On the backdrop of changes announced by the Ministry of Environment, Jayant Saha, a consultant takes stock of the situation and explains how the industry can prepare itself to face the challenge.
While using wastes from various sources, mainly as fuel, and also its inherent process requirements, cement plants face much adversity including environment pollution. This leads cement plants to undergo continuous technological advancements.
Most of the emissions are in the form of particulate matters, CO2, SOx, NOx and toxic matters containing mercury and other heavy metals and persistent organic pollutants. Almost all of chemical pollutants are generated in pyro section.
CO2 is produced through combustion, calcination, electrical energy consumption and indirectly through vehicles used by plant and plant personnel. To control CO2 generation BEE has introduced PAT scheme. These measures helped industry in reducing CO2 emission from 1.12 (in 1996) to 0.72 t of CO2 per t of cement. Some countries have taken mercury emission seriously and have started controlling it. Most mercury is present in gaseous phase as elemental or oxidized mercury – HgCl2. The common practice to reduce mercury is to increase the oxidized fraction by increasing chlorine content of fuel. Removal of oxidized mercury (typically 95 per cent) is easily done in wet FGD, SDA and CDS scrubbers. Removal of ~90 per cent of total mercury is possible by Br-PAC (Brominated Powdered Activated Carbon) injection added to the removal of oxidized Hg. PAC is injected into flue gas upstream of main filter. Mercury is absorbed on to the carbon and removed in a separate bag house to prevent recycling.
Recently, (August 25, 2014), the Ministry of Environment and Forests (MoEF) in India has introduced restriction on SOx (100 mg/Nm3) and NOx (600 mg/Nm3 for new plant – applicable from 01-06-2015 and 800 mg/Nm3 for old plants – applicable from 01-01-2016) emission. Monitored values are to be corrected to 10 per cent O2 and on dry basis. This led the need for special focus on cement pyro section technology/retrofit.
SOx Emission Control Technology
Emissions of SO2 are prominent in long kilns rather than energy efficient, dry process kiln systems.
Pyritic or organic sulphur gets burnt in the preheater upper cyclones at around 400-600oC forming SO2. Most of SO2 that escapes the preheater with dust is effectively collected if the gases are used in VRM and is re-introduced to the preheater with the kiln feed. Internal recirculation occurs when liberated SO2 gas in the kiln passes through the preheater and combines with the calcined raw meal and also alkalis in the lower cyclone stages, forming CaSO4 and alkali sulfates. Alkalis in excess of chloride combine with sulphur to form more stable alkali sulfates. Sulphur in excess of alkalis forms CaSO4 which has a higher evaporation factor. Optimum molecular ratio between sulphur and alkalis in the kiln system can be expressed as (SO3 / Alk)Optimum = (SO3/ 80)/((K2O/94) + 0.5 * (Na2O/62)) If the ratio exceeds 1.1 "excess" sulphur (E. S.) is available to combine with CaO. E. S. is expressed in grams SO3 per 100 kg clinker and calculated as:
E. S. = 1000 * SO3 GCo 850 * K2O – 650 * Na2O
For easy and hard burning raw mix, this figure should not exceed 600 and 250 gm SO3/100 kg clinker respectively to maintain smooth kiln operation.
The dissociation of alkali sulphate compounds can best be described as AlkGCoSO4 + heat = AlkGCoO + SO2 + ?O2
The equilibrium shifts to the left favouring the formation of Alk-SO4 with increasing O2 and SO2 partial pressure. For increasing oxygen content up to approximately 2 per cent, volatility of sulphur is progressively reduced while increasing the oxygen beyond 2 per cent has a limited effect.
CaSO4 starts to decompose slowly at temperatures above 1220oC.
CaSO4 + heat = CaO + SO2 + ?O2
In a reducing atmosphere (presence of C and CO), both alkali and calcium sulphates decompose releasing SO2.
Generated SO2 travels back to the preheater. With higher sulphur recirculation the plugging problems in the preheater increase significantly. The location of spreader box on kiln riser plays critical importance. If it is possible to place the spreader on the smoke chamber shoulder, the introduced hot meal will absorb the SO2 before it sticks to the riser wall. Thus a higher SO2 content in the smoke gas is allowable, which means that kiln can run with higher excess sulphur, sometimes up to more than 1000 gm/100 kg clinker; however, with consequences of increasing tendency to form dusty clinker.
The introduced sulphur ends up in the clinker if not removed elsewhere. Typically, the limit for sulphur in clinker is 1.6 per cent, as SO3, to assure good quality.
Removal of Sulphur Dioxide
- There are methods to remove and prevent the formation of SO2 by modifying or controlling conditions in the cement pyro-processing systems.
- Sufficient oxygen level can be maintained in exhaust gases to stabilise alkali and calcium sulphate compounds formed in the process.
- The burning zone flame shape can be modified to reduce the possibility of forming localised reducing conditions.
- Raw materials can be altered to affect the alkali/sulphur molar ratio and also to affect absence of sulphide sulphur, organic sulphide or carbon, may reduce SO2 emissions. Increasing alkali input may not be possible because of product quality limits on total alkali concentration in the cement.
- Addition of lime in kiln feed helps in absorbing released SO2 to form CaSO4 and gets back to the system.
- Installation of SOx reduction cyclone directs naturally occurring CaO present in the pyro system, especially in the calciner, to the upper stages. The dust laden gas from the calciner (near the outlet) is withdrawn and passed through an LP cyclone located towards the top of the preheater. The separated dust, rich in CaO, is fed to cyclone 1 or 2. The gas from this cyclone goes to stage two or three cyclone inlet.
