A. Cabanillas

Combustion behaviour of rice husk in a bubbling fluidised bed

Abstract The combustion of agro-industrial wastes in power plant seems to be an attractive possibility for the future. In Spain, the food industry generates around 618,000 tonnes of rice husk annually. The rice production is geographically focused in three regions: Valencia, Andalucia and Delta del Ebro. The rice husk is a suitable fuel for the fluidised bed combustors. An experimental combustion work using the rice husk as fuel was performed in a 30 kW th atmospheric bubbling fluidised bed pilot plant of CIEMAT. The influence of different variables such as temperature, fluidisation velocity on the combustion efficiency and CO emissions was investigated. Combustion efficiency in all test runs was higher than 97%. The characteristics of ash removed from the bed, cyclones and baghouse were studied. In the bed ashes, the potassium content increases along the operation hours.

Co-combustion of coal and olive oil industry residues in fluidised bed☆☆

Recently new environmental regulations of fossil fuels have further increased interest in the use of waste and biomass for energy generation. Co-combustion is generally viewed as the most cost-effective approach to biomass and wastes utilisation by the electric utility industry. The aim of this paper is to assess the feasibility of co-firing coal and a very specific biomass waste from the olive oil industry: foot cake, in a fluidised bed. This waste is quite difficult material to be used in combustion process, due to its high moisture content and alkaline content in ashes. Two different Spanish coals were selected for this study: a lignite and an anthracite. The combustion tests were carried out in the CIEMAT bubbling fluidised bed pilot plant. In order to study the effect of different parameters on the emissions and combustion efficiency, the tests were done using different operating conditions: furnace temperature, share of foot cake in the mixtures and coal type. The pilot plant tests show that the combustion of foot cake/lignite or anthracite mixtures in bubbling fluidised bed is one way to utilise this biomass residue in energy generation. The presence of foot cake in the mixtures has not any significant effect on the combustion efficiency. SO2 and NOx emissions decrease when the amount of foot cake in the mixtures increases, while N2O emission increases.

Circulating fluidized bed combustion in the turbulent regime: modelling of carbon combustion efficiency and sulphur retention

A model has been developed considering the hydrodynamic behaviour of a turbulent circulating fluidized bed, the kinetic of coal combustion and sulphur retention in the riser. The hydrodynamic characteristics of the turbulent fluidization regime were integrated together with the kinetic submodels of char combustion and sulphur retention by limestone. From the combustion of a lignite and an anthracite with limestone addition in a hot CBF pilot plant of 20 cm internal diameter and 6.5 m high, the effect of operating conditions such as temperature, excess air, air velocity, Ca/S molar ratio, coal and limestone particle size distributions on carbon combustion efficiency and sulphur retention were studied. The experimental results were compared with those predicted by the model and a good correlation was found for all the conditions used.

Modelling of sulfur retention in circulating fluidized bed combustors

The effects of operating conditions (linear gas velocity, CaS molar ratio, type of coal and limestone, etc.) on sulfur retention efficiency in a circulating fluidized bed combustor were studied. The operating conditions affect the sulfur retention differently. To explain this, a mathematical model was developed. To determine axial voidage profiles, the model uses a modified exponential decay hydrodynamic model, which divides the bed into a dense region at the bottom of the bed and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The SO2 retention rate is considered to depend on the bed height through the SO2 profiles generated by the coal devolatilization and char combustion rates, which change with the bed height. The model gives good predictions of the effect of the operating variables on sulfur retention in a CFB pilot plant of 20 cm i.d. and 6.5 m height, burning two different lignites and an anthracite with four types of limestone.

A model for prediction of carbon combustion efficiency in circulating fluidized bed combustors

Abstract The effects of operating conditions (coal particle size, temperature, excess air and linear gas velocity) on carbon combustion efficiency in a circulating fluidized bed combustor were studied. The operating conditions affected the combustion efficiency of each coal differently. To explain this, a mathematical model was developed. To determine axial voidage profiles the model uses a modified version of the hydrodynamic model of Kunii and Levenspiel, which divides the bed into a dense region at the bottom and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The model gives good predictions of the effect of the operating variables on carbon combustion efficiencies obtained in a CFB pilot plant burning two different lignites and an anthracite.

