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CONCENTRATED SULFURIC ACID PRODUCTION FROM NON-CONDENSABLE GASES AND ITS EFFECT ON ALKALI AND SULFUR BALANCES IN PULP MILLS

Andrés Mahecha-Botero1, Isabel M. C. L. Sêco2, Igor Aksenov1, C. Guy Cooper1, Jon Foan1,

Rohan Bandekar1, Kam Sirikan1, Jim Wearing1

1NORAM Engineering and Constructors Ltd., 200 Granville Street, Suite 1800, Vancouver, BC, V6C1S4, Canada, Phone: +1 604 681 2030, andresmb@noram-eng.com

2Now with Altri, SGPS, S.A.

SUMMARY

The pulp and paper industry often encounters challenges that require process improvements to remain competitive. These challenges may include the requirement to meet more stringent environmental regulations, stricter energy policies, or the need to improve product quality, increase production capacity and profitability.

As a result, the pulp mills of today have to focus on becoming more efficient by possessing an effective chemical recovery system and reducing chemical losses. The high degree of closure is beneficial for environment, water consumption and mill economy but can upset the Na/S balance and increase the build-up of non-process elements in the system.

Installing an acid plant to convert the sulfur containing Non Condensable Gases (NCG) into sulfuric acid will eliminate the NCG as a sulfur input to the recovery cycle, eliminate purchases of sulfuric acid, reduce caustic purchases, and produce additional steam that will positively impact the mill’s heat balance.

This paper provides an overview of the technology required to produce sulfuric acid in a pulp mill from NCG, presents some of the unique challenges related to feed variability, and discusses some of the technical features of NORAM’s sulfuric acid process technology and equipment.

Keywords: Non-condensable gases, sulfur-containing gas, sulfuric acid plants.

INTRODUCTION

Kraft process is a chemical pulping process in which the cellulose fibers are extracted from wood by using aqueous white liquor which is composed of the chemicals NaOH and Na2S. The sodium and sulfur balances impact pulp quality. The right Na/S balance in Kraft mills at first was achieved by replacing the chemical losses with sodium sulphate and sodium hydroxide. The typical chemical losses included brown stock washing losses, liquor spills, boiler and kiln emissions and purging of ESP ash or lime mud for NPE control. The process modifications over the years such as improved brownstock washing, partial or total recovery of bleach plant effluent, recovery boiler precipitator dust purification processes to selectively remove chloride and potassium, etc., have significantly reduced the typical chemical losses in a pulp mill. Furthermore the make-up chemicals used now, apart from NaOH and Na2SO4, include ClO2 generator by-product, sulfur input from a lignin extraction process, sodium and sulfur input from tall oil acidulation process, etc.

Mills that recycle bleach plant effluent to the kraft recovery system often see an excess of sodium/sulfur in the chemical balance of the mill. The alkali and sulfur compounds (NaOH, H2SO4, SO2) charged in the bleach plant effectively replace the make-up chemicals typically used in a scenario with no bleach plant effluent recycle. This results in a surplus of alkali and/or sulfur which then have to be controlled by purging ESP ash or ClO2 generator by-product. Table 1 below gives examples of

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sodium sulfur balance in mills that do and don’t recycle bleach pant effluent [1]. It’s apparent from the data that mills that recycle bleach plant effluent have to purge their ESP ash or invest in a technology that can deal with a situation of excess chemicals.

Table 1: Sodium/ Sulfur Balances [1]

No Bleach Effluent Recycle 

Total Bleach Effluent Recycle (PAA sequence) 

Total Bleach Effluent Recycle 

(Peroxide sequence) 

Total Bleach Effluent Recycle (Ozone sequence) 

Input Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Wood, etc 0 0.2 0 0.2 0 0.2 0 0.2

Chem. O stage 0 0.5 - - - - - -

Bleaching Chemicals

- 0 14 2 14 5 14 7

Make-up and Scrubber akali

9 0 0 0 0 0 0 0

Tall oil plant 0 2.8 0 2.8 0 2.8 0 2.8

Total 9 3.5 14 5 14 8 14 10

Output Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Pulp 4.5 1 1.5 0.5 1.5 0.5 1.5 0.5

