The main emissions from a flat glass furnace come from the combustion of fuel and the decomposition of the carbonate and sulphate raw materials. The raw materials are melted at a very high temperature so the process is extremely energy-intensive.
Emissions to air
The most significant pollutants from a glass furnace are: nitrogen oxides formed from the nitrogen and oxygen in the air at high temperature; sulfur oxides from the refining agent (and from oil fuel if that is used); and particulate matter derived from compounds volatilized from the molten glass. Carbon dioxide comes from both the decomposition of carbonates and from combusted fuels.
We are tackling pollutant air emissions with a range of primary and secondary abatement techniques. Primary methods, such as special burners or carefully-chosen raw materials, including more cullet (recycled glass), reduce pollutant formation and can also improve the thermal performance of the furnaces.
Secondary abatement removes the pollutants from the furnace waste gases before they are emitted through the chimney.
Until recently, the furnace emission pollution control plants in operation around NSG Group have all been based on standard technology. This removes the pollutants from the waste gases in three stages: alkali is injected into a scrubber to remove acid gases, the resultant reaction products and particulate emissions are then collected in an electrostatic precipitator, and finally nitrogen oxide is removed using ammonia injected into a selective catalytic reduction unit. However, the latest pollution control plant built by NSG Group uses a novel technique called a catalytic ceramic filter, or CCF.
The system is based on thousands of ceramic filters in a single unit. Alkali is added to the waste gas and the resultant reaction products and primary particles are collected by the filter. The novelty of the process is that the fibers of the filters are coated with a metal catalyst so when the gases pass through the filter structure and ammonia is added to the system nitrogen oxides are also removed. It is expected that CCF technology will become the standard for new furnace pollution control plant throughout NSG group.
A new NOx abatement unit – Selective Catalytic Reduction (SCR) unit – was retrofitted to the existing pollution control plant at the C2 Float glass factory in Caĉapava, Brazil. The De-NOx unit was designed and built by the Brazilian division of the German company GEA, based on functional specifications provided by NSG UK Engineering Department.
The new NOx abatement plant was required to comply with new emission limits enforced by the local environmental agency CETESBE, which enforced a maximum mass emission limit of 60 kg/h of NOx. The SCR unit will be capable of reducing over 75% of incoming NOx.
The installation of the SCR unit downstream of an existing semi-dry pollution control plant posed a few technical and technological problems, due to the relatively low temperature of the waste gases coming out of the electrostatic precipitator. Low-temperature and catalytic reduction of NOx with ammonia are very often difficult to deal with and require special attention in designing the SCR reactor and oversizing the catalyst.
Recycled glass to improve efficiency and reduce emissions
Glass for recycling is a valuable resource. Wherever quality allows, we recycle any glass off-cuts or cullet within our own glass melting lines. Glass from our downstream operations and from our customers represents a potentially useful resource to us. We gain a double benefit from the use of such cullet: its use to make glass reduces the requirement for raw materials and avoids disposing of what would otherwise be a waste material and closes the recycling loop.
Ten percent cullet use saves three percent furnace energy and leads to reductions in CO2 emissions. One of our sites has manufactured glass using only cullet.
Cullet return projects are ongoing to try to minimize the need for virgin raw materials. Landfill is the least favored disposal option.
If glass is produced that cannot be remelted on-site, it is sent, where practicable, for external recycling.