Black Liquor Droplet Research Articles

Direct Pyrolysis of Reed Black Liquor Droplets in a Fluidized Bed

Direct Pyrolysis of Reed Black Liquor Droplets in a Fluidized Bed

Black Liquor Droplet Swelling Model during Pyrolysis

Black liquor is a spent pulping liquor from kraft pulping process. The concentrated black liquor is combusted or gasified to recover the chemical energy and to recycle the pulping chemicals. It undergoes several processes, i.e. drying, pyrolysis, and combustion and/or gasification. The droplet swells slightly during drying. Conversely, it swells heavily during pyrolysis. It can swell up to 20-30 time its original volume. The swelling stops when the pyrolysis reaction ends and/or when the solid loses its viscoelastic properties. Swelling phenomena also happens in other system, e.g. polymer foam moulding and coal pyrolysis. Two key phenomena for swelling behaviour are: gas production inside a material that forms gas bubbles and the material viscoelastic properties. The bubbles grow and whole droplet then swells accordingly. For black liquor droplet swelling, some empirical models has been developed but none of them are based on the theoretical bubble growth process. In this paper, the black liquor droplet swelling modelling based on the bubble growth process is developed. The model generally in agreement with the black liquor droplet swelling experimental data from a single droplet Thermogravimetry Analyser (TGA).

Fuel and thermal NO formation during black liquor droplet pyrolysis with envelope flame

The behaviour of black liquor nitrogen under pyrolysis with envelope flame was investigated with experiments in single-particle combustion with measurements of CO and CO2 and NO. Char carbon and nitrogen retention after pyrolysis were also measured with CHNS analysis of char residue. The combustion environment had a temperature from 800 °C to 1000 °C with 0% to 15% O2 in N2 or Ar with a droplet size of 10 mg. Also, gas phase simulations of the enveloping flame surrounding the black liquor droplet using FLUENT and CHEMKIN software with detailed chemistry were also conducted to understand nitrogen volatile chemistry better. Thermal NO was observed at 1000 °C with O2 concentrations above 10% and at 900 °C with 15% O2. Fuel nitrogen conversion to NO averaged 25% of black liquor nitrogen. Simulations showed that the unaccounted nitrogen fraction in experimental results is most likely reduced to N2. A previously developed CFD boiler model was used to assess how much of black liquor pyrolysis occurs in conditions found to be conducive to thermal NO. Only 0.05% of black liquor droplets would pyrolyze in conditions were thermal NO would form. The boiler model also showed that 30% of black liquor droplets would pyrolize with an envelope flame, which makes fuel nitrogen reach the bulk gas phase of the recovery boiler as NO and N2 instead of NH3.

Role of lignin and sodium carbonate on the swelling behavior of black liquor droplets during combustion

AbstractA partial removal of lignin from black liquor (BL) by carbonation and lignin precipitation was studied. In lignin-lean BL droplets during combustion in a laboratory furnace at 800°C in stagnant air, the maximum swelling was decreased. This observation was interpreted as showing that the lignin content decrement is due mainly to removal of higher molar mass (HMM) lignin and that the Na2CO3content of the BL is increased. Stepwise precipitation experiments with industrial softwood and hardwood kraft BLs by carbonation (resulting in pH decrement from 13 to 9) indicated that a fraction of HMM lignin (MM >10 kDa) with a higher amount of carbohydrates precipitated more prominently and earlier than the fraction with lower molar mass (LMM) lignin (MM <10 kDa) containing less amounts of carbohydrates. Separate experiments with a 50/50 (by wt) mixture of different lignin fractions (1–5, 5–10 and >10 kDa) and BL-originated aliphatic carboxylic acids were performed and found that the mixture of medium MM fraction (MM 5–10 kDa) swelled more than the other lignin fractions. The addition of Na2CO3to BL also reduced the maximum swelling of a BL droplet.

Combustion properties of spruce black liquor droplets: Sulfur-free pulping and influence of hot-water pretreatment

Combustion properties of spruce black liquor droplets: Sulfur-free pulping and influence of hot-water pretreatment

Combustion behavior of kraft black liquor droplets from hot water pretreated hardwood and softwood chips

This paper describes the combustion behavior of birch and spruce kraft black liquors obtained from an integrated forest biorefinery concept in which a hot water extraction of chips was performed before pulping. This pretreatment, aiming mainly at the recovery of various hemicellulose-derived materials, increased the concentrations of lignin and hydroxy acids in black liquors, compared with those in the reference black liquors without any process modification. On the other hand, the pretreatment decreased the concentrations of volatile acids and other organics (extractives and hemicellulose residues). Because of these characteristic changes, the total burning times (pyrolysis time plus char burning time) of the reference black liquors were somewhat longer than those of black liquors from the modified cooking process. The novel biorefinery based black liquors also swell more than the conventional ones. This phenomenon was primarily associated with the combined effect of high-molar-mass lignin fragments and hydroxy acids. All of the detected changes in combustion behavior were more intense for birch black liquors than for spruce black liquors.