Bleached Rejects, Sludges,

Accidental Losses,

Condensates, etc

4.5 1.5 4.5 4.5 4.5 1.5 4.5 1.5

S gas - 1.0 - - - 0.5 - 0.5

Purges Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

Na, Kg/ADt

S, Kg/ADt

ESP ash - - 5 3 8 5.5 8 5.5

Soda (G.L) - - 3 - 0 - 0 -

S purge (Liquor Heat Treatment)

- - - 0 - 0 - 2

Total 9 3.5 14 5 14 8 14 10

Similarly in a softwood mill, an excess amount of sulfur usually exists and hence mills often tend to sewer the ClO2 by-product or recovery boiler ESP ash. In the process, sodium is lost which is usually made up by using expensive NaOH. Furthermore the ClO2 by-product has to be neutralized before being purged to the environment. ESP ash purge to the environment can be an important issue in the future when more stringent environmental regulations come into place. There are several methods that

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have been previously proposed to handle a situation of high sulphidity. The following examples illustrate some of the options:

Tall oil recovery with carbon dioxide. The use of carbon dioxide to replace a portion of the sulfuric acid required can provide significant benefits to the mill but sulfuric acid would still have to be purchased to achieve the low pH required for the process [2].

Electrolytic salt splitting to generate acid or a base from neutral salt using membrane electrolysis. This technology can be potentially applied to ESP catch from the recovery boiler to produce sulfur free alkali and sulfuric acid simultaneously. However the relatively low costs of sulfuric acid and caustic today make the operating and capital costs difficult to justify on economic grounds alone, except under certain circumstances [3].

Production of sulfuric acid from NCG. These gases are rich in sulfur and normally they are burned in an incinerator, lime kiln or recovery boiler. NCGs burning forms SO2 which can be sent to a sulfuric acid plant for acid production [4].

Generator acid purification (GAP) technology uses a short-column resin-bed technology to separate sodium sesquisulphate by-product from a ClO2 generator into sulfuric acid and sodium sulphate. This operation allows a reduction of caustic makeup while the sulfuric acid produced replaces the merchant acid in the bleach plant [5].

Bleaching with a combination of ozone, oxygen, hydrogen peroxide and chlorine dioxide to reduce the spent acid or saltcake produced in chlorine dioxide generation [2].

Adoption of improved chlorine dioxide generation technologies to reduce the saltcake production per ton of ClO2 produced [2].

The management of the chemical balances of a modern Kraft pulp mill is a challenging task due to the multiple variables to be controlled in the mill. There is increased pressure to reduce losses of chemicals to the environment by improving the efficiency of the chemical recovery system to comply with more stringent environmental regulations for emissions to air and water, and to reduce the make-up of chemicals. The high degree of closure is beneficial for environment, water consumption and mill economy but can upset the Na/S balance and increase the build-up of non-process elements in the system. There is a significant economic and environmental incentive in-situ sulfuric acid production. This paper provides an overview of sodium sulfur balance in pulp mills when integrated with in-situ sulfuric acid production. Moreover the technology required to produce sulfuric acid in a pulp mill is reviewed.

Mill Balances and System Integration

It is important to optimize the sulfur and sodium balances in the pulp mill. There are several disadvantages of running the mill at high sulphidity such as increased emission of NCGs throughout the recovery cycle, upsets in recovery boiler operation, increased corrosion throughout the pulping and recovery cycle, etc. For mills whose sulfur inputs must be limited or sulfur would need to be purged: the byproduct from the chlorine dioxide generator cannot be fully used as make-up chemical in the recovery cycle or part of the electrostatic precipitator ash from the recovery boiler would need to be dumped. Either way this represents a sodium loss in the recovery cycle that needs to be replaced by NaOH or Na2CO3 and therefore the cost of controlling the Na/S increases.

In the Kraft mill the chemical balance is of interest, both to minimize the production costs, and to control the process conditions. The Na/S balance has an influence on the pulping yield and pulp quality and emissions of sulfurous gases.

