Better Technology for
Hazardous Waste Treatment

By Michel Buron
December 2000

The Author is Asia Pacific Area Manager with SEGHERSbetter technology, based in Thailand. His company provides Fluidised Bed Solutions for Waste Gasification and Cleaning of Metal Parts.

Introduction

Some of the products used in our modern society are too toxic to be disposed of without particular treatment. Not only the industry generates such kind of poisonous and hazardous waste but also many household products fall under this category. If not disposed of correctly, some cleaners, solvents, pesticides, paints, etc. can contaminate a landfill, leak into the ground water or contaminate the ocean resulting in tremendous risks for the safety and health of human beings. For the respect of our environment, the basic 3R´s rule (reduce, reuse, recycle) should prevail as a prevention instead of curing. As good as it can be applied, this principle cannot totally avoid the generation of hazardous waste and long term solutions must be developed. Today treatment of hazardous waste includes biological treatment, chemical oxidation and reduction, neutralisation, stabilisation, incineration and energy recovery prior to landfill.

The role that combustion plays in hazardous waste management has changed dramatically over the past two decades. The recognition that land disposal of hazardous waste could present long-term pollution problems and the development of low emission incinerators prompted combustion to become the preferred method of waste management.

The increased use of incinerators to dispose of hazardous waste raised concerns about the proper role of combustion in waste management, as well as the safety of combustion. If conducted properly, the thermal processing of waste has several striking advantages.
First, it is a process that substantially and permanently reduces the toxicity and volume of virtually all organic-bearing waste streams, by destroying organic compounds.
Secondly, combustion devices can accommodate most types of waste, including liquids, solids, and sludge. Further, since combustion reduces a waste´s toxicity and volume, residues from combustion are generally more amenable to land disposal than the original waste streams.

Despite these technical attributes, controversy surrounds the use of combustion since hazardous wastes burned in combustion units often contain toxic organic chemicals, heavy metals, and chlorine, trace amounts of which may be released into the atmosphere in the form of emissions.

To address these concerns and better ensure safe combustion of hazardous waste, various governments have focused on key issues involving the role of combustion and alternative technologies, emission and control standards, risk assessments, permitting priorities, enforcement and compliance assistance, and public involvement in the permitting process. The U.S. Environmental Protection Agency (EPA) and the EEC are the most advanced and comprehensive directives available today.

Thermal processing of hazardous waste

We understand by thermal processing the combustion of waste in an enclosed area. Incinerators are used primarily for the destruction of waste with possible energy and material recovery; boilers and industrial furnaces burn waste not only for destruction but with main purpose the recovery of energy and material.

Comparison of standards for new sources:
(EPA- 40 CFR Part 60, et al. Hazardous waste combustors; revised standards: Proposes rules)
  EU HWCs¹ Proposed HW incinerator Proposed HW cement kiln
Dioxin/Furan: ng/dscm TEQ, and/or Total Congeners 0.19 TEQ. 0.2 0.2
PM, mg/dscm 13 (24-hr avg) 13-39
(30-min avg²)
69
2-hr avg
69
2-hr avg
Hg, µg/dscm 6.5 50 10-hr avg 50 10-hr avg
SVM, µg/dscm Cd: 3.25
Tl: 3.25
Pb: 65³
62 55
LVM, µg/dscm 585³ 60 44
CO, ppmv 52 (24-hr avg) 100 (1 hr avg) Wet and Long, Dry Kilns None
Kilns with By-pass, 100 in by-pass duct (or HC can not exceed 6.7) 1 hr avg.
HC 8 (24-hr avg) 12 (1 hr avg) Wet and Long, Dry Kilns 20 in main stack 1hr avg
Kilns with By-pass 6.7 in by-pass (or CO cannot exceed 100) 1hr avg
HCI and CI2, ppmv as
HCI equivalents5
8 (24-hr avg)
8-48 (30 min avg²)
67 67

Notes:

  1. The EU HWC guidelines have been corrected from the European basis of 11% O2 and O C to the US basis of 7% O2 and 20 C. Both are expressed on dry emissions.
  2. The EU HWC PM, HC, and HCI guidelines are based either 97% compliance with the lower number or 100% compliance with the higher number on a 30-minute average over a year.
  3. The EU LVM guideline is 650µg/dscm and includes Sb, As, Pb, Cr, Co, Cu, Mn, Ni, V, Sn. If all metals are emitted equally, their contribution to the guideline is 65µg/dscm. Pb, a SVM, was subtracted from this group, resulting in the 585µg/dscm level.
  4. The EU HWC CO guideline is based on either 95% compliance with the 156 ppm level on a 10 minute average or 100% compliance with the 104 ppm level on a 30-minute average in any day.
  5. The proposed MWC and MWI standards and the EU HWC guideline are for HCI only.

