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Incineration Fundamentals |
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INCINERATION APPLICATIONS:
Two classics and another coming down the road: Centralized incineration, hospital waste ("medwaste") and PFAs
For the purposes of preliminary sizing or design, it is advisable to characterize the application as best as possible, indicating:
load, e.g. kg/hr or tons/day and operating regime, e.g. 8, 12, 24 hours
nature and calorific value of the different load mixes (inevitable)
moisture content
ash content
requirements for the emissions control system (particulate matter, HCl,)
operating regime, e.g. excess air,
Many incinerator manufacturers in the 1970s and even into the 1980s used relatively low calorific values, on the order of 4,700 kcal/h (8,500 Btu/h), for their calculations. With the disproportionate increase in high calorific value materials, such as most plastics (2x, almost 3x), many incinerator operators, especially in the hospital sector, found it impossible to maintain the originally projected throughput due to the corresponding increase in air supply requirements. These materials, with their high combustion rate, end up raising the temperature excessively, causing slag formation, obstructing the air supply, hindering ash movement, and increasing particulate matter and carbon monoxide emissions.
In some ways, due to this significant change in the percentages of the main constituents, as well as the constant threat of accidental air infiltration through the loading doors, substoichiometric feeding operations have become difficult to maintain. Both the difficulty in preventing abrupt air intrusions and the high variability in the calorific value of different feedstocks have made operation in small systems challenging, even with sophisticated automatic controls. While batch feeding (as opposed to continuous feeding) can somewhat mitigate these obstacles, the effective throughput suffers due to longer burn cycles. In larger systems, e.g., 680 kg/h, the irregularities tend to self-balance, and operation can be acceptable.
To illustrate the calculations involved in a preliminary design, we can use the following hypothetical context, assuming the following specifications:
Load/hour: 100 tons/day of municipal solid waste
heating value 2,500 kcal/kg ( 4,500 btu/lb)
moisture content 25%
ash content: 20%
operating regime, e.g., 100% excess
In general, the higher the moisture content, the greater the amount of additional fuel (natural gas, fuel oil) required. With the chosen alternative (100% excess), we will have to bring the entire air mass to operating temperature , so our consumption and dimensions will necessarily be larger.
A simplified outline of the calculation involved can be seen in the insert , a purely illustrative historical design in which we have used linear interpolation to avoid having to work with enthalpy tables, which is notably more accurate, e.g., for predicting the flue gas discharge temperature . Updating this exercise would involve an appropriate characterization/composition of the contemporary waste, either by determining the "actual" calorific value or by using a full-scale boiler, calorimetric testing, or estimation using Dulong's formula for a known composition. In the case of municipal solid waste, the moisture content can vary between 18 and 25%, the ash content between 10 and 20%, with the remainder being combustible. For reference, the resulting calorific value for the volatile fraction will be between 8,000 and 12,000 Btu/lb (approximately 4,500 kcal/kg and 6,700 kcal/kg).
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In the specific case of the hospital waste category , the guideline could be:
Load/hourly: 200 kg/h
heating value 5200 kcal/kg (9360 btu/lb)
moisture content 9%
ash content: 8%
operating regime, e.g. controlled air: 70-80% in the first cell, 140-200% in the second cell.
As in the previous case, but especially aggravated by the characteristics of the type of solid waste in question (pathological/infectious/hospital), the need to anticipate (and design for!) significant presences of the following must be taken into account:
plastic material, in particular PVC (-> HCl)
presence of Pb, As and Hg (vapor) (-> include subprocess for their removal)
Formation and reformation of dioxins and furans
Except for mercury, toxic metals are associated with particulate matter that is mostly submicron in size. While a wet scrubber (particularly a high-energy venturi scrubber operating at approximately 65 inches of water column) can handle acid gases and dioxins, it is not currently the preferred method for meeting the 35 mg/Nm³ (at 7% O₂) requirement.
It should be noted that the system to be designed must be able to handle loads ranging from those that do not allow combustion without the aid of supplementary fuel (large content of anatomical parts) to loads with extremely high calorific value values such as polyethylenes and polystyrenes (calorific value up to more than 10,500 kcal/kg or 19,000 btu/lb).
The preferred alternative includes the injection of hydrated CaO for HCl removal and an additional reagent to inhibit dioxin formation. Energy recovery through steam generation is also frequently sought, or, less commonly, through dissipation using a gas-to-air heat exchanger.
Proposed Addition (Under Construction)
PFAs Destruction via Incineration and other alternatives
PFAS SCWO, HALT, and ECO refer to three prominent, cutting-edge technologies being deployed to destroy per- and polyfluoroalkyl substances (PFAS), also known as "forever chemicals". They permanently break down these toxic, highly persistent synthetic chemicals into harmless byproducts, providing an alternative to traditional landfilling or hazardous incineration.
1. PFAS SCWO (Supercritical Water Oxidation)
SCWO uses highly extreme temperature and pressure to destroy organic waste.
How it works: Water is heated to over 374 °C and pressurized to over 22.1 MPa (roughly 220 times atmospheric pressure), putting it into a "supercritical" state where it acts as both a gas and a liquid. When an oxidant (like pure oxygen) is introduced, this state dissolves organic pollutants and rapidly breaks the incredibly strong carbon-fluorine bonds in PFAS.
End products: It completely mineralizes PFAS into carbon dioxide, water, and inorganic fluoride salts.
Best used for: Highly concentrated PFAS waste streams, such as industrial wastewater, landfill leachate, and aqueous film-forming foams (AFFF).
2. HALT (Hydrothermal Alkaline Treatment)
HALT is a similar thermo-chemical method to SCWO, but it operates under slightly different conditions.
How it works: Like SCWO, it uses hot, compressed water, but it operates at a slightly lower temperature (around 350 °C ) and relies heavily on an added alkaline chemical reagent (such as sodium hydroxide) to facilitate a strong reaction.
End products: It breaks the carbon-fluorine bonds and turns them into inert fluoride salts.
Best used for: Sites requiring high energy efficiency and portability. HALT is effective at treating both highly concentrated industrial waste and large volumes of contaminated groundwater.
3. ECO (Electrochemical Oxidation)
ECO approaches PFAS destruction using electricity rather than high heat and pressure.
How it works: ECO passes an electrical current through a contaminated liquid solution inside an electrochemical cell. This generates highly reactive species (such as hydroxyl radicals) at the anode that directly degrade and destroy the PFAS molecules.
End products: It mineralizes PFAS into carbon dioxide, water, and free fluoride ions.
Best used for: Low-to-medium concentration PFAS in aqueous liquids, especially concentrates derived from preliminary filtering methods (like reverse osmosis or foam fractionation). It operates at ambient temperature and pressure, eliminating the need for complex, high-pressure equipment.
Sample keywords: hydrogen sulfide EGSB tannery (no commas!)