Table 2 presents an example of a Na/S balance for a Kraft pulp mill before and after conversion

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Table 2: Na/S balance to recovery cycle – Example for Mill A with and without a H2SO4 system from NCGs

Case at Mill A Base Case With H2SO4 System

chemical, kg/t Na, kg/t S, kg/t chemical, kg/t Na, kg/t S, kg/t

Inputs          

Wood 0.2  0.2 

ClO2 byproduct 7.3  2.4  1.6  10.4  3.4  2.4 

Purchased Na2SO4 0.0  10.2  3.3  2.3 

Caustic 10.0  5.8  0.0  2.5  1.4  0.0 

Totals 8.1  1.8  8.1  4.8 

Outputs

Sulfur in CNCG’s 3.0 

Other Losses 8.1  1.8  8.1  1.8 

Totals 0  8.1  1.8  8.1  4.8 

Because the sulfur balance should be kept undisturbed, the generation of sulfuric acid in the mill could be a solution to keep the Na/S ratio in balance and give a positive economic outcome to the pulp mill. Producing sulfuric acid from non-condensable gases is one possibility. These gases are rich in sulfur and normally they are burned in an incinerator, lime kiln or recovery boiler. NCGs burning forms SO2 which can be sent to a sulfuric acid plant for acid production.

By producing sulfuric acid from NCG gases, mills that sewer ClO2 by-product or ESP ash will be able to recover the chemicals completely. In addition the sulfuric acid produced can be used in the bleach plant and ClO2 generator, as shown in Table 2. Washing losses are constant and hence the total sodium makeup is the same for both the cases. Sulfur makeup, however, is increased by the amount withdrawn in the NCG. The sulfur intake will be therefore managed by recovering previously purged ClO2 by-product and adding inexpensive merchant saltcake. These increases of ClO2 by-product and merchant saltcake input bring in sodium as well and hence the need of sodium make-up as NaOH will therefore decrease resulting in significant savings.

Sulfuric Acid Production from Non-Condensable Gases

Non-condensable gases (NCG’s) are a byproduct that is collected from several unit operations in the pulp mill. NCG’s contain a variety of toxic and odorous sulfur-containing species that need to be abated to meet environmental regulations. NCG’s contain H2S, CH3SH, CH3SCH3, CH3SSCH3, and other species. NCG’s are typically burned to produce SO2 gas which cannot be vented to the atmosphere due to environmental reasons. The sulfur contained in NCG’s is typically fed back and fired in the recovery boiler, lime kiln or incinerator/scrubber systems.

By diverting the NCG’s from existing process and feeding them into an acid plant, to produce H2SO4 in-situ, the following could be achieved:

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Improved ability to control the Na/S balance.

Savings due to a reduction purchased NaOH (an expensive commodity) that otherwise would be needed to maintain the Na/S balance.

Revenue from H2SO4 product (for internal use or sales).

Revenue from steam production (for internal use or power generation). Steam can also be used for the recovery boiler sootblowing.

The main feed streams to the acid plant are: Ambient air, NCG gas, water, and sulfur. The main product streams to the process are: Sulfuric acid, steam and a clean gas stream to be fed to a stack or back into existing unit operations.

The production of sulfuric acid requires several process steps and considerations, as summarized in References [6-15]. In general, the following process steps must be performed:

Combustion of sulfur-containing species: A furnace is utilized to convert species into SO2. This step prepares the input stream to be fed into a catalytic converter by raising its temperature. It can generate energy in the form of steam, when utilizing a boiler.

H2S combustion

OHSOOSH 2222 2232 ( 0298H = -518 kJ.mol-1)

Sulfur combustion (Optional)

22 SOOS ( 0298H = -297 kJ.mol-1)

Other combustion reactions also take place in the furnace, converting all sulfur-containing species into SO2, H2O and CO2.

Reaction in a catalytic converter: The gas stream is then fed into a catalytic packed-bed reactor to convert sulfur dioxide into sulfur trioxide. The reactor utilizes a commercial high-Vanadium catalyst. Since the reactions taking place are exothermic, external cooling must be performed. The main reaction in the converter is:

322 2

1SOOSO ( 0

298H = -99 kJ.mol-1)

Hydration of sulfur trioxide: An SO3 absorber (typically a packed tower) is used to transform sulfur trioxide into sulfuric acid. The hydration reaction is given by Reaction 4 where the product sulfuric acid is made.