Combustion principle

Incineration is the controlled burning of substances in an enclosed area. During the process, the waste is fed into the incinerator´s combustion chamber and under the heat decomposes from solids and liquids into gases.
These gases pass through the flame and are heated further becoming so hot that the organic compounds in the gases are broken down into their constituent atoms. These atoms combine with oxygen forming stable gases that are further treated in the air pollution control devices before being released in the atmosphere.
Four main parameters influence the completeness of the combustion process and therefore the destruction of the waste: the temperature, the residence time in the combustion chamber, the turbulence (air/waste) and the size of the waste particles.
Efforts to achieve "low pollution" incinerators are mainly concentrating on the optimization of these parameters and the combination of air pollution control devices (APCD) to reduce emissions of particulate matter, metals and dioxins/furans, and acid gases. APCDs are grouped into three categories: wet, semi-wet and dry systems.
The gases produced by the combustion are primarily carbon dioxide and water vapors but small quantities of carbon monoxide, nitrogen oxides, HCl and other toxic gases may form due to the presence of pollutants in the waste. If the combustion is incomplete, compounds known as incomplete combustion (PIC) may be emitted as well.
Ash is another by-product generated by the combustion. It is an inert solid material composed mainly of carbon, salts and metals. Ash is usually collected at the bottom of the combustion chamber (bottom ash) and in the air pollution control device (fly ash) under the form of particulate matter (PM). Because of its characteristic or by the derived-from rule, ash is often a hazardous waste.

Typical hazardous waste incinerators include rotary kilns, cement kilns, liquid injectors, controlled air furnace and fluidised bed.

Rotary kilns were introduced about 30 years ago and developed according to the "all burn" principle. It consists of a rotating drum, supported by two massive rings carried in cradles at an angle of 2 degrees to the horizontal and rotating slowly (0.2 to 0.5 rpm).
Usually designed for capacities exceeding 2T/h, they can accept large lumps of solid waste, drums and containers, pasty, liquid and gaseous waste, which is fed at the front wall of the kiln via a chute.
Kilns usually operate in the primary chamber at temperatures above 1000C in order to achieve an acceptable burn out of the waste (poor contact air/waste). Complete burn out of the gases is achieved in the second combustion chamber at a temperature above 850C (up to 1200C) and with a residence time of at least 2 second. High energy and maintenance (seals, refractory) costs, fused slag formation unfit for direct landfill, difficult operation and high investment cost make this technology only interesting for very large plants (>5 TPH).

Cement Kilns facilities burning hazardous waste are normally equipped with either an electrostatic precipitator (ESP) or fabric filter (FF) to control emissions of particulate matter (PM). Typical wastes burned in kilns include paint, ink, spent halogenated and non-halogenated, still bottoms from solvent recovery operations, petroleum industry wastes, and waste oils.
Kilns burning hazardous wastes (especially with halogenated wastes) emit more particles than kilns burning normal, providing a pathway for metals to escape the incinerator in a form that is particularly dangerous to humans (attached to the outside of the fine particles).
The fly ash from kilns burning hazardous wastes is loaded with heavy metals and due to their high alkalinity (high pH) makes them more leaching than ash from normal hazardous waste incinerator. PICs are also created in lower-temperature parts of the kiln (APCD and stack) and escape to the atmosphere without treatment. Another source of problems may be chemical releases resulting from transportation often operated dangerously, in violation of applicable laws.

New trend

It is accepted that fluidized bed is the best available technology up to date for combustion. Its high contact surface waste/air, high heat transfer, turbulence and mixing properties confer to this type of reactor the best combustion efficiency. In order to take full advantage of a fluidized bed, waste should be fed preferably in small lumps (eventually a pre-treatment is required). Waste containing high salts content may cause formation of eutectics and risks of de-fluidization of the bed. These make the fluidized bed more suitable for "selected" waste streams at the opposite of "all burn" concept that was preferred in the past decades.
Illegal waste dumping by unscrupulous licensed contractors, liability of the waste generator and growing importance of a green image, etc. push the industry to rethink its waste disposal strategy.
Nowadays on site treatment if often preferred in order to control costs and disposal route (liability). Waste is not only treated but is also used as a source of energy, becoming a part of the production process.
Incinerators are tailor made to suit the requirements of the customer: the primarily function of the incinerator being efficient energy recovery from waste.

