New 3-in-1 combined process from Dürr for exhaust air purification

1. Current situation
In Germany, the draft bill of the new air pollution control regulations (Technische Anleitung zur Reinhaltung der Luft, TA-Luft) is currently under discussion, and new emission limits are now being formulated as recommendations. It may seem that these are only marginal changes to the state-of-the-art systems, but for many industries, the new limit values represent the starting point for considering acquiring additional equipment or alternatively implementing completely new exhaust air purification processes. The glass industry, among other industries, now needs to review the long-term suitability of existing technologies such as electrostatic precipitators.

Industrial growth in China continues to outpace that of Germany. Many firms, including those in the West, are investing in new production sites. Although GDP is no longer heading skyward at astronomical speed, the rate of growth is still three times higher than in Germany.
 
Pollution control in China cannot always keep up with such a fast pace GDP growth – and it has not been able to do so yet, which is evident when one takes a closer look at the current practices of many permit authorities in China. These days it is difficult to operate production facilities without an exhaust gas treatment system, especially in highly industrialized regions.

This was the case for one glassware manufacturer, who was threatened with the closure of his production plant in the greater Beijing area. Consequently, he was forced to install a suitable exhaust air purification system within a very short period of time.

2. Technology selection
In glass manufacturing, furnaces are used when the glass, made from various raw materials, especially cullet (recycled glass) is melted. These furnaces are heated by burners, which despite a high level of energy recovery still need large amounts of fuel (oil or gas). Because of the high temperatures in the glass tank, the exhaust gas flow contains high levels of nitrogen oxides (NO_x), sulfur oxides (SO_x), and dust.

When planning the system, it was important to reliably comply with current limit values and preferably take future tightening of limits into account as well.
 

First of all, a solution using the conventional technology was considered. This consists of a 3 part system, a Scrubbing Tower followed by an electrostatic precipitator followed by a catalytic NO_x abatement system employing a selective catalytic reduction (SCR) catalyst. While this SCR process would meet current limits, it would not meet the future stricter dust emission limit.

As an alternative, Dürr looked at using a filter with fabric hoses in combination with an Ecopure^® SCR. Since this type of filter can only be operated at a maximum temperature of 220°C due to the filter media used, the raw gas, which is hotter than 350°C, has to be cooled.

For the subsequent DeNO_x process however, the optimal temperature is around 350°C, which means the gas has to be heated again. The high investment and operating costs of a heat displacement system, that is the cooling/heating option, represented an excessive overall additional cost, so this technology was rejected.

3. Solution
Dürr’s Ecopure^® Catalytic Candle Filter (CCF) technology makes it possible not only to comply with emission limits without additional cooling or heating processes, but it actually keeps emissions more than 50% below limit values. As a result, the Chinese firm considers itself well equipped for future tighter limits.

With this new Dürr technology, three pollutants are eliminated simultaneously in one system, which translates into economic operating costs savings:

DeDust
Filtering exhaust gases at high temperatures was already an available technology for many years. Ceramic candle filters were used ten years ago in an exhaust air purification system at a hazardous waste incineration plant.
These candles are made from ceramic fibers that can withstand temperatures of up to 900°C. Since the filter wall is much thicker in comparison to fabrics, these filters are rigid. This results in an extremely long service life, as the deformation during cleaning, via a burst of compressed air which causes wear in fabric filters, doesn't occur. The filter’s rigidity means that a permanent filter cake forms on its surface. This contributes to better filter performance and significantly lower cleaned gas values especially for superfine particulates.

DeNO_x
In the established SCR process, nitrogen oxides are removed via a reaction between injected urea or aqueous ammonia with NO_x. Because of the catalyst, this reaction takes place at a relatively low temperature of 350°C.

Ecopure^® CCF filters are coated with this catalyst material. The candles therefore perform the same function as the coated ceramic honeycomb in an Ecopure^® SCR, another Dürr exhaust air purification system.

The doping of the fibres with catalytically active centers on their surface proves to be advantageous in this case. Unlike in conventional catalyst honeycombs, omission of gas phase mass transport of the reaction partners and products help to improve the filter’s performance.

The catalyst thereby is located on the inside of the filter wall (see figure 5) where it is dust-protected. The usual aging due to the clogging of pores and reduction of active surface area doesn't happen.

DeSO_x
Sulfur in many processes exists mainly as SO₂, which may be separated using wet or dry processes. With low pollutant concentrations, dry processes have become the preferred choice over highly efficient wet processes, due to their lower life-cycle costs. This technology is based on the reactivity of a sorbent such as calcium hydroxide (Ca(OH)₂) with acidic constituents in the exhaust gas such as SO₂, HCl and HF.

In many applications, the technology described above has proven effective both with electrostatic precipitators and with fabric filters. For the desulfurization process to achieve good separation efficiency, a temperature of up to 180°C and sufficient moisture are required. At higher temperatures, however, the reactivity of the calcium hydroxide decreases at first, before rising sharply again from about 300°C upwards. This selective temperature-dependent behavior makes the use of calcium hydroxide particularly suitable for separating acidic constituents from exhaust gas in a temperature range that is favorable for the Ecopure^® SCR DeNO_x process mentioned earlier.

3-in-1 technology
As all three processes are combined in just one unit, the setup is very compact and allows space-saving installation within existing production facilities. The high efficiency of the individual processes delivers maximum separation efficiencies for all types of pollutants, meeting the latest requirements of the forthcoming TA-Luft 2017 regulations particularly in respect of dust and nitrogen oxides.
Integrating the three individual processes into the Ecopure^® CCF system means lower maintenance costs and reduced space requirements, which results in lower operating costs.
The 3-in-1 technology has been well received, especially in overseas markets, and now it has been included in the official draft of the VDI 2578 standard in Germany.