Air pollution control and odor removal for the cement industry
The clinker burning process is the most important part of the cement production process in terms of the key environmental issues for cement manufacture – energy use and emissions to air. After milling, the raw meal containing limestone and clay as well as different additions such as quartz sand and iron ore is burned to clinker. In cement manufacture, fuel energy is generally used for burning the cement clinker. The cement industry is an energy intensive industry with energy typically accounting for about 40% to 60% of production costs. While there is a temperature of 2,000 °Celsius in the rotary kiln – an essential element of the clinker production process – the temperature in the downstream heat exchanger tower is well below. The type of fuel used is therefore decisive.
The characteristics of the clinker burning process itself allow the use of waste as raw materials and/or as fuels for firing the rotary kiln. Normally, various conventional fuels such as coal, petcoke, or heavy oil are used by about 40 percent. However, secondary fuels such as scrap tyres, waste oils, plastic waste or biogenic fuels (e.g. sunflower seeds pellets, recycled paper and animal meal) are used by more than 60 percent. In the cement industry, the key polluting substances emitted to air are dust, nitrogen oxides, sulphur dioxide, organic compounds and benzene.
In the U.S., emissions limits are based on pounds of pollutant per ton of clinker produced. For new major sources such as those which emit more than 10 tons per year of a single air toxic or more than 25 tons of a combination of air toxics (less than this is called an area source), the emission limits (averaged over 30 days) are 1.5 lb/ton of clinker for nitrogen oxides (NOx), 0.4 lb/ton of clinker for sulfur dioxide (SO2) and 0.01 lb/ton of clinker for particulate matter.
Beginning in September of 2013, the following restrictions will be added for existing and new major sources and area sources that were constructed, reconstructed or modified after 2008: for existing source kilns the emissions limits will be 55 lb per million tons of clinker for mercury, 24 parts per million by volume (ppmv) for total hydrocarbons, 0.04 lb per ton of clinker for particulate matter (as a surrogate for metals other than mercury), 3 ppmv for hydrochloric acid (major sources only). For new sources kilns the emissions limits will be 21 lb per million tons of clinker for mercury, 24 ppmv for total hydrocarbons, 0.01 lb per ton of clinker for particulate matter, 3 ppmv for hydrochloric acid. All emission rates and concentrations are averaged over 30 days.
Basically, it can be stated that despite the increasing usage of secondary fuels in the cement industry, emissions have been reduced. For quite some time now, it has been possible to precipitate dust. More and more cement factories are replacing electric filters with baghouse filters. The emission of acidic components in the waste gas (SO2, HCI, HF) is entirely unproblematic for most systems thanks to the use of limestone as the basic raw material.
Secondary measures such as dry sorption systems and wet scrubbers are used in some systems. Due to the high burning temperatures for clinker, nitrogen oxides occur in relatively high concentrations. They are removed mainly through selective non-catalytic reduction (SNCR).
Selective catalytic reduction technology (SCR) has recently been implemented in a few systems. At the moment, systems are in use before and after dust precipitation. Principally, it can be stated that the SCR technology in the cement industry is not yet state-of-the-art. Pollutants such as carbon monoxide, total organic carbon, benzene and odor cannot be reduced significantly with an SCR.
Years of research
Two Austrian companies – the environmental technology company CTP and the cement manufacturer Wopfinger – have taken an entirely new path. Wopfinger Baustoffindustrie GmbH is a third-generation family business with its headquarters in Wopfing, Lower Austria, where it employs 400 workers who produce cement, chalk, dry mortar, floor pavement and façade plaster. Wopfinger Baustoffindustrie GmbH is a subsidiary of Schmid Industrie Holding which also has its headquarters in Wopfing. The company has a workforce of around 5,000 employees worldwide and manufactures building materials in 19 countries.
The cement factory at Wopfing has a five-stage heat exchanger tower with a calciner. In it, a baghouse filter and a SNCR have been installed as a flue gas cleaning system. Given the companies odor emissions related to raw materials, Wopfinger has carried out years of research on the abatement of malodorous emissions and operated several pilot units in order to address the issue. CTP is one of the few leading companies worldwide in industrial air pollution control. Air pollution control systems that are generally based on regenerative thermal, catalytic and adsorptive processes are found in the world’s leading industrial companies – ranging from chemical and petrochemical, pharmaceutical industries to the aluminium industry and the automotive sector.
Together the two companies have developed a completely new and substantially improved process for emission control in cement factories – the world’s first regenerative thermal oxidation system in the field of cement production featuring an integrated denox system. The oxidizer consists of five structurally identical heat exchangers, in which waste gas is heated and cooled in cycles. The core of the heat exchanger consists of 25,000 ceramic honeycomb blocks which serve as heat storage units. By continuously switching between the exchangers, energy use is optimized so that practically no additional energy is needed to operate the system. Furthermore, natural gas consumption can be considerably reduced by means of the flammable waste gas components, thus making the installation of an air pollution control unit highly profitable.
The waste gas of the clinker burning process (including an adjacent raw material grinding system and a process of drying combustibles) is transported by a fan at a volume of 200,000 Nm3/h into a regenerative oxidizer, where it is then heated to 950 °Celsius and cleaned free of all residues. The regenerative thermal oxidizer features an integrated SNCR, capable of reducing nitrogen oxides in two stages. Diluted ammonia solution is injected directly into the unit through several lances. Nitrogen oxides and flammable substances in the waste gas can be simultaneously reduced in the oxidizer – a clear advantage to the pure SCR system. The system represents a classic end-of-pipe-solution – the production process is not affected. Raw materials and fuels can be selected freely, ensuring maximum flexibility.
The oxidizer has been in operation since August 2011. Since then, emission limits have been considerably undercut. CTP has already applied for a patent.