Key guidelines for recovery boiler upgrades.

The Recovery Boiler Air System Optimization "Hot Topics" Breakfast Roundtable at the 2004 TAPPI Fall Technical Conference, chaired by Jack Clement, developed these guidelines for a recovery boiler upgrade.

OPTIMIZATION

First, identify what you want to optimize Clearly define and agree on the objective(s) of a recovery boiler optimization and what is needed to meet the defined objectives.

STEP 1: Determine if the goal is a capacity increase, reduction in emissions, reduced carryover in the furnace, improved reduction, reduced dregs in the green liquor, and/or to extend operational periods. If the objective is to reduce emissions, determine if it is CO, NOx, S[O.sub.2], or particulate.

STEP 2: Establish quantitative goals for the major objectives. Which objectives are secondary? What operating level is needed?

STEP 3: Look beyond the "air system" and optimize the entire fuel preparation and delivery system. Fuel conditions and how black liquor is introduced to the furnace are as important as the entry of air to the furnace.

STEP 4: Establish the current operating baseline.

STEP 5: The data can be used for computer flow modeling (CFD) of baseline operation. Results should reflect the operators' knowledge of existing conditions. For example, if the boiler has a concentration of combustion gases on one side of the furnace, this must be reflected in the model.

AIR SYSTEM

Upgrading the air system to improve operation and/or increase boiler capacity produces several benefits. Modifications will help reduce emissions, carryover, and furnace exit gas temperature. Modeling can provide confidence that modifications to the physical plant will accomplish the objectives.

Today's norm is a minimum of four levels of air introduced to the furnace through relatively large ports on opposite furnace walls. The exception is use of small, closely-spaced primary air ports on four furnace walls. Boilers have different air port arrangements. They range from vertical alignment of ports at the various levels on opposite walls in three to seven columns; to ports in a horizontal array on opposite walls that are vertically offset for each level to induce recirculation; and levels above the liquor guns arranged with one level opposed on the front and rear walls and another level opposed on sidewalls or rear walls. Larger air port openings provide the desired penetration of the mass flow of air into the furnace at a lower static delivery pressure than small ports.

Air above the liquor guns is generally unheated, ambient air. Air heated using turbine extraction steam can be considered if there is strong emphasis on maximizing electrical generation. In this case, the positive or negative effect on recovery furnace operation must be evaluated.

The air system operates optimally when air port openings are maintained at a constant area. Openings that are reduced in area by smelt and char accumulation between roddings can produce a change in combustion. Automatic rodding provides combustion uniformity. Remember that plugage occurs even at the port levels above the liquor guns.

BLACK LIQUOR FUEL SYSTEM

The most significant development affecting black liquor combustion is the ability to evaporate liquor fuel to high dry solids content. A significant reduction in the water from the fuel to be burned can elevate furnace temperature, or final combustion of the liquor can be increased above the liquor guns. This provides flexibility in combustion system design. Some fundamentals are:

* Hardwood liquor burns differently than liquor from softwood pulping

* Unit optimization in a mill burning intermittently hardwood and softwood liquors represents a compromise

* Increasing the liquor solids reduces inflight drying time of the liquor droplets

* Higher solids increase the bed temperature, resulting in increased sodium fume to react with S[O.sub.2]

* Increased combustion higher up in the furnace moves molten deposits on the furnace walls to higher elevations, which can lower the gas temperature leaving the furnace; the heat absorption of the molten deposits is greater than that of the drier deposits

* High solids firing may require setting guns at a lower level.

STARTING POINT 

Clear identification of optimization objectives should be the starting point for any optimization project. Testing should establish a baseline for the present operation from which modifications can be evaluated.

ABOUT THE AUTHORS:

Joe Barsin was responsible for the upgrade and retrofit market while with Babcock and Wilcox and Kvaerner Pulping and is presently president of Technocrats Inc. He can be contacted at [e-mail].

Jack Clement is president of Clement Consulting Inc., specializing in recovery boiler engineering services by applying his 40 years experience in engineering, sales, and marketing with Babcock & Wilcox. Contact him by email at [e-mail].