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Maximizing Recovery Boiler Performance

February 1, 2007  By Pulp & Paper Canada

Upgrades to the combustion air system have helped to greatly enhance the performance of the boiler. Each boiler presents a unique set of operating conditions and limitations. Although they generally h…

Upgrades to the combustion air system have helped to greatly enhance the performance of the boiler. Each boiler presents a unique set of operating conditions and limitations. Although they generally have similar configurations, specific details change from unit to unit. Having an understanding of the current boiler operating conditions, practical limits and having combustion air design experience, are key factors in the success of the upgraded air system performance. In some cases, marginal changes to the existing equipment or to the operation can provide substantial gain in the unit performance.

Upgrade considerations


Traditional combustion air systems

Air system upgrades on recovery boilers have been proven to enhance the overall performance. As the requirements for the boiler throughput increase, careful consideration is needed to determine what changes will be required to help the boiler achieve the desired performance goals.

Traditional air systems have generally provided acceptable operation of the recovery boiler at original design Maximum Continuous Rating (MCR) or at minimal overload conditions. These designs consisted principally of two levels of air: a primary air level introduced at or near the char bed level, operating in a reducing zone environment; and a secondary air level, typically introduced above the level where liquor enters the furnace, in an oxidizing zone. Fundamentally, the principles of black liquor combustion using two levels of air performed adequately on units of capacities up to 1000 tonnes/day dry solids (2.2 million lbs/day).

As the units were pushed to higher firing rates and the demand for more combustion air to maintain high combustion temperatures and more turbulence increased, lower reduction efficiencies and increased mechanical carryover resulted. Emissions became more difficult to control and the effects of plugging in the convective zones limited the firing capacity of the boiler. The recovery boiler became the bottleneck in the mill and the demand for increased black liquor firing capacity for the existing boiler became the primary focus.

Boiler upgrade limits

Operating limits experienced on units with traditional two-level air systems were primarily due to combustion deficiencies, and resulted in increased emissions and fouling in the superheater and boiler bank regions. Many mills have endeavored to increase black liquor firing operation with the implementation of a modern air system design on recovery boilers. With the increase in black liquor firing capacity, further consideration was given to the boiler upper or ultimate limits. These operating limits related directly to the inability to maintain a continuous black liquor firing rate in the recovery furnace.

There are practical operating limits where the boiler performance and integrity can be compromised. They include:

1.Hearth Heat Release Rate (HHRR)

2.Black Liquor “Rain Density”

3.Chemical Ash Deposit Sticky Temperature

4.Combustion Rate (Volumetric Heat Release Rate)

5.Thermal Capacity (Fuel Heat Input)

6.Black Liquor Quality

7.Gas Side Velocities

8.Boiler Circulation and Steaming Rate

One or more operating limits can impede the boiler’s ability to perform continuously at higher loads. Items 1 to 4 are explained in more detail as they are limiting factors that are more commonly encountered.

The term HHRR is the furnace Hearth Heat Release Rate that is an indirect measure of the heat flux and gas velocities in the lower furnace. It is defined as the gross heat input from black liquor dry solids fired in the furnace over the furnace cross-section. Increased heat flux in the lower furnace affects boiler circulation and corrosion rates on the water wall surfaces. Increased gas velocities in the lower furnace can affect the overall combustion performance and particulate entrainment. This leads to increased fouling in the upper furnace and convective zones and can result in potential plugging and eventual downtime. HHRR is used to estimate the capacity of the lower furnace in terms of the ability of the boiler to effectively process the amount of liquor fired in the boiler in relation to the air introduced in the lower furnace. The furnace size, configuration and water wall construction are also determining factors in the recommended HHRR limits.

Recommended: 3.12 – 3.25 x 106 kcal/hr-m2

Maximum HHRR Limit: (1.15 – 1.2 x 106 Btu/hr-ft2)

Black liquor “Rain Density” is the term used to identify the loading of black liquor sprayed into the furnace over the full cross-section. An excessive amount of black liquor displaced in the lower furnace can have adverse effects on the char bed conditions, and contributes to additional carryover and incomplete combustion. Increases in the percentage of black liquor dry solids can effectively reduce the gas-fired black liquor loading or “rain density” in the furnace. Observations of units heavily overloaded on black liquor firing gave indications of a distinct upward trend in TRS emissions when black liquor firing loads exceeded the recommended “rain density” limit. The actual limit will change from unit to unit based on different black liquor conditions, but with tighter emission levels imposed on the mill, there becomes less margin or flexibility in the operation.

Recommended: 1367 kgs/hr-m2

Maximum Black Liquor Rain Density Limit: (280 lbs/hr-ft2)

Chemical Ash Deposit Sticky Temperature in the upper furnace convective regions contributes to the ash conditions whereby build-up of ash on the heat transfer surfaces can occur. This occurs where the gas temperatures operate in a range that affect the melting temperature of a given ash composition. The boiler designer reviews the ash characteristics and predicts the temperature range at which the ash becomes sticky to determine the potential for excessive accumulation on the heating surfaces. Variations in operating conditions and ash characteristics can occur during boiler operation. As such, a design margin is used on predicted gas temperatures in the upper furnace region. If the temperature margins are reduced, the risk increases for ash build-up in areas most prone to this, such as at the boiler bank and superheater. Excessive fouling in the superheater can affect the heat transfer and the ability to maintain steam temperature. The superheater is generally over-surfaced to ensure final steam temperature can be maintained during changes in heat transfer operation. The desuperheater attemperator helps to control the final steam temperature. However, as the superheater surfaces become increasingly fouled, the heating surfaces are less effective and the ability to maintain steam temperature is lost.

