INTRODUCTION
Commercial scale conversion of urea to ammonia has recently been reduced to practice in response to an increased need for ammonia, particularly in NOx reduction applications, and a growing public concern over ammonia transportation, transfer and storage. Utilities and companies under mandate to reduce NOx emissions have investigated the economics of utilizing urea to ammonia conversion resulting in 19 urea-based ammonia systems in operation or being installed as of this writing. The principal focus of this discussion is a comparison of the relative cost of a typical urea-based ammonia system to comparable anhydrous and aqueous ammonia systems.

It is important to note that the costs associated with permitting, evacuation planning, risk management and community relations are difficult to accurately anticipate as these tend to vary considerably by region, community and plant environs. Regarding permitting costs, $400,000 is indicated for an anhydrous ammonia system with $25,000 estimated for either a urea-based system or a 19% aqueous ammonia system. These costs are assumptions based on recent private conversations and should be considered rough estimates only.

	Urea Based Process	Anhydrous Ammonia
NH3 Use Rate	130 lb/hr	130 lb/hr
Storage	15 days	15 days
NH3 Present	NH3 in system <45 pounds Under EPA reportable spill threshold	NH3 storage ~25 tons Most spills reportable to EPA
Evacuation Zone	Risk area for catastrophic spill: Less than 0.0006 square mile	Risk area for catastrophic spill: Approximately 3 square miles

COMPARISON

Risk Assessment Comparison
Safety is the principal advantage of urea-based ammonia systems. Numerous accidents have occurred in the transport and handling of ammonia with news articles about evacuations, injuries and deaths appearing every year. In response to public concern, local authorities have placed restrictions on the transportation and use of ammonia in many locations. Aside from the potential dangers of transportation and transfer, the hazards associated with on-site storage require the planning of risk management strategies. The table above compares the risk issues of a typical urea-based ammonia system to an anhydrous ammonia system for an average SCR application:

Aqueous ammonia, often cited as a safer alternative to anhydrous, can be stored in a vented tank with very little loss due to evaporation at normal temperatures. Although this is true, as an additional precaution against the accidental release of even small amounts of ammonia vapor, some facilities store their aqueous ammonia in sealed pressure vessels either:

1. under ground


2. enclosed in steel or reinforced concrete “secondary spill containments” with solid covers or


3. in double-walled tanks.

While this degree of protection may far exceed the requirements of federal, state, county, or city laws or regulations for aqueous ammonia installations (even in urban areas) plants have consideredit prudent practice to allay local concerns over accidental release.

	Urea Based Ammonia Process	Aqueous Ammonia (19.5%)	Anhydrous Ammonia
SCR Ammonia Reagent	130 lb/h	130 lb/h	130 lb/h
Reagent Usage	230 lb/h	666 lb/h	130 lb/h
DI Water	41 gal/h	None	None
Total Steam (for calculation at $5.00/MMbtu)	0.68 MMbtu/hr	None	None
Electric Power (for calculation at $0.05/kwH)	9 kW	456 kW	36 kW
Capital Cost (based on a "typical" specification)	$750,000	$402,000	$280,000
Estimated Permitting, Evacuation Planning and Risk Management (actual costs are highly dependant on location)	$25,000	$25,000	$400,000 (Based on estimates made by various utilities ranging from $250,000 to $800,000)
Total Initial Cost	$775,000	$427,000	$680,000
Monthly Operating Cost (utilities and reagent)	$17,000	$48,000	$11,000
Ongoing Risk Management	Variable but SMALL	Variable but SMALL	Variable but SIGNIFICANT

Capital and Operating Cost Comparison
The capital and operating costs of anhydrous ammonia storage, vaporization and delivery systems are generally less than those costs for equivalent urea based ammonia systems. 19% aqueous ammonia, on the other hand, is considerably more expensive to operate than a ureabased ammonia system. The combined capital, permitting (at $25,000) and 5-year operating cost for a 130 lb/hr (as NH3), 19% aqueous system would be nearly 40% more than an equivalent urea based system.

The comparison preceeding is based on Wahlco’s typical designs and pricing for ureabased ammonia systems, and both anhydrous and aqueous ammonia systems. Since Wahlco, Inc. (a worldwide supplier of air pollution control systems) provides storage, transfer and ammonia vaporization and flow control skids for both anhydrous and aqueous ammonia, the preceeding table are designed to represent a fair comparisons. The underlying specification for the development of the unit price is based on typical specifications recently received from system buyers. The numbers reflect, as closely as possible, equivalent approaches to equipment quality, redundancy, contingency, controls and instrumentation, dilution air ratio, reagent delivery and storage, climactic conditions and load following requirements.

The system requirements and prices in the table are compared based on Ex Works, including ammonia flow control with dilution air at a production rate of approximately 130 lb/hr NH3.

RELIABILITY AND MAINTENANCE
Reliability and maintenance characteristics available to the author are specific to the Wahlco and Hamon Research-Cottrell’s patented U2A™ urea to ammonia conversion system. Other systems may have similar or different characteristics and care should be taken to study each system under consideration. Although much of the U2A system is made up of standard components, it utilizes some technology which is less mature than that found in ammonia systems but most of the operating urea-based systems have been shown to be equally reliable, with somewhat higher maintenance anticipated. Urea-based systems generally accumulate any dissolved or suspended solids in the converter. Routine cleaning of the converter is necessary in order to maintain efficient operation. The frequency of cleaning is a function of the grade of urea used and the water quality. To extend maintenance intervals, industrial grade urea is normally dissolved in deionized water. Experience with U2A systems indicate that when this combination is used optimal performance can be maintained with an annual cleaning.

CONCLUSIONS

1. Absent any unusually high costs associated with permitting, risk management and mitigation, anhydrous ammonia is the least expensive source of ammonia for SCR and SNCR processes. Savings are realized in capital, operating and maintenance costs.


2. Urea based ammonia systems ELIMINATE the possibility of transportation related ammonia spills and the attendant community reactions and costs.


3. 19% aqueous ammonia is significantly more expensive to operate than a ureabased ammonia system. Based on capital cost + 5 years operating life, 19% aqueous ammonia costs are nearly 40% higher than those of the equivalent urea based system.


4. Urea-based ammonia systems are proving to be safe, reliable and cost effective. Scalable from a few pounds to a few tons per hour, urea-based systems are both simple to implement and to operate. With an ammonia equivalence of approximately 50%, Urea provides lower operating costs when compared to aqueous ammonia and is a desirable alternative to the high risk of anhydrous ammonia.

About the Author
Jeffery E. Fisher is Vice President of NOx Systems at WAHLCO, INC. in Santa Ana, California. Prior to joining Wahlco, Mr. Fisher was President of Biognosis, Marketing Manager for Horiba Instruments, Chairman and President of Bestech Services, Inc., Vice President of Sales and Marketing for Analytichem International, and held senior management positions at Beckman Instruments, Inc. Mr. Fisher served 3 years on the Board of Directors of Independent Laboratory Distributors Association (ILDA). He received his Bachelor of Arts degree in chemistry from Linfield College, Oregon. He can be reached at jfisher@wahlco.com or www.wahlco.com.