DSS085: Preventing Dryer Fires and Explosions Using Superheated Steam with Prem Verma

In this episode of the DustSafetyScience Podcast, we interview Prem Verma, managing director at Swedish Exergy based out of Gothenburg, Sweden, about how facilities can use superheated steam to prevent fires and explosions.

It’s a technology that Prem and his team use in their dryers to prevent fires and explosions from happening, so he agreed to come on the podcast and answer the following questions:

What are the different ways that dryer fires and explosions happen in industry?
What is superheated steam?
What industries use superheated steam dryers?
What is the startup and shutdown process like?

What are the different ways that dryer fires and explosions happen in industry?

Prem explained that traditional drying technologies available on the market use air, which contains oxygen, as a drying medium. This means that the dryer has a combination of dust concentration and oxygen. The third component needed for a fire or explosion to occur is an energy source, which could be a small spark from a rotating machine part.

“So the ingredients needed for causing the fire explosion are present in traditional dryers all the time and that can happen at any time in the drying process, using air as a drying media, which traditional dryers do,” he said.

In these traditional systems, it is paramount to install fire and explosion protection in order to get the regulatory and legal approval to operate the dryer. The systems used in the market have the following:

Spark detectors at strategic points.
Extinguishing apparatus to quench any fire.
Explosion vents to vent the explosion safely outside without harming people or equipment.
An inerting system that activates if a spark is detected and prevents the explosion from happening.

“So you need quite a lot of expensive protection equipment for both fire and explosion,” Prem said. “And they call it nonproductive equipment because this equipment is not contributing to the drying process itself.”

These dryer systems are heavily regulated. They follow the ATEX standards and require independent third-party certification. Prem suggested that dryers in North America have to meet the NFPA standards and VDI is applicable in North America to a certain extent.

What is superheated steam?

When asked to explain the concept of superheated steam dryers, Prem pointed out that steam is an inert gas, which means that the drying media inside superheated steam dryers is 100% inert.

“That’s one aspect which makes the dryer completely inert, preventing any kind of fire or explosion,” he said. “Ingress of oxygen from outside also cannot happen in our dryer because it operates above atmospheric pressure. So we have a slight overpressure in the dryer from half a bar gauge up to three bar gauge, which means that if anything leaks from the drying system, it is steam leaking outside and not the air coming inside the dryer.”

These Swedish Exergy dryers also don’t require explosion vents and suppression because there is no oxygen in the system, which he described as a closed loop with airtight rotary valves on the inlet and exit. There may be some steam escapage as it comes out because the system is pressurized, but there is no air coming in.

The steam comes from the stock. During the start-up process, stock like biomass or sludge is fed through and water comes from it. The heating is indirect: whatever is used to heat the system is separate from the flow of the superheated steam. The result is a closed loop that dries the product, whether it be biomass or sludge.

What industries use superheated steam dryers?

Prem confirmed that superheated steam dryers are used in a wide range of industries. Swedish Exergy has supplied systems for peat drying, brick heating, pulp and paper production, sludge drying for municipalities, and even sugarbeet pulp.

“We are in discussion with the agricultural industry, especially the citrus and olive industry in Spain, which is seeing this technology of great interest because the waste from the industry gives them no value,” he explained. “But if they used our process to dry it, they can integrate the technology to produce high quality animal feed, which will give them a value addition.”

He added that one important aspect of the technology is that there is no emissions to the atmosphere, meaning that no particles or VOCs are emitted and no post-treatment equipment is necessary with the dryers.

The systems typically used today to clean the VOCs and particulates are RTOs (regenerative thermal oxidizers). They have electrostatic filters that are a significant financial investment as well.

“(These filters) are necessary to meet the regulatory requirements, but they don’t add to the production process in a positive way,” Prem said. “And the energy required (is) quite high. They have to be used all the time, like you use natural gas or other processes to just heat up the off gases […] which we do not need with our system.”

What is the startup and shutdown process like?

Superheated steam dryers are designed in a way that does not permit the dryer to have a lot of material inside. Even if a system is designed to process 50 tons of product through the dryer, there will only be around 100 to 200 kilos of material inside the dryer at any given time.

If the power fails and there is an emergency stop, everything stops working. The material inside the dryer will end up in the bottom of the cyclone because even if the power is cut, the fan keeps rotating for a few seconds, which will transport the product to the cyclone bottom.

“What we recommend is that if this happens, do not open the system while it is hot,” Prem said. “They need to allow it to cool down to under 100 degrees Celsius. Then the system can be opened. Because as soon as you open manholes or inspection holes, you allow the air to come inside. If you create two openings -one the bottom and one at the top- while the system is hot, you get a chimney effect, which sucks in air and creates a risk of pyrolysis or fire inside the dryer.”

He said that if the fault is inside the dryer, it can be restarted without opening anything. “This is because we start the outlet system and allow the product to go out and reheat the system again and restart the plant. That can be done. But when we start from cold -let’s say, during start up- the procedure is strictly defined. You then have to start the system and heat up the air inside the dryer, then inert the atmosphere with seam.” The air is then pulled out of the dryer until 100% steam remains.

Conclusion

Many facilities are slow to adopt effective fire and explosion protection because the cost is high and the equipment itself is nonproductive. With technologies like superheated steam dryers making it possible to recycle waste into a saleable commodity, the rate of adoption can increase and safety levels rise accordingly.

If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Prem Verna directly:

Email: [email protected]
LinkedIn: https://www.linkedin.com/in/vermaprem/

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.