Hazardous Waste Management - HW Combustion

HW Combustion

• Approximately 200 Treatment, Storage, and Disposal Facilities (TSDFs) utilize combustion for hazardous waste treatment and disposal.
• Combustion involves controlled burning within an enclosed area.
• Most facilities combust hazardous waste generated on-site, while some accept waste from off-site locations.

Advantages of Combustion

• Permanent destruction of toxic organic compounds:
  • Breaking chemical bonds to revert substances to constituent elements.
  • Reduces or eliminates toxicity.
• Volume reduction of hazardous waste requiring disposal:
  • Safer and more efficient land disposal of ash compared to untreated hazardous waste.

Combustion Reactions & Emissions

• A high-temperature flame zone breaks down organic and inorganic molecules.
• Reactions occur between hazardous waste components, oxygen, and nitrogen (N2) in the air.
• Predominant reactions:
  • Carbon and oxygen: produces carbon dioxide (CO2).
    C + O2 \rightarrow CO2
  • Hydrogen and oxygen: produces water vapor (H2O).
    2H2 + O2 \rightarrow 2H_2O
• Incomplete combustion of organics: produces some carbon monoxide (CO).
• Hydrogen reacts with organically-bound chlorine: produces hydrogen chloride (HCl).
  H + Cl \rightarrow HCl
• Oxygen reacts with sulfur and nitrogen compounds (and some N2 in air):
  • Produces sulfur oxides (SOx) and nitrogen oxides (NOx), respectively.
    S + O2 \rightarrow SOx
    N2 + O2 \rightarrow NO_x
• Oxygen and metals: produce metal oxides.
  Metal + O_2 \rightarrow MetalOxide

Efficient Combustion Factors

• High temperature: Every part of the gas stream must reach an adequately high temperature (e.g., 1800^{\circ}F for a rotary kiln).
• Sufficient residence time: Maintaining the required temperature for a sufficient period of time.
• Adequate mixing: Ensuring a proper mixture of fuel and oxygen.

Types of Combustion Processes

• Boilers:
  • Use controlled flame combustion to recover energy.
  • Energy is exported as steam, heated fluid, or heated gases.
  • Two main parts: combustion chamber and tubes/pipes holding fluid (e.g., water).
• Industrial furnaces:
  • Recover energy from hazardous waste.
  • Use hazardous waste as fuel to heat raw materials (e.g., cement kiln).
  • Recover materials from hazardous waste (e.g., lead smelter).
• Incineration:
  • Thermal destruction of primarily organic hazardous waste.
  • Uses controlled flame combustion to reduce waste volume to ash.
  • Lessens toxic gaseous emissions (e.g., rotary kilns).

Combustion By-products

• Primary by-product: ash (carbon, salts, and metals).

  • Bottom ash: collects at the bottom of the combustion chamber.

  • Fly ash: small particles carried up the stack with gases.

    • Fly ash is captured by air pollution control devices.

  • Ash may be considered hazardous waste:

    • Via the derived-from rule.
    • Due to exhibiting a characteristic of hazardous waste (e.g., metal content).

Rotary Kiln

• Temperature: 1800^{\circ}F maintained by the heat content of hazardous waste and/or supplemental fuel (e.g., natural gas).
• Liquid hazardous waste: injected into the primary combustion chamber via nozzle, atomized into droplets.
• Solid hazardous waste: fed in by conveyor or gravity feed.
• Kiln rotation: slow rotation ensures all sides of the hazardous waste are exposed to high temperature.
• Air supply: a fan draws excess air (containing oxygen) into the system.
• Conversion: hazardous waste is converted from solids or liquids into hot gases.
• Inorganics: metals not converted to gas drop out as bottom ash.
• Secondary Combustion Chamber (Afterburner):
  • Additional fuel increases temperature to 2200^{\circ}F.
  • Chemical bonds are broken, and atoms recombine.
• Air Pollution Control System (APCS):
  • Gases exiting the secondary chamber are cooled and cleaned.
  • Particles (fly ash) are removed.
• Computerized monitoring system:
  • Automatic adjustments are made as necessary.
  • Supplemental waste fuels are injected if temperatures drop.
  • Waste feeds are reduced if temperatures rise too high.
• Waste Feed Cut-Off (WFCO):
  • Automatically stops waste feed if key parameters fall outside operating requirements.

HW Incinerator Permit

• Operating permits are required from EPA or an authorized state agency.
• Permit application:
  • Detailed engineering design and operation plan.
• Trial burn:
  • Evaluates emissions from the process.
  • Demonstrates Destruction and Removal Efficiency (DRE).
  • Ensures protection of public health and the environment.
• Public hearings: held to gather views from those near the site.

