Frequently Asked Questions

Looking for answers about an SCR system for your stationary engines? Below, we answer the most common questions about selective catalytic reduction and NOx control. First, we explain how an SCR system works and how much NOx it can remove. Next, we cover data center generators, demand response, emissions permits, and fuels. Finally, we go through lead times, where our systems are built, DEF consumption, maintenance, and service. Can’t find your answer? Contact our team or Request a Consultation.

About AERINOX

AERINOX designs and builds SCR systems and exhaust aftertreatment that help stationary engines meet NOx and other emissions limits. Our work covers data center backup and prime power, peaking plants, gas compression, and combined heat and power (CHP). See our full range of emissions control systems.

AERINOX is a DCL Technology Group company. It’s also a joint venture between DCL International and H+H Engineering & Service. DCL brings 40+ years of emissions control experience, 400 employees, and 10 North American locations. In addition, DCL makes 500,000 liters of catalyst each year. H+H, meanwhile, brings decades of SCR system engineering.

AERINOX is headquartered in Johnstown, Colorado, where we build our control panels and assemble complete systems. Most catalyst housings come from DCL’s facility in Houston, with some from Toronto. When it gives a project better value or faster delivery, we also partner with other large manufacturers.

Together, the AERINOX and DCL team have installed thousands of SCR systems worldwide. For example, recent North American projects range from 24-engine prime power sites to a 42-engine Tier 4 data center backup fleet.

Three things set us apart. First, we size every SCR system to your engine and prove the design in CFD before fabrication. Second, we factory-test every control panel before it ships. Third, the majority of our catalysts come from our parent company, DCL. As a result, we control supply and lead time instead of waiting on a third party.

All of them. We supply owners directly, and we also work through genset OEMs, dealers, distributors, end-users, and EPCs. In addition, we coordinate commissioning with their installation crews.

SCR System Products & Technology

An SCR system uses selective catalytic reduction to remove NOx from engine exhaust. First, it injects a reagent, usually urea (DEF), into the exhaust. Exhaust heat then turns the urea into ammonia. Finally, the ammonia reacts with NOx on the catalyst to form nitrogen and water vapor.

We offer complete SCR systems, plus the emissions control components that go around them:

  • Oxidation catalysts
  • Diesel particulate filters (DPFs)
  • Silencers
  • Urea dosing and control enclosures
  • Turnkey pre-wired exhaust skids
  • Water-tube exhaust heat exchangers and turbine HRSGs
The SCR catalyst reduces nitrogen oxides (NOx). Most air permits cover more than NOx, so we build the rest of the control into the same system. An oxidation catalyst handles carbon monoxide (CO), volatile organic compounds (VOC), and formaldehyde. A diesel particulate filter (DPF) handles particulate matter (PM) on diesel engines. An ammonia slip catalyst keeps unreacted ammonia (NH3) under its limit. Every AERINOX proposal includes a guarantee table, pollutant by pollutant, sized to your permit. On recent natural gas projects, that’s meant 93 to 98% NOx reduction, up to 98% on CO and formaldehyde, and ammonia slip at or below 10 ppmvd at 15% O2.

An oxidation catalyst converts carbon monoxide (CO) and hydrocarbons into CO2 and water vapor. It also cuts VOCs and formaldehyde, which often appear as separate permit limits.

A diesel particulate filter (DPF) captures soot from diesel exhaust to meet particulate matter (PM) limits. You typically need one for diesel generator sets subject to EPA Tier 4 or state PM limits. Natural gas engines, however, generally do not require one.