- Scrubber technologies that capture SO2 after the kiln system can be divided into four classes, dry reagent injection, hot meal injection, lime/limestone spray dryer absorber, and wet scrubbers.
NOx Emission Control Technology
NOx (NO and NO2) is formed in cement pyro system by following mechanisms.
Thermal NOx Formation
Thermal NOx is formed at a temperature greater than about 1200?C by direct oxidation of atmospheric nitrogen. Since the flame temperature in cement rotary kiln is about 2000?C, considerable amount of thermal NO is generated.
The thermal reaction between oxygen and nitrogen to form NO takes place as per Zeldovic reaction:
O. + N2 ? NO + N.
N. + O2 ? NO + O.
NO formation increases exponentially with temperature and in the presence of excess oxygen. Factors affecting the concentration of NO in the kiln gases are:
- Maximum theoretical (adiabatic) flame temperature
- Flame shape (burner type)
- Excess air rate
- Maximum necessary material temperature
- Material retention time in burning zone
- Gas retention time in burning zone
- Kiln loading (TPD/ m3) Lower secondary air temperatures and presence of dust increases NOx formation. Dust reduces radiation from the flame which in turn reduces heat transfer to material.
Fuel NOx Formation
NOx also results from the oxidation of nitrogen compounds present in fuel, other than gaseous. The reaction normally takes place at relatively lower temperature, less than 1200?C.
Fuel NOx formation normally depends on:
- Nitrogen content in the fuel
- Volatile content in the (solid) fuel
- Oxygen level in the combustion zone
- Initial NO concentration in the combustion gas
- Temperature in the secondary combustion zone
A higher volatile content in the fuel reduces fuel nitrogen conversion to NO. At temperatures between 800 -?C and 1100 -?C, the following reactions may take place:
N + O ? NO (1)
N + NO ? N2 + O (2)
Since the rate of reaction 2 increases more rapidly than the rate of reaction 1 as the temperature increases, higher temperatures (between 800?C and 1100?C) may reduce NOx emissions in secondary combustion zones.
Prompt NOx Formation
Prompt NOx is formed by fuel-derived radicals, such as CH and CH2, reacting with N2 in hydrocarbon flames. The overall contribution of prompt NOx to total NO is relatively small.
In rotary kiln, thermal NOx generation is dominant whereas in the calciner and in the secondary combustion zone where combustion temperature is up to 1200 -?C fuel NOx is major contributor.
Influence of Kiln System on NOx Emission
Kiln system in cement plant is normally one of the following.
-Pre-heater kiln with grate or planetary cooler
-In-Line Calciner (ILC) kiln
-Separate Line Calciner (SLC) kiln.
In pre-heater kiln the NOx emission is determined exclusively by the condition in the kiln burning zone.
In ILC kiln system the kiln exit gases having NO pass through the calciner. CH radicals and nitrogen from the calciner fuel reacts with kiln NOx to reduce it to free nitrogen. Balance nitrogen compound in calciner fuel during combustion forms NOx. The result may be a net production as well as net reduction of NO in calciner.
In SLC kiln system the combustion in calciner takes place in pure air. When using solid fuel like coal up to 50 per cent of nitrogen compounds in the fuel may get converted into NO. Thermal NO from kiln leaves kiln string without any opportunity to reduce/reburn and gets added to NOx from calciner string. SLC kilns therefore, have higher NOx emissions from stack compared to ILC kiln system.
Control Techniques for NOx Reduction
Typical NOx emission in older technologies can be as high as 1800 – 2000 mg/Nm3, while average emission value in modern plants is around 1200 mg/Nm3.
The reduction of NOx emissions from cement pyro system can be done in two ways.
Primary NOx Reduction Measures
In primary reduction measures existing process is modified to reduce the formation of NOx, The following ways are very common.
- Optimisation of clinker burning process.
- Automatic kiln control system or Expert system.
- Use of Low NOx burner to allow low primary air and to control flame flow pattern.
- Addition of water to the flame or fuel of the main burner.
- Staged Combustion in Precalciner.
In calciner staged combustion, fuel is first burned under reducing conditions to reduce NOx and then remaining fuel burns under oxidising conditions to complete the combustion. Introduction of raw meal allows control of calciner temperature. Through these mechanisms, both fuel NOx and thermal NOx are controlled.
The reaction: 2CO+ 2NO ? 2CO2 + N2
Primary reduction measures can reduce NOx level up to 20 per cent.
Secondary NOx Reduction Measures (SNCR)
In Secondary reduction measure a separate gas cleaning unit is added. Selective Non Catalytic Reduction of NO with NH3 was developed by Exxon Research & Engineering Co., USA. The reagent, typically NH3 or urea, is injected into the kiln system at a location with an appropriate temperature window (870?C to 1100?C). The temperature is critical, at higher temperatures the reagents will form additional NOx whereas at lower temperatures the reactions proceed slowly and substantial amounts of unreacted ammonia will escape.
Ammonia and Urea Dissociation and Reduction Pathways have been shown in the Figure 1.
The performance of SNCR system depends on
-Residence time available at optimum temperature.
-Degree of mixing between injected reagent and combustion gases
-Uncontrolled NOx concentration and oxygen level.
-Molar ratio of injected reagent to uncontrolled NOx.
The performance also depends on the efficiency of installed injection system. ERC Chemtrol is one of the leading De-NOx system suppliers and claims a very high efficiency of their system.
SNCR system can easily be installed in pyro system and should be installed after taking primary reduction measures. This can reduce NOx up to 80 per cent.
Authored by Jayanta Saha, Cement Process Consultant (Freelancer) based in Navi Mumbai
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