METHOD FOR OBTAINING PRE-NEUTRALISED, REFUSE- DERIVED FUEL, FUEL OBTAINED IN THIS WAY AND ITS USE IN COMBUSTION

DESCRIPTION STATE OF THE ART

The present invention relates to a non-conventional fuel, neutralised before being fed to the combustor. More particularly it relates to MSW (municipal solid waste), RDF

(refuse-derived fuel) or industrial solid waste.

Waste, including MSW, contains high percentages of chlorine, bromine, fluorine and sulphur which are the main causes of acidity in combustion gasses and of the production of organic micropollutants such as dioxins and/or furans.

To reduce the acidity of the combustion gasses and mainly the hydrochloric, bromidric, hydriodic and hydrofluoric acid and the sulphur dioxide present in the combustion gasses, bases such as soda and/or calcium hydroxide and/or bicarbonates are used, injected in the various points of the system.

Experience teaches that the use of soda at a high temperature is advantageous, typically in the incineration furnace or after the same furnace, as in these conditions the soda is in the vapour phase and is highly efficient.

In the case of feeding in the furnace, the disadvantage consists of the interaction of the soda with the refractory of the furnace. Nebulisation after the furnace and before the boiler is a little less effective and requires a greater excess of reagent with consequent increase in fouling of the pipes of the boiler.

The patent EP 744208 of the same Applicant proposes feeding the

already vaporized soda after the boiler in order to have the advantages without the disadvantages of the previous methods yet with a slightly more complex system as embodiment.

All these methods are able to offer virtually complete abatement of the sulphur and an efficiency of abatement of the halogens of around 70%.

Calcium hydroxide or bicarbonates can advantageously be used at a lower temperature and typically around 160 - 180 ⁰ C in the end part of the systems before the bag filters on which they are deposited, continuing the reactions of neutralisation. The utilization factor of calcium is not high and yet it enables good efficiency of abatement of the residual acidity to below the legal limits.

For this reason calcium hydroxide is used in combination with soda in order to benefit from the advantages of both methods.

The problem of abatement of the organic micropollutants such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans is a little more complex since, with chlorine, they are produced during incineration, if combustion is not well controlled, but above all they are produced at a lower temperature in the boiler if not all chlorine has been eliminated before the boiler. The limits of emission laid down by the European Union are very restrictive and equal to 0.1 ng/Nm3, expressed as the toxic equivalent of 2,3,7,8 tetrachloro dibenzodioxin, an equivalent obtained as a sum of specific dioxins and furans normalised with a coefficient proportional to the specific toxicity of each one. At present the problem of the organic micropollutants present in the combustion gasses of incinerators is treated with special technologies, such as: - optimisation of the processes of combustion for example via fuels such as RDF with reduced organic residues;

- minimisation of compounds that can act as precursors in the lower temperature synthesis in the boiler, for example chlorine;

- the use of active carbons in powder form dispersed in the gasses, on fixed beds or in liquid suspension before emission into the environment.

As can be seen, the technologies for abatement of organic micropollutants partly overlap those of elimination of acid substances from the combustion gasses .

For example the patent IT 01306951 of the same Applicant uses RDF and soda fed directly into the incinerator of the RDF refuse for the purposes of efficient immobilisation of the chlorine and of other halogens. The combustion of organic matter is more efficient and the drastic reduction in the chlorine avoids the de novo synthesis of the dioxins in the boiler at around 600 ⁰ C. This technique, although very efficient both for the reduction of acidity and of micropollutants, has the disadvantage of exposing the refractory walls of the combustion furnace to potential attack by the soda nebulised therein. DISCLOSURE OF THE INVENTION

An object of the present invention therefore is a new refuse-derived fuel, neutralised at the origin before the feeding to the incineration furnace, which does not have the disadvantages of the prior art.

A particular object of the present invention is a method for producing said new fuel from a processed fuel such as an RDF obtained through bio-drying of MSW or from fuels with a higher heating value such as RDF, refined according to the method disclosed in the patent EP-A-706839 of the same Applicant.

A further object of the present invention relates to the use of this new fuel in incinerators with the purpose of reduction of the acid substances and of

the organic micropollutants in the combustion gasses.

