Joachim Werther

Sewage sludge combustion

In the current review paper, various issues related to the combustion of sewage sludge are discussed. After briefly explaining the formation and treatment of sewage sludge, current and future sludge production are discussed. Thereafter, the four sludge disposal methods which are currently used, i.e. recycling in agriculture, landfilling, dumping into sea and incineration, are examined, and the future trend presented showing the increasing role of sludge incineration. Thereafter, technologies for thermal processing of sewage sludge are presented. They are discussed in three groups, i.e. mono-combustion, co-combustion and alternative processes. Various mono-combustion incinerators, including multiple hearth, fluidized bed and smelting furnaces are briefly discussed, whereas for co-combustion, attention has been given to co-combustion with coals in pulverized and fluidized bed coal combustors, as well as co-incineration with municipal solid wastes in various furnaces. Where possible, data from large scale plants are presented. Currently being discussed in the sludge disposal cycles are the alternative thermal processes to sludge combustion. These include wet oxidation, pyrolysis, oil from sludge processes, and combinations of pyrolysis, combustion and gasification processes. Some of these alternative technologies are also briefly discussed. An important aspect during thermal processing of sewage sludge is its combustion mechanisms. Compared to coals, sewage sludge has very high contents of moisture and volatile matter which can affect the combustion process. The importance of the drying and devolatilization processes for sewage sludge combustion is thus examined. In a special case, the release and combustion of the volatiles during sludge combustion in fluidized bed combustors is analysed, and some information concerning the combustion of sludge char is presented. Another important issue of sludge combustion is the emissions of pollutants gases as well as the handling of solid by-products. Of concern include the heavy metals, mercury, dioxins and furans, acid gases, as well as NOx and N2O. These are also briefly discussed. A peculiar characteristic of sewage sludge is its high content of nitrogen, and attention has been given to see how this affects the N2O and NOx emissions. In a special case, emission performance of large scale combustors of sewage sludge is presented.

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.

Measurement techniques in fluidized beds

Abstract Quantities that need to be measured in gas fluidized-bed systems include solids volume concentrations, solids velocities and solids mass flows, the vertical and horizontal distribution of solids inside the system, the lateral distribution of the fluidizing gas, temperatures and gas concentrations. In the present paper an overview is given on available measuring techniques. In the first section techniques for industrial routine measurements are discussed. These are mainly temperature and pressure drop measurements. Practical applications and also the limitations of these techniques are outlined. In the second section more sophisticated techniques for local measurements inside fluidized bed systems, which have already proven their suitability in large-scale industrial reactors, are dealt with. Examples include suction probes for measurements of local solids mass flows, heat transfer probes for the detection of defluidized zones and solids flows inside fluidized-bed reactors and capacitance probes for solids concentration and velocity measurements under high-temperature conditions. The third section finally presents advanced techniques which are either still under development or which are particularly intended for academic investigations of basic fluidization phenomena. Examples include sensor techniques, imaging and tomographic methods.

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 characteristics of wet sludge in a fluidized bed: Release and combustion of the volatiles

Combustion characteristics of wet (i.e. mechanically dewatered) sewage sludge were investigated in both laboratory and semi-pilot-scale fluidized bed combustors. The aims were to determine the influence of the high water content of sludge on its combustion process, the distribution of sludge carbon between the volatiles and char, the location of the release and combustion of the volatiles and the concentrations of char carbon in the bed. The results showed that there is a significant overlap of the drying, devolatilization and char combustion processes, so that the three processes may be considered to run in parallel. Both drying and devolatilization take place at very low particle temperature. Up to 80% of the sludge carbon is volatile carbon and sludge combustion is characterized by very low char carbon concentrations in the bed. Therefore sludge combustion is dominated by the gas-phase combustion of the volatiles. The release of the volatiles takes place mainly in the bed. Freeboard combustion is dominated by CO oxidation and takes place mainly within 2.5 m above 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.

Jet attrition of catalyst particles in gas fluidized beds

Abstract The mechanism of jet attrition of catalyst particles in fluidized beds has been investigated both theoretically and experimentally for two different catalysts. A model is suggested which considers the efficiency of the comminution process by relating the surface energy created by this comminution process to the kinetic energy which has been spent to produce this surface area. The attrition rate, defined as the mass of attrited fines per unit time produced by a single jet, is found to be proportional to ϱ0do2u03 (ϱ0, jet gas density; do, orifice diameter; u0, jet exit velocity). The attrition rate depends on whether the jet is issuing into a pre-fluidized bed or otherwise into a non-aerated bed. The attrition effect of an upward jet is equal to that of a horizontal jet, whereas the attrition effect of a downward jet is significantly higher. The model is able to predict the attrition effect of a multihole gas distributor on the basis of a single jet attrition measurement.

Catalyst attrition in fluidized-bed systems

Catalyst attrition has often been a major obstacle in the development of new fluidized-bed processes and it is still of concern for existing processes whenever the catalyst is changed. The main consequence of attrition is the generation of fines and the resulting loss of valuable material. In this work the attrition of FCC catalysts in a fluidized-bed system consisting of a bubbling bed, cyclone gas cleaning, and external solids recirculation is studied. An approach developed describes the attrition-induced overall solids loss based on the main attrition sources: grid jet attrition, bubble-induced attrition, and attrition in the cyclone. Their respective contributions to the overall attrition rate are calculated from separately derived model equations that take into account the design of the system, the operating conditions, and the material properties of the catalysts. The solids entrainment from the bed into the cyclone is shown to have a strong effect on the attrition-induced loss rate and must therefore be taken into account for an a-priori prediction.

Visualization of flow structures inside a circulating fluidized bed by means of laser sheet and image processing

Abstract In the present paper, an attempt has been made to visualize local flow structures inside the upper dilute zone of a pilot-scale circulating fluidized bed (CFB) under realistic flow conditions, i.e. with cross-sectional average solids volume concentrations ranging from 0.1 to 1 vol.%. The high-speed video technique in combination with the laser sheet technique has been used. Since it is not possible to penetrate from outside with a laser sheet into the flow, an endoscope has been used to guide the laser light into the fluidized bed. Particles or loosely packed agglomerates passing the illuminated light sheet area reflect the laser light. This provides the possibility for an undisturbed observation of the local flow pattern via the camera endoscope. To understand the governing optical effects, a model has been developed which is able to estimate the influences of the solids properties and the operating conditions on the ratio of the received light intensity to the emitted one. Digital image processing was successfully applied to pre-process the obtained image sequences and to gain a deeper insight into the observed flow structures. Using specific image processing methods, shapes and velocities of the flow structure were calculated.

Axial pressure profiles and solids concentration distributions in the CFB bottom zone

Abstract The solids concentration distribution in the bottom zone of a cold model CFB unit was measured with fiber-optical probes and by γ-ray absorption. A higher solids concentration at the wall and a lower concentration in the center of the riser was found. Starting at the distributor, the time and cross-sectional average solids volume concentration increases slightly or remains constant with height up to a height H b from where it drops off. H b may be defined as the height of a bottom zone. The comparison with the axial pressure profile indicates the presence of strong local acceleration effects in the bottom zone. A core-annulus approach is presented to model the contribution of acceleration to the axial pressure drop.