Saturday, 24 December 2016
As heavy consumers of steam and natural gas, ammonia plants are major energy users so any opportunity to reduce the gas bill can provide a critical boost to thin operating margins.
Editor’s note: Material for this article was compiled by the U.S. Department of Energy (DOE) as part of its Save Energy Now program. See the sidebar on p. 48 of this issue.
As heavy consumers of steam and natural gas, ammonia plants are major energy users so any opportunity to reduce the gas bill can provide a critical boost to thin operating margins. With that in mind, Terra Nitrogen Co. took advantage of an opportunity to participate in a DOE “Save Energy Now” plant study at its ammonia and fertilizer plant in Verdigris, OK.
Terra Nitrogen Co. is a major producer of nitrogen fertilizer products with annual revenues of more than $400 million. The company’s facility in Verdigris, OK, is a highly integrated manufacturing site producing 2.2 million tons of urea ammonium nitrate solutions and 1.1 million tons of ammonia per year. Terra Nitrogen also operates shipping terminals in Blair, NE, and Pekin, IL.
Because natural gas is the primary feedstock for its hydrogen, which is combined with nitrogen to make ammonia, the plant requires significant amounts of natural gas for production. Moreover, natural gas is the primary fuel for the plant’s steam systems, which provide critical support to the ammonia production processes. As a result, natural gas costs account for most of the plant’s total expenses, and the Verdigris plant’s management is committed to improving its production and steam system efficiency. Natural gas cost for the Verdigris plant was approximately $7.00/MMBtu during the implementation period, which is the basis for savings calculations.
Terra Nitrogen undertook the study in early 2006 as part of the DOE assessment. Its main objective was to analyze natural gas use in the plant’s steam system and identify opportunities for energy savings. The assessment was performed by DOE energy expert Veerasamy Venkatesan of VGAEC, Inc., and it identified some important opportunities to improve efficiency with resulting reductions in gas use.
Verdigris plant personnel implemented several of the assessment’s recommendations to improve efficiency immediately. They upgraded two turbines, installed a loop dehydrator on an ammonia plant, and repaired failed steam traps and steam leaks. The aggregate annual energy and cost savings resulting from implementing these first-level measures is approximately 497,000 MMBtu and more than $3.5 million. With project costs of around $3.1 million, the plant achieved a simple payback of less than 11 months.
The energy assessment identified additional opportunities that are still being implemented. The assessment results were shared with three of the parent company’s U.S. plants.
Venkatesan is a qualified specialist in the use of DOE’s steam system assessment tool (SSAT) software. In addition to performing the evaluation, he introduced the SSAT software to two plant employees and encouraged them to use it in the future to analyze the steam system. This training enabled them to learn the software and review the data to determine how best they could improve the steam system’s efficiency.
Terra Nitrogen management has consistently encouraged the Verdigris plant’s efforts to improve its energy efficiency. Before the Save Energy Now assessment, plant management and personnel had already installed a loop dehydrator in ammonia plant 1, so they understood the potential for significant natural gas savings from this project.
Assessment analysis performed using the SSAT confirmed the scale of potential energy savings resulting from installing a loop dehydrator and retrofitting the back-pressure turbine with a condensing turbine in ammonia plant 2. When these and other opportunities were uncovered and quantified in the assessment, the plant’s personnel implemented them without hesitation.
After collecting data, the assessment team used the SSAT to analyze it and identify potential energy efficiency opportunities. Each opportunity was evaluated for technical and economic feasibility and grouped into near-, medium-, and long-term projects based on complexity and potential payback.
Recover flash steam from blowdown water—After flashing to a low-pressure header, a substantial amount of blowdown water was being sent to a cooling tower at 50 psig and 300 °F. Routing the blowdown water directly to a deaerator could help generate more than 1,200 lb/hr of flash steam for the plant. Estimated savings: 14,982 MMBtu or $105,000 per year.
Implement a steam trap maintenance program—Although a steam trap audit was not performed during the assessment, the team realized that some steam traps were poorly positioned and some were not even operating. Estimated savings of better trap installation techniques and maintenance: 12,264 MMBtu or $86,000 per year.
Implement a steam leak maintenance program—Although few leaks were found, the assessment recommended performing a leak audit and fixing all visible steam leaks. Estimated energy and cost savings: 876 MMBtu or $6,000 per year.