Trial Burn Requirements

• Destruction and Removal Efficiency (DRE): must be at least 99.99%.
• Many incinerators achieve a DRE of 99.999%.
• DRE of 99.99% means that 99.99% of the Principal Organic Hazardous Constituents (POHCs) have been destroyed or removed into ash or air pollution residue.
• Trial burn conditions:
  • Worst-case performance test: uses POHCs difficult to burn, and produces maximum ash and chlorine levels.
  • Tests the capability of the APCS to achieve particulate and hydrogen chloride (HCl) emission limits.
  • Incinerator operated at the edge of the expected range for critical operating parameters.

HW Incinerator Emissions & Monitoring

• Emission limits:
  • Not more than 4 pounds per hour of hydrogen chloride (HCl).
  • Not more than 180 milligrams of particulate matter per cubic meter.
• Continuous monitoring and recording of key parameters:
  • Combustion temperature
  • Waste feed rate
  • Combustion gas velocity
  • Carbon monoxide prior to release to the atmosphere
  • These parameters indicate proper incinerator operation.

Risk Assessment

• Required to ensure emissions do not pose a threat to public health and the environment.
• EPA sets a maximum acceptable risk level: 10 times more protective than other environmental permits.
• Risk assessment addresses:
  • Direct exposure: inhalation by the general public.
  • Indirect pathways: deposition of pollutants to local farmlands and waterways, and uptake in the food chain (cattle, fish, vegetables).
• Conservative assumptions:
  • Farmer resides at the same spot of maximum emissions impact for 30 years.
  • Farmer consumes 100% beef, fish, and vegetable products from that location.
  • Ensure conclusions are protective of public health.

MACT Standards

• Maximum Achievable Control Technology (MACT) standards under the Clean Air Act: required for all hazardous waste combustion devices.
• MACT standards can reduce metals, particulate, and dioxin emissions by 5 to 100 times.
• MACT intends to make hazardous waste combustion devices the lowest emitting industrial process.
• Risk estimates:
  • For modern, well-controlled incinerators, cancer risk estimates are generally small to negligible.
  • Lifetime cancer risk estimates are often below 1 in 100,000, even for the most-highly exposed persons (not workers).

Emissions Comparison

• Emissions from hazardous waste incinerators are the most tightly controlled and lowest among major manufacturing industries in the U.S.
  Example of typical HW incinerator emissions in Ohio, compared to other industrial emissions:

Dioxin/Furan Emissions

The Ohio hazardous waste incinerator test compared to other sources of dioxins and furans:

Dioxins are highly toxic, likely human carcinogens created by incomplete combustion of organics and chlorine, therefore low temperature combustion produces more of them i.e. municipal and medical waste incinerators

Air Pollution Control System (APCS)

• Particulate matter removal: electrostatic precipitators, fabric filters, or wet scrubbers.
• Hydrochloric acid (HCl) and sulfur dioxide (SO2) removal: wet scrubbers.
• Nitrogen oxides (NOx) removal: ammonia or urea injection.
• Dioxins and mercury reduction: activated carbon injection or carbon sorbent bed.
• Worker health risks: workers operating and handling residues may be at greater health risk.

HW Incinerator Emissions and Residues

• Incinerator design and operation:
  • Nearly complete combustion of the combustible portion of waste.
  • Low amounts of pollutants emitted under normal operating conditions.
• Residues generated:
  • Considered hazardous waste due to the “derived-from” rule.
  • Ash from HW combustion must be disposed of in a secure hazardous-waste landfill

Medical-waste incinerators (MWIs)

• Primarily used to destroy regulated medical waste that is potentially contaminated with pathogens
• EPA does not regulate infectious medical waste as hazardous waste
• Medical/infectious waste definition: any waste generated in the diagnosis, treatment, or immunization of human beings or animals, or research pertaining thereto, or production or testing of biologicals i.e. cultures, human pathological waste, and sharps that have been used in animal or human patient care or treatment.
• EPA emission limitations: particulate matter, carbon monoxide, dioxins and furans, hydrogen chloride, sulfur dioxide, nitrogen oxides, lead, cadmium, and mercury depending on whether it is a small, medium, or large quantity operation.

Medical Waste Incinerator Categories

• Divided into 3 source categories based on waste burning capacity:
  • Small: less than or equal to 200 lb/hr
  • Medium: greater than 200 to 500 lb/hr
  • Large: greater than 500 lb/hr