SEGHERSmulti waste degrader

SEGHERSbetter technology developed a fluidized bed to suit the requirements of this new era. Based on the fact that the customer is a specialist in its production process and not in waste incineration, SEGHERS combined the advantages of a fluidized bed with simplicity of use, easy maintenance and flexibility.

SEGHERSmulti waste degrader

Advantages

  1. Environmentally friendly concept
    This unique configuration of staged degradation with its multiple zone post-combustion allows for optimum control of each part of the process, which will result in extremely low emissions (CO, NOx, etc.).
    The state of the art ceramic fiber filter is regarded as the best available technology in hot gas filtration.
    The SEGHERSmulti waste degrader meets the highest air quality requirements and will also give maximum waste reduction.
    It can be best described as a Two Stage Fluidized Bed. The combustion is separated in two steps due to the oxygen content difference in the two zones of the fluidized bed:

    • the bed section is operating under oxygen lean conditions for the complete gazification of the organic part of the waste, at temperatures around 55O C. Solids, liquids or sludge are introduced in the fluidized bed by an adapted feeding system. Non-combustibles (glass, metal, etc.) are recovered from the bed in a basket and/or a tap point. The operating temperature is adjustable, usually being between 500 and 6OO C. Because a fluidized bed behaves as a moving boiling fluid, the sand mass retains a very constant temperature. Due to the low temperature in this first combustion stage, it is possible to recycle the glass and metal pieces in the waste as inert, completely free of organic. No refractory lining is used in the bed section reducing the maintenance problems. Risk of eutectics formation are eliminated as well (formation point above 650C).
      The organic part of the waste is gasified under oxygen lean conditions and is completely combusted in the second stage.
    • the post-combustion chamber is where the complete combustion takes place with sufficient excess oxygen. The chamber is sized to ensure at least 2 seconds (flue gas) residence time at 900 C or higher (up to 1200 C).
      The energy in the flue gases can be recovered in a waste heat boiler by bringing down the temperature as much as possible.

    The first step in the flue gas cleaning is the injection of high-reactive hydrated lime, possibly combined with active carbon for the abatement of heavy metals and dioxins.
    The reaction products and particles are separated and collected in a highly efficient ceramic fiber filter.
    The cleaned gases pass via the exhaust fan into the atmosphere through the stack.

  2. Flexible and trouble free operation
    The sand mass can contain the process heat for a very long time during standstill. The material choice in fluidized bed and post-combustion zone allow for quick start-up and quick shutdown without waste of energy.
    At the same time this fluidized bed reactor can cope with very wide variations in the sludge composition.
    A cooled fluidized bed distributor system never coming in touch with the sludge feed and the absence of moving mechanical parts or bottom grids in the hot zones will reduce maintenance drastically.
    The SEGHERSmulti waste degrader will beat any competing system when it comes to flexibility and trouble free, safe, economical and reliable operation.

  3. Proven Technology
    More than 500 installations with an identical concept are in use by our satisfied customers world wide. Applications range from bio-sludge, waste oil, bottoms, plastic residues, paint, chemical scaling, tar, pathologic wastes, etc.

SEGHERS fluidized bed principle

The SEGHERSmulti waste degrader basically consists of a reservoir filled with calibrated quartz sand. A gas-air mixture is blown into the sand bed at the bottom by means of a patented cold distribution system which accordingly fluidizes the sand mass. A pilot burner above the surface of the bed ignites the gas air mixture. As a result of the low air speed through the bed, the flame spreads across the whole surface of the fluidised bed. The resulting combustion energy is directly absorbed by the sand bed (patented principle of "direct heating") that heats up quickly and evenly due to the constant fluidizing.

SEGHERS fluidized bed principle
  1. Fluidized Bed
  2. Calibrated Quartz sand
  3. Air Supply
  4. Gas Supply
  5. Gas-Air Distributor
  6. Pilot Burner
  7. Direct Combustion
  8. Post Combustion Chamber
  9. Sludge Injection
  10. Process Gases
  11. Gas-Air Mixture
  12. Secondary Air Supply
  13. Exhaust off gases

Conclusion

Hazardous waste disposal is an issue concerning each of us. Public education will play an important role in order to establish a strong preference for source reduction over waste management. Development and enforcement of safety controls on hazardous waste disposal facilities, continuous scientific research to better understand the waste combustion issues, etc. are some of the top priorities that environmental protection agencies have to focus on, in order to preserve the health of our Children and Nature.

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