Scrubbing technology
When it comes to emission-reduction technology, how does a plant know it is selecting the right technology for the job? Redecam believes its DeSOx technology, known as RDS, provides the ideal solution for both the cement and the lime industry. Here it directly compares RDS with wet flue gas desulphurization (FGD). 

Environmental preservation is a global need, involving all countries in the world and all areas of human activity, from agriculture to industry. In terms of industry, there is a tendency to align emission requirements with the best performance achievable by technology. Therefore, it is vital that those companies operating in the industrial sector have as broad a spectrum of possibilities available to them when selecting the best emission-reduction technology for their plants. Several factors have to be taken into account during this selection process, including the possibility of achieving the required emission targets, the technology reliability, the interaction with other gas treatment technologies, technology flexibility (turn-down), byproducts recovery, the specific skills required to operate and maintain the emission reduction plant, as well as the operating and investment cost.

Building on circulating fluidised bed technology
One technology that meets many of these factors is the circulating fluidized bed (CFB) semi-dry reactor from Redecam, known as RDS. CFB scrubbing technology is based on the fluidized bed principle. Hydrated lime and water are injected into a reactor where the powder is suspended and mixed using a high-velocity stream of flue gas entering  from the bottom. The intense mixing between the acid gas, solid reagents and water, and the presence, for a given time, of a liquid phase over the reagent particles, enables SO2 reduction efficiencies of over 95 per cent. The system is also active for the removal of hydrogen chloride (HCl), hydrogen fluoride (HF), mercury and other trace metals, without using reagents other than lime. Once mixed and reacted, the gas flow carries the solids out through the top of the reactor into a fabric filter, which separates the dust from the flue gas. The dust is then recycled back into the  reactor with a recirculation rate in the hundreds with respect to the fresh lime injected, after which the clean flue gas is conveyed to the stack. Since water is injected directly into the reactor, hydrated lime is fed in powder form and no slurry handling is necessary.

Moreover, water is totally evaporated in the reactor, avoiding the need for wastewater treatment. As the final product evacuated from the filter is totally dry, drying equipment is also unnecessary.

The fabric filter is properly designed and sized for this application, due to the high dust load coming from recirculation and the relatively-low operating temperatures (usually 20°C above the dew point). Thanks to the bed mixing, the lime conversion rate is high, even when elevated acid gas reductions are required, so allowing a Ca:S molar ratio of around 1.5 or less.

The final byproduct, even if not usable to replace gypsum as a cement regulator, has a wide range of applications, including as a fertiliser in agriculture, as a building material (additive for the production of screed and mortar, additive for production of building bricks and lime sand brick, fibreboards, etc), in the production of binders, in road construction, in the field of surface and underground mining, for the conditioning of sewage sludge.

Potential benefits to the cement industry
This technology is already widely used in the power and waste-to-energy (WTE) industry, while in other sectors, such as cement and lime, it is not yet present, mainly because the kilns does not usually require DeSOx installations due to their natural scrubbing effect. However, there are several cases where the characteristics of the process, whether for raw materials or fuels, do require DeSOx treatment. Here, RDS technology becomes a competitive option, particularly where high reduction is necessary or where the operating cost of a dry sorbent injection (DSI) technology is not sustainable.

Cost comparison
Tables 1 and 2 compare RDS and wet FGD for a 3000tpd cement kiln where SO2 emissions need to be reduced by 95 per cent.

The operating cost was evaluated starting from a hydrated lime cost of €83/t and a power cost of €41.40/MWh. A higher power cost would further penalise the wet FGD technology, which is the most energy-demanding in this comparison. From a technical point of view, it should be noted that while the byproduct coming from the wet FGD is recoverable in cement grinding in the place of gypsum (the replacement percentage is very low), the byproduct of the RDS can be mixed with the kiln dust and recovered into the raw meal.

The operating cost of the two options is very similar. The higher reagent cost of the RDS is balanced by the higher power and maintenance cost of the wet FGD.

Of course, when the size of the plant decreases, the comparison goes in favour of the RDS, because maintenance cost is not linearly decreasing with plant capacity.

In terms of investment cost, RDS requires around half of the investment typically required for a wet FGD installation. Moreover, its footprint consumption is lower in comparison with a wet FGD as there are no sections for slurry treatment and byproduct drying. In addition,  it does not require specific skills or additional personnel for operation and maintenance.

Finally, due to the necessity to have a specific filter design, RDS is a very interesting option when it is required to improve both SO2 (or heavy metals, HCl, etc) and dust emissions.

A typical application could be when the existing dedusting is carried out by an ESP or old baghouse that has to be revamped.

In this situation, an RDS installed downstream, or in the place of the existing equipment, can solve both gaseous and powder emission issues without requiring heavy investment or big layout modifications. 

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EMISSIONS CONTROL
In many cases the latest round of stringent emission norms requires an upgrade of emissions control technology in which mechanical filtration is able to achieve a higher performance, usually lower than 10mg/Nm3 of dust at the stack. As a result, Redecam has been part of several projects converting electrostatic precipitators (ESPs) into hybrid or bag filters. One such market is India, where many cement plants require such upgrades, leading to the study of specific designs on a case-by-case basis. Often computational fluid dynamics (CFD) are a key tool in optimising gas distribution into existing ESP casings.