4223 SOHOHSO ( 0298H = -101 kJ.mol-1)

Producing H2SO4 in a consistent and reliable manner from NCG’s has a number of technical challenges that should be carefully observed during the design stages and technology selection. Some considerations include:

Variability of NCG gas conditions: The total amount of sulfur in concentrated NCG’s in a modern chemical pulp mill is typically in the range of 2 – 5 kg S/ADt. The quality of the wood feedstock and process conditions make the amount and concentration of NCG’s vary widely from mill to mill, and over time, and consequently the feed variability needs to be accounted for in the sulfuric acid plant design. It is known that the NCG gas composition and flow can vary significantly depending on the pulping conditions. Figure 2 shows some of the variability considerations.

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Figure 2. Variability of NCG’s conditions depending on pulping conditions [16].

Reliability of acid plant equipment under variable load: It is well known in the sulfuric acid industry that variable loads, if not dealt with correctly, will significantly complicate the plant operation, decrease the overall plant on-stream time and significantly reduce the equipment reliability. Some considerations include:

o Acid plant equipment (such as the catalytic converter) operates at high temperatures (typically 400 to 650°C). Variable loads subject the equipment to thermal cycling and significant thermal expansion, and reduce equipment life.

o Process gas in acid plants is highly corrosive. If the plant load changes, there is risk of corrosion due to unwanted condensation of sulfuric acid in ducting and equipment, which reduces the equipment life.

Periods of low SO2 concentration: The most critical parameter is the concentration of SO2 to the first bed of the acid plant catalytic converter, since it determines the amount of heat released in the oxidation reaction of SO2 to SO3. When the concentration of SO2 is low, there is not enough heat released to maintain adequate operation of the catalyst. When the acid plant converter is operated away from the required temperature, the emissions from the acid plant increase rapidly. If the catalyst beds were to fall below the catalyst ignition temperature, the acid plant would fail to do any SO2 abatement. The acid plant cannot operate correctly if the SO2 concentration in the feed gas falls below either one of the following two limits:

o Acid plant autothermal limit: This is the minimum feed SO2 concentration at which the acid plant can operate without external sources of heat. If the feed concentration is below this limit, the plant catalyst would cool down, the gas emissions would increase and eventually the plant would have to shut down. An alternative solution would involve supplementing heat through the combustion of hydrocarbons. This would increase the operating costs and CO2 emissions of the plant, and therefore is strongly preferred to operate in an autothermal regime.

o Acid plant water balance limit: This is the minimum feed SO2 concentration that the acid plant can treat while still producing strong sulfuric acid which is the least corrosive concentration (typically 93% or 98.5% w/w). If the SO2 concentration is too low, the plant may not produce strong acid, resulting in severe corrosion issues (because sulfuric acid is most corrosive in concentrations below 93%).

Operability of the acid plant: Variable loads require experienced operators to make the acid plant work adequately. This is particularly important during plant upsets and start-up scenarios.

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Economic considerations: The capital and operating costs of an acid plant are largely defined by the total gas volumetric flow. The low concentration of NCG’s require large volumetric flows and large equipment sizes for a fixed production capacity.

RESULTS AND DISCUSSION

Technology for Sulfuric Acid Production from Non-Condensable Gases

NORAM’s technology can be used in different configurations to convert NCG’s into sulfuric acid. Depending on the mill process conditions, one of the configurations may be preferable. The following variables determine the preferred approach:

Sulfuric acid production requirements: To increase the total H2SO4 produced by the acid plant, sulfur supplementation can be used. This is done by burning sulfur together or separate from the NCG’s to increase the net SO2 concentration and to increase the total H2SO4 production rate.

o Burning NCG’s without sulfur supplementation achieves low SO2 concentrations, typically in the range of 4 to 5% (vol/vol). The volume of the gas sets the size of the equipment.

o If sulfur is supplemented, SO2 concentrations of 8 to 12% (vol/vol) can be achieved, allowing for higher acid and steam production rates with a relatively small impact on capital expense. The production rate can be increased in the order of 2.5 times as the throughput of equipment is determined by the volume of the gas. This higher capacity allows the plant to produce acid for internal use as well as for use in other processes and for sales.

SO2 emissions requirement: Depending on the allowable SO2 concentration at the stack, the plant may use a different converter configuration.

o For example, if the pulp mill requires relatively low conversion efficiency, the acid plant may use a 1, 2 or a 3-bed converter configuration.

o Typically, the availability of a downstream sink for the process gas (such a scrubber, lime kiln or recovery boiler) provides more flexibility for the acid plant design. More stringent sulfur capture in the acid plant requires more catalyst beds, and higher catalyst loadings.

