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Ring formation in lime kilns is a common problem in pulp mills. Unstable burner operation that causes wide temperature variations within the kiln has been shown to be a main contributor. As rings grow thicker and longer, they are expected to affect the burner flame pattern, alter the temperature distribution, and further aggravate the problem. This study systematically examines the effect of rings on burner operation as a function of primary air and ring characteristics, using a laboratory mock kiln. The results show that primary air greatly affects the burner flame pattern and stability. Without a ring, the flame is brighter and shorter, with an increase in primary air (PA) up to 17%, and blows out at 20% PA or higher. In the presence of a ring, the flame becomes unstable and blows out when the ring is placed near the burner, but it is more stable when the ring is placed away from the burner. Thick and long rings destabilize the flame more than thin and short rings. Application: Pulp mill engineers may use the information in this study to help stabilize the lime kiln burner flame. [ABSTRACT FROM AUTHOR]

Electrostatic precipitators (ESPs) are used in most pulp mills to remove particulate from recovery boilers, power boilers, and lime kilns. As environmental regulations have become increasingly stringent in recent years, maintaining high ESP performance is of vital importance in mill operation. This paper discusses results of a literature review of the ESP technology used in industrial combustion units, including recovery boilers, as well as results of a parametric study using the well-known Deutsch-Anderson equation to correlate recovery boiler operating conditions with ESP collection efficiency. The results show that for particles up to about 0.3 μm, the ESP collection efficiency decreases drastically with increased particle size and with decreased temperature. For particles larger than 0.5 μm, however, the trend reverses; the collection efficiency increases with increased particle size and decreased temperature. The results also suggest that the particle concentration (or loading) in the flue gas has no effect on collection efficiency and that sodium chloride particles are more readily captured than sodium sulfate particles. The latter prediction, however, appears to be in contradiction with mill experience that sodium chloride particles are more difficult to capture. Application: Mills can maximize their boiler throughputs by better understanding key operating factors that affect electrostatic precipitator performance. [ABSTRACT FROM AUTHOR]

The presence of unreacted lime (free lime) in the lime mud can cause many problems in the chemical recovery operation of kraft pulp mills. There are currently no standards for free lime determination. Conventional methods used in mill laboratories require extensive preparations and give inconsistent and sometime erroneous results. A systematic study was performed to determine if zeta potential, a dispersion characteristic of the lime mud and liquor slurry, can be used to indirectly indicate the amount of free lime in the lime mud. Lime mud with different free lime contents was produced in the laboratory by causticizing green liquors with lime at different liming ratios. The results show that zeta potential changes from negative to positive when the free lime in lime mud exceeds a critical value. This unique characteristic enables a rapid detection of free lime. Application: This research can help mills take appropriate actions to avoid problems associated with "over-liming" in the recovery process. [ABSTRACT FROM AUTHOR]

Molten smelt normally flows smoothly down the smelt spout of a recovery boiler like water, but at times it suddenly becomes sluggish and forms a viscous blob on the spout trough that partially or completely blocks the smelt flow. This form of smelt is commonly referred to as “jellyroll” smelt. How such smelt forms has been a puzzle to boiler operators and mill personnel for years. Numerous mill observations and the results of a recent study performed on both smoothly flowing smelt and jellyroll smelt collected from a recovery boiler suggest that that jellyroll smelt can form through three main mechanisms: i) the freezing of the molten smelt, ii) the melting of fallen deposits, and iii) the inclusion of a large amount of unburned char in the molten smelt. These mechanisms are consistent with mill experience that jellyroll smelt tends to form in older recovery boilers burning liquor with low solids and low sulfidity. [ABSTRACT FROM AUTHOR]

Black liquor is often mixed with various types of materials before being burned in a recovery boiler to meet specific needs of kraft pulp mills. A systematic study was conducted using a thermogravimetric combustor to examine how added materials might affect the combustion behavior of black liquors obtained from several pulp mills. The results show that adding soap, caustic, white liquor, and sawdust significantly reduces the liquor swelling tendency, thereby requiring a longer time for the liquor to burn completely. Adding makeup saltcake, precipitator ash, sodium sulfate, and biosludge, on the other hand, has little or no effect on the liquor combustion behavior. Application: Mills that understand how each added material affects the combustion behavior of black liquor can optimize their recovery boiler operation. [ABSTRACT FROM AUTHOR]

Fireside deposits in recovery boilers are typically white, red, pink, grey, black, or occasionally yellow, depending on where they are in the boiler, the mechanisms by which they are formed, and the environment to which they are exposed. Although rare, blue deposits have been reported, and some were "bluer" than others. This study systematically examines the cause of the blue coloration of deposits in recovery boilers. The results show that for a deposit to become blue, it must a) contain sodium carbonate, b) contain a small amount of manganese, c) be molten or partially molten, and d) have exposure to an oxidizing atmosphere. Because deposits always contain sodium carbonate and manganese, these requirements suggest that blue deposits can form only in the superheater region of the recovery boiler when oxidizing conditions prevail. Blue coloration is thus more likely to be observed in boilers operating at a reduced firing load with a high excess oxygen target. [ABSTRACT FROM AUTHOR]

