DOC Catalytic Converter

How Industrial DOC Catalytic Converters Clean Up Generator Exhaust – What We've Learned from Real Installations

I've been to a lot of generator sites. Backup units for hospitals, prime power for factories, continuous run for telecom towers. One thing they all have in common – the engine runs steady. No stop‑and‑go. No idle for twenty minutes then full load. Just constant RPM, constant load, sometimes for days or weeks at a time.

You'd think that would be easier on a diesel oxidation catalyst than a truck engine. In some ways, it is. In other ways, it's harder.

We've supplied industrial DOC catalytic converters for dozens of generator installations. Some worked great. Some had problems we had to solve. Here's what we learned.

How a Generator DOC Is Different

A truck DOC has to handle thermal cycling – hot, cold, hot, cold. A generator DOC gets up to temperature and stays there. That's easier on the substrate. Less thermal stress, less cracking risk.

But a generator also runs for long stretches. A truck might run 8 hours a day. A prime power generator might run 24/7 for weeks. That means the diesel oxidation catalyst sees more total operating hours in a shorter calendar time.

And the flow rate is steady. No surges. So the DOC can be optimized for a narrow range of exhaust flow and temperature. That's actually good – you can dial in the cell density and coating exactly.

The challenge is often space. Generator enclosures are tight. The DOC has to fit in whatever space is left after the radiator, the alternator, the control panel. Sometimes that means a custom shape.

Real Installation – Hospital Backup Generator

We worked with a hospital in the Midwest. They had a 500 kW diesel generator for emergency backup. It ran a weekly self‑test for 30 minutes, plus occasional actual outages.

Their problem was smell. During self‑tests, the diesel exhaust would drift toward the emergency room intake. The hospital got complaints. They needed a DOC catalytic converter to cut the hydrocarbons.

We sized a 300 cpsi stainless DOC – 0.08 mm foil, heavy‑duty mat, standard precious metal loading. The exhaust temp during self‑test was around 350°C, plenty hot for the catalyst to work.

We installed it in the exhaust stack right after the turbo. The hospital did a before‑and‑after smell test. The difference was obvious. No more diesel stink near the ER intake. They've been running that DOC for three years now with no issues.

Real Installation – Factory Prime Power

Another customer ran a factory in Southeast Asia. The grid was unreliable. They had a 1 MW diesel generator running 18 hours a day, every day. That's a lot of hours.

Their problem wasn't smell – it was emissions compliance. The local environmental agency started enforcing limits on CO and hydrocarbons. The generator was old, no aftertreatment. They needed a DOC to get under the limits.

We spec'd a 200 cpsi diesel oxidation catalyst – lower cell density to keep backpressure down. The generator was already working hard. We didn't want to choke it. Foil thickness was 0.1 mm stainless. Heavy‑duty all the way.

We also added a temperature monitor before and after the DOC. The customer wanted to see light‑off time and conversion efficiency.

The DOC cut CO by 85% and hydrocarbons by 90%. The factory passed their emissions audit. The catalytic converter has been running for 8,000 hours now. Still working.

The Problem of Low Load

Not all generator applications are steady high load. Some generators run at low load for long periods – like a standby generator that only does light weekly tests.

At low load, the exhaust temperature might only be 200°C or 250°C. That's marginal for a DOC. The catalyst needs about 250–300°C to light off properly.

We had a customer with a generator that ran a 20‑minute self‑test at only 30% load. The DOC never got hot enough. Hydrocarbons weren't burning off. The smell was still there.

The solution was to either increase the load during self‑test or preheat the DOC with an electric heater. The customer chose to change their test procedure – they put a load bank on the generator during the weekly run. Got the exhaust temp up to 350°C. The DOC started working.

Lesson learned – a diesel oxidation catalyst needs heat. If your generator runs cold, don't expect miracles.

Fuel Quality Still Matters

Generator fuel is often delivered in bulk and stored in tanks for months. That fuel can degrade. It can pick up water or bacteria. And the sulfur content might be higher than on‑road diesel.

We saw a case where a generator DOC lost activity after only 1,000 hours. We cut it open. The washcoat had a grayish‑white deposit. Lab test said sulfur poisoning.

The customer tested their fuel. Sulfur was 800 ppm – way above the 15 ppm typical for on‑road diesel. They switched to a lower‑sulfur fuel and replaced the DOC. The new one is still going after 3,000 hours.

If you're running a generator on off‑road or bulk diesel, test your fuel. Sulfur kills DOC catalytic converters.

Sizing the DOC for Generator Flow

Generator exhaust flow is predictable. You know the engine displacement, the RPM, the load factor. So you can size the DOC precisely.

Too small and you get high backpressure. The generator loses efficiency, maybe even trips on high exhaust backpressure. Too big and the DOC takes too long to light off, and it wastes space and money.

We use a simple formula – face velocity around 2–3 meters per second at full load. That keeps backpressure low and light‑off fast.

For a 500 kW generator, that usually means a DOC about 10 inches in diameter and 12 inches long. But every engine is different. We ask for the engine specs before we quote.

Space Constraints in Generator Enclosures

Generator enclosures are packed. The diesel oxidation catalyst has to fit in the exhaust stack, which might have bends, silencers, and other components.

We've built DOC substrates in oval and rectangular shapes just to fit into tight enclosures. One customer had a generator with the exhaust pipe running between two structural beams. The only space was a 200mm by 150mm rectangle. We built a rectangular DOC to fit.

Custom shapes take more tooling time, but they work.

Maintenance and Monitoring

A generator DOC doesn't need much maintenance. But you should monitor a few things.

Backpressure. Install a pressure gauge before and after the DOC. If backpressure goes up, the DOC might be plugging with soot.

Temperature. A thermocouple before and after the DOC tells you when it lights off. If the temperature rise across the DOC drops, the catalyst might be losing activity.

Visual inspection. If you can, look at the DOC face during annual service. Soot buildup? Cracks? White deposits? All tell a story.

We provide customers with a simple log sheet. Record backpressure and temp rise every month. If numbers change, call us.

Bottom Line

Industrial DOC catalytic converters work well on generator exhaust – if you size them right, match the cell density to the load, and pay attention to fuel quality.

Steady temperature is easier on the substrate. But low‑load applications need preheating or higher test loads. Stainless foil, 300 cpsi, 0.08 mm thickness is a good starting point for most generators.

We've had successful applications on hospital backups, factory prime power, telecom towers, and even a landfill gas generator (special coating required). Every one taught us something.

If you have a generator that needs a diesel oxidation catalyst, tell us the engine model, the load profile, and the fuel you use. We'll spec a DOC that fits – and we'll help you avoid the problems we've already solved for other customers.