1. OverviewThe sludge contains hazardous substances including organic impurities and is classified as national hazardous solid waste. Toxic gases will inevitably be released during drying, hence closed-loop drying is required. Contaminants generated in the drying process shall be sent to an incinerator. Therefore, direct drying by tail flue gas is not suitable for the following reasons:
1. If tail flue gas is adopted for direct drying, the treated flue gas has to be recirculated back to the furnace. Massive internal flue gas circulation will cause unstable combustion in the incinerator, low boiler efficiency and higher energy consumption of fans. The fan performance curve must be verified and fans replaced if necessary.
2. The heat utilization efficiency is less than 40%. After passing through the dryer, the 130°C flue gas drops to 80°C, with a maximum heat efficiency of (130-80)/130=38.5%.
3. Low exhaust temperature at the incinerator outlet will easily cause corrosion on tail heating surfaces (economizer, air preheater, etc.) and the chimney, jeopardizing stable system operation.
For the above reasons, this solution adopts thermal carrier drying with steam as the heat carrier. Waste gas generated during drying circulates in a closed loop, and excess waste gas is delivered to the incinerator with no exhaust emission. Steam serves as the heat source, and hot water from the dryer outlet returns to the incinerator. Sludge is dried using the latent heat of steam vaporization, achieving a heat utilization efficiency of up to 85% (heat loss mainly comes from equipment heat dissipation and sensible heat carried by fluidizing air). If a wet chimney is used, the flue gas temperature can be properly reduced and steam flow increased. From the energy balance of the drying-incineration system, this is equivalent to recovering heat from tail flue gas.
Working PrincipleMechanically dewatered wet sludge is fed into the fluidized bed dryer via conveyor and suspended in a boiling state under high-temperature flue gas. Cross-counterflow heat exchange occurs between high-temperature flue gas and sludge for moisture evaporation. The specially designed internal structure enables continuous feeding of wet sludge from one end and continuous discharge of dried sludge from the other. Evaporated moisture is recovered by condenser and returned to the wastewater treatment system. Part of dried sludge is mixed with wet sludge in the mixer before entering the fluidized bed dryer.
Sludge pyrolysis refers to heating solids under oxygen-free or oxygen-deficient conditions to a specific temperature (high temperature: 500–1000°C; low temperature: below 500°C). The solids decompose into three combustibles: oil, non-condensable gas and char.
Main Features(1) Flexible operation and continuous running. By adjusting fuel/coal consumption, bed flow velocity and sludge feed rate, different solid contents of dried sludge can be obtained to meet final disposal requirements.
(2) Intense particle movement inside the fluidized bed enables fast mass and heat transfer between particles and gas, delivering high heat transfer efficiency and evaporation rate. High-moisture materials with moisture content over 80% can be dried in one pass to below 5% with low energy consumption. Moisture in sludge evaporates instantly, resulting in short drying time, uniform temperature distribution inside the dryer, consistent dryness and particle size of finished products.
(3) Compact design of fluidized bed dryer with large processing capacity per unit area and small footprint. No mechanical transmission parts, low failure rate and low construction & investment cost.
(4) The dried sludge features favorable physical properties (appearance, particle size, color, odor, hygroscopicity, etc.) and stable chemical composition (moisture, organic matter, total nitrogen and trace elements). No secondary pollution occurs for safe disposal or resource utilization.
(5) Customizable drying processes can be matched according to different requirements (e.g., rotary drum dryer, vibrating fluidized bed dryer, paddle dryer, belt dryer, etc.).
