QC 20100601

Carryover particles are partially/completely burned black liquor particles entrained in the flue gas in kraft recovery boilers. Understanding how carryover particles form and deposit on heat transfer tube surfaces is critically important in the design and operation of a recovery boiler. The tendency for a carryover particle to deposit on a tube surface depends on the particle temperature and composition at the moment of impact. This study was the first to examine systematically how carryover particle composition changes with the black liquor chemistry and burning conditions. The effect of black liquor composition and particle size, gas composition (O2 and SO2 concentrations) and temperature on the composition of carryover particles were studied using an Entrained Flow Reactor (EFR). Field studies were conducted on three operating boilers, where an air-cooled probe was used to collect carryover samples at the superheater entrance. The results show that the chloride (Cl) and potassium (K) contents in carryover particles were linearly proportional to their contents in black liquor. Cl and K were depleted during black liquor combustion due mainly to the vaporization of NaCl and KCl. The depletion of Cl is about three times greater than that of K. The significant depletion of Cl implies that carryover particles contain much less Cl, and hence, are less sticky than previously expected from black liquor composition. A dynamic model was also developed to predict the composition of carryover particles as a function of black liquor composition and burning conditions. Based on the data obtained experimentally in this study, the kinetic equations for the oxidation of sulphide available in the literature were modified and incorporated into the model to improve its sulphide and sulphate predictions. The model predicts the main components of carryover particles formed in both the EFR and three operating recovery boilers reasonably well, except for the K content, which is slightly over-predicted at high O2 concentrations (or high particle temperatures). Based on the predicted composition, it is possible to determine the thermal properties of carryover and to assess its fouling propensity in the boiler. The information helps boiler manufacturers and operators to identify locations in the boiler where massive carryover deposition may occur and to devise appropriate control strategies to minimize fouling and to improve boiler thermal efficiency.

Carryover particles are partially/completely burned black liquor particles entrained in the flue gas in kraft recovery boilers. Understanding how carryover particles form and deposit on heat transfer tube surfaces is critically important in the design and operation of a recovery boiler. The tendency for a carryover particle to deposit on a tube surface depends on the particle temperature and composition at the moment of impact. This study was the first to examine systematically how carryover particle composition changes with the black liquor chemistry and burning conditions. The effect of black liquor composition and particle size, gas composition (O2 and SO2 concentrations) and temperature on the composition of carryover particles were studied using an Entrained Flow Reactor (EFR). Field studies were conducted on three operating boilers, where an air-cooled probe was used to collect carryover samples at the superheater entrance. The results show that the chloride (Cl) and potassium (K) contents in carryover particles were linearly proportional to their contents in black liquor. Cl and K were depleted during black liquor combustion due mainly to the vaporization of NaCl and KCl. The depletion of Cl is about three times greater than that of K. The significant depletion of Cl implies that carryover particles contain much less Cl, and hence, are less sticky than previously expected from black liquor composition. A dynamic model was also developed to predict the composition of carryover particles as a function of black liquor composition and burning conditions. Based on the data obtained experimentally in this study, the kinetic equations for the oxidation of sulphide available in the literature were modified and incorporated into the model to improve its sulphide and sulphate predictions. The model predicts the main components of carryover particles formed in both the EFR and three operating recovery boilers reasonably well, except for the K content, which is slightly over-predicted at high O2 concentrations (or high particle temperatures). Based on the predicted composition, it is possible to determine the thermal properties of carryover and to assess its fouling propensity in the boiler. The information helps boiler manufacturers and operators to identify locations in the boiler where massive carryover deposition may occur and to devise appropriate control strategies to minimize fouling and to improve boiler thermal efficiency.

No description available.

