October 11, 2007
Catalyst Performance was the Hot Topic Yesterday
Lots of research and effort is resulting in improved performance for catalysts. Mark Ehrnschwender of STEAG, Stephen Guglielimo of Hitachi, Flemming Hansen of Haldor Topsoe, and John Cochran of CERAM were presenters during the Hot Topic Hour on October 11. Here is how they see some of the important issues.
Cofiring of biomass - Is there a problem with the catalyst?
Haldor Topsoe has experience with units co-firing both low and high percentages of biomass. Alkali build up can be a problem with high percentages. Also particulate fouling has to be avoided through proper sizing of the biomass and efficient combustion.
Year round operation vs. ozone season
Flemming Hansen recommends soot blowing rather than sonic horns for cleaning if the plant is operating year round. He says diligent flow modeling is required for high availability.
SO2 to SO3
Variables such as coal type and temperature affect the performance of a specific catalyst in terms of conversion of SO2 to SO3. CERAM has ten U.S. high dust references operating with a guarantee of less than 0.5 percent. These range from 3.5 to 5.5 percent sulfur. Design temperatures range from 640-800°F. Hitachi at a large coal-fired power plant is achieving less than 0.1 percent conversion at 700°F with three layers while achieving 90 percent NOx removal and limiting NH3 slip to less than 2 ppm.
Mercury
The ability of the catalyst to oxidize mercury is not only a function of the catalyst design but also the halogen content of the flue gas being treated. CERAM is testing at three major utilities. They say the oxidation is a function of the halogen content, temperature, and ammonia distribution. Flemming adds space velocity to this list. He also presented a chart showing that as the vanadium content increases so does the Hg oxidation.
The new Hitachi catalyst combines low SO2 to SO3 conversion with high mercury oxidation. A pilot unit operated at Pleasant Prairie even with low HCl concentration was able to achieve 80 percent mercury oxidation. A full layer of mercury oxidizing SCR catalyst will be installed in the Unit 2 SCR at Pleasant Prairie in 2008.
Efficiency
Pressure is being placed through the permit process to increase SCR efficiency. How much more would it cost to move to levels higher than 95 percent? John Cochran tackled this issue by pointing out that catalyst is not the limiting factor. It is difficult to achieve high efficiency with lower NOx inlet values. Limiting factors are allowable ammonia slip, ammonia grid tuning limitations and control system variability.
Catalyst life and cleanliness
Catalyst life and cleanliness has not met expectations. As a result, in situ and ex situ cleaning has become common place. How does the renovated catalyst compare to the original? Mark Ehrnschwender of STEAG says that full mechanical strength can be retained by avoiding unwanted removal of critical compounds and controlled recalcination after re-impregnation of catalytically active substances.
How can the life of the original catalyst be extended? Flemming says the deactivation is due mainly to particulate fouling and not poisoning by arsenic. CERAM says that with adequate mechanical design it is possible to regenerate catalyst up to five times. John advises that original catalyst life can be extended by monitoring and minimizing the big five: deactivation, plugging, erosion, sintering, and mechanical damage.
Hitachi is regenerating catalyst commercially in Europe and Asia. The company uses a mobile cleaning and regeneration unit. Costs are lower than with off-site regeneration.
Delivery problems or is there enough capacity?
Hitachi recognizes the problem and is planning to add more capacity. CERAM and Haldor Topsoe say their plants are very busy but that they are confident they can handle increased activity.
Based on this discussion the structure of the NOx Decision Tree may be altered slightly to reflect goals, options, and process variables as listed below.
Goals
Efficiency now
Efficiency later (CO2 ready)
Operation time (year round or ozone)
SO3 oxidation minimization
Hg oxidation maximization
Pressure drop minimization
Maximum interval prior to cleaning
Minimize deactivation
Ammonia slip minimization
Catalyst options
Type-honeycomb or plate
Pitch
Percentage of vanadium and other active ingredients
Hg conversion
Number of layers
Process variables
Coal type
Supplemental fuels
Boiler type
Boiler cycling
Boiler operation
Temperature
Uniformity of gas flow
To view the individual power point presentations which are now incorporated in the NOx Decision Tree utilize the links provided below.
John Cochran - CERAM
Catalyst Performance on NOx, SO3 and Mercury - Hot Topic Hour October 11, 2007
Fleming Hansen – Haldor Topsoe
Topsoe SCR Experience - Fleming Hansen - Hot Topic Hour October 11, 2007
Steve Guglielmo – Hitachi
Catalyst Performance on NOx, SO3, Mercury - Hot Topic Hour October 11, 2007
Mark Ehrnschwender – Steag LLC
Catalyst Performance on NOx, SO3, Mercury – Hot Topic Hour October 11, 2007