Advanced fuel burning technologies in cement production

In the recent climate summit at Paris, the cement sector has backed a plan to shift toward the use of de-carbonisation technologies. Notwithstanding to the commitment given over there in the summit, use of waste materials as fuel is a very significant step to reduce consumption of fossil fuels and it is going to increase in our country in the coming years. Prof JD Bapat takes stock of the design of the burners for effective use of waste or alternate fuels with less gaseous pollution.

The high cost of energy has increased focus on energy efficiency in cement production. The concern for GHG emission has lead to greater preparedness for increased utilisation of alternative fuels in the Indian cement industry. The modern multi-channel burners satisfy both the requirements of energy efficiency and GHG reduction. While the modern cement plants have multi-channel burners, the older plants are gradually shifting from uni-flow (or mono-channel) burners to multi-channel burners.

The alternative fuel utilisation is still low in India. The average thermal substitution rate (TSR) is less than 1 per cent. The multi-channel burners effectively manage the variety and complexity of alternative fuel burning. Compared to a conventional burner, modern multi-channel burner offers much better possibilities for flame shape control, high momentum and the flexibility to use different types of fuels, such as hazardous chemicals or solid biomass. The advanced burners reduce the loss in production during kiln disturbances and also reduce NOx in the burning zone as the primary air ratio is low. The NOx emissions can be reduced as much as 30-35 per cent over emissions from a typical direct fired, uni-flow burner. Better flame properties with the multi-channel burner improve combustion efficiency and eliminate flame impingement on refractory.

While multi-channel burners are becoming increasingly common in the Indian cement industry today, they might need further innovation and better design to suit the requirements of an increased TSR of 30 per cent or more; latest research on plasma burners, for example. The anticipated benefits with advanced burning technologies are: thermal savings: 3 to 5 kcal/kg clinker, electrical savings: 0 to 0.5 kWh/t clinker and CO2 reduction: 2 to 4 kg/t cement.

Modern Burners
The Fig 1 and Fig 2 illustrate the combustion air flow and its distribution in a cement kiln burner. Primary air is defined as air passing through the burner. It consists of axial-air, radial-air and fuel-conveying air. The percentage of primary air in the required combustion-air is called primary air ratio. Low heat value of fuel increases fuel consumption and the requirement of air for combustion. The effective air for combustion is composed of primary air, hot secondary air from the clinker cooler (supplied through the rotary kiln hood) and false air infiltrating through openings and sealing. Almost all equipment installed for surveillance or operational purposes give rise to false air. Primary air should be low and false air completely avoided, from thermal efficiency point of view. Excess air (above the stoichiometric requirement, expressed as ratio) is required to ensure complete combustion. A high excess air ratio reduces the energy efficiency and the increased exhaust gas amounts may limit the production capacity.

Evolution of modern burner
The purpose of the rotary kiln burner is to produce flame to provide the thermal energy to the raw materials enabling a temperature increase from about 900 to 1,450 ?C, to facilitate liquid phase formation and clinkerisation reactions, in the burning zone. The ideal characteristics of the rotary kiln burner are: (a) to provide a short, narrow, highly radiant flame to enable efficient heat transfer to the clinker bed, (b) to ensure complete burning of solid fuels while suspended in the flame, (c) to produce minimum of CO and NOx, (d) to ensure a stable coating formation in the burning zone, (e) to operate with a minimum of primary and transport air, (f) to operate with a minimum of excess air and (g) to be able to handle a flexible choice of both conventional fossil fuels and alternative fuels.

The flame properties are of importance for the clinker quality and for the pyro-system stability and efficiency. Inefficient heat transfer to the clinker bed can result in high amounts of free lime in the clinker, hence lower alite (C3S) content and a lower strength of the cement product. Long, high temperature flame may cause unwanted crystal growth of the clinker minerals which reduces the clinker grindability and increases the energy consumption for grinding. Too wide flame can cause impingement on the clinker bed increasing sulphur evaporation and flame impingement on the kiln walls may lead to coating breakdown which may shorten the lifetime of the kiln refractory. The modern rotary kiln burner has evolved through several stages; the Fig 3 gives a summary illustration.

Burner design parameters
The burner momentum (Ia, N/MW) describes how well the hot secondary air, at about 100, is mixed with the cold (ambient) primary air. The primary air is added to the process at high pressure and velocity (150 to 250 m/s). There is a difference between radial and axial momentum but when momentum is mentioned it normally refers to the total momentum. The most useful and easiest definition for the momentum includes the product of primary air mass flow (ma, kg/s) and velocity (va, m/s) at the burner tip, divided by the thermal energy input (H, MW).

