A. P. Watkinson

Biomass gasification in a circulating fluidized bed

Abstract This paper presents the results from biomass gasification tests in a pilot-scale (6.5-m tall × 0.1-m diameter) air-blown circulating fluidized bed gasifier, and compares them with model predictions. The operating temperature was maintained in the range 700–850°C, while the sawdust feed rate varied from 16 to 45 kg/h . Temperature, air ratio, suspension density, fly ash re-injection and steam injection were found to influence the composition and heating value of the product gas. Tar yield from the biomass gasification decreased exponentially with increasing operating temperature for the range studied. A non-stoichiometric equilibrium model based on direct minimization of Gibbs free energy was developed to predict the performance of the gasifier. Experimental evidence indicated that the pilot gasifier deviated from chemical equilibrium due to kinetic limitations. A phenomenological model adapted from the pure equilibrium model, incorporating experimental results regarding unconverted carbon and methane to account for non-equilibrium factors, predicts product gas compositions, heating value and cold gas efficiency in good agreement with the experimental data.

Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier

Abstract A non-stoichiometric equilibrium model based on free energy minimization is developed to predict the performance of gasifiers. The model considers five elements and 44 species in both the gas and solid phases. The gas composition and heating values vary primarily with temperature and the relative abundance of key elements, especially carbon, hydrogen and oxygen. Pressure only influences the result significantly over a limited temperature range. The model predicts the onset of formation of solid carbon where the gas composition becomes insensitive to additional carbon. The carbon formation boundary is plotted in C–H–O ternary diagrams as a function of temperature and pressure. When the experimental carbon conversion is introduced, the kinetically modified equilibrium model gives good predictions of the gas composition from an air-blown pressurized circulating fluidized bed gasifier for two coals. The role of water, including both fuel moisture and steam injection, is examined based on a water balance on the feed and product gas to evaluate the steam demand.

Heat Exchanger Fouling: Mitigation and Cleaning Strategies

Heat exchangers are the workhorse of most chemical, petrochemical, food-processing, and power-generating processes. The global heat exchanger market is estimated to top a total of

Fouling of heat exchangers-new approaches to solve an old problem

12.7 billion by 2012, with an increase of 3–5% per annum [1]. Despite this very positive market outlook, manufacturers are under increasing pressure to produce heat exchangers that are more efficient in terms of heat recovery and use of material, while at the same time being faced with fluids that are increasingly difficult to process. One major problem directly related to these requirements is the deposition of unwanted materials on the heat transfer surfaces, which occurs in the majority of heat exchangers [2]. Conservative studies estimated that heat exchanger fouling leads to additional costs in the order of 0.25% of the gross domestic product (GDP) of industrialized countries, and that it is responsible for 2.5% of the total equivalent anthropogenic emissions of carbon dioxide [2, 3]. Therefore, efficient mitigation and cleaning methods must be available to safeguard the operation of heat exchangers. Two basic approaches are possible to combat heat exchanger fouling, namely, mitigation (including on-line cleaning) and off-line cleaning techniques. The general criteria for the selection of any of these strategies are: • Dominant fouling mechanism. • Severity of fouling. • Type of heat exchanger.

Fouling of some Canadian crude oils

For most engineering graduates, the severely fouled equipment shown on the cover page of this issue of Heat Transfer Engineering bears no relation whatsoever to what they have learned in their undergraduate courses on heat transfer and heat exchanger design. Nevertheless, this is reality! As reported in a survey [1] and shown in Fig. 1, the majority of industrial heat exchangers suffer from deposition problems and must be designed with some allowance for the resulting reduction in thermal and hydraulic performance. To date, the formation of deposits on heat transfer surfaces is the least understood problem in the design of heat exchangers. Well-proven codes and correlations are now available for standard heat exchanger design, and computational fluid dynamics simulations can be performed for complex, single-phase flow conditions. However, all of these sophisticated calculations are offset by the current practice of adding constant, crudely estimated, experienceor imagination-based fouling resistances (less accurately termed fouling factors), which may increase the heat transfer surface calculated for clean conditions by 20– 200%. Even worse is the situation for the prediction of pressure drop. While more heat exchangers are taken out of operation due to excessive, fouling-related increase in flow restriction, there is virtually no information about the potential effects of deposits on pressure drop. Considering the fact that heat exchangers are the workhorse of most chemical, petrochemical,

