Controlling gaseous emissions

In addition to the normal products of combustion, a typical waste stream can produce the following gaseous species:

Acid gases (HCl, SOx, NOx);

Unburned hydrocarbons – VOCs, dioxins;

Heavy metals – Pb, Cd, Hg, As.

When the legislation was introduced in the late 80s, there was little available gas cleaning expertise in the UK and it was necessary to look to Europe and Scandinavia for established systems. It is these systems, or their derivatives, which have been adopted.

Pollutant formation

Acid gases are produced in most combustion processes although they are not all formed in the same way. For instance, SOx production is directly related to the concentration of sulphur in the waste. Sulphur is present in the ash or bound organically. This also applies to chlorine which appears as hydrogen chloride (HCl) in the flue gas. NOx differs in that it is formed by two main methods; from the mixing of the nitrogen bound chemically in the waste (fuel NOx) and/or by direct combination of the nitrogen and oxygen in the air at high temperatures (thermal NOx).

Unburned hydrocarbons are produced as a result of poor combustion except for dioxins which are principally formed by de-novo synthesis at temperatures between 200-400oC after the combustion step. Setting the combustion parameters, to ensure that all of the waste is burned completely, is not straightforward due to its varying calorific value and ash content. Incinerators are designed to provide a two second residence at a temperature in excess of 850oC and to operate with a high amount of excess air to ensure that the material is burned properly. This ensures that emission levels of CO and VOCs are low (0-100mg/Nm³ concentration range) but emission problems can arise during start-ups or due to other plant perturbations.

Heavy metals are produced typically from batteries in MSW streams, sharps from clinical waste streams and sewage sludge which can contain a multiplicity of metallic elements/compounds. They are present in the flue gas as sulphates, chlorides, oxides etc. and pass through the incinerator in all phases – solid, liquid and gaseous but, by the time they reach the dust collector, most have condensed on the surface of the fly ash. The exception is Hg which can still be in the vapour form at temperatures below 200oC.

There are several ways in which emissions can be controlled from waste combustion:

Careful selection/blending of the waste streams (if possible);

Good combustion and control (where possible);

End-of-pipe treatment.

Ideally, if the waste stream was devoid of the precursors to the pollutant gases then none would be formed and the pollutant problem would not exist. Unfortunately, in most cases, this is not possible as the main purpose of incineration is to reduce the volume of waste for landfill purposes; segregation of waste streams not always being an option.

Good combustion control, however, can be achieved by blending wastes of varying calorific value in order to produce a more uniform feedstock. High levels of turbulence in the furnace to promote mixing of the combustion air with the waste will ensure that most of the volatiles are burned. Incinerator design is obviously very important in these circumstances.

Nowadays, with the implementation of tight emission limits, neither of the above can be considered as the total solution. End-of-pipe treatment has to be incorporated as part of the solution. The options available are described in detail below.

An end-of-pipe solution is the terminology used to describe a system which treats the flue gas just prior to it being emitted to atmosphere.

Acid gases – wet or dry? The normal way of removing acid gases (except NOx) from flue gases is by gas scrubbing. There are two basic categories of scrubbing system – wet and dry – although there are other variants in between called semi-dry or semi-wet systems. Historically wet scrubbing was the favoured option as the acid gases are soluble in water and react readily with sodium, calcium and ammonium hydroxides. Wet scrubbing removes particulates at the same time which made it a particularly attractive option. The main disadvantages are the add-on cost of treating the wet residue and the production of a visible plume which often needs reheating to rise. The latter can be avoided but, because of these problems, dry scrubbing (and its variants) is now dominating the scene although wet scrubbing is still the preferred option in certain applications.

Dry scrubbing systems exist in many guises but generally comprise a scrubbing tower into which the sorbent powder is injected, bag filter and sorbent/ash handling systems. The sorbents are typically calcium hydroxide or sodium bicarbonate. The choice of which to use is based on several criteria. Calcium hydroxide is cheaper and as such is preferred but, in applications where there is a chlorine present, it reacts to form calcium chloride which creates a gel-like deposit which is difficult to filter. In these circumstances, sodium bicarbonate is the preferred option.

Sorbent cost and the disposal of the spent sorbent are the main operating costs of a dry scrubbing system. To keep these costs to a minimum, it is important to ensure that all of the sorbent injected reacts. To achieve this, many systems incorporate a recycle loop where the partly reacted sorbent collected by the bag filter is recycled. This is done several times to ensure sorbent utilisation is high. The totally reacted sorbent is normally sent to landfill or recycling into other processes.

NOx cannot be removed by standard scrubbing techniques and requires a slightly different approach. The cheapest way is to arrange the combustion in such a way as to avoid the production of NOx in the first place. Techniques such as air staging and flue gas recycling can be employed and although effective do not remove all of the NOx . Where their performance is considered inadequate, there is a choice of other techniques such as Selective Catalytic Reduction and Selective Non Catalytic Reduction (SCR and SNCR). Both are costly, and bring with them a series of additional operational problems.

Dioxins and furans

Unburned hydrocarbons? If combustion is good then the issue of unburned hydrocarbons should not be a problem. Most operators will make every effort to ensure this is the case. Poor combustion will result in higher plant corrosion rates, more outages and higher maintenance costs. The exception to the rule is dioxins and furans which, as already indicated, are formed by de-novo synthesis in the 200-400oC temperature range downstream of the combustion chamber. One way to prevent them forming is to quench the flue gas rapidly through this temperature window. But if the incinerator has heat recovery, this is not possible and experiments have shown that metallic surfaces can actually act as a catalyst in their formation.

In these cases, the end-of-pipe solution is the only available option using activated carbon as the adsorbent. There are essentially two types of removal system: one involves using a fixed or circulating bed of activated carbon through which the flue gas is passed and the other is where the activated carbon is injected as a powder on to the filter bags of the already existing dust collector. The latter is less expensive and is therefore considered the more attractive option.

Heavy metals? Fortunately, most heavy metals condense on fly ash at some point through the incinerator and heat recovery sections. Therefore, by the time the flue gas reaches the chimney, most will be in the solid phase and can be captured by the dust collector. The exception is the chlorides of mercury which will still be in the vapour phase.

Ideally, waste streams containing mercury should be segregated and disposed of separately, but where this is not possible, capture by activated carbon is the prescribed route. The systems which are available are the same as for dioxin removal and there are activated carbons on the market which will adsorb both dioxins and mercury vapour.

To comply with current emission standards, end-of-pipe solutions are applied except for NOx control where combustion techniques are used. This trend is set to continue with ever increasing system complexity. The cost of these techniques will need to be met by higher waste gate fees and returns from NFFO and the sale of heat (hot water/steam). Economics will dictate that this will lead to a small number of large incineration plant. Given that motor vehicle emissions contribute some 40% of the UK’s total NOx production, the transporting of waste by road to a small number of large incinerators with DeNOx equipment may well not reduce countrywide NOx production. Could it be that smaller, more local incinerators with lower specification DeNOx equipment may, in overall terms be a better environmental option?