The SCR chemistry is the same for both fuels. However, the equipment around it and the operating conditions differ:

  • Particulate: Diesel exhaust carries soot, so diesel systems add a DPF or a combined SCR-on-filter (SCRonF). Gas engines don’t need particulate filtration.
  • CO, VOC, and formaldehyde: Gas systems place an oxidation catalyst ahead of the SCR, since these are common gas engine permit limits. On diesel, the DPF stage also oxidizes CO and hydrocarbons.
  • NOx load: Diesel engine-out NOx is typically much higher. As a result, diesel systems use more reagent, larger tanks, and more catalyst.
  • Duty cycle: Diesel backup gensets start cold and often run at light load. So we size for low exhaust temperature and insulate the exhaust. By comparison, gas prime power runs at a steadier load, which makes high reduction easier to hold.
  • Engine type: SCR is for lean-burn gas engines. Rich-burn gas engines typically use a three-way (NSCR) catalyst instead.

Both designs can include an ammonia slip catalyst after the SCR to keep NH3 slip under permit limits.

Our standard SCR system design delivers up to 98% NOx reduction. When a permit calls for more, we design for 99%+ reduction and outlet NOx below 2.5 ppm at 15% O2.

We run computational fluid dynamics (CFD) in-house on every custom geometry. With CFD, we check mixing and ammonia distribution at the catalyst face. We also predict NOx conversion before fabrication starts.

We typically design every SCR system for under 10 in. w.c. of back pressure. Our quoted ceiling of 16 in. w.c. carries a 20% margin, so installed systems typically land under 13 in. w.c. If there is a back pressure concern, we can design the system to account for it. 

With a single ammonia slip catalyst layer, we keep slip well under 5 ppm (corrected to 15% O2). That meets the 5 ppm limits we expect to see in new rules.

Urea dosing starts once the catalyst inlet reaches 450°F, and we prefer 500°F at startup. Below that, urea doesn’t fully convert to ammonia and can leave deposits. DCL SCR catalyst reaches full design conversion at around 660°F at the inlet.

When exhaust runs hotter than the design window, we blend in tempering air to protect the catalyst. In addition, insulation blankets ship standard to hold temperature at light load.

A diesel SCR system requires ultra-low sulfur diesel (ULSD) with under 15 ppm sulfur, because sulfur poisons SCR and DPF catalysts. For gas engines running field gas or biogas, please send a fuel analysis with your RFQ.

The control panel reads NOx before and after the SCR, along with engine run and load signals. It then meters reagent to hold your outlet NOx target. We offer four panels, and the right one depends on how low your NOx limit is:

  • NOxLess Panel (emergency backup only): a single panel with NOx sensors before and after the SCR. It controls NOx down to 30 ppm.
  • Mini Panel (continuous duty): a single panel with NOx sensors before and after the SCR. It controls NOx down to 20 ppm.
  • Analyzer Panel with IR analyzer: a pre-SCR NOx sensor, a post-SCR IR analyzer and a separate dosing panel. It controls NOx down to 6 ppm.
  • Analyzer Panel with chem cell analyzer: a pre-SCR NOx sensor, a post-SCR chem cell analyzer and a separate dosing panel. It controls NOx down to 1.5 ppm.

Your quote recommends the right option based on your permit limit and duty cycle.

SCR System Questions for Data Centers

Large generator fleets can push a site over permit thresholds, even when each engine is Tier 2 or Tier 4 certified. Aftertreatment such as an SCR system lowers emissions from each engine. As a result, you can run more engines for more hours and still stay within your permit. Learn more about our data center emissions solutions.

Yes. Monthly exercise and load-bank testing are easy on the catalyst. However, the harder part is cold exhaust at light load. So we size for your actual load profile and ship 1-inch insulation blankets standard to hold temperature.

Depending on engine load and ambient conditions, the SCR system can be on and at temperature within 15 minutes to 1 hour.

We recommend running the engine periodically above regen temperature for at least an hour every few start-and-stop cycles.

Yes. We supply SCR and oxidation catalyst skids for natural gas prime power. For example, recent projects include large, multi-engine sites in North Carolina and Ohio. Read more about distributed power generation.

The SCR system needs no special layup. Pumps cycle to keep reagent in solution, and lines air-purge at every shutdown. We also set freeze protection for your climate.

Yes. Our open controls architecture maps to the site BMS/DCIM and exchanges status with genset controls over Modbus TCP. We also test that integration at the factory.

