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Unconventional, efficient and profitable

For van Roje, a German sawmill and pellet producer, being less conventional is a way of striking a balance between technical, logistical, environmental, and economic efficiency. So it makes perfect sense that they should use off-spec fuel and install a novel flue gas condenser.

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Stuart Lloyd, boiler house manager at van Roje, explaining the touch screen control for SaveEnergy flue gas condenser unit.
Stuart Lloyd, boiler house manager at van Roje, explains the touch screen control for SaveEnergy flue gas condenser unit. Stuart Lloyd, boiler house manager at van Roje, explaining the touch screen control for SaveEnergy flue gas condenser unit.

Set in the rolling foothills of the Rhine-Westerwald national park, about 30 km northeast from the city of Koblenz, is Oberhonnefeld-Gierend where I. van Roje & Sohn Sägewerk und Holzhandlung GmbH & Co. KG is located.

As the name suggests van Roje is a family-owned firm founded in 1929 as a small sawmill producing pit props for the mining industry.

In 1996 the current sawmill at the 24 ha Oberhonnefeld site was built consolidating operations that had been carried out across several sites previously. Today the company processes over 500 000 m3 of roundwood per annum.

Custom production

The species mix is 85 percent spruce (Picea abies) 15 percent Douglas fir (Pseudotsuga menziesii) and sourced within about 100 km radius of the surrounding hinterland.

Almost all the logs are supplied to the mill as tree length and on arrival at the log yard, a gantry crane operator sorts the logs according to length and grade. The logs are also scanned for metal as shrapnel from World War II can still be found.

All production is strictly to order and van Roje offers about 5 000 different dimensions, right up to 350 x 350 mm and 14 m in length.

Two dry kilns have a combined capacity to dry 120 000 m3 per year, about half the annual production, the remainder is supplied to clients unseasoned. On average 70–80 percent of production is exported.

100 percent utilization

The company consistently works with trying to make the best economic use of the logs including each residue stream. In 2007 the company set up Energiepellets Oberhonnefeld investing EUR 6 million into building a 45 000 tonne-per-annum pellet plant next to the sawmill to use the sawdust.

We produce 6 mm pellets and are DIN plus certified. We do bulk only and have limited on-site storage which is now full so we would like the heating season to get going, said Gerhard Hauschulte, Production Manager at van Roje during a round tour of the facilities.

In the meantime, the company is busy installing a new hammermill for grinding woodchips as there is not enough sawdust.

Off-spec fuel

In 2008 a new 8 MW biomass-fired heating plant was commissioned to supply heat for the sawmill and pellet facility. Although the moving grate boiler was originally designed to use waste wood and bark, van Roje purposely uses off-specification fuel as the bark is sold for gardening and landscaping purposes.

We buy in shredded green wood waste instead of using our own residues since it is 50 percent cheaper than what we get for our bark and woodchips, explained Stuart Lloyd, responsible for the boiler house operations.

The boiler uses about 40 000 tonnes of fuel a year. Being freshly cut, green waste typically contains a percentage of leaves that increases the chlorine content.

As the boiler was not designed for such fuel, maintenance and replacement costs are significant.

Although we may spend EUR 100 000 to 200 000 on boiler maintenance annually, it still makes around EUR 1/2 million difference to use green waste as fuel. And it keeps me busy, said Stuart Lloyd.

Novel heat recovery

Apart from saving on fuel costs the company also keeps an eye on energy efficiency measures. This is in order to safeguard future energy requirements avoiding, for instance, a situation whereby pellet production becomes hampered because heat is a limiting factor.

A view of the skid-mounted SaveEnergy flue gas condenser unit from the front (Photo SaveEnergy). A view of the skid mounted SaveEnergy flue gas condenser and energy recovery unit from the front (photo courtesy SaveEnergy).

At the end of 2012, van Roje removed the original heat recovery unit in the boiler house as it too suffered from the use of off-spec fuel. It was replaced by a new flue gas condensation unit supplied by IS SaveEnergy AG, a Swiss company specializing in developing, distributing, and maintaining robust solutions for economical and environmentally efficient flue gas cleaning and heat recovery systems.

This novel unit recovers the heat from the hot and moist flue gases while at the same time it brings down the emissions of particulate matter (PM).

The recovered low-temperature heat is fed into a Swiss Combi belt-dryer to dry the sawdust down to 10 percent moisture content at the pellet plant.

Another pay-off from using such wet fuel is that this flue gas condenser unit extracts the low-grade heat we want for the belt-dryer and removes particles. Furthermore, its own power consumption is scalable. The unit boosts the efficiency of the entire boiler house by over 25 percent, said Stuart Lloyd.

The output of the flue gas condenser is limited by the quality of the fuel but according to Adam Sitka, business development manager, SaveEnergy, can “reach over 50 percent of the recovery of a given boiler capacity.”

Droplets form the exchanger

The heat from the flue gas is extracted by injecting process water into a specially designed chamber through which the flue gas passes. The sum total of all the water droplet surfaces assumes the function of a mechanical heat exchanger.

This gives a heat transfer surface area the size of several football pitches enabling a high level of efficiency in heat recovery. The heat from the flue gas is absorbed by the liquid and emitted via a plate heat exchanger to the heat consumer cooling the flue gas down to 2–3 °C above the temperature of the heat consumer.

Schematic of how the SaveEnergy unit works (graphic courtesy SaveEnergy).

The droplets continuously reform this heat exchange surface and cannot become soiled nor corroded. By cooling to below the dew point, the water from the flue gas condenses.

Closed-loop scrubber

The pH level of the condensate is neutralized and cleaned from the solids by a water treatment system also supplied by SaveEnergy. Only the cleaned condensate excess is discharged into the recipient system so the condenser does not require any fresh water.

With the intensive injection of process water into the flue gas, the flue gas condenser doubles as a flue gas scrubber. Here, coarse dust particles in the flue gas are bound to the surface of the liquid by the inertial forces and then transported into the water treatment system via the process water.

Monitoring the pH of the process water. Monitoring the pH of the process water.

The condensate is cleaned in a two-stage process. In the first stage, condensate is passed through a lamella separator. Gravity causes the majority of the solids to be deposited before stage two in which the pre-cleaned condensate seeps through a sand filter to remove the remaining solids.

The sand filter is cleaned automatically and the cleaned condensate is collected in the clean water tank with excess discharged to the recipient system.

In short, the SaveEnergy unit is really a compact, robust and clever piece of self-contained kit. It came modular on a skid and we lowered it into place through the roof. Connection and set-up was almost plug’n’play as it was pre-designed to our circumstances, concluded Stuart Lloyd.

This article was first published in Bioenergy International no. 69-2013. Note that as a magazine subscriber you get access to the e-magazine and articles like this before the print edition reaches your desk!

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