Waste heat boiler

01

Characterise the gas and its lowest safe outlet temperature

The US Department of Energy's waste heat recovery report (2008) states that the heat in a waste stream depends on its mass flow rate and specific enthalpy, and classes waste heat by temperature: high at 649 °C (1,200 °F) and above, medium from 232 to 650 °C, and low at 232 °C (450 °F) and below. Because heat transfer follows Q = UAΔT, a small temperature difference between gas and water needs a larger heat exchanger. [1]

The same report explains that flue gases cooled below their dew point condense and deposit corrosive substances, so exchangers are designed to keep gas above the condensation point: gas from natural gas might be cooled to about 120 °C (250 °F), while gas from coal or high-sulphur fuel oil may be limited to about 150–175 °C (300–350 °F), and sulphates in glass-furnace exhaust deposit below about 270 °C. It calls 300 °F (150 °C) "a common design point used to avoid condensation with many waste gas streams" and notes that fouling remains an unresolved problem. [1]

DOE's steam tip sheet on economisers gives the same fuel-based limits for boiler flue gas: 250 °F (about 120 °C) for natural gas, 300 °F (about 150 °C) for coal and low-sulphur fuel oils, and 350 °F (about 175 °C) for high-sulphur fuel oils, set to prevent condensation and possible stack corrosion. [2]

Give the supplier the gas source, flow (mass or normal volume), inlet temperature range, composition including H₂O, SO₂/SO₃ and HCl, dust load and stickiness, allowable gas-side pressure drop, required outlet temperature, steam pressure and temperature or hot-water conditions, feedwater temperature and quality, auxiliary firing, space, and design code.

02

Estimate the heat recovered and the steam it makes

DOE describes waste heat boilers as water-tube boilers that use medium- to high-temperature exhaust gases to generate steam, available for gas intakes from 1,000 to 1 million ft³/min; auxiliary burners or an afterburner can be added where waste heat is insufficient, and superheated steam needs an external superheater. [1]

In DOE's economiser example, 45,000 lb/h of 150 psig saturated steam (enthalpy 1,195.5 Btu/lb) made from 117 °F feedwater (84.97 Btu/lb) is a boiler output of 50 million Btu/h, and DOE states that "generally, boiler efficiency can be increased by 1% for every 40°F reduction in flue gas temperature". [2]

Thermax (TBWES) lists waste heat recovery boilers for incinerators that cool flue gas from burning waste gases, liquids and solids and raise steam for power or process, at capacities up to 15 TPH and pressures up to 25 kg/cm² (g), to ASME, IBR, TR CU or PED. Its options include water-tube or fire-tube construction, bare or extended heat-transfer surface, natural circulation, single- or multi-pass gas paths, membrane-wall or refractory-lined construction and an application-specific cleaning mechanism. [3]

Editorial calculation: 25 kg/cm² (g) is about 24.5 bar(g) or 2.45 MPa(g) (25 × 0.981). [3]

Editorial calculation: 20,000 kg/h of exhaust gas cooled from 500 °C to 180 °C, with an assumed mean specific heat of 1.1 kJ/kg·K, gives up 20,000 / 3,600 × 1.1 × 320 ≈ 1,956 kW. Using DOE's enthalpy rise of 1,195.5 − 84.97 = 1,110.5 Btu/lb (≈ 2,583 kJ/kg) for 150 psig saturated steam from 117 °F feedwater, that heat would make about 1,956 / 2,583 ≈ 0.76 kg/s, or about 2.7 t/h of steam, before heat losses and blowdown. Gas flow, temperatures and specific heat are assumptions; use measured values and your own steam conditions. [2]

03

Chinese boiler makers listing waste heat units beside fired boilers

Six companies are shown below; Zhengzhou Boiler is the only one whose card gives a range, 10–75 t/h up to 2.5 MPa, and the others list waste heat boilers next to coal, biomass, CFB or oil-gas boilers.

Listings include waste heat boilers alongside fired, biomass and fluidised-bed boilers, with published capacity and pressure ranges from some makers, and waste heat and incineration boilers with pressure parts. Ask each for heat-balance calculations on your gas, not catalogue ratings.

04

Match the boiler type to gas flow, gas pressure and solids load

Rentech Boiler Systems lists watertube and firetube waste heat boilers for flue gas capacities from 50,000 to 1,000,000+ lb/h (22,000 to 450,000+ kg/h), steam capacities from 10,000 to 400,000+ lb/h (4,500 to 182,000+ kg/h), saturated or superheated steam, and pressures from 200 to 1,500 psi (14 to 105+ bar). It names high flue gas pressure, high solids content and corrosive gases as design conditions, and says its refractory linings can comply with API 936. [4]

For dusty gas it offers an open bottom for removing ash and solids, or ash hoppers attached under the boiler that connect to a solids-removal system supplied by the customer. Its single-drum firetube boilers suit flue gas at relatively high pressure, in one-, two- or three-pass vertical or horizontal arrangements; elevated-steam-drum firetube boilers with natural circulation serve higher steam output and pressure; and process gas firetube boilers can have an internal bypass to hold the required gas exit temperature. [4]

Editorial calculation: Rentech's upper figure of 105+ bar is about four times the 25 kg/cm² (g), about 24.5 bar(g), listed for Thermax's incinerator units (105 ÷ 24.5 ≈ 4.3). [4] [3] Give each supplier your gas pressure, solids load and corrosive constituents as well as the steam pressure.

05

Evaporator, drums, economiser, superheater, auxiliary burner and level control

The superheater and auxiliary burner are options that DOE describes for gas that is too cool or too little.

06

Steam capacity, pressure, gas temperatures and materials for the quotation

Gas flow, composition and steam conditions come from your process. DOE and Thermax figures appear in the middle column as limits and examples, not as a rating for the unit offered.

07

Send the gas, steam and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Gas: [source; flow; inlet temperature range; composition; dust]
Limits: [outlet temperature; pressure drop]
Steam: [pressure; temperature; quantity needed]
Feedwater: [temperature; quality]
Options: [economiser; superheater; auxiliary burner]
Site: [space; code; cleaning method]

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