Heat Exchanger Fouling: Environmental Impacts

A thermal fouling study was undertaken using three sour Canadian crude oils. Experiments were carried out in a re-circulation fouling loop equipped with an annular (HTRI) electrically heated probe. Fluids at pressures of about 1000–1340 kPa under a nitrogen atmosphere were re-circulated at a velocity of 0.75 m/s for periods of 48 hours, and the decline in heat transfer coefficient followed under conditions of constant heat flux. Bulk temperatures were varied over a range of 200–285ˆC, and initial surface temperatures ranged from 300–380ˆC. Heat fluxes were in a range of 265–485 kW/m2. Surface temperature effects on fouling of the three oils were compared, and fouling activation energies were estimated. For the lightest oil, a more detailed study of velocity and bulk and surface temperature effects was carried out. The fouling rate decreased slightly with increasing velocity but increased with both surface and bulk temperatures; a rough correlation was developed using a modified film temperature weighted more heavily on the surface temperature. Deposits showed high concentrations of sulfur and minerals, indicating the importance of iron sulfide deposition.

Fouling Characteristics of a Light Australian Crude Oil

Fouling of heat exchangers in processing industries is a chronic operational problem that compromises energy recovery and environmental welfare. Mitigation of fouling is frequently restricted by the inadequate knowledge of the underlying mechanisms, and the transient and fluctuating impacts of fouling on heat exchanger performance. Fouling in tubes, flow channels, or other locations in processing equipment may create one or more of several major operating problems: (1) loss of heat transfer, (2) under-deposit corrosion, (3) increased pressure loss, and (4) flow maldistribution. All this will have severe negative impact on cost, safety, health, and environmental aspects:

Pyrolysis of pitch

Abstract Australian crude oils, which generally contain little asphaltenes, nevertheless give rise to fouling in refinery pre-heat trains. In this research, the fouling of a series of such crude oils and their blends is being assessed. The present work focuses on thermal fouling resulting from heating Gippsland crude oil at moderate temperatures. The oil is maintained under nitrogen at a pressure of 379 kPa and re-circulated at bulk temperatures of 80–120ˆC through an electrically heated annular probe at velocities in the range 0.25–0.65 m/s with surface temperatures from 180–260ˆC. Experiments are run for periods up to ninety hours at constant heat flux. Fouling is detected by the increase of wall temperature of the probe. The oil is characterized by its filterable solids content, density, and viscosity both before and after the fouling run. The trends in fouling rates are compared to predictions of the threshold-fouling model proposed by Ebert and Panchal [6]. Data on deposit composition are presented, and the fouling mechanism is discussed.

Fouling of augmented heat transfer tubes

Abstract Kinetics of thermal pyrolysis of pitches from two heavy oil upgrading processes have been studied using thermogravimetric analysis. Experiments were carried out at atmospheric pressure with heating rates of 25–150°C min −1 to a final temperature of 800°C. At temperatures below 150°C, there is little devolatilization of either pitch. At higher temperatures, the pyrolysis takes place in two stages. Heating rate had a minor effect on the weight loss at a given temperature. Single-stage, first-order reaction methods, including models which incorporate a Gaussian distribution of activation energies, were found to be inadequate to interpret the thermogravimetric analysis results. A two-stage model which reflects changes in the chemical constitution or structure as conversion prcoeeds was necessary to describe the dependence of devolatilization rates on remaining volatile content. The transition between these two stages is a sharp one, occurring at about 450°C for both pitches. The magnitudes of the activation energies suggest that both stages are kinetically controlled.

A nonisothermal model of a spouted bed gasifier

Comparisons are made between fouling of augmented and plain heat transfer tubes. Results from the literature are analyzed on a consistent basis to emphasize differences between extended and enhanced surfaces. Tubes with internal and external fins are considered, as well as different types of enhanced tubes. Based on the analysis of this data, recommendations are given for assigning fouling resistances for augmented tubes in terms of the corresponding plain-tube fouling resistances.