Boiler Stack Loss and Condensate Recovery Decision
A 15 t/h coal-fired packaged boiler at a textile process supplies steam at 10 bar(g). Coal consumption is 2.6 t/h; coal GCV is 4,100 kcal/kg. Feedwater is 85°C and condensate recovery is only 35%. The boiler has no economiser or air preheater. Recent measurements: flue-gas temperature 235°C, O2 8.5% dry, CO 120 ppm, stack draft stable. Blowdown is manual, about 6% of feedwater, and boiler feedwater TDS is 1,450 ppm against an allowed maximum of 3,500 ppm. Steam traps were last surveyed four years ago. The plant wants to invest in an economiser and air preheater before the next inspection shutdown. Production runs 7,200 h/yr. Coal costs Rs 7,500/t. Management asks whether the proposals are justified and in what order.
1. Using the stack and O2 data, identify the dominant losses and decide whether the economiser or air preheater should be first.
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O2 at 8.5% dry implies excess air roughly 68% (21/(21 − 8.5) = 1.68), well above good coal firing; CO at 120 ppm is low, so combustion is complete but there is too much air. Stack loss dominates: 235°C flue gas with high excess air carries sensible heat, and feedwater at 85°C with no economiser is a large heat sink. An economiser is usually first because it recovers heat to feedwater, lowers stack temperature and is independent of combustion air. But the final gas temperature must stay above acid dew point; coal sulfur is missing, so sample sulfur and set the outlet above 150°C or use corrosion-resistant material. An air preheater can recover more but risks cold-end corrosion and higher NOx; it is not first without sulfur and dew-point data. Condensate recovery and a trap survey are no-regret measures before both.
2. Quantify the condensate opportunity if recovery rises from 35% to 70%; is it bigger than the boiler capital projects?
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Feedwater ≈ 15/(1 − 0.06) = 15.96 t/h. Condensate now = 0.35 × 15.96 = 5.59 t/h; at 70% it is 11.17 t/h, so extra condensate = 5.59 t/h. With condensate at about 90°C replacing makeup at 25°C, heat saved ≈ 5.59 × 1,000 × 65 = 363,000 kcal/h. At 75% boiler efficiency and 4,100 kcal/kg coal, coal saved ≈ 363,000/(4,100 × 0.75) = 118 kg/h. At 7,200 h/yr that is about 850 t/yr, worth Rs 63.8 lakh/yr. This is likely cheaper and faster than an economiser. It also reduces blowdown and makeup treatment. Verify the trap survey first, because condensate may be lost through failed traps; otherwise the 70% target is not achievable.
3. What data and safeguards are needed before buying the economiser?
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Need coal sulfur and ash, flue-gas dew point, feedwater temperature, load profile, boiler turn-down and space for the economiser. Size it for 7,200 h at average 2.6 t/h coal, not peak; include bypass and isolation for maintenance. Specify an outlet gas temperature above acid dew point, corrosion-resistant tubes, soot blowers and drains. Check feedwater pressure and economiser pressure drop; ensure it does not cause steaming. Add continuous O2/CO trim to hold excess air at the correct coal-firing level. Verify savings with flue-gas temperature, feedwater flow and coal flow before/after. If sulfur is high, consider an air preheater only with corrosion protection, or avoid it. Without sulfur and dew-point data, the economiser design is not complete.