Sludge Drying and Incineration: Where the 13,510 kJ/kg Autothermal Threshold Comes From

1. Calorific Value: Where Autothermal Operation Begins

Table 2-2 of Chapter 2 of the Technical Guideline for Sludge Treatment and Disposal of Municipal Wastewater Treatment Plants (trial), Jian Ke [2011] No. 34, gives sludge calorific values on a dry sludge basis as follows.

• Primary sludge: 15 to 18 MJ/kg.
• Mixed primary and waste activated sludge: 8 to 12 MJ/kg.
• Anaerobically digested sludge: 5 to 7 MJ/kg.

Calorific value is governed by volatile solids. The same document gives volatile solids fractions of 50 to 70 percent for primary sludge, 60 to 85 percent for waste activated sludge, and 30 to 50 percent for anaerobically digested sludge - which is exactly why digested sludge has the lowest calorific value: the volatile fraction has already been decomposed.

Laboratory results are usually reported as the air-dried basis net calorific value Qad,net. The guideline converts this to the net calorific value Qar,net of wet sludge at moisture content Mar using equation (5-1), which involves the air-dried basis moisture Mad, and states explicitly that if the laboratory result is the net calorific value of absolutely dry sludge, Mad equals zero. This step is the starting point of every heat balance; using a dry-basis value directly as the heat input to the furnace is the most common error in such designs.

2. Sludge Quantity after Drying: Equation (5-2)

Equation (5-2) of the guideline gives the mass balance across the dryer: A2 = A1 x (100 - M1) / (100 - M2), where A1 and M1 are the wet sludge feed rate (kg/h) and moisture content (percent) before drying, and A2 and M2 are the corresponding values after drying. This expression fixes the mass to which all subsequent heat calculations apply, and it also determines how much volume reduction has actually been achieved.

3. Heat Consumption of Drying: Equation (5-3) and Its Stated Constants

Equation (5-3) estimates the heat consumption qgh of the thermal drying system, and the guideline lists the values to be used.

• Cv, mean specific heat of water, taken as 4.187 kJ/(kg.degrees Celsius).
• T1, initial sludge temperature, normally taken as 20 degrees Celsius.
• T2, vaporisation temperature of water, taken as 100 degrees Celsius at atmospheric pressure.
• Latent heat of vaporisation of water at T2, 2,261 kJ/kg at atmospheric pressure.
• eta_gh, thermal efficiency of the dryer.

In other words, drying heat consists of sensible heating plus latent heat of vaporisation, divided by thermal efficiency. This is also why heat consumption rises almost linearly with each further point of moisture reduction, while the scope for improving thermal efficiency is limited.

4. Heat Released by Incineration and the Autothermal Criterion

Equation (5-4) gives the heat produced by the incinerator: qgl = A2 x Qar,net (as fired) x eta_gl, where A2 is the sludge feed rate (that is, the dried sludge rate in kg/h), Qar,net is the net calorific value of the sludge as fired, and eta_gl is the thermal efficiency of the incinerator.

The criterion is stated plainly.

• If qgl is greater than qgh, no auxiliary fuel is required.
• If qgl is less than qgh, the auxiliary heat required is qgh minus qgl (kJ/h), from which fuel consumption follows using the auxiliary fuel calorific value.

The guideline then provides a worked example that can be cited directly: with dewatered sludge at 80 percent moisture, dried to 40 percent moisture before firing, and with both dryer and incinerator thermal efficiency at 85 percent, only a sludge dry-basis net calorific value of about 13,510 kJ/kg (that is, 3,227 kcal/kg) or higher avoids the need for auxiliary fuel.

Setting this threshold against Table 2-2 yields a very practical conclusion.

• Primary sludge at 15 to 18 MJ/kg (15,000 to 18,000 kJ/kg) exceeds 13,510 kJ/kg and can be autothermal.
• Mixed primary and waste activated sludge at 8 to 12 MJ/kg does not reach it.
• Anaerobically digested sludge at 5 to 7 MJ/kg falls well short.

This is the fundamental reason why most sludge drying and incineration projects in China must either provide auxiliary fuel or dry the sludge to a lower moisture content. A design document that simply says "combined drying and incineration, energy self-sustaining" without performing this check is not acceptable.

5. Hard Parameters for the Incinerator Itself

Chapter 5, Section 2 of the guideline gives typical technical indices for the fluidised bed incinerator, currently the principal sludge incineration equipment.