Energy recovery requirement: Depending on the value of steam for the mill, the steam equipment configuration may be optimized. Acid plants can produce high-pressure superheated steam if needed. In some cases, it may only be economical to produce medium pressure saturated steam. Some examples include:

o Production of superheated steam at 60 bar(a) @ 480○C.

o Production of saturated steam at 12 bar(a).

o Typical steam to acid ratio in the range of 3.1 ton/ton (tons of steam per ton of acid produced).

Availability of an existing NCG incinerator: If the pulp mill has an existing NCG incinerator, it may be used as part of the acid plant configuration. NCG gas may be burned separately from other sources to minimize the changes to existing equipment and reduce the cost of acid plant equipment. It also allows for continued operation, even when the acid plant is shut-down (e.g. for maintenance).

Sulfuric acid concentration requirements: Dry conversion acid plants typically produce 98.5% (w/w) H2SO4. However, other acid strengths may be accommodated depending on the water balance of the plant. Some options include:

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o Production of 98.5% (w/w) H2SO4 available for sale.

o Production of 93% (w/w) H2SO4 in some areas where acid may freeze in the winter.

o Use of wet conversion acid plants to avoid water balance issues (no drying required).

Figures 3, 4 and 5 provide examples of the implementation of our technology, depending on different plant requirements. These schematics only show the gas-side of the plants. The H2SO4 product is produced in the absorption steps, and exported from the acid plant. The following process steps are carried out:

NCG Burning: gas is fed together with ambient air into a burner where the sulfur containing species are converted into SO2 gas while heat is released. The NCG burning can be carried out with or without sulfur supplementation in one or two burners.

Energy Recovery: Process gas is cooled and heat is recovered as steam. Depending on the site requirements, a steam system is designed to achieve the required steam specification. Energy recovery uses economizers, boilers and superheater (if required).

Quenching, Cooling, and/or Drying: These process steps may be required for part or all the process gas to reduce the amount of water fed into the converter. This is required for dry conversion only.

Blowing: A main blower is typically used to convey the process gas.

Conversion: Process gas is fed into a catalytic converter (typically 1 to 3 beds) where SO2 is converted into SO3.

Absorption or Condensation: Process gas is fed into an absorption tower where the SO3 gas is converted into sulfuric acid product.

Tail gas: Tail gas can be sent to a stack or fed to a scrubber or into other units of the pulp mill.

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Figure 3. Example 1 of NORAM NCG sulfuric acid technology using: dry conversion, 2-bed converter and sulfur supplementation.

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Figure 4. Example 2 of NORAM NCG sulfuric acid technology using: dry conversion, 3-bed converter, sulfur supplementation, and an NCG incinerator.

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Figure 5. Example 3 of NORAM NCG sulfuric acid technology using: wet conversion and a 1-bed converter.

Desired features of a Sulfuric Acid Plant by a Pulp Mill

It is important that the process design maintains ideal process conditions for the acid plant, since it can control the total gas flow and SO2 concentration to the converter. It is not recommended to process the NCG gas by itself due to its low O2 content, low sulfur concentration, process variability and low acid production. Some desired features of a sulfuric acid plant include:

Ability to deal with feed variability: Optimal design which is not affected by feed variability and that can control the process gas composition at the acid plant itself. Hence effectively decoupling the acid plant performance acid production rate and stack gas emissions from the NCG process conditions.

Ability to maintain constant SO2 concentration to acid plant: Figure 6 provides an example of a supplementation strategy to avoid these problems. The design achieves:

o Minimization of thermal cycling: By maintaining a stable SO2 concentration and temperature profile, the thermal cycling is minimized.

o Minimization of risk of corrosion due to acid condensation: The design stabilizes operation and minimizes the risk of corrosion due to unwanted acid condensation.

o Operation above the water-balance limit. The design maintains operation above the water-balance limit producing acid of high concentration and low corrosion.

o Operation above the autothermal limit. The design maintains operation above the aerothermal limit allowing for continuous operation without external fuel/heat input.

Good operability of the acid plant: The design continuously adjusts the sulfur feed to get a constant SO2 concentration to the converter bed 1, allowing for simpler operation.

