CHON composition of raw gas from biomass gasifier

For unattended biomass gasification processes, rapid methods for monitoring the elemental composition (CHON) of the raw gas leaving the gasifier are needed. Conventional methods rely on time-consuming and costly laboratory procedures for analysing the condensable part of the raw gas. An alternative method, presented in this work, assesses the CHON composition of raw gas in a "one step" analysis without the need to previously characterise its chemical species composition. Our method is based on the quantitative conversion of a raw gas of complex chemical composition into CO2, H2O, and N2 in a small combustor. The levels of these simple species can be measured with high accuracy and good time resolution, and the CHON composition of the raw gas can be determined from the mass balance across the combustor. To evaluate this method, an online combustion facility was built and used to analyse the raw gas from the Chalmers 2-MWth dual fluidised bed steam gasifier. Test runs of the developed facility demonstrated complete combustion of the raw gas and the measurements were both fast and reliable. The new method used in combination with zero-dimensional reactor modelling provides valuable data for the operational monitoring of gasification processes, such as the degree of fuel conversion, composition of the char exiting the gasifier, oxygen transport by catalytic bed material, and amount of condensables in raw gas

Volatile gases from biomass pyrolysis under conditions relevant for fluidized bed gasifiers

The pyrolytic volatiles released from a converting biomass particle are investigated in this work through laboratorial fluidized bed experiments simulating conditions typical of large-scale gasifiers. Two types of wood (eucalyptus and pine) and two types of pellets (forest residues and wood) with particles of 6-8 mm in diameter are fed over the hot bubbling bed at temperatures within 600-975 degrees C. The resultant major pyrolytic products (char, soot, liquids and permanent gas) are collected to verify the overall mass balance, and the composition of the permanent gas is resolved in C3H8, C2H6, C2H4, CH4, CO2, CO, and H-2. Primary pyrolysis of the parent fuel particles is essentially complete at 600 degrees C and further increase of the temperature mainly leads to a progressive change in the composition of the volatile gas mixture. Although the gas release does not attain thermodynamic equilibrium under the conditions tested, our results show that the yields of CO2 and light hydrocarbons go through maxima as temperature increases to give rise to CO and H-2 as the preferable species at high temperatures. As a whole, the gas composition evolves in such a way that the corresponding lower heating value steadily increases with temperature increase, from about 11 MJ/kg at 600 degrees C to above 17 MJ/kg at 950 degrees C. Furthermore, the yields of key gas species were found well correlated to each other (C2H4 vs. CH4, CH4 vs. CO and H-2 vs. CO), with the relation between the yields of H-2 and CO being slightly dependent on the composition of fuel