Improve boiler efficiency and recover water from flue gas

Rising energy costs have made it imperative of companies within the process industry to periodically review their energy consumption and diverse methods to improve energy efficiency to improve operating margins. This case study highlights the advantages of a condensing boiler in reducing energy cost.

Energy conservation initiative

Natural Gas ( NG) is a very common and clean fuel which is widely used by various industries for steam generation. Natural gas is also well known for its combustible behavior and the most nonpolluting and easy-to-combust fuel due to its low C/H ratio. Therefore, it has always been in great demand not only for various industrial uses but also for many domestic applications including usage in car, bus etc to meet stringent norms of the environment. Moreover, it is easy to handle.

In a usual operation of any medium pressure boiler designed for 15 bar saturated stream generation, there are mainly three types of heat losses; conductive and convective heat loss through insulation and radiation, heat loss through blow down and heat loss through exit flue gas. Though, energy loss through insulation , radiation and blow down is minimal, large and sizeable amount of heat energy is lost with the flue gas exited in the atmosphere at high temperature. Recovering this heat energy from this flue gas from various types of boilers is a real challenge (Table 1).

Normally, net calorific value ( NCV) of natural gas varies is in the range of 8500- 8600 Kcal/Sm3 . Having flue gas temperature in the range of 110-115°C in a natural gas fired boiler, boiler efficiency varies in the range of 91-92%. Flue gas temp of different conventional steam boilers using different fuel is given in Table 1 based on our operational experience.

Due to change in Govt policy and huge demand supply gap, natural gas price had radically increased from beginning 2009. Price increase was further coupled by the volatile price of liquefied natural gas (LNG) which was being imported to meet the excess demand. Atul Ltd ( Aromatics Division) being an export oriented unit, was badly affected and started losing business mainly on account of higher input energy cost to the product though our atom efficiency is at par or in some cases it is even better than the global bench mark. In Indian scenario, both supply and cost of natural gas had changed considerably due to dramatic increase in the consumption in various segments. This had widened the gap of demand and supply leading to significant price increase of natural gas.

Therefore, there was no other option, but to think differently to reduce energy cost the overall. Boiler being one of the major energy consumption points, the primary focus was on increasing the boiler efficiency and reducing natural gas consumption and therefore the energy cost for steam generation to retain the leadership position. To achieve the stated objective, M/s Transparent Energy System Pvt Ltd (TESPL), a Transparent Group Company & known for innovative solutions in various fields, was invited to explore and provide a techno-commercially feasible solution for having improved boiler efficiency. After rigorous discussion and technical appraisal, installation of a condensing boiler (the patented technology of TESPL) was proposed by TESPL. Atul was convinced with the techno-commercially feasible solution suggested by TESPL and decided to go for installation of a 24 TPH (tonnes per hour) boiler designed at 14 bar with 3 stages economizers system. This entire condensing boiler was successfully designed, manufactured and commissioned by TESPL in mid 2014 and helped us to achieve the energy conservation objectives.

Heat recovery from flue gas from different types of boilers

No Steam Boiler using different fuel Flue Gas Temp
1 Coal Fired Boiler 160-170°C
2 Furnace Oil Fired Boiler 170-180°C
3 Conventional Natural Gas fired Boiler 110-115°C
3 Condensing Boiler, Natural Gas fired 53-56°C

Boiler efficiency

Any petroleum fuel contains constituents like Carbon (C), Hydrogen (H), Oxygen ( O) , Sulphur (S), Nitrogen (N) and water or moisture ( H2 O ) etc. in different proportions. Combustion of C,H,S in presence of air releases energy due to exothermic nature of the reaction. Gross Calorific Value ( GCV) is defined as the energy released due to combustion of per unit mass of fuel. It is generally expressed in Kcal/Kg or Kcal/Sm3

Combustion of Hydrogen (H) present in fuel forms water which remains in vapor form in flue gas; let’s call this mass as x1 . Moisture/water present in the fuel also evaporates and remains in water vapor form in flue gas; let’s call this mass as x2 . x2 is normally very less in natural gas. Combustion also carries atmospheric moisture along with air which also remains in flue gas in vapor form. Let’s assume this mass as x3 . Therefore, total water vapour present in the flue gas is x1 +x2 +x3 .

For evaporation of this total water and reach to flue gas temperature requires energy, known as enthalpy, which is taken away from the gross energy released during combustion process. As a result, heat carried away by the water vapour in the flue gas is not available for steam generation. It is directly lost in the atmosphere. The air and the combustion products other than water (CO2 , CO, NOx ) and nitrogen also take away a significant amountof heat.

Hence, GCV minus heat lost in the water vapour, inerts and combustion products per unit mass of the fuel is called Net calorific Value (NCV). Boiler efficiency is calculated based on NCV of the fuel as heat carried or lost with water vapour is any way not available for heat transfer. It is NCV which is available for heat transfer in the boiler for steam generation.

Condensing boilers use heat from exhaust gases that would normally be released into the atmosphere as flue gas. To use this latent heat, the water vapour from the exhaust gas is turned into liquid condensate through heat exchange of cold demineralized feed water which is preheated in a specially designed condensing economizer. Due to this process, a condensing boiler is able to extract more heat from the fuel it uses than a standard boiler. It also means that less heat is lost through the flue gases.

Flue gas temp coming out from the boiler at 330-340°C is passed through the 1st stage non-condensing economizer and decreased to 170-175°C. Then it is again passed through a 2nd stage non-condensing water pre-heater where temp is reduced to 80-85°C. Finally it is passed through a 3rd stage condensing economizer where final temp of the flue gas is further reduced down to 53-55°C by preheating the boiler feed make up water fed at 31°C. Reducing the flue gas temp from 33° to 55°C is very unique in industrial boiler operation. This serves two purposes at a time.

