Rotary Kiln

Sludge is a semi-solid residue produced during wastewater treatment. It is mainly composed of water (about 60-80%), organic matter (about 60-80% dry weight) and inorganic matter (about 20-40% dry weight). Organic matter includes microbial residues, cellulose and protein, etc.; inorganic matter includes sand, metal salts and clay, etc. Sludge also contains nutrients such as nitrogen (2-5% dry weight), phosphorus (1-2% dry weight) and potassium (0.5-1% dry weight), as well as heavy metals and pathogens. The specific composition of sludge varies depending on its source and treatment method. For example, industrial sludge may contain more heavy metals, while domestic sewage sludge contains more organic matter.

The amount of sludge produced worldwide each year is about 30-40 million tons of dry weight. Among them, developed countries account for a large proportion. For example, the United States produces about 7.5 million tons of dry sludge each year, the European Union produces about 10 million tons, and China produces about 6 million tons. The main sources of sludge include:
Municipal sewage treatment plants (accounting for 70-80%)
Industrial wastewater treatment facilities (accounting for 15-20%)
Agricultural and livestock wastewater treatment (accounting for 5-10%)

Other sources, such as ships and small community sewage treatment systems (accounting for less than 5%) With the acceleration of global urbanization and the improvement of environmental protection requirements, it is expected that by 2030, global sludge production will increase to 50-60 million tons of dry weight/year.

Sludge incineration kiln is a device that oxidizes and decomposes organic matter in sludge through high-temperature combustion, while achieving reduction, harmlessness and resource utilization. Its working principle mainly includes:
Sludge drying: reduce the moisture content to 30-50%
High temperature combustion: burn at 850-1000℃ for 2-3 seconds
Flue gas treatment: remove particulate matter, acidic gases and heavy metals
Energy recovery: use thermal energy to generate electricity or heat
Main types include:
Fluidized bed incinerator (accounting for about 60%): suitable for sludge with high moisture content
Rotary kiln incinerator (accounting for about 25%): suitable for various sludge types
Multi-layer furnace incinerator (accounting for about 10%): suitable for sludge with low moisture content
Other types (accounting for about 5%): such as gasification furnace, plasma furnace, etc.

Main emissions:
Flue gas: 95-98% of the total, including CO2, H2O, NOx, SO2, etc.
Fly ash: 1-3% of the total, containing heavy metals and trace organic matter
Slag: 1-2% of the total, mainly inert inorganic matter
Flue gas treatment:
Dust removal: bag filter, dust removal efficiency>99.9%
Desulfurization: wet desulfurization, desulfurization efficiency>95%
Denitrification: selective catalytic reduction (SCR), denitrification efficiency>80%
Heavy metal removal: activated carbon adsorption, removal efficiency>90% Two more

Significant reduction effect: After incineration, the volume and mass of sludge can be reduced to 10-20% of the original, greatly reducing the need for subsequent disposal and landfillHarmless treatment: High-temperature incineration can completely kill pathogens and harmful organic matter in sludge and achieve harmlessness.

Energy recovery: The heat energy generated during the incineration process can be used for power generation or heating. Sludge incineration can be used as an alternative energy source to achieve resource utilization.Reduce landfill pressure: The ash produced after incineration is small in volume and stable and inert, which can effectively reduce the demand for land for sludge landfill.

Disadvantages:High energy consumption: The sludge has a high moisture content and needs to be pre-dried, and the energy consumption during the incineration process is relatively large.Secondary pollution: The flue gas generated by incineration may contain harmful substances such as sulfur dioxide, nitrogen oxides and heavy metals, requiring an efficient flue gas treatment system, which increases the cost.

Ash treatment: The fly ash and slag produced after incineration contain heavy metals and harmful substances, which need to be solidified or safely landfilled, which increases the difficulty of treatment.High equipment investment: The investment cost of sludge incineration equipment such as rotary kilns and fluidized bed furnaces is high, which is suitable for large-scale centralized treatment, but has poor economic efficiency for small sludge treatment projects.

The energy recovery rate of sludge incineration is usually between 30% and 50%, depending on factors such as the moisture content of the sludge, the type of incinerator and the way the heat is used. Lower moisture content (30-50%) can improve the energy recovery rate of the incineration process because more organic components can be fully burned and release heat when the moisture content is reduced. Fluidized bed incinerators usually have a higher energy recovery rate and are suitable for treating sludge with high moisture content, while rotary kiln incinerators are suitable for a variety of sludge types, but their energy recovery rate is relatively low.

In addition, the way the heat is used will also affect the energy recovery rate. For example, if the heat generated during the incineration process is directly used for steam power generation or district heating, its recovery efficiency will be higher. If it is only used for sludge drying, the energy recovery rate may be lower. To improve the energy recovery rate, it is usually necessary to match an efficient waste heat recovery system and energy management technology, and optimize the sludge pretreatment process to reduce the moisture content.