Overliming and the consequent presence of unreacted lime (or free lime) in the lime mud are commonly believed to be the cause of many problems in the operation of causticizing plants and lime kilns in kraft pulp mills. The free lime content in lime mud is typically determined in mill laboratories using a so-called ammonium chloride (NH4Cl) method and in commercial laboratories using a thermal decomposition (TD) method. Over the years, we analyzed many lime mud samples from mills and found that the free lime content was consistently low, < 3 wt% calcium hydroxide (Ca(OH)2), even in cases where overliming was suspected to have caused problems. A systematic study was therefore conducted to investigate the validity of free-lime measurement methods, the reason for the consistently low free lime content in lime mud, and if free lime values can be used to indicate overliming. The results show that the NH4Cl method is not suitable for determining free lime. The TD method is good, but the possible interference of magnesium hydroxide (Mg(OH)2) must be taken into account. Since most pulp mills perform their free lime analysis on mud samples collected from pre-coat filters which have been washed, the resulting free lime value is low, and thus, cannot be used to assess the extent of overliming in the causticizing plant. Application: Mills should be aware that the conventional NH4Cl method for measuring the free lime content in lime mud does not work. It gives a free lime value that is low and cannot be used to assess the extent of overliming in the causticizing plant. [ABSTRACT FROM AUTHOR]

Lime kilns are used to calcine lime mud to produce lime for reuse in the causticizing plant of pulp mills. With significant increase in fossil fuel costs and the sustained drive to minimize carbon footprint, there has been a great deal of research and development of the biorefinery concept and production of biofuels for use within pulp mills. Several new biofuels will become available, together with traditional mill derived fuels such as tall oil. In considering use of biofuels for replacement of fossil fuels in the lime kiln, each fuel has a different impact on kiln performance. An assessment on kiln performance can be achieved and compared to either heavy fuel oil or natural gas. In most cases, the lime kiln fuels with high oxygen content and moderately low calorific values burn in a similar manner as natural gas; hence, in assessing impact on kiln performance, production and induced draft fan limits together with kiln exit gas temperatures need to be considered. Application: This information will be useful to pulp mills wanting to assess the impact of replacing fossil fuels with biofuels on lime kiln performance. [ABSTRACT FROM AUTHOR]

For kraft pulp mills that have thermally limited recovery boilers, lignin removal from black liquor has become an attractive option for increasing pulp production by allowing more black liquor to be processed through the boiler. This study systematically examined the combustion characteristics of lignin-lean black liquor and precipitated lignin from three kraft mills using a thermogravimetric combustor. The results confirm that adding lignin-lean black liquor to its original black liquor decreased the heating value and the degree of swelling of the mixed liquor. The effect on liquor swelling, however, was insignificant for mixed liquors that contained less than 20 wt% of lignin-lean liquor. As with other biofuels, the combustion of precipitated lignin was found to occur through three main stages: drying, volatile burning, and char burning. During the volatile burning stage, hardwood lignin swelled significantly, softwood lignin did not swell much, and mixed hardwood and softwood lignin was somewhere in between. Although the char content in lignin was about half of the volatile content, it took 10 times longer for the char to burn compared to the volatiles. Application: Understanding the combustion behaviors of lignin-lean black and precipitated lignin is of great importance for pulp mills that plan to burn lignin in their lime kilns or power boilers. [ABSTRACT FROM AUTHOR]

In kraft recovery boiler operation, fly ash or dust generated from black liquor combustion is mixed with the virgin black liquor in a mix tank and returned to the boiler with the as-fired black liquor. This internal recycled dust stream varies widely from boiler to boiler and from time to time and can have a great impact on the asfired black liquor flow and properties and, ultimately, on the boiler thermal performance. A new method has been developed to quickly and accurately determine the amount of internal recycled dust in recovery boilers. The method is based on the difference between the total organic carbon content of the virgin black liquor and that of the as-fired black liquor. Tests using the method were performed on recovery boilers at three of Fibria's mills in Brazil. The results show that while the specific virgin black liquor solids produced at these mills were about the same, the internal recycled dust load varied widely, from as low as 4 wt% of as-fired black liquor solids fired in the boiler at one mill to as high as 15 wt% at another mill. Instead of total organic carbon values, heating values may also be used, but the result is not as accurate. [ABSTRACT FROM AUTHOR]

