The solubility of ESP dust

Written by Roxare on April 6th, 2010

Chloride and K are enriched in the electrostatic precipitator (ESP) dust of the recovery boiler. Most methods used for removing these two elements from the recovery cycle rely on the treatment of ESP dust. As such, ESP dust is dissolved in water to near the saturation level, or to make slurry. It is then treated using one of the commercially available systems for removing Cl and K. In a recent paper [1], the solubility of ESP dust was predicted using OLI, a commercial thermodynamic program. The data obtained helps maximize Cl and K removal, while minimizing Na and S losses.

The input to the OLI program were the amounts of dust (or its salt components) and water, and the solution temperature. The outputs were the amounts and compositions of the solids and liquids, and the solution temperature. The program was used first to predict the solubility of pure compounds, for which the corresponding literature data were available. It was then used to predict the solubility of a mixture of salts. The agreement between the measured and calculated values was very good.

The solubility of two ESP dusts, one with a high carbonate and lower sulfate content, were measured at three different temperatures (10, 25 and 85 C). Since there were no data available in the literature, continuous conductivity measurements were used to obtain the solubility of the dust. The maximum solubility occurred at the maximum conductance of the solution.  There was good agreement between the OLI and conductivity data.

Simulation data showed that, for the range of 60-100 C, temperature had little effect on the solution composition of the slurry. At 85 C, the effect of dust concentration on the composition of the solution phase of the slurry showed that there was little change in Na, K and CO3 content; by contrast, the SO4 content decreased with increasing dust concentration and the Cl content increased.

Reference:
1.    D. Saturnino and H. Tran, Prediction of the solubility of recovery boiler precipitator ash, Tappi Int. Chem. Rec. Conf., 2007


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Posted in Chloride, ESP dust, Potassium, Recovery Cycle | No Responses »

Effect of Ammonia Injection on NOx Emissions for a Recovery Boiler

Written by Roxare on January 17th, 2010

For modern recovery boilers, which are designed for low NOx emissions, the conversion rate of N2 to NOx can be lower than 15-20%. The recovery boiler operating conditions such as excess air, furnace temperature and air staging have a direct effect on the emission rates [1]. Selective non-catalytic NOx reduction (SNCR) has been used in power boilers to reduce NOx emissions. This technology was borrowed for a series of tests on a 2000 tds/d recovery boiler built in 2001 by Metso Power, equipped with quaternary air [2]. The SNCR technology injects ammonia into the flue gas stream at a certain temperature range to reduce NOx into N2 gas.  Critical factors are good mixing and sufficient residence time at the right temperature. At too high a temperature, the ammonia oxidizes to NOx whereas, at low temperatures, the reactions are too slow and ammonia leaves the boiler un-reacted (ammonia slip).

A series of tests were carried out with ammonia injections, and the gas and deposit compositions were monitored extensively. The injection took place through lances, at three different injection levels, with air atomized spray nozzles. Carrier air was provided around each lance at the screen level to improve penetration in the furnace. At the lowest level, the quaternary air was used as carrier air. Long term tests at the quaternary level were carried out at 62% load while, at the injection level below the screen tubes, tests were conducted at 74% load or higher.

During the trials, FTIR was used for monitoring the emission composition. The flue gas temperature at the injection position was also measured.  Deposit probes were used in the superheater and economizer areas, with temperatures controlled at 450, 250 and 150 C. Based on the data obtained, a significant reduction in NOx emission was achieved (30-50%).  For other gaseous compounds, the CO emission did not change much, although it was difficult to operate the boiler under exactly controlled conditions.  Other species, such as SO2, HCl, HF and N2O were not clearly impacted by the addition of ammonia. In nine out of twelve tests the reduction in NO was followed by a reduction in NO2. Only in one case was there a significant increase in NO2 which was due to very low levels of NO2 in both cases, i.e., with and without the SNCR turned on. In this study, the ammonia slip was typically less than 20 ppm. A slip much higher than this cannot be tolerated, for environmental and economical reasons.

CFD simulations showed that the penetration of the SNCR jets into the recovery boiler gas steam was poor, indicating that a limited number of injection ports were not adequate to ensure an even distribution. This suggests that significant improvements to the injection system can be made.

The compositions of the ESP ash and deposits collected on the probes showed that ammonia injection had little impact on the properties of the ash. Thermodynamic calculations confirmed that ammonium salts would not form in the superheater region. The formation of liquid NH4HSO4 in the cooler part of the flue gas channel, for boilers with high SO2 emission, is a possibility. However, due to the significant amount of sodium present, most of the liquid salt will be in the form of NaHSO4 and the effect of NH4HSO4 is expected to be negligible.