The Fate of Char Nitrogen in Black Liquor Combustion—Cyanate Formation and Decomposition

The main gaseous nitrogen emissions from a Kraft pulp mill are in the form of NOx and NH3. In black liquor combustion roughly 60% of the black liquor nitrogen is released during the devolatilization phase and finally forms N2 or NO. The remaining 40% is retained in the char and forms sodium cyanate during char conversion. A part of the sodium cyanate decomposes in the lower furnace, while the rest exits the recovery boiler with the smelt. Sodium cyanate decomposes to NH3 in green and white liquor, which can contribute to the overall nitrogen emissions of a pulp mill. The purpose of this work is to better understand the formation and decomposition of cyanate during thermal conversion of black liquor. The formation and stability of cyanate was studied during thermal conversion of black liquor droplets in different gas compositions, and the formation was also studied by controlled char gasification in CO2 atmosphere. The decomposition was studied by exposing laboratory made black liquor smelt to various gas atmospheres at different temperatures. The results show that gas atmosphere and temperature play a role in how much cyanate is formed as well as the rate of decomposition. The share of black liquor nitrogen converted to cyanate is clearly higher if the gas composition contains CO2 or O2 mixed with N2. Less cyanate is formed if the surrounding atmosphere is pure N2 gas or contains water vapor. The formed smelt cyanate is stable at 800 °C in atmospheres containing only CO2 and N2, whereas it decomposes slowly at 900 °C in atmospheres containing only CO2 and N2. The cyanate in smelt decomposes quickly in oxygen and water vapor containing atmosphere compared to pure N2 atmosphere. Also, importantly, cyanate leads to very little NO formation (less than 5% of the original black liquor N) in all of the conditions tested. This information will be utilized in the future to develop a simplified char-N model for implementation in computational fluid dynamics (CFD) with the ultimate goal of finding new ways of further lowering NO emissions from recovery boilers without downstream NOx reduction technologies.

Combustion Experimental Study of Reed Black Liquor (RBL) Droplets and Particles in Fluidized Bed

The experiments were carried out to investigate combustion characteristics of RBL droplets (RBLD) and RBL particles (RBLP) in fluidized bed. The combustion experiments were focused on the influence of six different bed temperatures in the ranging from 530 to 780°C on gas production rates, yields of product, carbon and hydrogen conversions, releases of Na, K and Cl and emissions of NOxand SO2. The experiments results indicate that the combustion behaviors of RBLD and RBLP mainly depend on bed temperatures. The conversions and emissions have linear relation with bed temperature, respectively. And denitrification and desulfurization processes must be had during incinerate RBL in industry application.

Mathematical Modeling of Drying of Black Liquor Droplets in Recovery Boilers

Mathematical Modeling of Drying of Black Liquor Droplets in Recovery Boilers

Kinetics of Devolatilization of Black Liquor Droplets in Chemical Recovery Boilers - Pyrolysis of Dry Black Liquor Solids

Black liquor is a by-product of the pulping process in the manufacture of paper. It is a complex mixture of both organic and inorganic chemicals with both chemical value and energy content. To recover these two quantities, the black liquor is concentrated to around 60 - 85 % solids concentration and fired into a recovery boiler. In the boiler, the black liquor is sprayed in the form of droplets which fall through the atmosphere of the furnace to the bottom. During this process, the droplet undergoes drying, devolatilization and char burning in succession before coalescing with the smelt at the bottom. The modeling of these processes during the movement of the black liquor droplet is quite complex and in this paper, we attempt to simulate the weight loss during the devolatilization stage by studying the pyrolysis of dry black liquor solids in a muffle furnace. Experiments were carried out at temperatures varying from 973 to 1273 K and for residence times varying from 30 to 180 s. The kinetic scheme of dry black liquor solids decomposition by a single reaction giving gaseous volatiles and char is used. The proposed single reaction model is simulated and the best values of the kinetic parameters i.e. activation energy and Arrhenius constant are found by using the two-dimensional surface fitting non-linear regression algorithm. The results indicate good agreement between predicted and experimental data.

