April 15, 2007

Focus on O&M  (April 2007)

EMISSIONS CONTROL

Control pollution and slagging on a shoestring

A patented twist on an old technique, flue gas recirculation (FGR), helps prevent slagging in the upper furnace and convective pass. The technology—along with a companion or separate patented technology for furnace sorbent and urea injection for SO₂ and NO_x control—could help owner/operators of smaller, older coal-fired plants comply with emissions limits fairly inexpensively. The process could save the day for the 300 coal-fired units in the U.S. rated between 100 MW and 300 MW that their owners may simply shut down, rather than make the exorbitant capital investment needed to bring them under the latest SO₂ and NO_x emissions caps.

Modeling work and preliminary engineering assessments by Aptech Engineering Services Inc. (www.aptecheng.com), the holder of the patents, suggest that at least 50% and possibly up to 80% reduction of both NO_x and SO₂ can be achieved on a 300-MW unit for around $9 million. That's at least an order of magnitude less than the cost of buying and installing a scrubber and a selective catalytic reduction (SCR) system.

Simplicity is at the heart of the process (Figure 1). Flue gas extracted downstream of the flyash collection device (or air, if a slight heat rate penalty is acceptable) is ducted (recirculated) to the upper levels of the furnace and injected through special venturi-type registers and ports into the main flue gas flow. The point of injection is chosen so the flue gas is cooled to below the ash sintering temperature, preventing or dramatically reducing slagging and fouling in the upper furnace, superheater, and reheater surfaces.

1. New approach. Dotted lines indicate the additions needed to modify a coal-fired unit to use flue gas recirculation (FGR) to reduce slagging and fouling of its boiler. Source: Aptech Engineering Services Inc.

The use of regulating sleeve dampers allows controlled reductions in temperature of up to several hundred degrees Fahrenheit. Water can also be injected to further reduce flue gas temperatures, but in most cases it shouldn't be required, avoiding a heat rate penalty. The main impact on operation will be slightly higher pressure differential and power consumption by the induced- or forced-draft fan.

To control SO₂ and SO₃, an aqueous solution of a suitable sorbent is injected as well. This makes good use of the flue gas residence time in the furnace and keeps the sorbent cool, avoiding sintering and ensuring that the sorbent reaches the critical region so it can react with sulfur. Modeling suggests that 50% sulfur reduction is a "no brainer" and that up to 75% SO₂ removal and close to 100% SO₃ removal are achievable at a calcium-to-sulfur molar ratio of 2.0 or less. Incorporating urea into the process offers the potential of achieving up to 80% NO_x removal. This is possible since the reaction temperature windows for urea and sorbent nearly overlap.

The process can be considered a new generation of technologies that have previously been demonstrated for their respective functions: furnace sorbent injection for SO₂ removal, urea injection for NO_x removal (also called selective noncatalytic reduction), and FGR for controlling superheater and reheater temperatures and achieving more uniform temperature distributions in the boiler. State-of-the-art computer modeling (either cold flow modeling or via computational fluid dynamics) is used to determine the optimum locations for injected recirculated flue gas, sorbent, and reagent.