References:
1.    Distribution of Black Liquor Nitrogen Between Smelt, NOx and Flue Gases in Recovery Boilers, K. Saviharju et al, TAPPI 2007 Chemical Recovery Conference, Quebec City, QC, Canada.

2.    Effect of Ammonia Injection on Black Liquor Recovery Boiler NOx Emissions and Ash Chemistry, M. Lundberg et al, TAPPI Engineering, Pulping & Environmental Conference, Aug. 24-27, 2008, Portland, Oregon, USA.


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Posted in NOx, Recovery Boiler | No Responses »

NOx Emissions

Written by Roxare on October 17th, 2009

One of the drawbacks in high solids firing is the increased rate of NOx emission. Since NOx contributes to acid rain formation, a reduction in its emission is desirable. To control NOx emissions, knowledge of nitrogen chemistry in the recovery cycle is required. A study of three European mills showed that about 95% of the N2 in wood is released during pulping, with 10-15% being converted to ammonia. The remaining nitrogen is in the form of organic compounds; these compounds, and about half of the ammonia, end up in the black liquor (BL) [1]. Char gasification tests indicate that most of the nitrogen in the char is converted to cyanate during char oxidation, and that about 30% of it ends up in the smelt. Cyanate reacts to form ammonia in alkaline solutions such as green and white liquors. This, in turn, is vented from the liquor storage tanks or returned to the digester with WL. The ammonia is then vented from the digester blow tank or the early evaporator’s effects. When BL is concentrated to about 30%, some additional ammonia and a small amount of volatile organics are released into the foul condensate.

A more recent study [2] of 18 European mills over six years of operation, with recovery boilers manufactured or retrofitted by Andritz, shows that nitrogen distribution in the recovery cycle can vary widely between different mills with different recovery boilers (RBs).  The conversion to NOx was about 12-46%, while the conversion into N2 varied between 40-77%; the amount of nitrogen in the smelt was 8-28%.  For modern RBs, the conversion of nitrogen to NOx was about 20-30% of the total nitrogen content of BL. For RBs which are designed for low NOx emission, the conversion can be lower than 15-20%.  For these RB’s, the breakdown was 15% into NOx, 10% into smelt and 75% into N2 in the flue gas.  These boilers operate in steady conditions, with continuously cleaned air port openings and liquor guns, and well controlled excess air. The flue gas NOx emission rates correlated with excess air, furnace temperature, air staging and, in some cases, with the amount of carryover.

Lower NOx emissions may increase SO2 emissions if sulfidity is high and the furnace hearth temperature is low. The operating conditions for low NOx emissions may lower the melting point of carryover, resulting in fouling and corrosion in the superheaters, lower power generation capacity and increased steam consumption for soot blowing.
For some boilers, the high carryover resulted in a lower concentration of nitrogen in the smelt. This is because char particles burning in the upper furnace produce NOx, reducing the amount of nitrogen in smelt.  High liquor loading and high reduction efficiencies also decreased the nitrogen content of smelt. NOx emission did not appear to be connected to the concentration of nitrogen in smelt.

According to the results of this study, the key factors affecting the conversion of nitrogen into NOx are: furnace loading, design of the air and liquor systems, excess oxygen in firing, liquor dry solids, droplet size and design of the NCG burners.  Both the lower and upper furnace zones affect the conversion of nitrogen into NOx. A 1% unit decrease in excess oxygen in flue gas would reduce the NOx emission by 20 ppm.  The flue gas temperature in the final oxidation area has a direct effect on the conversion factor.
The nitrogen in char in the lower furnace, under reducing conditions and high temperature, forms N2 instead of NOx.

References:

1.    Distribution and Release of Nitrogen Compounds at Kraft Pulp Mills-A Survey of Three European Mills, N. DeMartini et al, Tappi 2004 Chemical Recovery Conference, June 6-10, Charleston, SC, USA.
2.    Distribution of Black Liquor Nitrogen Between Smelt, NOx and Flue Gases in Recovery Boilers, K. Saviharju et al, Tappi 2007 Chemical Recovery Conference, May 29-June 1, Quebec City, QC, Canada.


