Sep-2013

Improving reaction furnace performance with the Blasch VectorWall mixing checkerwall

Operators of large refractory lined combustion chambers, whether used as primary reaction furnaces, thermal oxidisers in tail gas units or miscellaneous incinerators, all rely on stable, consistent operation, and the efficiency of the combustion reaction taking place within the vessel to sustain economical operation, or to keep other areas of their plants operating normally.

Jeffrey J Bolebruch
Blasch Precision Ceramics, Inc

Article Summary

Often, however, these pieces of equipment can be subject to variable feeds, periodic downstream upsets and ignition problems; and stable consistent operation alone can be a challenge. Add to that the limitations imposed by reaction kinetics, compounded by the penalties of inefficient combustion, and the result easily leads to very costly consequences.

Frequently, some type of structure will be placed in the furnace in an attempt to influence process efficiency by creating specific conditions in certain parts of the furnace, or some type of mixing downstream. The results, however, are generally short of optimal, both mechanically, and in terms of process impact.

The business challenge is to develop a checkerwall-type product that would prove to be mechanically stable under all imaginable conditions and would include some mechanism that would allow for a customised mixing solution based on the specifics of the customer’s process, and to do so in an easy-to-install, cost-effective design.

Description of the solution Improving mechanical stability
Address stability issues with a design that would be refractory friendly: stable within the plane of the wall during times of high-temperature excursions and thermal cycling, and resistant to lateral movement axial to the reactor during times of upset. As is apparent from the photos below, stability of these large, partially open walls can be problematic. The effects of high-temperature creep and delayed ignition can each negatively impact these walls in two very different, but equally catastrophic, ways.

High-temperature creep
Creep is defined as follows: “The tendency of a solid material to move slowly or deform permanently under the influence of stresses. It occurs as a result of long-term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near melting point.”¹

Not being able to greatly affect the rate of softening of the material, the operating temperature or the length of exposure, we are left to influence the applied structural load. Design input provides the mechanical solution.

Flat spans
Flat spans that undergo creep are put into tension as they soften and gravity pulls them downward. They sag and the bottom surface bows outward. As these unsupported spans deform they destabilise the points of contact at either end, damaging the mortar bed holding the brick in place and destabilising the entire wall. Brick checkerwalls and Matrix Walls are constructed of flat spans.

Arches
An arch is a structure that spans a space and supports a load. The arch is significant because it provides a structure that eliminates tensile stresses in spanning a great amount of open space. All the forces are resolved into compressive stresses. This is useful because several of the available building materials such as stone, cast iron and concrete can strongly resist compression but are very weak when tension, shear or torsional stress is applied to them.⁴

Delayed ignition
A delayed ignition in a reaction furnace occurs much the same way as it does in your gas grill and with the same result: a large “woof” and a resulting pressure wave that impacts the wall. This force can easily blow structurally questionable walls over. Stacked cylinder walls, for example, are loosely stacked in the furnace and quite susceptible to this. The Blasch VectorWall mixing checkerwalls are formed with a series of tabs and slots on each of the six external surfaces that mechanically engage to resist lateral movement. The Blasch walls have demonstrated 100% structural reliability with no collapses in several operating installations.

Increasing process efficiency
To address efficiency issues in terms of combustion efficiency/reaction kinetics and to develop a geometric component that could be used in conjunction with the existing block design that would impact flow as it passed through the wall and induce mixing downstream of the wall in some manner consistent with these principles.

Combustion efficiency is often defined in terms of the three Ts: Time, Temperature and Turbulence.

Time refers to the optimal amount of time that the reactants need together in order to completely react. This is not necessarily the amount of time in the furnace, but the amount of time properly mixed so that the required reactions can proceed.

Temperature refers to the optimal temperature to drive the reaction taking place, and the assumption is that this temperature will exist consistently all throughout the furnace. Turbulence refers to the mixing that must occur to put the reactants in intimate contact and when and how completely it occurs.

In the Claus sulphur recovery process, where our initial experiences lay, existing reaction furnace technology approaches mixing from two different philosophical directions. The use of checkerwalls is only one of them. The other is a feature called a choke ring and that is a device that reduces the ID of the furnace at a specific point between the burner and the outlet. The idea is that the choke ring reflects back part of the flow toward the burner and creates a bit of back mixing. Back mixing is defined as the tendency of reacted chemicals to intermingle with unreacted feed in reactors, such as stirred tanks, packed towers and baffled tanks. This raises another mixing related issue. What is the optimal mixing scheme in this or other types of reaction furnaces, and what is the mechanism for determining it?