Pulp and paper mills are high power consuming industries. Pulp and integrated mills produce power via steam turbines in recovery boilers. Due to high power prices and the fact that biomass combusted in the recovery boiler is considered as green energy, there is today a desire to always increase the power generation when investing in new recovery boilers. In order to increase power output from the steam turbine the steam data (i.e temperature and pressure) needs to be increased. With higher steam temperature follows a higher risk of corrosion due to the non process element potassium in the boiler fuel. The uncertainties of high temperature corrosion and the unpredictable environment in the furnace makes it difficult to design recovery boilers. This results in higher investment cost and could lead to less profit for the mill buying the boiler. The question then stands whether the revenue obtained from the higher power generation, is higher than the investment made for the upgrade in order to produce the higher steam data over a certain time. And more specifically what steam data will be the most economical, when comparing revenue from power generation with investment cost? In this study, together with ÅF Industry AB, four boilers with different steam data (Boiler A: 38.5 bar, 450°C; Boiler B: 92 bar, 480°C; Boiler C: 106 bar, 500°C; Boiler D: 115 bar, 515°C) were compared. The boilers were compared for four potassium levels: 1.0wt%, 1.5wt%, 2.5wt%, 3.5wt%. And two values of power were used, 300 SEK/MWh and 700 SEK/MWh. The marginal differences between the boilers were: the amount of material used in the superheaters in order to produce different steam data; the type of material used in the superheaters and the furnace; whether an ash-treatment system was needed (in order to purge potassium from the process); the turbines and generators; whether a feed water demineralization equipment was needed; the yearly cost for make-up chemicals (due to usage of an ash-treatment system) and the amount of power generated. The boilers investment cost and net yearly revenue were compared in order to determine the marginal pay-off in years. The most economical choice of boiler for the different potassium levels for 300 SEK/MWh: 1.0wt%, Boiler D; 1.5wt%, Boiler C; 2.5wt%, Boiler B; 3.5wt%, Boiler D (A). And for 700 SEK/MWh: 1.0wt%, Boiler D; 1.5wt%, Boiler C; 2.5wt%, Boiler D (B); 3.5wt%, Boiler D. The conclusion in this thesis was that the deciding factor is whether the boiler is in need of an ash-treatment system. Higher steam data is preferable as long as ash-treatment can be avoided. However, when comparing two boilers with ash-treatment the one with higher steam data is more feasible. Low steam data, such as boiler A, will never be feasible, regardless of potassium level and value of power.

Black liquor, a byproduct from pulp production, is burned in a recovery boiler to generate electricity and recover inorganic materials. Black-liquor-droplet combustion is fundamental to understanding and potentially improving recovery boiler performance. This investigation reports comprehensive experimental data collected for a suite of five liquors. All the data were collected with newly-designed laboratory equipment, several aspects of which are unique. Single droplets were suspended on a thermocouple and heated in a furnace. Simultaneous diameter (size and shape), internal and surface temperatures, surface emissivity, and mass were measured under various furnace temperatures and gas compositions. The images recorded by the cameras show the droplet swelling characteristics during combustion. Droplets swell more in inert environments and with low heating-rates than in oxidizing environments and with high heating-rates. Softwood liquors show greater swelling than softwood/hardwood mixed liquors. The internal temperature measured by the thermocouple is lower than the surface temperature measured with the optical pyrometer. This temperature gradient between the droplet interior and surface can be larger than 300 K. Molecular-beam mass spectrometry data were used to determine droplet mass loss and off-gas composition. These mass-loss data differ by about 10% from the balance mass data. These data provide a self-consistent and reasonably comprehensive description of black-liquor-droplet reactivity. A one-dimensional transient droplet model predicts droplet temperatures and mass loss comparable to the data. The combustion of volatiles, droplet swelling, and enthalpy of devolatilization were proved to be significant. Intermediate-sized particle (ISP) formation was investigated with a suspended-droplet experimental technique. The images indicate that a small amount of particles are ejected during drying. Insignificant ISP formation during devolatilization occurs. Char burning and smelt oxidation produce the largest amount of ISP. Quantitative analyses indicate that both droplet solids-content and liquor type impact ISP formation. Longer burning times produce more ISP, with total ISP formation being 0.2-2% of dry solids. Sampled ISPs show consistent trends in surface structure and particle sizes under a scanning-electron-microscope, independent of the experimental conditions.

Electrostatic precipitators (ESPs) are commonly used to control particulate emissions from recovery boilers in the kraft pulping process. The electrical resistivity of entrained particulates is known to affect the performance of ESPs. There are many factors which influence resistivity of particulate matter, such as field strength, time of exposure, particle size, particle composition, temperature, and flue gas composition. The objective of this study was to identify the ideal operating conditions and dust particle characteristics which lead to an optimum dust resistivity and maximized ESP efficiency. The effect of these factors was examined through a variety of experimental and analytical techniques on kraft recovery boiler ash samples and synthetically prepared samples. It was concluded that particle composition, gas composition, and ambient temperature had the most pronounced effects, while the other factors, such as field strength and exposure time, had a smaller impact on particle resistivity.