No production of by-products or waste: The design should not produce a scrubbing waste that is proportional to the plant rate and inlet concentrations of SO2, SO3 and H2SO4.

No consumption of chemicals: The design should not consume scrubbing chemicals in proportion to the plant rate and inlet concentration of SO2, SO3 and H2SO4. Examples of these chemicals are caustic, ammonia, hydrogen peroxide, milk of lime, limestone, etc.

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Figure 6. Example of SO2 supplementation to deal with variable loads scenarios (this is a typical example, actual operation may require a larger ratio of supplementation to feed rate).

CONCLUSIONS

The proposed configuration of a sulfuric acid plant has the following potential benefits and features:

Improved ability to control the Na/S balance.

No odors and improvement of environmental performance.

Potential savings due to a reduction purchased NaOH (an expensive commodity) that otherwise would be needed to maintain the Na/S balance.

Revenue from H2SO4 product (for internal use or sale).

Revenue from steam production (for internal use or power generation). Steam can also be used for the recovery boiler sootblowing.

Ability to deal with feed variability while maintaining high reliability.

Ability to maintain constant SO2 concentration to acid plant to eliminate issues related to thermal cycling, corrosion, water-balance, and autothermicity.

Improved operability of the acid plant.

Significant increase in production of sulfuric acid product and steam, with a moderate increase in capital cost due to the potential use of sulfur supplementation and increased gas strengths.

Proven equipment designs and long track record in the sulfuric acid industry.

No production of byproducts or waste, and no consumption of chemicals.

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REFERENCES

1. Bjorn Warnqvist. “Handling Alkali and Sulfur Balances In Closed-Cycle Bleached Kraft Pulp Mills – Options, technology and Costs, Minimum Effluent Mills Symposium Proceedings 1996.

2. S. E. Johnson and P. Nicolas, Tall Oil recovery with carbondioxide – A cost effective solution to reduce sulfur input to the kraft recovery cycle, Emerging Technologies Conference, March 1997, Orlando, USA.

3. A. Burns and C. Brereton, Electrolytic Salt Splitting for Sulfuric Acid and Caustic Recovery: Can It Be Cost-Effective? , Extraction, 1867 - 1882 (2018)

4. Andrés Mahecha-Botero, Isabel M. C. L. Seco, C. Guy Cooper, Jim T. Wearing, Jon Foan, “Use of Non-Condensable Gases in Pulp Mills to produce Concentrated Sulfuric Acid and Steam” International Forest Pulp and Paper Conference, Aviero, Portugal, October (2018).

5. Wayne Bucher, Dennis Carwile, James Lockhart and Jim T. Wearing, “Commercial recovery of ClO2 generator sesquisulfate by-product using short column resin bed technology” Tappi Engineering Pulping and Environemental Conference, Memphis, Tennessee, October (2009).

6. Louie, D.K. Handbook of sulfuric acid manufacturing. DKL Engineering. Richmond Hill, Canada (2008).

7. Andrés Mahecha-Botero, Kim Nikolaisen, Brad Morrison, C. Guy Cooper, Brian Ferris, Nolan Okrusko. “New Emissions Reduction Process for Side-by-Side Plants”. in Book of Proceedings for Sulfur 2017, CRU British Sulfur: Sulfur, Sulfuric Acid and Sulfur Dioxide. Pages 245-260 (2017).

8. Andrés Mahecha-Botero, C. Guy Cooper, Kim Nikolaisen, Brad Morrison, Brian Ferris, Hongtao Lu, J.P. Sandhu, Victor Lourenco, Igor Aksenov, Tony Mah, Bryan Chow, Lina Li, John Orlando, Davood Faraji, Mo Mohsin. “Sulfuric Acid Plant Modernization Projects: Project Execution Strategy And Use Of Improved Technologies” in Book of Proceedings for Sulfur 2017, CRU British Sulfur: Sulfur, Sulfuric Acid and Sulfur Dioxide. Pages 283-296 (2016).

9. Andrés Mahecha-Botero, Brad Morrison, Brian Ferris, Hongtao Lu, J.P. Sandhu, C. Guy Cooper, Nestor Chan, Grace Juzenas, Tom Hamilton. “Upgrading a sulfuric acid plant: project execution strategy and performance evaluation”. in American Institute of Chemical Engineers (AIChE) Conference, Chapter of the 40th International Phosphate Fertilizer & Sulfuric Acid Technology Conference. (2016). www.aiche-cf.org.

