Thermo Fab Engineers has supplied more than 15 numbers of steam Boilers in India ranging from 1 TPH to 80 TPH with a pressure range of 10.5 Bar to 87 Bar.

Petcock fired Boilers are characterized with high carbon content, high heating value and high sculpture contents, the sculpture content ranges between 6-7%. The SO2 concentration in the flue gas after the boiler can reach very high value of 11000 mg/nm3 downstream. Electrostatic Precipitators is required (ESP) for dust removal to maintain overall SPM limits.

Recently new emission regulation has been introduce by CPCB. According to this, the boilers in the Industry have to be retrofitted with FGD (Flue gas De-sulfurization) Plants. To comply with the new emission standard, a FGD technology has to be ensured which ensures a very high SO2 removal of 99.5 %.

Thermo Fab has used its competency and expertise to identify a suitable FGD concept and shall prepare the detail design, supply & commission of the complete project.The technologies will be assessed with respect to:-

 1). Achievable SO2 Degree.

2).Space Demand.

3).Consumables & Residues. 

4).Cost Aspects. (Cape & Open). 

The current Center and state Pollution Board norms has to be taken under consideration.

Old Vs New Environmental Norms




NEW NORMS (mg/Nm3)




















BEFORE 31.12.2003

31.12.2003 AND UPTO























< 500 MW

> 500 MW

< 500 MW

> 500




































No Standard
















150 mg/Nm3








For 210 MW








and above








No Standard








There is different FGD process available mainly, belonging to the main categorized as:-

>Dry Process

>Conditioned dry Processes (also called semi- Dry Process).

>Wet Processes

>Regenerative Processes and others.

The separation of gases substance takes place via adsorption over a solid or via adsorption into a liquid. Chemical of Physical adsorption agent are usually brought in to contact with the flue gas depending on the system , the reaction products accumulate in the form of dissolved salts flue gas treatment for SOx/ KNOx make use of wet and dry systems . The Dry and Wet systems usually consist of a reaction area with additives supply, possible a conditioning phase and a dust collector. (ESP, Bag House). Wet air pollution control system for the reduction of HCL, HF & SOx emission operates by means of adsorption using scrubbers of various designs. Each of the system has its own advance and disadvantage and literacy depending on location and specific circumstances.The Basic feature of both Wet and Dry system are compared in *Table 1A. These features are important  for the selection of the most appropriate system.

Wet System

Dry System

Expensive Additives ( NAOH)

Cheap & easy available Ca(OH)2

High Separation Capacity for Relative High Boilers capacity (And Vice Versa) (99.9%)

High Separation capacity of relatively Low Boiler capacity. (99.7%) 

Multi Stage system

Single stage system

Complex effluent treatment and disposal

Simple and robust components

Usually requires dust Pre-Separation

Requires No Dust Pre-Separation

High Areola leads to high Maintenance

Low Maintenance

PCDD/PCDF separation in the conjunction with packed scrubber and HOK doses

Very good separation of PCDD/PCDF via HOK dosing. 

Greater Space requirement

Less Space Requirement

Larger amount of Water is required

No Water required

Downstream water treatment plant is required

No Treatment Plant is required

Slurry Management shall be required

No Slurry hence no additional cost and burden

Relatively cheap then Dry Process.

Relatively costly then Wet Process

High Power Consumption required

Relatively Low Power Consumption Required


Process Description:      Two Stage Process (From 11000- 450 mg/Nm3)

1). First Stage ( Reduction of SOx From 11000 - 5000 mg/Nm3)

In Furnace sorbent injection is injected directly into the section of the furnace in the range of 5-7% by weight of Fuel Qty. After attaining heat the lime stone breaks up and expand it and become a sorbent (Sorbent – The substance which has the property of collecting the molecule of another substance). The Reactive sorbent captures the Sox in suspension to form calcium sulphate. The Solid particles are then captured by the ESP part of the lime dosed reacts with Sox and balance un-reacted sorbent is again injected in the furnace to reduce the stoichiometric factor from 1.8 to nearly 1.2.

2). Second Stage (Reduction of SOx from 5000 to < 450  mg/Nm3)  

After removing the PM, the Separation of Acidic flue gas constituents via Dry adsorption system is a simultaneous and adsorptive gas / solid reaction which take place in the sorbent employed in the process. In the Process the gaseous pollutants are bound to be surfaced of the introduced solid, The Additives can then be separated from the flue gas together with the dust particle via an Absorber.

It is advisable for deployment of an ESP before the Injection of Sorbent .The Injection formulates  a filter cake inside the Absorber which is necessary for the dusting process to function successfully , as it influence the reaction time of the addictive and , hence the contact time of the addictive and the pollutants in the flue gas .

The Dry system are characterized on the additive applied they are accordingly referred to sodium based / Lime based system.

Owing to their single stage construction and low operative requirement with regard to spaces and maintenance, Dry flue gas cleaning system are comparatively inexpensive in OPEX mode .

The large Quantity of the additive that this type of Process consumes in counter balanced by the low purchase price of the said additive additionally the by product can also reduce the cost substantially, and therefore also makes this type of process interesting from an economic point of view.

Energy consumption level is likewise low when compared to the energy level of Wet Flue gas systems.

3). Third & Final Stage (Further reduction SOx from 450 to < 50  mg/Nm3))

By addition of Activated Carbon( AC)  based Absorber along with its accessories it Is further possible to reduce the SOx from 450 mg/Nm3 to further < 50 mg/Nm3) . Here HOK is used to Adsorption of SOx . The Module can be made scalable to the requirement of future SOx limits. Contaminated HOK is further utilized in the system by firing in the Boiler hence leaving no residue.

Hydrated Lime Based Process:-

The Separation of the Pollutants takes place via absorption on the surface of Calcium Hydroxide particles. These particles are brought in to contact with the flue gas in a variety of forms. The Primary reaction takes place accordingly to the following equations:-

1). SO2 + Ca (OH)+ ½ H20         =             CaSO3 + H20.

2). SO3 + Ca (OH)2                          =             CaSO4 + H20. 


                    1). No Re- heating is required after pre-cleaning.

                   2).  No Water Consumption.

                    3). SO3 concentration is significantly reduced therefore risk of corrosion from Suppler acid is


    4). Dry – Pre-cleaning stage material: Carbon Steel.

   5).  By Product therefore easier to handle. Further processing required. 

For Example Process data considered for 52 TPH Boiler ( But considering revised capacity of 60 TPH after modification ) :-

             Incoming gas Volume of Flue Gas after ESP          :               97,600 m3/hr (For 60 TPH)

                Average Temp of Flue Gas after ESP                      :               145 Deg C

                Incoming SOx                                                          :               11000 mg/Nm3

                Outgoing SOx                                                           :               <450 mg/Nm3(After Stage I & II)

                Ca (OH)2 Injection                                                  :               1650 Kg/hr (For 60 TPH)

                By Product Produce (Gypsum)                                :               2300 Kg/hr (For 60 TPH)

               Cost of Ca (OH)2                                                      :               Less then Rs 6 per Kg/-(Approx)

                Cost of Salable (Gypsum)                                       :               Depends on Quality Rs 6 per Kg /-            

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