• Sludge throughput shall meet design requirements, with a preferred fluctuation range of 65 to 125 percent.
• Dense-phase zone temperature of the fluidised bed should be 850 to 950 degrees Celsius.
• Flue gas exit temperature above 180 degrees Celsius (because sludge incineration flue gas has high moisture content, this prevents ash deposition and corrosion).

The design and process control clause (1.5) further requires:

• design service life not less than 100,000 operating hours, with appropriate redundant capacity and adjustable feed rate;
• the incinerator shall be fitted with an explosion door or other explosion protection;
• automatic control and monitoring systems shall be provided, displaying operating conditions and flue gas emission parameters on line;
• oxygen content in the flue gas at the incinerator outlet shall be 6 to 10 percent (dry gas).

6. The Three Safety Factors in Drying: the Section Most Often Overlooked

Clause 4.1, item 3 of Chapter 4 Section 4 states this firmly: the three principal factors relating to explosions in drying equipment are oxygen, dust and particle temperature. The safety elements that must be controlled are:

• oxygen content below 5 percent for fluidised bed and vertical disc dryers; below 10 percent for belt, paddle and horizontal rotating disc dryers;
• dust concentration below 60 g/m3;
• particle temperature below 110 degrees Celsius.

Two further operating controls matter equally: methane concentration in the wet sludge silo shall be kept below 1 percent, and dried sludge particle temperature in the dry sludge silo shall be kept below 50 degrees Celsius. Wet sludge shall be conveyed into the dryer sealed, by pump and pipeline; the dryer outlet shall be provided with an emergency storage bin or emergency discharge opening for use during shutdown or abnormal operation. Where sand content is high, wear allowance shall be increased and rotating part speed reduced - particularly important when thermal oil is used as the heat transfer medium.

7. Dryer Selection and Energy Use: Table 4-4 and Unit Capacities

The guideline lists six drying technologies. Energy consumption (Table 4-4) is:

• Fluidised bed: heat 720 kcal/kg water evaporated, power 100 to 200 kWh per tonne of water evaporated.
• Belt: 760 kcal/kg, 50 to 55 kWh/t.
• Paddle: 688 kcal/kg, 50 to 80 kWh/t.
• Horizontal rotating disc: 688 kcal/kg, 50 to 60 kWh/t.
• Vertical disc: 690 kcal/kg, 50 to 60 kWh/t.
• Spray: 850 kcal/kg, 80 to 100 kWh/t.

Unit evaporation and throughput capacities (sludge throughput all at 80 percent moisture): fluidised bed 1,000 to 20,000 kg/h and 30 to 600 t/d; belt dryer low-temperature below 1,000 kg/h and below 30 t/d, medium-temperature up to 5,000 kg/h with full drying up to about 150 t/d; paddle up to 8,000 kg/h and about 240 t/d; horizontal rotating disc 1,000 to 7,500 kg/h and 30 to 225 t/d; vertical disc 3,000 to 10,000 kg/h and 90 to 300 t/d; spray 5 to 12,000 kg/h and up to 360 t/d.

Heat source cost from low to high: flue gas, coal, steam, fuel oil, biogas, natural gas. The guideline recommends indirect heating and states that thermal drying should be combined with waste heat utilisation and should not be installed as a standalone process. For off-gas: water vapour is condensed and treated, non-condensable gases (odour) are discharged; the dryer must be fully enclosed and both the dryer interior and the drying hall kept at slight negative pressure; odour control follows GB 14554 and discharged wastewater follows GB 8978. Investment, where waste heat is available and domestic equipment is used, is 100,000 to 200,000 yuan per tonne of sludge at 80 percent moisture, rising to 300,000 to 400,000 yuan per tonne with imported equipment.