High-speed interferometric measurement and visualization of the conversion of a black liquor droplet during laser heating

Black liquor is a mix of organic and inorganic materials that is left after the kraft pulping process. In a modern pulp mill the pulping chemicals and the energy in the black liquor is recovered and used in the pulping cycle by burning the black liquor in a recovery burner. An alternative to the recovery boiler is to gasify the black liquor to produce an energy rich synthesis gas that can be upgraded into synthetic fuels or chemicals. Characterization of black liquor has mostly been done under conditions that are relevant for recovery boilers but the conditions in a gasifier differ significantly from this. In particular the droplets are much smaller and the heating rates are much higher. This paper presents an optical interferometric technique that has the potential to produce data under relevant conditions for gasification. In the paper, results are measured at atmospheric conditions and with relatively low heating rate. However, the method can be applied also for pressurized conditions and at heating rates that are only limited by the frame rate of the digital camera that is used to capture the transient event when the droplets are heated. In the paper the dynamic properties of the gas ejected from and the swelling during conversion of a single droplet are measured.

Influence of droplet size on the release of atomic sodium from a burning black liquor droplet in a flat flame

The influence of initial droplet size on the release of atomic sodium from black liquor solids (BLS) during each stage of black liquor combustion has been assessed using a planar laser-induced fluorescence (PLIF) technique. Three different initial diameters of black liquor droplets, 1.3, 1.7 and 2.2 mm were burned in a flat flame at equivalence ratios of 0.8, 0.9 and 1.25. The temporal release of the atomic sodium under fuel rich conditions was found to be different from that under fuel-lean conditions, especially during the smelt coalescence stage. For each stage of black liquor combustion, the measured release rate of atomic sodium increases with decreasing d i. This implies that significant release of atomic sodium could occur during the in-flight combustion of small droplets, which are known to be generated in recovery boilers from either the carryover or the ejecta.

CFD-Based Modeling of Kraft Char Beds • Part 1: Char Bed Burning Model

Understanding the physical and chemical phenomena governing char bed burning is important for stable and efficient operation of the recovery boiler. Stand-alone char bed models have been developed and evaluated to increase knowledge of the complex char bed conversion processes. Available computational fluid dynamics (CFD)-based recovery furnace models previously have been applied to evaluate lower and upper furnace processes rather than to investigate the behavior of the char bed. In this first of two papers, a stand-alone char bed model for predicting the chemical processes of char bed burning was modified and implemented into a CFD-based furnace model. The char bed model, which is fully coupled with the gas phase and a simplified black liquor droplet model, solves equations describing the mass and energy balance on the surface of the char bed and includes carbon conversion via direct oxidation, gasification reactions, and sulfate reduction. The model was tested by simulating a 3150 tons dry solids/day recovery boiler. Simulations for two different primary air distributions and using two sulfate reduction degrees in droplets arriving to the bed were performed to evaluate the char bed model. The effects on bed conversion processes were clearly revealed by the new char bed model. The model gives a reasonable description of the chemical processes occurring on the char bed in steady state. In the second part of this work, the model is applied to study the effects of droplet size and bed shape.

Simultaneous measurement of the surface temperature and the release of atomic sodium from a burning black liquor droplet

Simultaneous measurement of the concentration of released atomic sodium, swelling, surface and internal temperature of a burning black liquor droplet under a fuel lean and rich condition has been demonstrated. Two-dimensional two-colour optical pyrometry was employed to determine the distribution of surface temperature and swelling of a burning black liquor droplet while planar laser-induced fluorescence (PLIF) was used to assess the temporal release of atomic sodium. The key findings of these studies are: (i) the concentration of atomic sodium released during the drying and devolatilisation stages was found to be correlated with the external surface area; and (ii) the insignificant presence of atomic sodium during the char consumption stage shows that sodium release is suppressed by the lower temperature and by the high CO 2 content in and around the particle.