10. Andrés Mahecha-Botero, Kim Nikolaisen, Kyle Loutet, C. Guy Cooper. “SO2 supplementation to stabilize metallurgical plants”. in Sulfur 2015, CRU British Sulfur: Sulfur, Sulfuric Acid and Sulfur Dioxide. Oral presentation in: Toronto, Canada. November 9-12 (2015).

11. Kim Nikolaisen, Andrés Mahecha-Botero, C. Guy Cooper. “Strategies for reducing start-up emissions from sulfuric acid plants”. in American Institute of Chemical Engineers (AIChE) Conference, Chapter of the 39th International Phosphate Fertilizer & Sulfuric Acid Technology Conference (2015). www.aiche-cf.org. Oral presentation in: Clearwater, USA. June 5-6 (2015).

12. Andrés Mahecha-Botero, C. Guy Cooper, Kim Nikolaisen. “The suitability of double-absorption and scrubbing technologies to meet new emission standards”. in X Round Table of Sulfuric Acid Plants (Mesa Redonda de Plantas de Ácido Sulfúrico), pages 251 –266. Oral presentation: Punta Arenas, Chile. November 16-20 (2014).

13. Andrés Mahecha-Botero, Kim Nikolaisen, C. Guy Cooper, Victor Lourenco, Igor Aksenov. “Strong sulfuric acid system upgrades – opportunities in acid plant retrofits”. in Sulfur 2014, CRU British Sulfur: Sulfur, Sulfuric Acid and Sulfur Dioxide, pages 445-457 (2014). Oral presentation in: Paris, France. November 3-6 (2014).

14. Andrés Mahecha-Botero, C. Guy Cooper, Igor Aksenov, Kim Nikolaisen. “Debottlenecking metallurgical and sulfur-burning sulfuric acid plants to increase capacity and reduce emissions”. in Sulfur 2013, CRU British Sulfur: Sulfur, Sulfuric Acid and Sulfur Dioxide, pages 157-174 (2013). Oral presentation in: Miami, USA. November 4-7 (2013).

15. Andrés Mahecha-Botero and C. Guy Cooper. “Clean and efficient sulfuric acid and sulfur dioxide manufacture”. in American Institute of Chemical Engineers (AIChE) Conference, Chapter of the 36th International Phosphate Fertilizer & Sulfuric Acid Technology Conference, (2012). www.aiche-cf.org. Oral presentation in: Clearwater, USA. June 8-9 (2012).

16. S. -H. Yoon, X. -S. Chai, J. Y. Zhu, J. Li and E. W. Malcolm, In-Digester Reduction of Organic Sulfur Compounds in Kraft Pulping, Advances in Environmental Research, 5 (1): 91 -98 (2010)

Gateway to the Future

Use of NCGs to Produce Concentrated Sulfuric Acid and Steam

Rohan Bandekar, MScrbandekar@noram‐eng.com

Slide 1

TT1 Thanh Trung, 9/11/2019

WHY INSTALL A SULFURIC ACID PLANT•Challenges faced by our industry

•More stringent environmental regulations• Stricter energy policies •Drive to improve profitability

•Become more efficient•Optimize chemical recovery cycle•Reduce chemical losses / capture more NCG sources

•All of these affect the Na:S balance

PROCESS CHANGES AFFECT SODIUM:SULFUR BALANCES

• Reduction of air emissions o Reduces sulfur losses

• Burning petroleum coke or fuel oil in the lime kilno Increases sulfur inputs

• Adding lignin plant / NCC plant• Increases sulfur inputs

• Burning of NCG in recovery boilero Reduces sulfur losses

SULFURIC ACID PLANT BENEFITS

• Eliminates the NCG as a sulfur input to the recovery cycle

• Eliminates or reduces purchase of sulfuric acid

•Reduces caustic or soda ash make‐up

•Produces additional steam that will positively impact the mill’s heat balance 

•Reduces transportation risk•Reduces inorganic discharge

Gateway to the Future

Slide 5

INTEGRATED PROCESS DIAGRAM

NCG

Gateway to the Future

CNCG ‐ Overview   Hazardous ‐ Emitted from digester and evaporator areas Hydrogen Sulfide, Methyl Mercaptan, DMS,DMDS 2‐5 Kg of S in NCG/pulp ton

