Solid Waste Thermal Reduction Rate (Loss on Ignition) Testing

Thermal reduction rate, internationally known as loss on ignition (LOI), is a key indicator of combustion completeness and regulatory compliance for solid waste incineration residues. This technical article explains the definition, calculation, testing samples, methods, and applications of solid waste thermal reduction rate testing in accordance with national and international standards such as GB 18485, GB 18484, and ASTM D7348.

Technical Overview

The thermal reduction rate, widely referred to internationally as loss on ignition (LOI), is defined as the percentage of mass lost by a pre-dried solid waste sample when it is ignited in a muffle furnace at a specified temperature, typically 600 °C ± 25 °C, until constant mass is reached. The mass loss corresponds primarily to the combustion of residual organic matter and unburned carbon remaining in incineration residues.

The parameter is calculated using the formula: P = (m1 − m2) / m1 × 100%, where m1 is the mass of the oven-dried sample before ignition and m2 is the mass of the residue after ignition to constant weight. Results are expressed as a percentage, generally on a dry-mass basis.

The thermal reduction rate is a legally binding performance indicator for waste incineration facilities. The national standard GB 18485-2014, Standard for pollution control on the municipal solid waste incineration, stipulates that the thermal reduction rate of bottom slag shall not exceed 5%, while GB 18484-2020, the standard for pollution control on hazardous waste incineration, requires a value below 5% for incineration residues. A low value indicates complete combustion, efficient energy recovery, and a reduced risk of persistent organic pollutants in the residue.

• Quantifies unburned combustible matter in slag, ash, and other solid wastes
• Serves as a compliance parameter for incineration permits and acceptance testing
• Reflects combustion efficiency and supports process optimization
• Influences the suitability of bottom slag for resource utilization as aggregate

Testing Samples

Bottom Slag from Municipal Solid Waste Incineration

Bottom slag discharged from the grate of municipal solid waste incinerators is the most frequently tested sample type. Its thermal reduction rate directly demonstrates whether the furnace achieves complete burnout and is a mandatory indicator under GB 18485-2014.

Fly Ash from Flue Gas Cleaning Systems

Fly ash collected from boiler hoppers, bag filters, and flue gas treatment units can be characterized by loss on ignition to assess carryover of unburned carbon, which affects subsequent stabilization, solidification, and disposal decisions.

Residues from Hazardous and Medical Waste Incineration

Slag and residual ash from rotary kilns, liquid injection incinerators, and pyrolysis systems treating hazardous or medical waste are tested against the thermal reduction rate requirement of GB 18484-2020 to verify destruction efficiency.

Other Combustion Residues and Solid Wastes

Biomass ash, coal combustion residues, gasification slags, dried sludge, cement kiln bypass dust, and solid recovered fuels may also be evaluated using loss on ignition methods such as ASTM D7348 to estimate combustible or organic fractions.

Testing Items

Core and supporting determinations typically offered by accredited third-party laboratories include:

• Thermal reduction rate (loss on ignition) at 600 °C ± 25 °C
• Moisture content determined at 105 °C for dry-basis conversion
• Ash content and combustible matter content as supplementary indicators
• Supporting characterization such as total organic carbon (TOC), heavy metal content, and leaching behavior for integrated residue assessment

Testing Methods

Testing is performed with reference to the methods cited in GB 18485-2014 and GB 18484-2020, together with international loss-on-ignition practice described in ASTM D7348 for solid combustion residues. Sample collection and preparation follow solid waste sampling specifications such as HJ/T 20 to ensure representativeness.

Sample Preparation

• Reduce the gross sample by coning and quartering to obtain a representative laboratory portion
• Dry the sample at 105 °C in a forced-air oven to constant mass and record the dry mass
• If required for homogenization, grind the dried sample to pass a sieve of approximately 1 mm and mix thoroughly

Ignition Procedure

• Pre-ignite a porcelain or quartz crucible at 600 °C, cool it in a desiccator, and weigh it accurately
• Weigh typically 10 g to 20 g of the dried sample into the crucible and record the combined mass
• Place the crucible in a muffle furnace stabilized at 600 °C ± 25 °C and maintain the temperature for at least 3 hours
• Transfer the crucible to a desiccator, cool to room temperature, and weigh; repeat short ignition cycles until the mass change meets the constant-weight criterion
• Calculate the thermal reduction rate from the mass loss and report the result on a dry basis

Quality Control

Method reliability is assured through duplicate determinations with controlled relative deviation, calibrated analytical balances with 0.1 mg readability, verified muffle furnace temperature distribution using calibrated thermocouples, empty crucible blanks, and, where available, certified reference materials. Test reports state the ignition temperature, holding time, basis of reporting, and measurement uncertainty.

Applications

• Regulatory compliance monitoring of bottom slag from municipal solid waste incineration plants
• Acceptance and commissioning tests for newly built or retrofitted incineration lines
• Performance verification of hazardous waste and medical waste incineration systems
• Operational optimization of grate speed, excess air ratio, and primary air distribution based on burnout quality
• Qualification of bottom slag for reuse as road base aggregate and cement raw material
• Research on combustion modeling, unburned carbon estimation, and residue stabilization
• Due-diligence and environmental audits by operators, regulators, and accredited third-party laboratories

FAQ

What is the difference between thermal reduction rate and volatile matter?

Volatile matter is measured under standardized pyrolysis conditions, usually at 900 °C with limited oxygen, and reflects the volatile fraction of a fuel. The thermal reduction rate measures the actual residual combustible matter left in a combustion residue at 600 °C, so it evaluates burnout quality rather than fuel devolatilization behavior.

Why is an ignition temperature of 600 °C specified?

At approximately 700 °C and above, carbonates such as calcium carbonate begin to decompose and release carbon dioxide, which would artificially inflate the result. Ignition at 600 °C ± 25 °C selectively measures unburned organic carbon and combustible matter while minimizing interference from inorganic decomposition reactions.

What regulatory limits apply to the thermal reduction rate?

GB 18485-2014 requires that the thermal reduction rate of bottom slag from municipal solid waste incineration not exceed 5%, and GB 18484-2020 requires a value below 5% for hazardous waste incineration residues. Specific permits may impose stricter or additional requirements.

On what basis are results reported?

Results are conventionally reported as a percentage on a dry-mass basis, meaning the moisture content determined at 105 °C is removed from the calculation. Reporting on an as-received basis is possible but should be clearly stated in the test report.

What causes elevated thermal reduction rates?

Common causes include insufficient residence time on the grate, poor air distribution, feedstock with excessive moisture or incombustible content, and overloading of the furnace. Corrective actions typically involve adjusting grate speed, secondary air injection, and combustion control parameters.

How much sample is needed and how long does testing take?

A representative sample of several hundred grams is generally sufficient, from which 10 g to 20 g portions are ignited in duplicate. Because drying, ignition to constant weight, and cooling are sequential, routine testing is usually completed within a few working days after sample receipt.