1. It helps in improving thermal efficiency by lowering NG consumption for unit production of steam

Fig 1 : Graph of Thermal Efficiency vs Flue gas temerature reproduced from brochure of TESPL

A schematic diagram of condensing boiler showing reduction in flue gas temp in stages is given below. Efficiency of the natural gas fired boiler at different flue gas temperature is shown in the Fig 1. At 55°C flue gas temp, boiler efficiency is ~ 100 % compared to 91-92 % in a conventional boiler having flue gas temp of 110- 115°C. At 45°C flue gas temp, boiler efficiency can be as high as 103% setting a new bench mark

Estimated natural gas saving per year at different capacity condensing steam boiler is given Fig 3 based on actual operational experience of condensing boiler of Atul Ltd. Min 3 Sm3 natural gas can be saved per MT of steam generation Vs conventional boiler. NG saving can be increased upto 4 to 5 Sm3 per MT of steam generation depending of the lower flue gas temp and efficient design. Installation of 24 TPH condensing boiler had led to saving of 550000 Sm3 natural gas per year @ 80% load and value addition of more than ` 1.7 Cr on account of only energy conservation.

2. Recovering water from the flue gas

Due to low C/H ratio, burning of 1 mole methane (Molecular weight of methane is 16) generates 2 moles of water (Molecular weight of water is 18). Therefore, combustion of 16 kg methane generates 36 kg water on complete combustion with sufficient air which is carried away with the flue gas in the form of vapour. This water is generally lost in the atmosphere. Cooling down the flue gas temp below dew point and recovering the water from the flue gas directly leads to conservation of natural resource.

Latent heat of condensation of water from flue gas is indirectly used for preheating the boiler feed water entering @ 31°C. About 60,00,000 litres water is recovered per year by condensation from flue gas at flue gas exit temp of 55°C. This is almost 40 % water recovery from the flue gas. Condensed water is little acidic and pH varies in the range of 3-4 . This water is used and recycled in the process for suitable use. Acidity is mainly due to presence of mainly weak carbonic acid formed by scrubbing of carbon di-oxide gas with condensed water at lower temp. This CO2

Fig 2. Schematic diagram reproduced from presentation of TESPL

Fig 3. Estimated natural gas savinf per year at different capacity

Fig 4. Installation of 100% thermal efficiency condensing boiler by TESPL at Atul Ltd.

Scenario before and after installation of 24 TPH condensing boiler at Atul Ltd (Aromatics Div)

No Description Before After
1 Flue Gas Exit Temperature in Chimney 115°C 55°C
2 Water Recovery from flue gas, per annum (L) 0 60,00,000
3 Natural Gas Consumption per MT of steam, Sm 74 70
4 Natural Gas savings per annum, Sm³ 0 5,50,000
5 Energy Cost Saving per annum, lacs 0 500
6 Average reduction in carbon dioxide emission MT/annum 0 1600
7 Boiler Thermal Efficiency on NCV of Fuel 92% 100%

absorption process directly reduces greenhouse effect in the atmosphere. The entire operation of 24 TPH condensing boiler is automatically controlled and monitored from a dedicated PLC system. Consumption of both natural gas and water recovered is measured real time using a flow meter installed on-line. The overall efficiency of the condensing boiler of Atul Ltd has been audited by Petroleum Conservation and Research Association (PCRA) in operating conditions. They have certified that the overall thermal efficiency is 100.62% based on NCV @ flue gas temp of 52°C and natural consumption is 69.6 Sm3 /MT of steam generation at 14 bar.

Challenges in installation of condensing boiler:

  • Selection of proper metallurgy for designing the condensing economizer for long life as it leads to low end corrosion due to acidic nature of the condensate and condensation of other acidic vapour such as NOx or SOx if any.
  • Chimney should be with proper MOC i.e. either FRP lined or AISI SS 304.
  • Independent condensate collection system and suit able use in the process with the by-pass arrangement of the condensing section to have flexibility in flue gas shift.
  • Pressurized de-aeration tank to remove dissolved oxygen from feed water by steam sparging.

Benefits of installation of condensing boiler:

  • Boiler efficiency can be increased > 100% depending of the temperature of condensate return.
  • Minimum 3 Sm3 natural gas can be saved per MT of steam generation Vs conventional boiler. NG saving can be increased upto 4 or 5 Sm3 per MT of steam generation depending of the lower flue gas temp and efficient design.
  • Highly energy efficient & eco-friendly.
  • De mineralized water recovery from flue gas leading to conservation of natural resource.
  • Reduction in CO2 emission in the atmosphere.
  • Low ambient temperature due to lower flue gas temp in chimney ( 50-55°C).

Installation of condensing boiler has gained the momentum in all developed countries to reduce energy cost. Population of such boiler installation has been limited in India. However, it is always better late than never and therefore, Atul did it for a larger goal.

Syamal Kumar De

Syamal Kumar De is the General Manager (Technology & Operation) in Atul Ltd’s Aromatic Division. De is an M Tech Chemical Engineer from IIT, Bombay. He has been serving Atul Ltd in the field of design & development, technology innovation & transfer, efficiency & productivity improvement for the past 21 years.

Original Publisher:

Syamal Kumar De, "Improve boiler efficiency and recover water from flue gas", Chemical Industry Digest, Page 72, April 2016