8. Three Boundaries for Co-Incineration

• Co-processing in thermal power plants: co-firing should be carried out on coal-fired boilers of 35 t/h and above; for existing plants converted to co-processing, the sludge input should not exceed 8 percent of the coal feed (new boilers designed for sludge co-firing are not subject to this limit); wet sludge should be fed from the furnace top, and mid-furnace feeding requires water cooling of the feeder; furnace temperature after co-firing shall not fall below 850 degrees Celsius; for co-firing after drying, the sludge must be dried to semi-dry state (below 40 percent moisture) with a loose structure, and indirect drying equipment is recommended.
• Co-processing in cement kilns: municipal sludge at 60 to 85 percent moisture can be incinerated directly in cement kilns. Five modifications are required: refractory in the kiln inlet chamber changed to spalling-resistant castable; compressed air cannons added in the kiln inlet riser duct to remove build-up; the inlet of the precalciner restriction adjusted accordingly; the kiln inlet process dust collector modified; and kiln ventilation area enlarged by 5 to 10 percent. Sludge containing low-temperature volatile components such as hydrocarbons must be fed into the high-temperature zone of the kiln system.
• Co-firing with municipal solid waste: dewatered sludge should not be mixed directly with municipal solid waste; sludge moisture should first be reduced by drying to a level comparable to the waste; waste incineration waste heat should be prioritised as the drying heat source; fluidised bed incinerators should be selected; co-firing temperature shall not fall below 850 degrees Celsius; and the final air pollutant emission limit shall be the lower of the limits for standalone sludge incineration and standalone municipal solid waste incineration.

On flue gas treatment: sludge has lower chlorine content than municipal solid waste, so dioxin formation is typically lower. The main dioxin control methods are improving the "3T" effect (turbulence, temperature, time), maintaining adequate combustion temperature with a residence time of more than 3 seconds, and injecting powdered activated carbon into the flue gas. Bag filters are recommended for dust removal. Fluidised bed sludge incinerators normally meet NOx limits without additional denitrification; where further control is needed, SNCR is recommended, achieving 30 to 70 percent removal. Desulfurisation may use in-furnace desulfurisation combined with semi-dry desulfurisation. Flue gas emission concentrations shall meet GB 18485, Standard for Pollution Control on the Municipal Solid Waste Incineration. Bottom ash and fly ash shall be collected separately, with fly ash disposed of after identification under GB 5085.

9. Sludge Quality Standards and Policy Position

• Clause 1.6 of the guideline requires that "sludgequality for incineration meet CJ/T 290" - the reference as at its 2011 publication. The later GB/T 24602-2009, Disposal of Sludge from Municipal Wastewater Treatment Plant - Quality of Sludge Used in Separate Incineration, is the national standard for the same purpose: issued 15 November 2009, effective 1 June 2010, under the Ministry of Housing and Urban-Rural Development with execution by the National Technical Committee on Urban Water Supply and Drainage Standardization, and confirmed as continuing valid at the review of 4 August 2025. Both documents show as current on the standard information platform, so design documents must state which is applied and confirm this with the competent authority.
• GB 18485-2014 has an amendment jointly issued by the Ministry of Ecology and Environment and the State Administration for Market Regulation (Announcement No. 56 of 2019, effective 1 January 2020); citations should note that the amendment is included.
• Implementation Plan for Harmless Treatment and Resource Utilisation of Sludge (28 September 2022), article (6): cities with large sludge generation, scarce land, high population density and good economic conditions are encouraged to build centralised sludge incineration facilities; sludge containing heavy metals or toxic and refractory organic compounds should preferentially be treated by centralised or co-incineration; for standalone incineration, combined drying and incineration is encouraged, with optimised design, high-efficiency equipment and waste heat recovery to improve thermal energy utilisation; local waste incinerators, thermal power plants and cement kilns should be used for co-processing, with contingency plans for kiln maintenance and shutdown. Article (3) is explicit: except for incineration, mixing industrial sludge that does not meet sludge quality control indices with sludge from municipal wastewater treatment plants is strictly prohibited.
• Order-of-magnitude cost reference: with both drying and incineration using domestic equipment, investment is 300,000 to 500,000 yuan per tonne of sludge at 80 percent moisture, rising to 500,000 to 700,000 yuan per tonne with imported drying equipment; operating cost is about 170 to 250 yuan per tonne with an imported fluidised bed dryer and domestic fluidised bed incinerator, and about 120 to 200 yuan per tonne with a domestic hollow paddle dryer and domestic fluidised bed incinerator (both at 80 percent moisture, excluding depreciation), of which coal and electricity account for roughly 55 to 70 percent.

Sources: Table 2-2 of Chapter 2, Chapter 4 Section 4 (including equations 5-1 to 5-4, Table 4-4 and clause 4.1 design and process control) and Chapter 5 Section 2 of the Technical Guideline for Sludge Treatment and Disposal of Municipal Wastewater Treatment Plants (trial), Jian Ke [2011] No. 34; the standard information page of GB/T 24602-2009; the announcement issuing the amendment to GB 18485-2014 (Announcement No. 56 of 2019); and the Implementation Plan for Harmless Treatment and Resource Utilisation of Sludge (28 September 2022).