Yes. We engineer the housing and stands to your space envelope, including roof-mounted skids. Send us a generator STEP file, and we’ll confirm access-panel and door clearances during design.

Demand Response for Emergency Generators

Utilities and grid operators pay large power users to take load off the grid when it’s stressed. A facility with on-site generators starts its gensets, drops grid demand, and gets paid for the capacity it commits and the energy it delivers. However, program rules and payments vary by utility and regional grid operator.

On a limited basis. EPA allows emergency engines up to 50 hours a year of non-emergency operation under a financial arrangement. Also, those hours count inside the 100-hour annual allowance for maintenance and testing. The program must also meet 5 federal criteria. For example, the local balancing authority or transmission operator must dispatch the engine to prevent a local outage. Peak shaving and most economic DR programs don’t qualify.

EPA is currently reviewing these provisions, so confirm the current rules with your air consultant before you enroll.

The engine loses its emergency status and must meet non-emergency requirements. For most large diesel gensets, that means Tier 4 Final emission levels, performance testing and a revised air permit. After that, plan to treat the engine as non-emergency for good.

The engine keeps its Tier 2 certification. However, AERINOX adds aftertreatment that brings the genset’s exhaust to Tier 4 Final emission levels. Stack testing confirms those levels, and your air permit documents them. In fact, air agencies have permitted this approach for Tier 2 standby gensets.

A typical retrofit combines three technologies:

  • Selective catalytic reduction (SCR): cuts nitrogen oxides (NOx) using a reagent (typically urea).
  • Oxidation catalyst (Oxicat): cuts carbon monoxide and hydrocarbons.
  • Diesel particulate filter (DPF): captures soot.

For gensets above 900 kW, Tier 4 Final takes NOx from Tier 2’s 6.4 g/kW-hr (NOx + NMHC) to 0.67 g/kW-hr. AERINOX will design the system to meet your permit limits. 

Yes. SCR needs exhaust heat to work, and DR dispatch runs gensets at meaningful load. As a result, that load gives the catalyst the temperature it needs. We also size the system and dosing controls around your engine’s load profile, including low-load testing runs.

Tier standards apply to diesel engines. However, gas engines fall under separate EPA rules, and CHP units usually already operate as non-emergency engines. AERINOX SCR and Oxicat systems help gas engines meet tighter state and local NOx and CO limits.

First, AERINOX reviews your engine data sheet and site layout and sizes the system. Next, we support your air consultant with the permit application. Then we fabricate, assist with installation, and commission the SCR system, and we support the compliance stack test. Start with a consultation.

Emissions Regulations & Permit Compliance

Our systems have met permit conditions under EPA Tier 4, CARB, SCAQMD, Bay Area AQMD and Pennsylvania RACT, among many other federal, state and local programs. They are also ready for the EPA Good Neighbor rule. Covered pollutants include NOx, CO, PM, VOC, and formaldehyde.

The air permit is often the longest-lead item on a power project. Getting your engines to permittable emission levels early helps keep construction on schedule. It also avoids friction with the local community.

Yes. Our control panels and enclosures carry UL/cUL 508A listing. Exported systems carry CE marking, and all welding follows documented ASME/AWS procedures.

Yes, as a quoted option. A CARB-verified product lets you avoid separate PM source testing.

Yes. EPA test ports come standard at 3 to 4 in. NPT. We set them 90 degrees apart to satisfy testing agencies.

Our housings usually ship with room for a spare catalyst layer, typically 3 active layers in a 4-layer housing. So if limits tighten, you add a layer of catalyst instead of replacing the housing.

Yes. We map your jurisdiction’s limits to the right technology and quantify fleet emissions for your permit application. That includes unit conversion. For example, analyzers report NOx in ppm, while permits often set limits in g/bhp-hr or in ppm corrected to 15% O2. We use EPA Method 19 to put test data, guarantees and permit limits on the same basis. However, your permitting consultant or counsel makes the final regulatory call.

