A little over a year ago I blogged about how boiler optimization can help power producers better manage Selective Catalytic Reduction (SCR) systems. Today, CAIR is in full effect, and we’re seeing more plants grappling with reducing NOx while simultaneously managing the financial and operational side effects of SCR systems. At NeuCo, we’re answering more customer questions about boiler optimization and SCRs. So I thought a blog post that answered some of the most common SCR-related questions would come in handy.

Q. If we’re reducing NOx with an SCR or SNCR, why do we need boiler optimization?
Using boiler optimization to reduce NOx can provide many additional and SCR-related benefits, including less ammonia use,  better catalyst management, improved plant availability, and sophisticated balancing of the complex chemical, thermodynamic and emissions tradeoffs.

Lowering boiler NOx through optimization reduces ammonia consumption by an equivalent percentage. And by reducing NOx at the source, combustion optimization lowers the NOx entering the SCR system by 10-20 percent – meaning fewer reagents and emissions passing through the SCR. Boiler optimization can also help to avoid ammonia slip and extend effective catalyst life.

Q. Isn’t the complexity associated with an SCR such that I should focus boiler optimization on units without post-combustion technology?
A. NOx regulations have driven extensive investments in unit hardware modifications. Each addition moves the generating unit farther from its original design state, and creates tradeoffs with efficiency and operational tolerance and complexity.

Unlike hardware, optimization can be used to balance complex systems where dozens of variables can impact an objective or a constraint.  Optimization technology based on adaptive neural networks, model predictive control, and condition-based expert rules is ideally suited to the complexity, the system interactions, the departure from design conditions and the difficult tradeoffs inherent to post-combustion emissions control.

Q. Is there a payback associated with reducing NH3 through boiler optimization?
A. Removing one ton of NOx from the furnace exit gas requires approximately $350 worth of ammonia.  For a base-loaded 600MW coal-burning unit generating 6,500 tons of NOx per year, ammonia costs can run about $2M per year. Reducing boiler NOx through optimization reduces ammonia consumption by an equivalent percentage.  A 15 percent reduction in boiler NOx and thus ammonia in the unit described above is worth over $350,000 year in variable operating dollars.  In the case of plants using AOD, the value of a 15 percent reduction would be even higher, due to the increased cost of generating ammonia. 
 
Q. Can boiler optimization impact catalyst costs?
A. Annually, catalyst costs are roughly the same as NH3, with current pricing at about $400/ft3.  Since the volume of catalyst required in a high-dust SCR can easily be 25,000 ft3, the initial charge can run $10M.  Assuming one-third of the catalyst volume is changed every three years, catalyst change out costs can easily run $1M per year. 

Reducing boiler NOx also directly impacts catalyst longevity.  Typical catalyst management practice employs ammonia slip levels as the metric for catalyst change out.  A fresh charge of catalyst is most effective at removing NOx from the furnace gas.  As catalyst poisoning and masking occur, the amount of exposed active catalyst drops and the net ability to remove NOx degrades.  If less NOx is removed, less ammonia will be consumed.  While hundreds of ppm of ammonia are added upstream of the SCR and react with NOx, ammonia slip of just 5ppm is the usual catalyst change out constraint. By decreasing boiler NOx, optimization enables a smaller amount of exposed catalyst to meet the NOx reduction goals for which the SCR was designed, thereby extending the catalyst life.  A 15 percent reduction in boiler NOx can delay hitting the ammonia slip limit by 9 months, worth about $200,000/year.

Q. Will emissions market conditions impact the best use of optimization regarding SCR operations?
A. Yes. Instead of extending catalyst longevity, reduced boiler NOx obtained through boiler optimization could be used to drive the stack NOx levels lower than the initial SCR design values call for, thus allowing greater NOx credit attainment. In this scenario, a 15 percent drop in boiler NOx from combustion optimization could manifest itself as an additional net 1.5 percent NOx removal by the SCR beyond its design set point.  Decreased stack NOx with allowance prices at $4,000 per ton for the 600 MW unit would be worth an additional $400,000 per year.

We’ll be talking more about SCRs, optimization, and CAIR at NeuCo’s upcoming Users Summit.