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Posted in Black liquor, NOx, Recovery Boiler, Recovery Cycle | No Responses »

Measurement of Residual Effective Alkali in Black Liquor

Written by Roxare on August 5th, 2009

Data from the measurement of black liquor properties are used to gain insight into and to optimize, the operation of the recovery boiler and evaporators. One important parameter in measuring the extent of reaction in pulping and optimizing black liquor processing in terms of viscosity, is the residual effective alkali, or REA. In North America, it is normally expressed as % Na2O/black liquor solids. Many mills have used old procedures developed by PAPTAC (J15P) and TAPPI (625 cm-85) based on acidimetric titration.  However, these methods overestimate the true concentration of alkali due to interference from dissolved organic compounds. The SCAN method (SCAN-N 33:94) provides a more accurate measure of the alkali content of a black liquor. In this method, potentiometric titration with 1M HCl to the first inflection point is used for black liquors with an initial pH of at least 11 and concentrations not exceeding 20% solids. If the first inflection point is not visible, the volume of HCl at a pH of 11.5 is used instead. A correction factor is applied to obtain the true value of the REA.

A more recent work [1] compares different methods for measuring REA in black liquor, and recommends measuring the alkali content with potentiometic titration at a dilution factor of 4 for strong black liquor, using 1M HCl to a pH inflection point between 11 and 11.5.  The measured REA is then corrected using a factor. The correction factor is obtained from the linear regression equation of measured REA versus true REA. The true REA is measured by a capillary electrophoresis method. If an auto-titrator is not available, or a distinct inflection point is not observed, the REA is calculated at a fixed pH end-point of 11.3. Again, a correction factor is used to obtain the true REA. The difference between the fixed end point of 11.5 in the SCAN method and 11.3 in the more recent work is that the latter uses a dilution factor of 4, whereas the SCAN method uses a dilution factor of 10 and adds carbonate to the black liquor sample.

Another issue that affects the expressed value of REA in black liquor is the measurement of its solids content. Because strong black liquor forms a skin when dried, it traps some residual water, affecting the accuracy of any solids measurement.  TAPPI method 650 om-89 dries the liquor sample in a standard oven at 105 C, using sand as a surface extender to ensure complete removal of water. Oven drying provides consistently reproducible results, but it has some drawbacks. Volatile organics such as methanol, turpentine and reduced sulfur compounds are also evaporated around 105 C. Although these compounds are not dried solids, they affect the liquor’s material balance. Unless oxygen is excluded during the drying process, the sulfide content of the black liquor is oxidized to thiosulfate, increasing the apparent dry solids by about 3-5% [2]. This error can be corrected when the sulfide content of the black liquor is known.

References:

1.    Improved Methods for Measuring Residual Effective Alkali in Kraft Black Liquors – Part 2: Comparison of Titration Methods, T. Radiotis, J.L. Sullivan, M. MacLeod, S. Syed and T. O’Hagan, FPInnovations, Paprican, TAPPI Engineering, Pulping and Environmental Conf. 2007.

2.    Black Liquor Properties, Chapter 3, W.J. Fredrick, Kraft Recovery Boilers, Ed., T.N., Adams, TAPPI Press, 1997.


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Posted in Black liquor, Recovery Cycle, Residual Effective Alkali | No Responses »

Energy Issues and the Biorefinery

Written by Roxare on June 7th, 2009

With the increasing cost of energy and mounting environmental concerns about emissions, a few pulp mills are installing biomass gasifications as an alternative source of energy. One recent example is the Kruger tissue mill in New Westminster, BC. The mill is installing a gasification system to convert locally –sourced wood residue into syngas, which will be fired into a boiler, replacing natural gas. The system start-up is scheduled for December 2009, and will produce 40,000 lb/hr of process steam and replace around 445,000 GJ of natural gas per year [1]. The gasification process uses 20-30% of the oxygen required for complete combustion. Through pyrolysis, a syngas is produced which mainly consists of carbon monoxide, hydrogen and methane. The main advantage of syngas compared to hog fuel is that it is a clean fuel; hog fuel generates a significant amount of particulate emissions. It is estimated that New Westminster installation will save millions of dollars in energy costs.

Other related projects in the pipeline are an ethanol production plant in Westbury, Quebec that will produce ethanol from old utility poles, and a plant in Edmonton, Alberta, which will produce ethanol from municipal solid waste. These projects indicate another option for wood residues in the pulp and paper industry. Click to continue »


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Posted in Biorefinery, Energy | No Responses »

Modern Recovery Boilers

Written by Roxare on April 5th, 2009

New recovery boilers are being installed around the world which are bigger, have higher efficiencies and produce more electricity. Producing green electricity for sale based on renewable resources has put a new focus on the recovery operation. About 80% of the 20 most recent recovery installations have higher pressures (>85 bar) and higher temperatures (>480o C). New recovery boilers have capacities of more than 5000 tds/day, with some evaporation units exceeding 1000 t H2O/h water removal capacity [1].