Recommended: 28C “Delta T”

Maximum Ash Sticky Temperature Range: (50F “Delta T”)

Combustion Rate is defined as the furnace volume heat release rate. It is the amount of heat input from the black liquor fuel released into the furnace divided by the furnace volume in which the char particles burn out. Typically, recovery boilers have quite a large furnace, so the combustion rates are generally lower than other boiler applications burning fossil fuels. However, some smaller recovery boilers have been overloaded to the point that combustion rates exceed normal recommended limits, thus limiting the ability to complete combustion and control ash fouling in the superheater and boiler bank regions.

Recommended: 133,500 – 178,000 kcal/hr-m3

Maximum Combustion Rate: (15,000 – 20,000 BTU/hr-ft3)

Improved combustion

Experience has shown that improved combustion methods, through the use of modified or enhanced air system designs, have provided appreciable benefits in terms of increased black liquor firing capacity with minimal changes to the boiler. In several cases, air system upgrades alone have enabled the recovery boiler to operate continuously above original design conditions. The value of improved operation with
the enhanced air system resulted in an overall cost benefit to the mill. In several cases, only partial changes to the existing air system were required to achieve the performance goals. The level of required increase in performance or operating capacity dictates the degree of changes or modifications.

The important factor in an air system upgrade is maintaining a fundamental approach to the design so that performance expectations are attained. Each air level is designed to serve a distinct function and additional levels of air should compliment the others to further enhance the operation (see Figure 1).

The standard approach has generally been to upgrade the recovery boiler air system to a modern 3-level air design. Further enhancements, such as a fourth level of air, are usually determined by the limitations of the current air system or by the need for further improvements on NOx and other emissions. The question can be asked, how many levels of air are required; three levels, four levels or even more air levels. This question can usually be answered by evaluating the unit performance expectations or requirements versus the invested cost to achieve the future performance goals. The best technical solution is generally not the most cost-effective approach. The combustion air system upgrade requirements to achieve the performance expectations of the upgraded boiler generally depend on:

* Boiler Upgrade Performance Requirements

* Boiler Configuration and Size

* Existing Air System Configuration

* Current Boiler/Air System Performance

* Boiler and Equipment Operating Limitations

* Economic Constraints

Practical applications

The following are examples of boiler upgrades, with each offering a somewhat different approach while achieving performance expectations. All three cases involved an increase in black liquor firing capacity.


Capacity increase, improved emissions, addition of dilute NCG’s

The improvements to the boiler performance were achieved through the following modifications:

* As a first phase, the traditional 2-level air system was modified to a modern 3-level air system. The modifications involved primarily the addition of a new Secondary Air level along with some minor modifications to the existing air levels.

* Changes to the black liquor firing pattern were made, utilizing a single splashplate nozzle on each wall, for a total of four (4) nozzles.

* As a second phase, a complete, separate new interlaced Quaternary Air level was provided. The HVLC stream was incorporated into the existing Tertiary Air level.

The concentric mixing profile generated at the Tertiary Air level coupled with that of the interlaced Quaternary Air level ensured effective destruction of the NCG compounds in the furnace. This combination also proved to be very effective in reducing combustible and sulfur emissions to minimum levels, and maintaining low carryover rates. Boiler cleanability has been good on this boiler with 12-month continuous operation between shutdowns.


Capacity increase, improved emissions and boiler cleanability

The improvements to the boiler performance were achieved through the following modifications:

* A new partial-interlaced Secondary Air level was added, along with a new interlaced Tertiary Air and Quaternary Air level. Some modifications to the existing primary air level were also made.

* Changes to the black liquor firing pattern were made, utilizing eight (8) splashplate nozzles (2 per wall) instead of ‘full bore’ nozzles.

This case provided evidence of the role that black liquor firing plays in helping to optimize recovery boiler performance at increased loads. Considering the heavy loading of black liquor in the furnace and the current fuel conditions, the 4-level air design provided the necessary combustion air required above the liquor firing level to ensure complete combustion. In this case, it was observed that combustion and carryover rates did improve with higher firing solids (71.5%).


Capacity increase, improved emissions and boiler cleanability

The improvements to the boiler performance were achieved through the following modifications:

* As a first phase, the original 2-level air system was modified to a bi-level primary 2-level air system. Unfortunately, as the unit was being pushed to higher loads, the boiler required water washing every 60 days.

* An additional row of sootblowers was added in the boiler bank region. The unit was then able to run continuously for 90 days between water washing.

* Air system was upgraded to a 4-level design.

In all three cases, the installation of the upgrades provided a cost benefit to the mill in terms of installed cost as well as savings in continuous operating cost.

Dave Burton is the principal technologist at Alstom. For more information, please contact Denise Levesque, product manager, Industrial Boilers, at 613 747-5739 or denise.m.levesque@power.alstom.com. The phone number for Alstom is 613 747-5222.

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