Influence of stoichiometry on the release of atomic sodium from a burning black liquor droplet in a flat flame with and without boron

The influence of boron on the temporal release of atomic sodium during a burning black liquor droplet has been assessed using planar laser-induced fluorescence (PLIF) technique. Single 10 mg droplets were burned in a flat flame at temperatures ranging from 1600 to 1725 °C with an equivalence ratios of 0.8, 0.9 and 1.25 and excess oxygen concentrations of 3.7, 1.8 and 0 (vol%), respectively. Under these conditions, the magnitude and temporal release of atomic sodium for the liquors with and without boron were not significantly influenced by the boron. However, the addition of boron to the liquor clearly reduced the total atomic sodium released, especially during the char consumption stage.

Assessment of the release of atomic Na from a burning black liquor droplet using quantitative PLIF

The quantitative measurement of atomic sodium (Na) release, at high concentration, from a burning black liquor droplet has been demonstrated using a planar laser-induced fluorescence (PLIF) technique, corrected for fluorescence trapping. The local temperature of the particle was measured to be approximately 1700 °C, at a height of 10 mm above a flat flame burner. The PLIF technique was used to assess the temporal release of atomic Na from the combustion of black liquor and compare it with the Na concentration in the remaining smelt. A first-order model was made to provide insight using a simple Plug Flow Reactor model based on the independently measured concentration of residual Na in the smelt as a function of time. This model also required the dilution ratio of the combustion products in the flat flame entrained into the plume gas from the black liquor particle to be estimated. The key findings of these studies are: (i) the peak concentration of atomic Na from the combustion of the black liquor droplets is around 1.4 ppm; (ii) very little atomic Na is present during the drying, devolatilisation or char combustion stages; and (iii) the presence of atomic Na during smelt phase dominates over that from the other combustion stages.

Nitrogen Oxide Emission Formation in a Black Liquor Boiler

This study of nitrogen oxide (NO x) formation in a recovery boiler uses computational fluid dynamics (CFD) combined with a detailed reaction mechanism. The model is used to reveal where NO x is formed in the recovery boiler and how different process parameters affect the emission. The CFD investigation includes a detailed model for black liquor droplet conversion and a model for the char bed behavior. The gas phase reactions are described using a reaction mechanism consisting of 21 species involved in 54 reactions.

Surface temperature and time‐dependent measurements of black liquor droplet combustion

AbstractBlack liquor droplets, initially 1–3 mm in diameter, were burned while simultaneously measuring surface and internal temperature, mass loss, and diameter to gain insight into the physical processes of single droplet combustion. Data were collected by weighing a droplet hanging on a thermocouple wire within a heated furnace. Two‐color images of the droplet were obtained to determine a surface temperature and particle size. The data suggest that models of droplet combustion should treat drying, devolatilization, and char oxidation as occurring simultaneously, though typically in different regions of the particle. Isothermal models should be avoided as temperature differences in excess of 300 K exist between the surface and interior, with larger gradients along the surface. Droplet swelling increased at lower ambient temperatures, and is greater for softwood than hardwood liquors. Additional unidentified factors also influence swelling, meaning it is still highly unpredictable and may be a function of initial droplet shape, size, and chemical composition. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Characteristics of black liquor sprays from gas-assisted atomizers in high-temperature environments

Black liquor droplets in an entrained-flow gasifier that are too large or too small can cause problems in the chemical recovery process. It is therefore important in gasifier design to understand the nature of the atomized gas spray. We used high-speed imaging to study black liquor sprays in cold and hot environments. Significant conclusions are that: 1. the droplet size distribution width is linked to the mean droplet size as for other sprays in the literature, necessitating a gasifier design that is tolerant of the distribution width associated with the target droplet size; 2. the shape of black liquor droplets is highly non-spherical, necessitating consideration of shape in addition to mass; 3. black liquor has exceptional ability to attach to the nozzle and thereby to form larger-than-desired fragments of liquor; and 4. the furnace environment has a measurable impact on droplet formation, making cold-spray-chamber test results difficult to apply to practical in-furnace spray performance. It had previously been assumed that the furnace environment does not affect the spray.

Experimental Investigation of Black Liquor Pyrolysis Using Single Droplet TGA

Black liquor pyrolysis is studied in a single droplet gasification reactor. The reactor is equipped with online analytical instruments to measure mass, temperature, and concentration of released gases during the process. It is also possible to follow visually and to record droplet geometric changes through a glass window. The pyrolysis parameters measured in this study are: the droplet swelling profile, pyrolysis gas released, and temperature inside the droplet. The gas detect-ed by the analytical instrument can be deconvoluted to obtain the gas released at the actual time of droplet pyrolysis, thus minimizing the effect of transportation time. These data are important for studying the kinetics of black liquor droplet pyrolysis.