Waste Gas Heat Recovery (also commonly referred to as off-gas waste heat recovery or exhaust heat recovery)    is an engineering approach that captures useful thermal energy from industrial exhaust streams and converts    it into reusable heat carriers such as hot water, hot air, or steam. Properly designed systems can reduce    fuel and utility consumption, lower operating cost, and improve overall plant energy efficiency—while    supporting stable and compliant environmental performance.

1. Overview of Waste Gas Heat Recovery

In many industrial processes, exhaust streams (waste gas / off-gas) leave equipment at meaningful    temperatures but with limited remaining fuel value or limited feasibility for further combustion.    If released directly, the sensible heat (and sometimes latent heat) is lost. Waste gas heat recovery    systems use heat exchangers or waste heat boilers to transfer that energy to a usable medium.

This topic is broader than HRSG (Heat Recovery Steam Generator). HRSG typically refers to a specialized    steam generator used primarily behind gas turbines in combined-cycle power plants. Waste gas heat recovery,    by contrast, is a general industrial concept covering a wide range of sources, temperatures, and recovery    targets (hot water, hot air, low/medium-pressure steam, process preheating, etc.).

2. What Is “Off-Gas” or “Waste Gas” in Industrial Practice?

In industrial contexts, “off-gas” or “waste gas” generally refers to exhaust streams that:

• have completed their primary reaction/combustion duty and are no longer intended as a heat source,
• contain limited fuel value or are not suitable for re-combustion,
• still carry significant thermal energy (commonly 120–600 °C, sometimes higher), and
• may include corrosive, dusty, or moisture-laden components that influence recovery design.

Typical sources include:

• Boilers, heaters, furnaces, incinerators, and thermal oxidizers (e.g., RTO/RCO) outlet gas
• Process vents and off-gas from chemical, coking, metallurgy, and pharmaceutical operations
• Post-treatment exhaust from VOC control systems and related environmental units
• Dryers, reactors, and other thermal process equipment

3. Core Objectives and Value Drivers

A well-scoped waste gas heat recovery project is not merely “cooling the exhaust.” It is typically designed    to achieve one or more of the following:

1. Energy reuse: Convert exhaust sensible/latent heat into hot water, hot air, or steam for plant use.
2. OPEX reduction: Reduce consumption of fuel, steam imports, electricity, or other utilities.
3. Overall efficiency improvement: Enable heat cascading and better integration with upstream/downstream processes.
4. Operational stability: Support downstream environmental systems by lowering exhaust temperature and stabilizing flow.

4. Common Waste Gas Heat Recovery Configurations

4.1 Gas-to-Water / Gas-to-Air Heat Exchange

The most common configuration uses an exchanger to transfer heat from waste gas to water or air,    producing hot water or hot air.

• Typical equipment: shell-and-tube heat exchangers, economizers, air preheaters
• Best suited for: low-to-medium temperature exhaust, stable continuous operation

4.2 Waste Heat Boiler (WHB)

When steam is the most valuable recovery medium, a waste heat boiler generates saturated or low/medium    pressure steam using the exhaust stream as the heat source.

• Recovery outputs: saturated steam, low/medium-pressure steam
• Key notes: design must address corrosion, dust/ash fouling, and allowable backpressure

4.3 Condensing (Low-Low Temperature) Heat Recovery

For moisture-rich exhaust, additional energy may be recovered by condensing water vapor to capture    latent heat—provided corrosion and condensate management are properly engineered.

• Technical focus: corrosion-resistant materials, dew point control, condensate drainage and treatment
• Best suited for: lower-temperature, high-humidity exhaust streams

4.4 Integrated Process Heat Recovery

Recovered heat can be integrated into plant utilities or process duties such as:

• feed or process fluid preheating
• adsorption/desorption regeneration heating
• district/plant heating and domestic hot water
• other auxiliary thermal demands

5. Typical System Architecture and Key Components

A complete waste gas heat recovery package typically includes:

• Gas take-off and bypass arrangement (for startup, upset conditions, or turndown)
• Heat recovery equipment (heat exchanger / economizer / waste heat boiler)
• Anti-fouling and maintainability features (soot blowing, access doors, removable bundles, online cleaning)
• Corrosion control design (material selection, coatings, dew point management)
• Condensate and blowdown handling (where relevant)
• Instrumentation, controls, and interlocks (temperature, pressure, DP, flow; safety logic)
• Inspection and maintenance provisions (platforms, manways, isolation, drains/vents)

6. Critical Engineering Design Considerations

Waste gas heat recovery is highly site-specific. A robust design typically starts from an accurate gas profile    and then addresses the main risk drivers:

6.1 Gas Composition and Corrosion Risk

• Check for acid-forming species and corrosion triggers (e.g., SO_x, HCl/Cl⁻, H₂S, NH₃ slip, etc.).
• Evaluate dew point behavior (acid dew point vs. water dew point) and select materials accordingly.

6.2 Minimum Stack Temperature and Dew Point Control

• Define the minimum allowable exhaust outlet temperature to avoid condensation-induced corrosion.
• Where condensing recovery is required, implement a dedicated corrosion strategy and condensate handling.

6.3 Pressure Drop and Backpressure Constraints

• Confirm allowable pressure drop to avoid impacting upstream fans/blowers and process stability.
• Use conservative fouling allowances to prevent DP escalation over time.

6.4 Fouling, Ash, and Plugging Mechanisms

• Assess dust loading, particle size, stickiness, and potential for salt crystallization or tar deposition.
• Specify soot blowers, rapping, online washing, or maintenance access to sustain performance.

6.5 Turndown and Load Variability

• Consider bypass and control strategies to handle start/stop and fluctuating flow/temperature.
• Check thermal stress, expansion, and fatigue under cycling conditions.

7. Typical Applications by Industry

• Coking: off-gas heat recovery after primary cooling, desulfurization, and related units
• Chemical & Pharmaceutical: reaction vent/off-gas, solvent recovery exhaust, dryers
• VOC Control: RTO/RCO outlet gas heat recovery for utilities or process duties
• Metallurgy: sintering/cooler exhaust, reheating furnace exhaust, process off-gas
• Energy & Environmental Retrofits: boiler exhaust heat recovery and efficiency upgrades

8. Summary

Waste gas heat recovery is an effective pathway to capture “hidden” energy from industrial exhaust streams,    provided that corrosion, fouling, backpressure, and load variability are properly engineered. By converting    exhaust heat into hot water, hot air, or steam—and integrating that energy into the plant’s heat consumers—    facilities can reduce operating cost and improve overall efficiency while supporting stable environmental    performance.