A scaled-down experimental apparatus was built to examine smelt shattering during typical recovery boiler operations. Water-glycerine solutions and air were used in place of smelt and steam. A high-speed camera and image processing software were used to record and quantify liquid shattering in terms of droplet number and size distributions, as a function of air velocity, air nozzle position, liquid flow rate, and liquid viscosity. The results showed that increasing shatter jet velocity reduced average droplet size, increasing the liquid flow rate increased droplet size, and placing the shatter jet nozzle closer to the liquid stream decreased droplet size. These results were all as expected. The effect of liquid viscosity (1-50 cP) depended on the shatter jet velocity. At high air velocities, even the viscous liquid was well shattered, but at lower velocities, the effect of viscosity on shattering was significant. [ABSTRACT FROM AUTHOR]

A systematic study was performed in the laboratory and in the field to examine the effect of magnesium oxide additive on deposit accumulation and removal. Laboratory results show that the additive has little effect on the amount of deposits collected on a probe, but it can make deposits easy to remove if a coating layer thicker than 30 µm can be effectively applied on the probe surface. The results also show that mixing the additive with black liquor or injecting it separately has no significant effect on deposit accumulation and removal. Tests performed in a recovery boiler where the additive was continuously injected show no evidence for the existence of the coating layer. The magnesium oxide content in the boiler deposits varied between 0.1 and 0.4 wt%, which is several times lower than the amount that was found to be effective in laboratory tests. Nonetheless, the additive was shown to be effective in facilitating deposit removal if it can form a coating layer on deposit/tube surfaces. This may be possible by periodically injecting a large amount of additive into the boiler for a short period and reducing the black liquor firing rate at the same time. [ABSTRACT FROM AUTHOR]

The article presents a study concerning the optimization program at Irving Pulp and Paper Ltd. to increase boiler production in Saint John, New Brunswick. It states that Irving Pulp and Paper is a kraft mill which aims to increase the output of black liquor dry solids from 1100 metrics per day to 1680 metric tons per day. It mentions that the goal of the program is to overcome the high total reduced sulfur (TRS) levels.

The article discusses the study on the use of principal component analysis (PCA) in analyzing high resolution operational data from three recovery boilers to identify major operating variables that contribute to fouling and plugging. The study shows that PCA can be used to visualize the variability of long-term fouling trends in boilers and distinguish changes of boiler fouling condition. The result will help create operating variables based on PCA that would minimize and monitor fouling.

The article examines the technical feasibility of alternative fuels, including petroleum coke, directly fired wood residues and precipitated lignin. It cites the advantages and disadvantages of burning each fuel before using it in the lime kiln. Petroleum coke is the most common alternative fuel in the U.S., but one of its disadvantages is high sulfur input to mill recovery system. Lower heating values relative to fossil fuels is a disadvantage of gasified wood residues and directed fired wood.

The article discusses a study that focuses on the reduction of dead load chemicals in order to improve the chemical recovery operations of kraft pulp mills. It provides some guidelines for minimizing the effects of dead load chemicals. It talks about the effect of dead load chemicals such as sodium carbonate, sodium sulfate, and thiosulfate in the kraft pulping and chemical recovery system of kraft pulp mills.

The article presents the study which aims to examine the solubility in water of various precipitator ash samples. The researcher describes the program and its capabilities as well as discusses the results obtained from the program prediction and their implications. The result shows that the predicted values by the program agrees with literature and experimental data and notes that between 30-100 degrees Celsius has little effect on the solubility of precipitator ash and on the composition of the liquid portion of the slurry.

The article presents a study on the impact of chloride (Cl), potassium (K) and sulfate on the causticizing reaction in the kraft recovery process in the paper mill industry. Using an advanced thermodynamic program for equilibrium computations in aqueous solutions of ionic salts, the study examines the effects of Cl, K and sulfate on the distribution equality of the causticizing reaction Na2CO3 + Ca(OH)2=2NaOH + CaCO3. Details of the study are included, citing no significant effect or little effect of Cl, K and sulfate toward the causticizing equilibrium.

The article examines the correlation between nodule size and residual carbonate content in lime kilns. According to the report, the study was performed on several limes kilns to establish a correlation between the size of reburned lime nodules and their residual carbonate content. It determines that nodules with a diameter smaller than 20-30 mm have a low, uniformly distributed residual carbonate content. It also specifies that larger nodules have an uncalcined core and thus contain significantly more residual carbonate.

The article presents the study that evaluates the effectiveness of the bulk thermal stresses for the kraft recovery boiler fireside deposit cleaning in pulp mills. The removal of fireside deposits from the heat transfer tubes in the boilers were carried out by sootblowing and by chill-and-blow practices. During the later practice, black liquor flow is lessen to easily cool the deposits. However, thermal stresses are produced in the deposit as a result of thermal shock which can effectively weaken the deposit. Furthermore, the mechanical weakening effect of thermal stresses is developed by measuring the decrease in biaxial flexure strength of a preheated sodium chloride disk.