Modify synthesis loop—The assessment found that the plant 2 synthesis loop was operating inefficiently, requiring large amounts of high-pressure steam. Reversing the circulation in the ammonia condensing loop would improve the ammonia plant’s efficiency and reduce high-pressure steam demand. The assessment estimated that this measure would increase the ammonia plant’s efficiency by 0.4% and lower high-pressure steam demand by approximately 20,000 lb/hr. Estimated energy and cost savings: 0.4 MMBtu per ton of output, yielding natural gas savings of 274,000 MMBtu or $1.9 million per year.
Turbine upgrade—Ammonia plant 2 uses two back-pressure turbines to let down 545 psig steam to 50 psig used for some low-pressure steam applications. The back-pressure turbines powered methyldiethanolamine pumps and were supplemented by hydraulic turbines. The assessment found that excess 50 psig steam was being vented and recommended that the existing turbines be upgraded with more efficient condensing turbines. The recommended condensing turbines could reduce high-pressure steam demand and low-pressure venting. Estimated energy and cost savings: 178,000 MMBtu or $1.2 million per year.
Improve operation of condensing turbines—The vacuum in the surface condensers of the condensing turbines in ammonia plant 1 is maintained at between 24 and 26 in. of Hg, depending on the season. Installing an absorption chiller powered by low-level waste heat to cool the supply-side cooling tower water could increase the vacuum by an additional 0.5 in. of Hg. Estimated energy and cost savings: 170,000 MMBtu or $1.2 million per year.
Build a high-pressure natural gas pipeline—The plant’s local utility delivers natural gas to the Verdigris plant at 185 psig. Because the plant requires natural gas at 550 psig for its processes, it operates steam-driven gas compressors to raise the level. The assessment explored the possibility of building a high-pressure gas pipeline from the plant and connecting it to a high-pressure pipeline owned by the plant’s natural gas utility to eliminate the compressors. Estimated energy and cost savings: 851,000 MMBtu or nearly $6 million per year.
Improve auxiliary boiler efficiency—Efficiency of the auxiliary boiler in ammonia plant 1 could be improved by reducing the stack temperatures from 400 to 320 °F. This could be done by installing an air preheater on the stack to recover some heat. Estimated annual energy and cost savings: 135,000 MMBtu or $945,000.
Verdigris plant personnel implemented two of the most important recommendations in the assessment soon after it was conducted and then began working on several others.
They upgraded the back-pressure turbines with condensing turbines and installed a loop dehydrator on ammonia plant 2. Each of these two measures resulted in annual energy savings of 228,000 MMBtu, for a combined savings of 456,000 MMBtu per year. The annual energy cost savings resulting from implementing the two measures is just under $3.2 million.
In addition, the plant hired a consultant to audit and repair broken or poorly functioning steam traps, and purchased an infrared leak detector to detect and repair steam leaks.
These “low hanging fruit” projects were easy to implement with straightforward solutions and had very short payback periods. Others were not so clear once costs and other trade-off issues were included, but still merited additional thought in hopes of finding creative but practical approaches.
After Verdigris plant personnel carefully reviewed other opportunities uncovered in the assessment, they decided to pursue some additional steps to improve steam system efficiency. They examined the boiler in ammonia plant 1 and found that the boiler’s coils were dirty and one was leaking. Cleaning and repairing the coils could improve process efficiency by an estimated 0.3 MMBtu/ton.
After evaluating the condensing turbines in ammonia plant 1, they decided to defer installing an absorption chiller, and instead do an overhaul of the condensing turbines by changing rotors, cleaning cooling units, and replacing low-pressure steam ejector nozzles during a 2007 plant maintenance shutdown. Other recommended measures either had lengthy paybacks or were too difficult to implement. For example, many difficult permitting and right of way issues were associated with the high-pressure natural gas pipeline, and it would have required renegotiating the plant’s contract with its natural gas utility.
Independent, alternative evaluations of industrial process systems can confirm many known opportunities to reduce energy consumption, as well as uncover important additional opportunities that help to achieve significant energy cost savings. Employees at Terra Nitrogen’s Verdigris plant were highly knowledgeable about the steam system’s energy use, and they regularly performed steam balance analyses. They also knew from prior experience that efficiency gains could result from upgrading steam system components. However, a system-level analysis using the SSAT quantified these and similar opportunities and uncovered new ones that were highly compelling.This energy assessment’s results persuaded management to carry out many of the recommendations and encouraged performance of more rigorous future analyses. DOE software tools such as the SSAT and AIRMaster+, the Fan System Assessment Tool (FSAT), MotorMaster+, the Process Heating Assessment and Survey Tool (PHAST), the Pumping System Assessment Tool (PSAT), and 3EPlus can all be used to analyze industrial systems and processes and generate energy efficiency opportunities
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