Yes. We typically guarantee stack emissions for a defined period, such as a set number of operating hours, specified months after the initial performance test, or a set time after delivery, whichever comes first. Terms depend on operating conditions, engine data and maintenance, and each quote states them. Compliance testing is the owner’s responsibility. However, AERINOX support during testing is available as an option.

Engines, Fuels, and Applications

We cover reciprocating engines from 50 kW to 3,000 kW+ on natural gas, diesel, biofuel and hydrogen blends. We also cover gas turbines.

Our SCR system and catalyst projects span:

Caterpillar, Cummins, Guascor, Rehlko, Waukesha, Siemens, Clark, Cooper Bessemer, and GE turbines, among others. We size each SCR system to the specific engine model from its data sheet.

Yes. When required, we offer a full Class I Div 2 package. For example, it can include Rosemount instrumentation and Pepperl+Fuchs isolators. We work with you to design the package to your specifications and C1D2 compliance requirements. Please send the area classification with your RFQ, since enclosures are larger.

Usually not. With insulation blankets, 30 to 50 ft runs stay above catalyst light-off temperature. Run length only starts to matter when you need more than about 90% NOx reduction.

Yes. Retrofits take more planning than new builds, because the space, structure, and exhaust weren’t designed for aftertreatment. We engineer the housing to the existing envelope and keep back pressure inside the engine OEM’s limit.

Quoting, Lead Times, and Installation

We need three things: the engine data sheet, your emissions target or permit limits, and your site constraints. In addition, STEP files and a satellite image with your preferred equipment location marked help us quote faster. Ready to start? Request an SCR system consultation.

First, our sales and application engineers size the SCR system and confirm scope. Then we send a formal quote with price, lead time and terms, valid for 30 days. Once you order, we acknowledge your purchase order within 1 to 2 business days.

For systems using DCL Metalcor® catalysts, typical lead times are:

  • Standard single-engine SCR systems: 14 to 16 weeks
  • Engineered turnkey skids: 16 to 24 weeks
  • Engineered Tier 4 combination housings: 16 to 28 weeks
  • Spare parts and sensors: 2 to 4 weeks

Lead times are typical and subject to change and begin after engineering approval. Your quote then confirms the lead time for your project.

The clock starts at PO acknowledgment, receipt of approved drawings, and receipt of any payment due at order under the payment terms in your quote. Design, drawing submittal, and approval typically take 8 to 10 weeks.

Yes. Every control panel and dosing skid goes through a written factory acceptance test (FAT). The FAT covers:

  • Point-to-point wiring checks
  • Staged energization
  • Witnessed I/O
  • Fault injection
  • Wet testing

You receive signed records, including a Modbus TCP integration test against your controller.

Plan to provide two utilities:

  • Instrument air: 100 psi, regulated to 65 psi, at 5 to 10 CFM. If this isn’t available, we can supply our own source of air. 
  • Power: 120, 208, or 480 VAC, about 600 W

At low reagent volumes, an electronic injector removes the need for compressed air.

Very little. The field connections are two interconnect points plus a load and run signal from the engine. Power distribution, tank heaters, pumps, and internal wiring all stay inside the package. Pre-fabricated harnesses and optional preloaded catalyst also cut field labor.

Yes. Nearly all of our stands carry a PE stamp for seismic and wind loads.

Manufacturing: Where Is Your SCR System Built?

We assemble and test AERINOX systems at our facility in Johnstown, Colorado. Large one-box systems ship directly from our Colorado fabrication partner, while we build and ship their controls from Johnstown.

AERINOX control systems are engineered, assembled, and tested in the USA. Major components come from North American manufacturing, primarily in the US, with some from Canada.

AERINOX builds and tests every control panel in-house. A Colorado manufacturer fabricates the skids and ships them to Johnstown. There, we assemble them with the control panels, housings, and catalyst into a complete SCR system.

DCL’s Houston facility fabricates the majority, and DCL in Toronto supplies the rest. If your project requires a specific origin, we can specify it.

