E.-U. Hartge

Combustion of agricultural residues

In the current overview paper, various issues related to the combustion of agricultural residues are discussed. Attention has been given to the problems associated with the properties of the residues such as low bulk density, low ash melting points, high volatile matter contents and the presence of nitrogen, sulfur, chlorine and sometimes high moisture contents. Consequently the issues discussed include densification, the combustion mechanisms of agricultural residues, problems of low melting point of ash such as agglomeration and fouling, emissions and co-combustion. Further, design considerations of facilities for the combustion of agricultural residues are discussed.

SOLIDS CONCENTRATION AND VELOCITY PATTERNS IN CIRCULATING FLUIDIZED BEDS

Fiber optic probes were used to measure local instantaneous solids concentrations and local solids velocities inside a 0.4 m diameter bed. The velocity information was obtained from cross-correlation calculations. A detailed signal analysis yielded information about the local distributions of solids concentrations as well as of the local solids velocities. The measurements lead to a detailed picture of the flow structure of the circulating fluidized bed which consists essentially of two phases, i.e. a dense phase and a lean phase. A model is suggested which interrelates solids concentrations, solids velocities, and gas velocities in the individual phases on the basis of the measurements presented here. Integral values of the net gas velocity and net solids mass flow obtained by averaging the local values over the beds cross-sectional area are in good agreement with the fluidizing velocity and the solids mass flow introduced into the bed.

Combustion of coffee husks

Combustion mechanisms of two types of coffee husks have been studied using single particle combustion techniques as well as combustion in a pilot-scale fluidised bed facility (FBC), 150 mm in diameter and 9 m high. Through measurements of weight-loss and particle temperatures, the processes of drying, devolatilisation and combustion of coffee husks were studied. Axial temperature profiles in the FBC were also measured during stationary combustion conditions to analyse the location of volatile release and combustion as a function of fuel feeding mode. Finally the problems of ash sintering were analysed. The results showed that devolatilisation of coffee husks (65–72% volatile matter, raw mass) starts at a low temperature range of 170–200°C and takes place rapidly. During fuel feeding using a non water-cooled system, pyrolysis of the husks took place in the feeder tube leading to blockage and non-uniform fuel flow. Measurements of axial temperature profiles showed that during under-bed feeding, the bed and freeboard temperatures were more or less the same, whereas for over-bed feeding, freeboard temperatures were much higher, indicating significant combustion of the volatiles in the freeboard. A major problem observed during the combustion of coffee husks was ash sintering and bed agglomeration. This is due to the low melting temperature of the ash, which is attributed to the high contents of K2O (36–38%) of the coffee husks.

ANALYSIS OF THE LOCAL STRUCTURE OF THE TWO PHASE FLOW IN A FAST FLUIDIZED BED

ABSTRACT The aim of the present work was to investigate the local hydrodynamics of fast fluidized beds, the scale of the equipment being sufficiently large for scale-up purposes. Two fluidized bed installations were used. The laboratory scale unit comprised a 5 cm diameter fluidization column whereas a 40 cm diameter fast fluidized bed was taken to be representative for the pilot scale. Fine quartz sand with a surface mean diameter of 0.056 mm was used in most of the experiments. The average solids holdup was measured by means of γ-ray absorption at different levels above the gas distributor. These measurements were found to be in fair agreement with solids concentrations derived from pressure drop measurements. The vertical solids concentration profiles were similar in both units and exhibited the characteristic shape with a dense region at the bottom and a lean region at the top of the bed. Local values of the solids holdup inside the bed were successfully measured by fiber optic probes, the capacitance probes being severely influenced by electro-static charging in the pilot-scale bed. Characteristic profiles of the lateral distribution of the solids concentration were found in the 5 cm unit as well as in the 40 cm diameter bed, the voidage ɛ being always higher in the center of the bed than near the wall.

The role of mixing in the performance of CFB reactors

A direct consequence of the complex fluid mechanics of the circulating fluidized bed (CFB) is its complex mixing behavior. For both, solids and gas mixing, different scales of the mixing processes may be distinguished, namely the micro, meso and macro scales. While the micro scale is connected to the molecular scale and is mostly described by transfer coefficients, the meso and macro scales, are directly related to the flow pattern inside the CFB. On the meso-scale mixing effects are associated with the formation, disintegration and movement of clusters and strands. Macro-scale mixing is introduced via the existence of large-scale flow patterns, e.g. the core annulus structure. The present paper intends to give an overview on the mixing in a circulating fluidized bed and on its effects on the performance of CFB reactors. Special attention will be given to the interdependence between the reaction rate and the requirements for mixing. The combustion process in a CFB will mainly be used to exemplify this interdependence.

Three-Dimensional Simulation of Temperature Distributions in Large-Scale Circulating Fluidized Bed Combustors

Circulating fluidized bed combustors (CFBC) of industrial scale have sometinIes diameters of more than 10m and heights exceeding 40 m. Depending on the intensity of the reactions and the location of the insertion of reactants into the bed, temperature effects can not be neglected. A simulation tool for the investigation of these large-scale systems including sub-models for fluid dynamics, dispersion, reactions, mass balances and an enthalpy balance has been implemented. The simulation results provide three-dimensional distributions of temperatures inside the combustion chamber. The simulation results have been validated with results of local measurements in a 235 MWe CFBC of Elektrownia Turow in Poland.

Gas Concentration Measurements in the Combustion Chamber of the 235 MWe Circulating Fluidized Bed Boiler Turow No. 3

Local measurements of concentrations of O2 , CO2 , CO, NO and SO2 were carried out inside the 235 MWe circulating fluidized bed boiler no. 3 Turow power plant. The combustion chamber had a cross-sectional area of 21.1 × 9.9 m2 and a height of 43 m. Water-cooled probes with a length of 4.7 m were used to take samples from inside the boiler. 20 ports in 5 different heights were used to introduce the probes. The penetration depth inside the boiler was up to 3 m. The sampled gas was led to online analyzers. Even though the number of ports and the penetration length was not sufficient to get a full 3-D mapping of the concentrations the measured horizontal and vertical gas concentration profiles of NO, CO, CO2 , O2 and SO2 clearly indicate a core/annulus structure with a wall layer thickness of about 0.5–1 m. Significant differences are observed between gas concentrations near the front wall and those near the rear wall which indicate an uneven distribution of fuel. One consequence is the formation of plumes with high concentrations of CO, NO, CO2 and SO2 near the front wall which extend up to the exit region. The fact that nevertheless the stack emissions are still below the legal limits may be attributed to the excellent performance of the cyclones as gas mixers and post combustion reactors.Copyright

Sensitivity analysis in the simulation of complex solids processes

Abstract In the present work a method to quantify the range of uncertainty of selected stream values and to detect the most influential parameters within a given flowsheet has been developed for the flowsheeting of solids processes. By using heuristic optimization methods the range of uncertainty of relevant stream values can be determined. Distributions of attributes of solids can be handled and shape and location of distributions as well as characteristic parameters like the Sauter diameter can be predicted. Furthermore the most influential parameters for the units or the models in the range of uncertainty can be detected by investigating the difference quotients in different operating points. With this information the time required to improve the accuracy of prediction of the simulation results can be reduced significantly. As an example the simulation of a hydrocyclone is presented. The influence of the parameters of an empirical separation model on the solids mass flow collected with underflow is demonstrated and the range of the product particle size distribution depending on uncertain parameters is examined.