Electrostatic precipitators (ESPs) are commonly used to control particulate emissions from recovery boilers in the kraft pulping process. The electrical resistivity of entrained particulates is known to affect the performance of ESPs. There are many factors which influence resistivity of particulate matter, such as field strength, time of exposure, particle size, particle composition, temperature, and flue gas composition. The objective of this study was to identify the ideal operating conditions and dust particle characteristics which lead to an optimum dust resistivity and maximized ESP efficiency. The effect of these factors was examined through a variety of experimental and analytical techniques on kraft recovery boiler ash samples and synthetically prepared samples. It was concluded that particle composition, gas composition, and ambient temperature had the most pronounced effects, while the other factors, such as field strength and exposure time, had a smaller impact on particle resistivity.

O avanço da capacidade do processamento dos computadores e do desenvolvimento de métodos numéricos tem proporcionado ferramentas de modelagem, projeto e otimização cada vez mais eficientes para a constante melhoria de caldeiras de recuperação química Kraft. Dentre as técnicas utilizadas, os modelos CFD abrangentes tem sido empregados nos últimos anos para representar a operação da caldeira de recuperação considerando o maior número possível processos vinculados ao escoamento dos gases de combustão. O presente trabalho busca desenvolver um modelo CFD abrangente considerando o escoamento turbulento dos gases de combustão, o arraste das gotas de licor, a combustão homogênea dos voláteis em fase gasosa, as etapas de combustão heterogênea do licor preto e uma representação simplificada do leito carbonizado. Os resultados se mostraram coerentes com o comportamento apresentado na literatura. A simulação do modelo permite análise dos projetos e das operações da caldeira. / The advance of computer processing power and the development of numerical methods promote more efficient tools for modeling, design and optimization aiming to increase improvements in Kraft recovery boilers. Among the available techniques, the comprehensive CFD models have been applied in the last years to represent the recovery boiler operation considering as much as possible processes linked with flue gas flow. The objective of present work is the development of a comprehensive CFD model considering turbulent flue gas flow, black liquor droplets drag, volatiles homogeneous combustion in gas phase, the steps of heterogeneous black liquor combustion and the simplified interaction between flue gas flow and char bed. The results have good agreement with boiler behavior presented in the literature. The simulation of the comprehensive model allows the analysis of design and operations of the boiler.

O avanço da capacidade do processamento dos computadores e do desenvolvimento de métodos numéricos tem proporcionado ferramentas de modelagem, projeto e otimização cada vez mais eficientes para a constante melhoria de caldeiras de recuperação química Kraft. Dentre as técnicas utilizadas, os modelos CFD abrangentes tem sido empregados nos últimos anos para representar a operação da caldeira de recuperação considerando o maior número possível processos vinculados ao escoamento dos gases de combustão. O presente trabalho busca desenvolver um modelo CFD abrangente considerando o escoamento turbulento dos gases de combustão, o arraste das gotas de licor, a combustão homogênea dos voláteis em fase gasosa, as etapas de combustão heterogênea do licor preto e uma representação simplificada do leito carbonizado. Os resultados se mostraram coerentes com o comportamento apresentado na literatura. A simulação do modelo permite análise dos projetos e das operações da caldeira. / The advance of computer processing power and the development of numerical methods promote more efficient tools for modeling, design and optimization aiming to increase improvements in Kraft recovery boilers. Among the available techniques, the comprehensive CFD models have been applied in the last years to represent the recovery boiler operation considering as much as possible processes linked with flue gas flow. The objective of present work is the development of a comprehensive CFD model considering turbulent flue gas flow, black liquor droplets drag, volatiles homogeneous combustion in gas phase, the steps of heterogeneous black liquor combustion and the simplified interaction between flue gas flow and char bed. The results have good agreement with boiler behavior presented in the literature. The simulation of the comprehensive model allows the analysis of design and operations of the boiler.