A Colorado supplier fabricates AERINOX one-box systems. However, we still build and test their controls in-house in Johnstown.

DCL’s facility in Toronto manufactures most of our catalysts, using precious metals sourced from the US.

It means shorter lead times, less exposure to overseas shipping risk, and direct access to the engineers who built your SCR system. For data center projects on tight schedules, that matters.

SCR System Operation, Maintenance, and Service

An SCR system can use aqueous urea (DEF), aqueous ammonia or anhydrous ammonia. Urea is the default: a 32.5% aqueous urea solution meeting ISO 22241-1, delivered in bulk by an industrial supplier. Aqueous ammonia can suit sites with very long idle periods or systems with low exhaust temperatures.

No. Instead, buy urea solution in bulk from an industrial supplier, delivered by tanker or tote into your tank. The concentration is the same as retail DEF. However, jugs and open handling bring contamination that can poison the catalyst, and retail volumes cost far more. In addition, multiple bulk suppliers serve most US markets. For aqueous ammonia, go through an industrial gas supplier.

It depends on how much NOx you need to remove. Each pound of NOx removed takes about 2 lbs of DEF (32.5% urea). In other words, 1 gallon of DEF removes roughly 4.5 lbs of NOx.

We typically size tanks for 48 to 72 hours of runtime, from 21-gallon day tanks to 20,000-gallon bulk tanks. We also confirm exact consumption in every quote.

Use these four steps:

  1. Take engine-out NOx in g/bhp-hr from the engine data sheet.
  2. Multiply by engine bhp and your required reduction (e.g., 0.90). Then divide by 454 to get lb/hr of NOx removed.
  3. Multiply by 2 for lb/hr of DEF, then divide by 9.1 lb/gal for gal/hr. That’s the theoretical minimum. Quoted design rates carry margin and run higher, so size tanks and supply from the quote.
  4. Multiply gal/hr by the runtime you want between fills to size the tank.

Example: A 2 MW diesel genset (about 2,800 bhp) at 4.5 g/bhp-hr engine-out NOx and 90% reduction removes about 25 lbs/hr of NOx. That takes about 50 lbs/hr, or 5.5 gal/hr, of DEF before margin. By comparison, a natural gas 3508-class engine uses about 0.15 gal/hr, because its engine-out NOx is far lower.

We oversize pumps 2 to 3x, and they return excess flow to the tank for agitation. Lines also air-purge at shutdown, and air cools the injection lance. Finally, we specify tank heaters for your climate, since DEF freezes near 12°F.

The SCR system alarms to warn you that the tank needs refilling. If the tank reaches an empty level, the pumps shut down safely and the SCR system turns off.

We typically size catalysts for 8,760 operating hours per year for prime or peaking power, then add a design margin. For backup generators, we typically size them for 500 operating hours per year. However, actual life depends on the application, runtime, and avoiding poisons and contaminants in the exhaust.

It’s a one-person job, and you don’t need a lift or crane. AERINOX systems typically use DCL catalyst, made in North America by our parent company. DCL elements weigh about 18 to 20 lbs and sit in gasketless metal trays, so you replace only the failed trays. Because the elements share a standard size, one spare set covers every engine on the site. Learn more about our catalyst inspection and replacement.

Yes. Our services include on-site and remote commissioning, operator training, and scheduled service at 3-month, 6-month, annual and 2-year intervals. We also offer service agreements, plus spare parts and catalyst supply. Learn more about our commissioning and on-site training.

Yes. We can see live system data, and we fix 9 out of 10 field issues remotely without sending a tech to site. Your team can also get the same dashboard view. Where firewalls block network access, a self-contained cellular gateway connects the SCR system without touching your network. 

Learn more about our online emissions monitoring.

The SCR system logs data for your reference. However, we don’t intend it to serve as your compliance record.

Still Have Questions About Your SCR System?

Every site is different, so the best answers start with your engine data sheet and permit limits. Send them to our application engineers, and we’ll recommend the right SCR system for your engines. Request an SCR system consultation or contact our team.

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