Heat Pump Used to Drain Contaminated Sludge in a Steel Factory


Courtesy of Hydro-Québec


In 1987, new regulations required that factories dispose of their contaminated water and waste themselves. Making galvanised steel wire, the Stelfil factory produces sludge contaminated with lead and zinc. Stelfil spent thousands of dollars to bury this waste and evaluated three systems to remove most of the water: a gas dryer, an induction heated screw conveyor and a heat pump system. Even though it had never been used in those particular conditions, the heat pump (HP) system was chosen and demonstrated good results. Energy savings of 300 MWh were achieved as well as economical savings of CAD 50,000 per year.

- Savings of CAD 50,000 per year
- Waste weight reduction of 40%
- Reduction of greenhouse gas emissions
- Savings of 300 MWh

Aim of the Project
Stelfil produces about 70,000 tons of galvanised steel wire every year. The process uses significant quantities of toxic substances that are rejected in the wastewater from the factory, thereby threatening the environment.

Following new regulations, all factories have to manage their own waste. They are responsible for eliminating residual waste and for treating their used water. In order to satisfy these new environmental regulations, important changes were made to the process to minimise waste production. However, Stelfil still had to dispose of the contaminated sludge. Spending tens of thousands of dollars to bury the contaminated sludge produced by the water process, an economical solution had to be found. Because contaminated sludge contains water at 50% concentration, it was decided to implement a drying system to minimise both the volume and weight.

The Principle
The heat pump system is equipped with a compressor, a heat exchanger and a ventilator. The process is quite simple: dry air is injected and circulated through the sludge compartment where it becomes moist. It is then sent to a condenser where water is removed by lowering the temperature. Finally, it is heated back to its original temperature via a heat exchanger. While air is reprocessed through the cycle, water is completely discarded, treated and recycled.

This process is kept in a closed cycle in order to avoid toxic gas emissions. To isolate untreated sludge, three flexible pipes are directly connected from the tank where sludge is previously stored, to the cabinet where it is processed. Once sludge is drained, it is transferred to a landfill site.

The alternative for this system was a gas dryer where moisture is removed and then treated by a scrubber. The numerous moving parts required make this system more difficult to maintain than the heat pump option (which does not need a scrubber for instance).

The Situation
Stelfil had to make rapid changes in their way of making galvanised steel wire after new environmental regulations from the Montréal Urban Community were put in place in 1987. Producing large quantities of lead, zinc, sulphuric acid and chloride acid, it was decided to minimise the use of these substances by a new line of pickling and hydraulic jump tank to clean the steel before the galvanisation process. Moreover, Stelfil build its own facilities to treat used waters. But the problem of disposing of the 350 tons of contaminated sludge remained.

Since the sludge contained 50% water, three drying systems were evaluated: a traditional gas dryer, an induction-heated screw conveyor, and a heat pump system. The first option was too costly and seemed to be cumbersome and difficult to install. The induction-heated screw conveyor, recently developed by the LTEE of Hydro-Quebec, was promising but needed further enhancement before being installed in such a facility as the Stelfil factory.

The heat pump option was interesting both because of its low initial cost compared to the other two options and its energy efficiency: heat pumps can recover thermal energy with a ratio of nearly 98% (compared to 50% with a gas dryer). Even though it had never been used yet to drain that kind of sludge, the simplicity of installation and the compactness of such a system made its implementation possible without any important modification to the factory.

The actual system showed very interesting results. Sludge weight was cut by 40%. In more concrete terms, a reduction of 150 tons of sludge per year was achieved. Compared to a gas dryer (operating with natural gas), the system avoids emissions of 51 tons of CO2 per year. This is calculated without the reduction of toxic gas emissions due to the drying process.

The monthly consumption of the heat pump system is 10,300 kWh. Compared to a gas dryer system that consumes approximately 35,000 kWh, annual energy savings of 300 MWh are achieved. Because the only moving part of the HP system is the ventilator, it is easier to maintain, and thus, more economical.

The Company
As a branch of Stelco, Stelfil Ltée is one of the largest galvanised wire producers in North America. With its head and sales offices in Lachine, Quebec, on the island of Montreal, it has been producing wire and wire products for over 100 years, operating a 37,000-square-meters (400,000-square-foot) manufacturing facility. Stelfil has an annual production in excess of 90,000 tons. Both low and high carbon wires can be produced in a variety of sizes and shapes including aluminium-conductor steel reinforcing wire, telephone wire, armour wire, mechanical spring wire and industrial-quality wire.

The cost of the gas dryer was CAD 120,000, including the installation cost of CAD 20,000. The system had a thermal energy recovery ratio of only 50%.

In comparison, the total cost of the heat pump system was of CAD 45,000 and its ratio was nearly 98%. It allowed disposal cost for the sludge to be cut by CAD 38,000 per year (on the basis of CAD 250 per ton with a reduction of 150 tons per year). Moreover, the energy cost of the gas dryer was CAD 18,900 per year compared to 5,600 for the heat pump system. Calculated over a year, this is equivalent to annual savings of more than CAD 50,000, including energy savings of 300 MWh and the cost reduction for disposing of the sludge.

Thanks to financial support from Hydro-Quebec, the result was a simple payback period of less than one year.

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