Ia = ma . va / H
The thermal energy (H) input from burner for kiln capacity of 3,000 to 12,000 t/d ranges between 63 to 250 MW (5.4 to 21.5×107 kcal/h). The lower and upper limits may vary substantially depending upon the burner design and efficiency. A typical 1,200 t/d rotary kiln, burning mineral coal, required burner momentum of 6 N/MW, which was increased to 10 N/MW, to burn plastic pellets.

The momentum formula is useful and makes it possible to compare different burners. Higher momentum is not always better; it is necessary to find optimum momentum.

Keeping the burning zone in the right temperature range and maintaining the flame position is crucial and requires a high burner momentum, adapted to the conditions in the kiln system. High burner momentum has several advantages:

• Stable kiln operation and improved fuel efficiency
• Short burning zone and improved clinker reactivity
• Consistent clinker granulometry leads to efficient cooler operation
• Low volatility and recirculation of sulphur leading to overall improvement of the kiln condition
• Decreased tendency to form build-ups and rings
• Increased kiln capacity due to better operation

Burning alternative fuels
The introduction of alternative fuels may influence emissions, cement product quality, process stability and process efficiency. Worldwide the alternative fuel usage in cement production in 2010 constituted about 12.5 per cent of the thermal energy of which about 77 per cent was from waste derived fuels and 23 per cent is from biomass. In some European countries, the alternative fuel substitution in the calciner unit has reached close to 100 per cent.

In some cement plants, the alternative fuel firing also takes place at the rotary kiln materials inlet-end or at a mid-kiln position or in a separate combustion unit where large-size solid fuel is injected, substituting a fraction of the calciner firing.

Challenges and solutions for alternative fuel firing
Alternative fuels may differ from conventional fossil fuels in combustion behavior, due to differences in physical and chemical properties and reaction kinetics.

Particle size and moisture content: Alternative fuels are ground coarser, to save the cost of comminution. While solid fossil fuel (mineral coal) introduced in the burner typically has a size of 5 per cent retained on 90 micron sieve, ground alternative fuels may have size ranging in millimeters. Dried sewage sludge and municipal solid waste may be available with moisture contents from a few to more than 50 per cent mass percentage. Large local and seasonal variations may also occur. Excess air ratio and applying Oxygen enrichment is required for full conversion of the large fuel particles.

Circulation of chlorine and sulphur compounds in pyro-system: Alternative fuels may contain high proportion of chloride and sulphur compounds, which get volatilised at high kiln temperature. Installing kiln bypass is commonly applied to reduce the process challenges related to volatile circulation in the pyrosystem. The bypass for kiln gases is located immediately after the kiln gas outlet and acts as a valve. The extracted gas is quenched to condense the chlorine compounds and to a lesser extent sulphur compounds. The dust may be reused in the cement production or may require disposal. Bypass increases the specific energy consumption by 1.5 to 3 kcal/kg clinker per percentage removal of kiln gas. Typically up to 15 per cent kiln gas may be bypassed to obtain an effective chloride removal, whereas sulphur removal may require higher exhaust gas amounts removed and will require subsequent scrubbing to avoid SO2 emissions.

Conclusions
1.The multi-channel burners effectively manage the variety and complexity of alternative fuel burning. The anticipated benefits with advanced burning technologies are: thermal savings: 3 to 5 kcal/kg clinker, electrical savings: 0 to 0.5 kWh/t clinker and CO2 reduction: 2 to 4 kg/t cement
2.Keeping the burning zone in the right temperature range and maintaining the flame position is crucial and requires a high burner momentum, adapted to the conditions in the kiln system.
3.In some cement plants, the alternative fuel firing also takes place at the rotary kiln materials inlet-end or at a mid-kiln position or in a separate combustion unit where large-size solid fuel is injected, substituting a fraction of the calciner firing.
4.The most important challenges of alternate fuel firing are: particle size and moisture content, circulation of chlorine and sulphur compounds in pyro-system.

References

• Existing and Potential Technologies for Carbon Emissions Reductions in the Indian Cement Industry, Cement Sustainability Initiative (CSI), World Business Council for Sustainable Development (WBCSD), January 2013
• European Cement Research Academy (ECRA) Newsletter, 2/2010
• Gronwall, F., ?Optimization of Burner Kiln 7, Cementa Slite?, SLU, Swedish University of Agricultural Sciences, Department of Energy and Technology, Uppsala 2010
• ?Combustion of solid alternative fuels in the cement kiln burner?, Linda Kaare Norskov, Industrial PhD thesis, Department of Chemical and Biochemical Engineering, Technical University of Denmark (DTU)

Fig 2: Typical combustion-air distribution