INTRODUCTION

Frankly, we would have liked to polish a little bit more the content and presentation of this digital booklet, at one time the thirteenth chapter of the James C. Young Environmental/ Balestie & Balestie series. Most incineration project requests we received over many, many years were based on dated design information anticipating big time failure.

Three main points motivated our decision to significantly advance the release of these technical support notes. 

1. reviewing the requests we've received recently on this topic, we noticed a dangerous lack of clarity in the specification or selection of the load's calorific value .It has been very common to receive requests basically inspired by EPA publications, references from Calvin Brunner , the now legendary Penn State graduate, or similarly qualified sources, but extrapolating from characterizations of municipal solid waste incinerators (known as "mass burn" in industry jargon) and assimilating the design with some modifications.What is the consequence of this?   

Let's assume the specification plans for the incineration of 9 tons/day of hospital-type waste, under a continuous operating regime.   Having examined the database of applications received, we can see that there is a very widespread trend of using calorific value values ​​averaging 4800 kcal/kg (c. 8500 btu/h), “classic” values ​​from most incinerator manufacturers in the 1970s and even the 1980s.   

With the drastic increase in the proportion of plastics, e.g., 30% (e.g., 4 times the calorific value in some cases), the net result is that the forecast of air to be supplied, and therefore the capacity of the incineration plant, plummets drastically; i.e., it cannot maintain the original planned throughput and triggers the whole series of known problems, in no particular order:  

 incomplete/irregular combustion,

uncontrolled emission of particulate matter and carbon monoxide,

nozzle clogging by slag, and, of course,

all the problems of dioxins, furans, lead, and mercury.   

In short, it's a case of what's coming out of the plant being sadly worse than what's going in ! And to top it all off, they're relentlessly pushing us towards the agreed-upon 9 tons every day. In short, we're still obligated to receive what was agreed upon, the plant's operation has become chaotic, and the press is giving us front-page coverage.

However, it's obvious that all of this could have been perfectly avoided : time passes, compositions change, but so do resources and experience. It's incredible that in this often controversial but somewhat "routine" field, undertaking a project based on design information from just a few (e.g., 2-3) years ago can be disastrous: you end up building something that doesn't fulfill its intended purpose at all, amplifying the negative impact all of this has on society .

2. The second motivation for bringing this material forward was the recent updates to acceptable emissions requirements in Europe, Mexico, and the USA.   Looking back at the history of this technology, we see how modular or starved air units (SAUs) were able to replace units operated with excess air. Their competitive advantage—provided they were well-designed—was their ability to meet the 0.08 g/cu.ft. (approximately 182 mg/m3 for particulate matter) emissions standard without the use of scrubbers or fabric filters to treat the flue gas.   Under the new sanitary framework (0.015 g/cu.ft. or 30-35 g/m3), not only are the two incineration technologies now on par with the rotary kiln category in terms of the obligation to add controls, but even the popular venturi (“high energy venturi scrubber”), a successful design when particulate emission standards were 0.044 g/cu.ft. (100 mg/m3), is out of the running, unless a condenser scrubber is incorporated in conjunction with a cooling tower downstream to achieve nucleation in the venturi: a venturi scrubber operating at 40-50 inches of water column achieves only 70 mg/m3, double the amount allowed in the new standards of 34 mg/m3. All of this forces us to abandon the high energy venturi scrubber or at least significantly displace it as an alternative, especially if we add the inherent need for the treatment and disposal of the effluents generated by the scrubber.     

3. The third motivation, and unfortunately no less serious in terms of the situation's gravity, is the traditional use of activated carbon as a prescription for the dioxin problem.   We all know that combustion, especially of waste, is one of the largest generators of dioxins.   Simultaneously, we all know that high-temperature incineration is actually the main method for dioxin destruction. So how can this apparent dilemma, this seemingly intractable contradiction, be resolved?   In the case of an incinerator with good combustion in the secondary chamber, the concentrations of organic compounds are very low, practically at detection levels.   Theoretically, given the short time the gas remains in the boiler and ducts, it seems difficult to imagine dioxin formation.   However, reality shows us that not only do they form, but they form in large numbers!

For traces of organic material to react and ultimately produce dioxins, they must be concentrated in some way. This concentration occurs as the combustion gas cools (through simple adsorption). In this context, we have two candidates to promote adsorption: iron oxides and the suspended ash generated ("fly ash"). Let's examine this in more detail.     

As the flue gas cools, e.g., below 300–400°C, organic compounds adsorb onto the ash, producing dioxins by reacting with the metal chlorides present in it.   Metal chlorides, particularly ferric chloride, are continuously formed by the reaction of chlorine (from the HCl gas present) and metal oxides.   Some of the dioxins formed in the ash vaporize and are carried to the exhaust duct; the rest remain in the ash.   This generation process is chronic and/or perpetual as long as the ash is retained in the system and exposed to HCl.   By the time many books and professionals prescribe neutralizing the HCl at the lower temperatures to stop the process, it is too late , and the damage has already been done.   Treatment systems using an electrostatic precipitator for collecting the ash generated in suspension create the worst possible context since they optimize the conditions for dioxin formation: prolonged mutual exposure of the constituents of the suspended ash set, organic compounds and gaseous masses with HCl content.

In installations with highly corrosive systems, iron oxides act as the primary adsorbents and generators of dioxins.   In most cases, the amount of iron oxides exceeds the amount of suspended ash.   Furthermore, because they remain in the system for a much longer time, they significantly outweigh ash as a source of dioxins.   Since, as we all know, iron oxides react very readily with HCl to form ferric chlorides, dioxin generation in poorly maintained systems (i.e., high levels of corrosion) can be simply spectacular .   Iron oxides not only give rise to this "cascade" but also simultaneously create reservoirs   or memory pools for guaranteed dioxin emissions .

Finally, and directly related to this third point, we stated earlier that for traces of organic material to react and ultimately produce dioxins, they must become concentrated.   It's natural to assume that dioxin generation will increase as the presence of these two components—i.e.,   iron oxides and fly ash—increases.   In other words, a greater quantity of oxides or a greater quantity of ash will result in a greater quantity of dioxins.   On the other hand, we all know that the injection of powdered activated carbon (PAC) is the traditional method for inhibiting or reducing the amount of dioxin generated.   However, we also know that PAC is generally manufactured from coal.   The resulting product can contain ash percentages of up to 20-25%. This added ash provides all the reactive oxides necessary for conversion to chlorides through contact with HCl, which is inevitably followed by the chlorination of all the adsorbed organic compounds. In other words, PAC injection results in dioxins, on the order of five times more, according to EPA trials!   

It is in consideration of these three realities that we prefer to advance the material of the site, intended to be basic or introductory   in nature, and include our apologies simultaneously beyond technical details or characteristics of equipment or machines easily found in manufacturers' manuals, classic literature on the subject or journals of different types.

Like all previous digital booklets, we hope it will provide genuine assistance to designers and field professionals, as well as students and staff at research centers, technology laboratories, universities, and related institutions. 

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