A new paper outlines what it calls a simple-to-use technique which enables companies to optimize the performance of industrial boilers and reduce their main emissions.

Obviously there aren't going to be any new super-efficient combustion techniques any time soon -   combustion is what it is - but using a new oxidation method with an improved catalyst is a different story.

A catalyst is a substance which increases the speed of chemical reactions, without taking any apparent part in the reaction, or which modifies the parameters of the reaction (temperature, pressure, concentration, etc.).  In the case of catalytic combustion, the main parameter is its speed of reaction given by the equation for the speed of the flame front UL:

UL = Ö(DT.Vreaction) where the Arrhenius expression is UL = U0 .e(-Ea/2 kT)

In the above, DT is the coefficient of thermal diffusion, Vreaction is the speed of reaction, U0 is the initial speed of reagents, Ea is the activation energy, k the Boltzmann's constant and T the absolute temperature of the flame.

The speed of reaction therefore depends on the following factors:

- The nature of the reaction,
- The temperature,
- The concentration of reagents,
- The physical state of the body,
- The nature of the catalysts present.

The acceleration of the kinetics of combustion favours the complete reactions and may be compared to an increase in the temperature of the fuel/combustion agent mixture. The catalyst sets up a different reaction mechanism, which demands less energy. It should be remembered that the activation energy is the energy that must be given to the molecules of fuel in order to enable them to be transformed into molecules of a complex activated by the oxygen in the air.

The energy-level diagram of a chemical reaction with (40 kJ) or without a catalyst (55 kJ) is given below.  The reformulation of the previous equations demonstrates that the activation energy is inversely proportional to the speed of reaction: the more this reaction is accelerated, the less significant the activation energy.

Ea = -2.k.T.ln(Ö(DT. Vreaction)/U0)

The catalysis of the combustion reaction increases the reactivity of the fuels present. This modification is beneficial since it occurs on entering the system (fire box), in the zone of contact between the combustion agent and the fuel (primary air), and may be obtained in post-combustion (secondary air).

The calculation of the heat balance is based on the calorific value of a fuel, which is directly influenced by the quantity of nitrogen in the combustion air: indeed, nitrogen has an extinguishing influence on the combustion reaction (nitrogen deprives the fire box of oxygen by forming NOx and thus reduces the energy produced). The excess air, therefore, has a negative impact on combustion and overall efficiency.

When the combustion reaction occurs in the presence of a catalyst, the formation of stable C-O bonds (see formation of CO2) takes priority over the formation of N-O and S-O bonds. Therefore, there is a reduction in the concentration of NOx and SOx and an increase in that of CO2, which is characteristic of a complete combustion reaction (stoichiometric conditions).

The catalyst that was identified and chosen, after a long, in-depth investigation, is MMT (Methylcyclopentadienyl manganese tricarbonyl), selected thanks to the authors' close collaboration with AFTON Chemical. The choice is justified by the previous analysis, since it is the only one that satisfies the specifications, every step of the way. The levels of performance achieved are unparalleled, in spite of on-going research into new formulations.

For several decades, the MMT molecule has been used as a catalytic additive in motor fuels (http://www.aftonchemical.com/Products/MMT/index.htm) - the formula of which is C9H7MnO3. MMT has three groups of oxidants (carbonyl) capable of improving the supply of oxygen to reducing zones. It is polarised and oriented, which enables quicker and more selective reactions on the carbonaceous chains (catalytic cracking). This organo-metallic compound is organised around the central metallic atom - manganese - which is one of the transition metals commonly used in catalysis.

Technical description

To be efficient, the use of the catalyst must guarantee the catalytic action in homogeneous phase. Accordingly, the catalyst is mixed in the form of catalytic vapours with the combustion agent(s), so as to combine a gas (MMT vapours) with another gas (combustion air).

The catalyst diffusion equipment includes a storage cell, which ensures its confinement in the liquid state. This cell, which is a parallelepiped, contains a succession of metal plungers, which are partially immersed in the liquid. They are covered with a technical textile which absorbs the liquid and impregnates a certain surface area by the process of capillarity. A fraction of the preheated combustion air passes through this cell.

The adopted temperature is adapted according to the MMT vapor pressure, so as to ensure its evaporation. The flow rate of air passing through it is generated by the vacuum pressure of the fan which draws in the combustion air. The quantity of air depends on the force exerted and on the opening of the control orifice.

The arrangement of the plungers, the total surface area coated with catalyst, as well as the flow rate of air and its temperature, determine the quantity of catalyst (its concentration) and the dosage of its vapours required for the equipped boiler. The equipment designed in this way provides all the flexibility required for most of the aforementioned cases. Their design, together with their automatic set points, guarantees the use of the catalysts in complete safety (no leaks, losses or dissemination are possible).

Furthermore, the usage method associated with these devices reinforces the on-going health, safety&environment (HSE) requirements:

• MMT is supplied in sealed, non-pressurised standard IATA cylinders,
• Users cannot come into contact with the product, either in liquid or vapor form.

Following more than 20 tests and industrial-scale pilot units, in many different fields of activity and on various boilers, the main technical performances achieved - validated, on several occasions, by independent bodies - are as follows:

Improvement in efficiency: from 4% to 8% (extremes of 2% and 60% - at partial or full load, on production boilers, with liquid and solid fuels).

Reduction in emissions:

o Of dust: from 30% to 80% (e.g.: on leaving the stack after filtration from 45mg/Nm3 to 20mg/Nm3, biomass boiler);
o Of NOx: from 20% to 50% (e.g.: on sub-bituminous coal from 650mg/Nm3 to 380mg/Nm3);
o Of SOx: from 10% to 50% (e.g.: on low-sulphur content fuel oil: half the threshold value imposed by the regulations);
o Of unburned gaseous residues: up to 80% and 90% (e.g.: on CO from 1g/Nm3 to 0.3 g/Nm3).

Reduction of unburned residues:

o Proportion of residual C in the ash: a factor between 2 and 5 (e.g.: on coal ash of 19% to 4% in dry weight);

o Opacity index (Baccarat): a factor between 1.5 and 4.

Reduction/elimination of preventive/curative maintenance operations:
o Virtually total elimination of sweeping operations;

o Increase in machine availability rates, up to the limit.

The technical successes obtained during the trials, as well as the favorable reaction of potential users during market research, have convinced the authors to commit commercially to the project.

On ‘large systems', the thermal outputs are of hundreds of MWth to GWth (as found in power plants), using pulverization techniques, while on ‘small systems' the thermal outputs are of several KWth to hundreds of KWth (as found in domestic and residential boilers using rechargeable cartridges).