Co-Combustion of Biomass and Coal in Circulating Fluidized Bed: Modeling and Validation

In this work carbon combustion efficiencies in circulating fluidized bed combustion (CFBC) when co-firing biomass and coal mixtures were studied. Experimental results were obtained from the combustion of two kind of coals with a forest residue (Pine bark) in a CBF pilot plant (0.3MWth ) with 20-cm i.d. and 6.5-m height. The effect of operating conditions such as percentage of biomass in the feed, temperature, excess air, air velocity and percentage of secondary air on carbon combustion efficiency was studied. A mathematical model for the co-combustion of coal and biomass in a circulating fluidized bed boiler has been developed. The riser is divided in three zones with different hydrodynamic characteristics: bottom, splash and freeboard. The bottom bed has a constant voidage, determined by a modified two-phase theory. The solids are considered in perfect mixing and the gas in plug flow. The voidage in the splash region follows an exponential decay model. In the freeboard region, the solids and the gas are in plug flow, and a core-annulus structure is considered. Devolatilization of solid fuels is modeled with a particle reaction model which allows to determine the volatiles generation rate as a function of time and operating conditions. Kinetics of char combustion is modeled with the shrinking particle model with mixed control by chemical reaction and gas film diffusion, assuming that the ashes separate once formed. To consider that the char particles are a mixture of coal and biomass char particles, a weighted average combustion rate is defined taking into account the individual combustion rates. Population balances of char particles in the different regions were developed to calculate carbon concentrations. The developed model can predict the different gas concentrations along the riser, such as oxygen, SO2 , CO, CH4 , etc..., and the carbon combustion efficiency. The experimental results of carbon combustion efficiencies and gas emissions were compared with those predicted by the model and a good correlation was found for all the conditions used.Copyright

NOx and N2O Emissions During Fluidized Bed Combustion of Leather Wastes

Transformation of hide (animal skins) into leather is a complicated process during which a significant amounts of wastes are generated. Fluidized bed combustion has been extended to burn different wastes that have problems with their disposal showing its technical feasibility. Considering the characteristics of the leather waste, especially the heating value (12.5–21 MJ/kg), it is a fairly good fuel. Moreover, leather waste has a high volatile matter, 65%, similar to other biomasses and an unusual high nitrogen content, 14%. The first objective of this work is to study the fluidized bed combustion of leather wastes presenting experimental results regarding NOx and N2 O emissions. A series of experiments have been carried out in a fluidized bed pilot plant to understand the importance of operating parameters such as furnace temperature, oxygen content in gases, staged combustion and residence time on the NOx and N2 O emission level. Despite having high nitrogen content, low conversion of N-fuel to NOx and N2 O was measured during the combustion of leather waste in BFB. Bed temperature and oxygen content were found as the most important single parameters on N2 O emission and only oxygen content has a significant influence on NOx emission. Leather waste exhibits a great NOx/O2 trend; NOx decreases as the oxygen concentration decreases and the effect of the combustion temperature on NOx is insignificant. Staged combustion does not give a reduction in NOx.Copyright

Study of Cr(VI) in Ashes from Fluidised Bed Combustion of Leather Waste: Applicability of different Speciation Methods

Leather waste originated from shoe industry is considered potential hazardous waste as a consequence of the chrome tanned leather process. On the other hand, the leather waste has an inherent value as fuel that makes feasible its use as a very interesting potential source of energy; nevertheless ash residues are produced during the combustion process. This waste disposed in landfills can cause leachate problems. For that reason, determination of hexavalent chromium is considered as an essential input parameter for the safe disposal of these residues. However the current methods provide ambiguity in the results. The objective of this work was to evaluate two different speciation methods: Method SLC 22 (IUC 18) called Method I, used for leather technicians and Method II, developed by the Analytical Chemistry Laboratory of CIEMAT, to analyse the Cr(VI) content in ashes from fluidised bed combustion of leather waste. Both methods provide appropriate results for these kinds of samples when a non-reducing environment is present.