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102 % INDIAN COMPANY PUTS ITS NEW BOILER TO THE EFFICIENCY FIELD TEST
Fig 1 : A Schematic Diagram Showing Construction Of The Steamstar Superplus™
The Steamstar SuperPlus™ is a packaged, natural gas fi red, condensing steam boiler having 102% thermal effi ciency on NCV of natural gas. To achieve maximum heat recovery, this packaged boiler consists of six passes of flue gas, namely three passes in the evaporator, and a single pass in the economizer, non-condensing water pre-heater, and condensing water pre-heater.
Combustion products of natural gas comprise gases such as carbon dioxide, water vapor, nitrogen, and traces of other inert gases. The latent heat of condensation of the water vapor is recovered in condensing sections of Steamstar SuperPlus™ making it a 102% thermally efficient steam boiler. The report provides details of the STEAMSTAR SUPERPLUS™ indigenously developed and installed by Transparent Energy Systems PLtd. at the fully owned subsidiary of a German company in India. Performance parameters of the boiler were certifi ed by Petroleum Conservation Research Association (PCRA), India, per BS-845-Part I.
CONSTRUCTION OF STEAMSTAR SUPERPLUS™
As shown in Figure 1, the Steamstar SuperPlus™ comprises three passes of heat recovery in a smoke-tube type boiler, followed by a fourth pass of heat recovery in the economizer, a fifth pass of the noncondensing water preheater, and sixth pass of the condensing preheater. On the flue gas side, the condensing preheater is followed by an externally supported stack with material of construction, carbon steel with internal FRP lining, or stainless steel conforming to IS 6533.
Steamstar SuperPlus™ is equipped with a reliable flue gas side condensate handling system. It comprises a SS- or FRP-lined condensate collection and neutralization tank equipped with condensate transfer pump and automatic level controller. When flue gas is cooled below dew point, the flue gas water vapor is condensed in the condensing preheater. The pH of condensate from flue gas from natural gas combustion has pH values in the range of 3.0–4.5.
To achieve longer life of heat transfer surfaces, a suitable corrosion resistant grade of stainless steels, AISI 304, is used for construction of flue gas path and heat transfer surfaces. Also, adequate access is provided to facilitate maintenance, cleaning, and repairing of heating surfaces. In order to reduce corrosion of heat transfer surfaces, a pressurized deareator tank is provided to remove dissolved oxygen from feed water by steam sparging. Also, an automatic continuous chemical dozing system is provided.
The boiler can be designed to suit conditions such as 0 to 100% condensate return from process. The boiler has flexibility in operation so that the same boiler could be operated through a fuel shift from natural gas to FO/LDO/HSD by bypassing condensing preheater and by operating the flue gas diverter valve and feed water bypass arrangement.
| Instrument | Parameter | Medium |
|---|---|---|
| Flow meter with totalizer | Flow | Steam, Feed water, Fuel Flue gas condensate (optional) |
| Thermocouple | Temperature |
Steam Feed water at inlet to economizer Feed water at inlet to boiler Water at inlet to deaerator tank Flue gas at boiler outlet, economizer outlet & water preheater outlet Water at inlet & outlet to water preheaters |
| Pressure gauge | Pressure |
Steam at outlet of evaporator Feed water at inlet of economizer Feed water at inlet & outlet of water preheaters |
| Oxygen sensor | Oxygen | Flue gas oxygen content |
Table 1. Details of performance measuring system
| MODEL | STEAMSTAR SUPERPLUS™ 6000 |
| FUEL | NATURAL GAS |
| CAPACITY | 6000 KG/HR (F&A 100) |
| SAFETY VALVE SET PRESSURE | 10.54 KG/CM2(G) |
| APPLICATION | STEAM FOR PROCESS USE |
| THERMAL EFFICIENCY ON NCV OF FUEL | 104% |
Table 2. Technical specifications of the installation.
The same boiler could be operated through a fuel shift from natural Gas to FO/LDO/HSD by bypassing the condensing preheater and by operating the flue gas diverter valve and feed water bypass arrangement.
WORKING PRINCIPLE
Boiler feed water is preheated in three stages: condensing water preheater, noncondensing water preheater and economizer. Flue gas at the outlet of a smoke tube type evaporator section passes through the economizer section to heat the boiler feed water preheated in the non-condensing water preheater. Furthermore, the flue gas passes through the non-condensing water preheater to heat the boiler feed water preheated in the condensing water preheater such that the flue gas is cooled just above its dew point temperature of 57°C. Then the flue gas passes through the condensing water preheater to heat the boiler feed water from 30°C to 35°C such that flue gas is cooled below its dew point temperature up to 47°C.
In this way, the latent heat of condensation of water vapors present in flue gas is recovered to enhance the thermal efficiency of the boiler up to 102%. Optionally, when the air preheater is provided, it further improves the efficiency of the Steamstar SuperPlus™ up to 103%.
The flue gas from combustion of natural gas has better characteristics as compared to flue gas from fuel oils:
- Higher percentage of water vapors in flue gas. This allows higher heat recovery in condensing preheater.
- Higher flue gas dew point. This offers a thermodynamic advantage in recovering heat in multiple stages such as condensing preheater, non-condensing preheater, and economizer.
- Higher pH of flue gas condensate.
- High pH value of condensate does not demand usage of expensive corrosion resistant materials for construction of heat transfer surfaces.
| Temperature of flue gas at the outlet of heat recovery UNIT (DEG C) | Overall thermal efficiency, on lhv basis in % | Overall thermal efficiency, on hhv basis in % |
|---|---|---|
| 183 | 92 | 83.1 |
| 120 | 95 | 85.8 |
| 99 | 96 | 86.7 |
| 78 | 97 | 87.6 |
| 57 | 98 | 88.5 |
| 55 | 99 | 89.4 |
| 53 | 100 | 90.3 |
| 51 | 101 | 91.2 |
| 49 | 102 | 92.1 |
| 47 | 103 | 93 |
| 44 | 104 | 9 |
Table 3. Overall thermal efficiency vs outlet temperature of flue gas.
PERFORMANCE MEASUREMENT
The Steamstar SuperPlus™ is equipped with a continuous online real time measurement and recording system comprised of instruments shown in Table 1.
The performance tests of Steamstar SuperPlus™ can be carried out by two methods:
- Thermal efficiency measurement by direct method of enthalpy difference across boiler (steam to fuel ratio).
- Thermal efficiency measurement by indirect method as per BS-845- part I & II.
CASE STUDY
Based on above mentioned principles and mode of construction, a commercial-scale Steamstar SuperPlus™ was built in year 2011 for a fully owned subsidiary of a German pigment processing company having a process plant in Gujarat, India. Table 2 gives the details of the installation.
The performance trails were conducted by both direct as well as indirect method. The Table 3 accompanied by the graph shows the performance of the Steamstar SuperPlus™ in terms of overall thermal efficiency vs outlet temperature of flue gas.
The thermal efficiency measurement of Steamstar SuperPlus™ was made using indirect method as per BS-845-part I. The performance trials were witnessed and certifi ed by the Petroleum Conservation & Research Association (PCRA) of India.
Original Publisher:
Transparent Energy Systems Pvt Ltd., "102 % Indian company puts its new boiler to the efficiency field test", Today's Boiler, Magazine of American Boiler Manufacturers Association, Page 14, Fall 2013