New recovery capacity is being built in Asia and South America, where half of the new mills produce HW pulp (Eucalyptus). Europe and North America account for less than half of total new recovery capacity. In Europe and North America, there are a lot of old recovery boilers, with an average age of 30 years. Since the life expectancy of a recovery boiler is about 30-40 years, significant investment in older mills is required within the next 10 years. The alternative is mill closure. Click to continue »


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Posted in Recovery Boiler, Recovery Cycle | 2 Responses »

Controlling Black Liquor Viscosity

Written by Roxare on January 18th, 2009

The viscosity of black liquor affects its handling properties and is a critical parameter for the control of a recovery furnace, in terms of the spray characteristics and the char bed size and shape. Many mills have increased the as-fired black liquor solids in order to improve their boiler’s thermal efficiency, increase liquor throughput and reduce environmental emissions. However, black liquor viscosity increases significantly at solids concentrations over 70%. If a mill is not equipped with a pressurized liquor handling system, in which high temperatures can be used to reduce viscosity, another viscosity reduction method such as alkali profiling may have to be used to facilitate high solids firing. There are other methods to reduce viscosity, such as liquor heat treatment or high temperature oxidation, but these involve capital cost. For more information on black liquor viscosity see my posts of May 26, June 9 and June 28, 2007. Click to continue »


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Posted in Black liquor viscosity, Recovery Boiler | 3 Responses »

SO2 emission dependency on recovery boiler operation

Written by Roxare on November 16th, 2008

Modern recovery boilers (RB) operating at high bed temperatures have, typically, a low SO2 emission. These RBs have a modern air system and, by firing liquor at high solids concentrations, create adequate burning intensity and mixing in the lower furnace. Older boilers operating at high sulfidity, low dry solids and/or a not sufficient air system will have a high SO2 emission rate, i.e., around 1000 mg/Nm3 [1].

The molar ratio of S/Na in black liquor (BL), along with furnace operating conditions, determine the chemistry of the S and Na in the flue gas. Based on the results obtained from a flue gas survey of different boilers, the range for Na and S emissions has been examined. For RBs with a low S/Na ratio in the flue gas (high burning intensity) and a hot bed, more Na is released than needed to bind with S, resulting in little or no SO2 emission. The fly ash will contain a high Na2CO3 concentration, corresponding to a high pH. RBs with a high S/Na ratio in the flue gas have low burning intensity and colder beds that do not release enough Na to capture S, resulting in more SO2 emission and the formation of acidic sulfates, which lower the sticky point of the ash and its pH. Click to continue »


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Posted in Recovery Boiler, SO2 emission | 4 Responses »

Review of sodium salt scaling in the recovery cycle

Written by Roxare on October 13th, 2008

I have covered this subject under different postings in the past [March and April 2007]. However, this is an up to date review with some new information regarding the nature of scaling in black liquor evaporators [1].

Sodium salt scaling takes place in black liquor [BL] evaporators, smelt dissolving tanks and green liquor lines. Scaling occurs when crystals nucleate from solution on pipe walls or heat transfer surfaces. In a solution containing no precipitated salt, nucleation occurs when the solution become supersaturated to the point where the metastable limit for the system is exceeded. The level of supersaturation depends on the solution, the process, the number of hetero-nuclei, the heating rate, the residence time and, sometimes, the shear rate [1]. Click to continue »


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Posted in Evaporator scaling, Sodium salts scaling | 2 Responses »

Capturing value in pulp and paper manufacturing

Written by Roxare on September 15th, 2008

Here is a summary of a recent article by J.D. McNutt [1] which I think is timely and interesting.

We all know that the pulp and paper manufacturing is an industry in difficulty. A new integrated, virgin fibre-based mill has not been built in North America (NA) in more than 17 years. Value creation and stocks have gone nowhere over the past decade, making new investments unlikely. However, in the past, some really bad industries have turned around with strong asset management discipline and consolidation; an example is the steel industry. The possibility exists for the pulp and paper industry to turn around, judging by investments made by some private equity firms. Private equity is paying about 30-40% of invested capital costs for assets. With a new focus on capital asset management, older assets (capacity) are being permanently retired, resulting in both lower capacities and improved asset quality. This constrained new capacity investment and focused asset management climate should help paper companies to reach healthier returns on invested capital. Click to continue »


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Posted in Value | No Responses »

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