CNCG ‐ Destruction Lime Kiln Power Boiler Recovery Boiler Dedicated Incinerator

CONCENTRATED NCGs

Gateway to the Future

VARIABILITY OF NCG CONDITIONS DEPENDING ON PULPING CONDITIONS

S. H. Yoon, X. ‐S. Chai, J. Y. Zhu, J. Li and E. W. Malcolm. “In‐Digester Reduction of Organic Sulfur Compounds in KraftPulping”. Advances in Environmental Research, 5 (1): 91 ‐98 (2001).

EXAMPLE OF SO2 SUPPLEMENTATION TO DEAL WITH VARIABLE LOADS SCENARIOS

Gateway to the Future NCG Acid Technology

Process• Combustion of Sulfur-Containing Species

• Reaction in Catalytic Converter (High-Vanadium Catalyst)

• Hydration of Sulfur Trioxide

OHSOOSH 2222 2232

22 SOOS

1298 518 molkJH

322 2

1SOOSO

4223 SOHOHSO

1298 297 molkJH

1298 99 molkJH

1298 101 molkJH

NORAM NCG SULFURIC ACID TECHNOLOGY: CONVERSION AND SULFUR SUPPLEMENTATION

NCG Acid TechnologyProcess

• The NCG gas characteristics - total volume to be treated sets theminimum equipment size for the Acid Plant.

• By supplementing sulfur, SO2 concentrations of 8+% (vol/vol) canbe achieved.

• The production rate can be more than doubled for a givenequipment size without significantly changing the total capitalinvestment.

• This higher capacity allows the plant to produce acid for internal useas well as for use in other processes and for sales.

NCG Acid TechnologyAdvantages

• High strength commercial grade sulfuric acid 93/98.5% which reduces corrosion risk and allows use of carbon steel equipment

• Ability to deal with feed variability while maintaining high reliability

• Ability to maintain constant SO2 concentration to acid plantMinimization of thermal cyclingMinimization of risk of corrosion due to acid condensationOperation above the water-balance limitOperation above the autothermal limit

NCG Acid TechnologyAdvantages

• Good operability

• Savings due to a reduction in purchased NaOH

• Revenue from commercial grade H2SO4 product

• Savings from steam production

• Proven equipment designs and long track record in the sulfuric acid industry.

Mill Chemical Balance• Washing losses are constant.

Therefore, the total sodium makeup isthe same before and after the project.

• Sulfur makeup, however, is increasedby the amount withdrawn in the NCG.This allows more free ClO2 byproductand inexpensive merchant saltcake tobe utilized

• These increases of Na2SO4 input bringin sodium as well, thus reducingcaustic demand significantly.

CaseatMillA BaseCase WithH2SO4 Systemchemical,kg/t

Na,kg/t S,kg/t

chemical,kg/t

Na,kg/t S,kg/t

SulfuricAcidProductionsulfurinCNCG’s 3.0 3.0

Purchasedsulfur 3.1

Sulfuricacidproduced 0 18.7

Steamproduced 0 56.1

MakeupClO2

byproduct 7.3 2.4 1.6 10.4 3.4 2.4PurchasedNa2SO4 0.0 10.2 3.3 2.3Caustic 10.0 5.8 0.0 2.5 1.4 0.0Totalsofelements 8.1 1.6 8.1 4.6

ACID TOWER

CONVERTERS

COMPLETE PLANTS

SummaryNo odors and improvement of environmental performance

Improved ability to control the Na/S balance

Savings due to a reduction in purchased NaOH

Revenue from commercial grade H2SO4 product

Savings from steam production

TT4

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TT4 Make these text boldThanh Trung, 9/11/2019

SummaryAbility to maintain constant SO2 concentration to acid plant andhence eliminate issues related to thermal cycling, corrosion, water‐balance and autothermicity.

Ability to deal with feed variability while maintaining highreliability

Proven equipment designs and long track record in the sulfuricacid industry

TT5

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TT5 Make these text boldThanh Trung, 9/11/2019

THANK YOU