Expanding business in India
India has seen a boom in infrastructure and housing projects in recent years and cement producers have expanded their production base accordingly. This drive continues and the country aims to achieve a staggering 600Mta of clinker production by 2022. To improve its service to this key market, Italy-based Redecam Group SpA created with its joint-venture partner Isgec Heavy Industries, a new company, earlier this year a new company with Isgec Heavy Industries called ISGEC Redecam Enviro Solutions Pvt Ltd, which is headquartered in Noida, Delhi. With this joint venture, both partners are geared up to be a complete provider for flue gas treatment systems for the cement, power and metals industries.

“Air pollution is one of the major environmental issues facing India and the rest of Asia today. It is a serious problem with the major sources being industrial emission and biomass burning, vehicle emission and traffic congestion,” said Niccolò Griffini and Suman Jain, board members at Isgec Redecam Enviro Solutions. They added: “In an effort to reduce the country’s air pollution and to help companies to grow with a sustainable development approach, Redecam and Isgec aim to build a strong business in Asia, drawing upon Redecam’s global expertise combined with the skills and knowledge of Isgec, a strong partner headquartered in India.”

ESP-to-baghouse conversion
Case study: Cemex’s Caracolito plant, Colombia

In this case study Cemex required the reuse of the ESP casing to reduce costs of the ESP-to-baghouse conversion project at its Caracolito plant in Colombia. Redecam saw this as an opportunity to provide the customer with a highly-effective yet lowercapex solution.

The existing ESP with capacity of 800,000Am3 /h was transformed into a Redecam DPT model bag filter, including a Simple Pressure System (SPS). The SPS bag fixation system guarantees that the tightness of the casing between the dusty and clean sides is 100 per cent effective. It prevents dust leakage at weak points as a result of two main design features:

1. Individually-punched and -drawn tube sheet holes increase the contact surface area of the bag against the tube sheet by extending and contouring the tube sheet opening. Therefore, the surface contact is not limited to the pure thickness of the plate but is extended to the entire internal surface of the drawn hole.

2. The pressure of the bag collar on the drawn edge of the tube sheet hole is increased, firmly securing the bag’s cloth. The collar is also designed to take advantage of the temperature. The tube sheet is carbon steel and the collar is aluminium, resulting in higher useful pressure being generated on the sealing surface. The filter bag cages were designed to maximise the lifespan of the filter bags. It is crucial that the filter bag fits perfectly around the support cage, because if the bag is too big, it will rub on the cage during cleaning, causing premature wear and tear. The bag cages for this customer were made of carbon steel with cataphoresis painting and were tailored to fit with fibreglass bags with PTFE membrane stitching. The baghouse was engineered to include a jet-pulse Bi-Jet bag cleaning system. This system has a dual venturi arrangement, minimising the dispersion of compressed air during the injection phase and thus increasing the volume of air forced into the bag. In turn, this reduces the quantity of air needed to pulsate the bag and achieves a higher flow velocity than in systems equipped with one venturi. The dual venturi also ensures more accurate air pulses and therefore, less wear on the bags due to misaligned equipment.

Redecam supplied Cemex Caracolito with an online model, which provides less stress on mechanical devices (since there are no compartments to be closed for cleaning operations) and consequently reduces power consumption. The maintenance of a steady pressure drop value across the filter further contributes to reduced mechanical stress on the bag and eliminates a peak of negative pressure over the exhaust fan, which in turn lowers power demand. Redecam also provided and installed the new dust transport and compressed air systems, and revamped the ID fan.

Reducing capex
In addition to transforming the ESP into a baghouse, Redecam designed other parts of the system to reduce costs. The ID fan downstream of the baghouse needed a new impeller, due to the increased pressure drop of the new filtration system. As much of the existing, ductwork was reusable, the existing raw gas duct reaching the baghouse and the existing stack were kept. While this added complexity to the installation, it also reduced its direct costs.

Installation and commissioning
Redecam was instructed by Cemex Colombia to perform an ESP conversion to baghouse filter in 21 days “flame to flame” shutdown time, which gave the Redecam crew 19-20 days to achieve the result. During the kiln outage, Redecam used a 750t crawler crane to perform the ESP dismantling and installation of the new components that conform to the Redecam baghouse filter.

The Redecam team included:

• 109 full operative workers on site
• four site engineers from local erection contractor
• one head of site management
• one mechanical supervisor
• one fan engineer
• one commissioning engineer.

The 50t Clean Gas Plenum lift was executed during a late Sunday afternoon with all the major components already installed and pretested on the ground (dampers, electrical panels, compress air collectors, solenoid valves). Installation of the 3500 bags and cages was achieved in a total of 42h, thanks to a dedicated crew that worked on a 24h basis. Before the first passage of fumes a fluorescent powder test was performed to check the perfect sealing of the clean gas plenum. The kiln start-up was then successfully performed within the guaranteed time.

“All these activities were carried out in the middle of the rainy season,” said Cristian Guadagnin, Redecam’s head of site management, “which affects the schedule due to the impossibility of performing several working tasks such as welding and major lifts. The great effort of the Redecam team with the collaboration of our main contractor was the key to this successful project.”

Conclusion
Since its foundation in the 1980s, Redecam has been committed to providing tailormade solutions to customers’ needs. Being flexible and finding innovative solutions is a must to remain focused on this approach. These projects, which transform underperforming ESPs into bag filters, are perfect examples of saving considerable capex costs, while potentially meeting India’s emissions reduction requirements.

Read the article here: http://www.redecam.com/wp-content/uploads/2018/01/ENVIRONMENTAL-Redecam.pdf