POST COMBUSTION TECHNOLOGY

NOx Abatement from Thermal Power Plants

In the case of coal-fired thermal power plants in India, the focus at present is on control of particulate emissions. However, it is expected that increasingly stringent norms on invisible and harmful NOx emissions will require catalytic control technologies. In view of this, the indigenous technology for 'Selective Catalytic Reduction' adaptable to both low and high levels of NOx emissions developed by ACC's R&D Division at Thane, is a significant step. A patent application has been filed in India on the technology.

The National Thermal Power Corporation (NTPC) participated in the technology demonstration and joined the TIFAC expert committee for the project by providing their facilities and support at the Badarpur Thermal Power Station, near Delhi (Fig. 10).

Fig.10, Demonstration of NOx Reduction System Development by ACC
at Badarpur Power Station

The pilot plant systems have been conceived, designed, installed and commissioned by the team of R&D engineers within ACC and involved significant innovations. Some difficulties encountered in curing the extruded catalyst section have been successfully overcome. The team observed the performance of the catalyst using 100 litres of catalyst with sample flue gas tapped from a chimney at the Badapur Thermal Power Station. The analysis has been carried out using two separate analysers, one at the inlet to the catalyst and the other at the outlet (which was also monitored for oxygen and ammonia slippage). The technology has given excellent results of >95% NOx reduction.

In the earlier phase of the project, the technology demonstration was carried out at the Caprolactum Plant of FACT (The Fertilizers & Chemicals Travancore Ltd.), Kochi (bench scale level of Catalyst volume 4 litres).

SELECTIVE CATALYTIC REDUCTION (SCR) TECHNOLOGY

SCR technology was initially developed in Japan during the late seventies as a post combustion NOx control technique. The technique is preferred as the Best Available Control Technology (BACT) as it is superior to several other primary and secondary NOx control measures available today. The NOx reduction efficiency through SCR technology is more than 95%. In the SCR technology, stoichiometric quantities of ammonia (NH₃) are injected into the flue gas over a catalyst at temperatures ranging from 300° C to 400° C to reduce NOx into harmless nitrogen and water. The reduction occurs even in the presence of large excess of oxygen (O₂) as follows:

4 NO + 4 NH₃ + O₂ ----------à 4 N₂ + 6 H₂O

2 NO₂ + 4 NH₃ + O₂ ----------à 3 N₂ + 6 H₂O

A mixed metal oxide system, primarily containing titania-vanadia along with promoters is used as the SCR catalyst. At ACC, the indigenously available titanium dioxide has been suitably modified to yield the high surface area titania which is the catalyst support. This can be cites as high technology application of the indigenous titania extracted from the rich deposits of Indian Institute. The formulation extruded into honeycomb monoliths offers minimum pressure drop to the large volumes of flue gas in the operating systems. Special binders, plasticizers and extrusion aids have been identified and used in the extrusion technology.

Under Home Grown Technology (HGT) of Technology, Information, Forecasting and Assessment Council (TIFAC), the technology for manufacture of honey-comb catalyst has been scaled up to semi-commercial levels of operation using a high vacuum, high-pressure extruder procured from Germany. The complex dies used for the honeycomb extrusion are designed and fabricated locally. Cell configurations developed vary from 12 cells per square inch for use in high dust atmospheres to 60 cells per square inch for use in dust-free stack gases. The catalyst development is a result of the interactive efforts of scientists from the disciplines of catalysis, ceramics and chemical engineering.

RESULTS OF THE PILOT PLANT TRIALS

In pilot plant trials, the performance of the catalyst has been monitored over several weeks and no deactivation is observed. At optimized conditions of operation, the performance obtained is given in the table in the next column:

Biotechnology has been identified the world over as the discipline that can make the most significant contributions towards the future development of sustainable solutions. Essentially, biotechnology involves the conversion of biological wealth, through biological tools into bio-products. It is crucial to recognise both the need for and the significance of indigenous technologies in view of the fact that such climatic conditions as temperatures, humidity, dust, etc. have a profound effect on biological processes.

Despite the imminent need for indigenous technologies, it has been seen that a large number of indigenously developed techniques are abandoned at the laboratory stage, for want of the funds that are required to transform the technique into technology.

Operating conditions

Linear velocity (Nm/s) 2.4

Space velocity (h^-1) 5,000

Temperature (° C) 350

NH₃/NOx ratio 1.0

Catalyst performance

Total NOx in outlet (ppm) <15

NH₃ slippage (ppm) <10

DENSE PHASE COAL ASH SLURRY DISPOSAL SYSTEM

The disposal of bottom ash and fly ash from thermal power station in the form of high concentration slurries has been demonstrated to have significant advantages over disposal with traditional lean phase slurry or dry placement methods. These advantages include fully contained, dust free handling, high tonnage per disposal hectare, dense deposit and a low life cycle cost per tonne of material handled.

AUSTA energy, an Institute under Queensland Electricity Commission, Australia first demonstrated High Concentration Slurry Disposal (HCSD) Technology at Stanwell Power Station in the year 1992. With the increase of flyash concentrations in the transport water, the amount of free water i.e. the water released from the body of the flyash decreases rapidly as shown in Fig. 11.

For solid concentrations in the range of 60 to 75 percent by mass the amount of free water is less than 10 percent of the transport water. The slurry behaves like non-Newtonian fluid with the flow in the deposit area being self limited. The settling of the flyash occurs as a uniform mass which result in a high density deposit over a limited area, thus making large deposits possible in a small units. The operational advantages of HCSD technology are:

• No free water at discharge
• No Ash Dam Required
• Use of Existing Ash Dams and Retro-fit Situations
• Self Distribution of Slurry
• In-Situ Deposit Density
• In-Situ Permeability
• Deposit Drying
• No Generation of Fugitive Dust

The specific energy consumption compared to lean phase and dry disposal systems for a 2x350 MW Station, transporting 445,000 tonnes per annum of ash over a distance of 1400 m, is shown in Fig. 12.

The cost comparison between HCSD and other options are given in the following table.