The object therefore of the present invention is that of providing the neutralising agent at the time of combustion, thus avoiding at the outset the production of acid fumes to be neutralised in a subsequent phase. A further object is that of reducing direct contact of the soda with the refractory of the furnace, the potential cause of its more rapid wear.

These objects are achieved by the invention which proposes a method for the production of refuse-derived fuel with the features of the annexed independent claim 1 , and such a fuel derived from refuse having the features of the annexed independent claim 7.

Preferred embodiments of the invention are disclosed by the dependent claims.

It has in fact been found that bio-dried waste absorbs aqueous solutions intensively, up to 60% of their weight, and that therefore, when said waste is placed in contact with appropriately batched solutions of caustic soda, it adsorbs it completely.

The bio-dried waste normally has a water content of between 17 and 20% in weight and consists of plastic, wood, paper, dry food residues and mineral and metallic inert materials. The fine screened product and the metals were removed from the product obtained from the bio-dried waste through refining and it was ground to a granulate size of around a few centimetres.

These materials also contain 0.5 - 0.8% in weight of halogens and typical chlorine and 0.2 - 0.4% sulphur. The stoichiometric soda quantity to be added is therefore very small and around 10 - 20 grams per kilo of fuel or alternatively 35 - 65 grams of 30% aqueous solution of soda.

The soda, if dispersed in an aqueous solution evenly over the material,

diffuses in the cellulose compounds, hydrolysing them, reacts with the fatty acids originally present in the waste or with the acid substances produced by aerobic digestion and dehalogenates the halogenated organic matter that may be present. The result of this operation is that the soda is bound in the form of organic and inorganic sodium salts and the pH is around the value of 10, typical of a solution of a salt of a strong base with a weak acid.

The operation of neutralisation of the RDF can be carried out either at the production plant or at the incinerator. In order to mix the waste with the soda in solution it is possible to use various methods such as for example nebulisation of the solution on the pile of waste, aerated and stored at the bio-drying plant or at the incinerator, or continuous nebulisation on the conveyor belts, etc., although it has been seen preferable to spray the solution in the end mill for grinding of the bio-dried waste or the refined bio-dried product.

This machine is already available in the bio-drying system, as described in EP-A-706839 and in greater detail in IT 01297234 of the Applicant, and allows optimal dispersion of the soda on the waste for the action of mixing which this machine exerts on the waste during nebulisation. The waste neutralised in this way is stored in order to mature in a pit while awaiting compacting in 20 - 30 tonne containers in order to be transported to the incinerator.

The reaction of soda with the lignin, cellulose substances and acid and chlorinated organic substances in the waste continues both during the transport and in the holding pit of the incinerator.

It should also be noted that, although the bio-dried waste is free from pathogens as claimed in EP-A-706839, the treatment with soda offers a further guarantee of hygienisation of the same waste in the case of production batches

not to Standard.

The treatment with soda in solution is therefore integrated in the process of production of the bio-dried product that involves:

- rough grinding of the MSW; - aerobic digestion in a closed space and in a flow of air; final grinding to medium size (one decimetre) with the addition of soda;

- compacting of the waste for transport.

It is also integrated in the more complete process of production of the refined bio-dried product that involves:

- rough grinding of the MSW;

- aerobic digestion in a closed space and in a flow of air; screening to separate the fine screened product;

- removal of metals; - final grinding to a fine size (a few centimetres) with the addition of soda; compacting of the waste for transport.

It is possible to batch the soda so as to neutralise the total acidity present in the waste and also to over-batch the soda according to the needs of the incinerator, for example in the case wherein co-combustion of the bio-dried product with another waste, not initially neutralised, is foreseen.

In any case the basicity of the waste is reduced, the quantity of soda fully buffered by the excess of reactive substances present in the waste itself. In order to understand better the performances of the new fuel that is the object of the present invention and to put the same into practice, the following are some non-limiting examples of application. Example 1 The plant at Corteolona (PV) has capacity for treating 60,000 t/a of

MSW and produces approximately 30,000 t/a of refined RDF.

The MSW is composed on average of paper, cellulose products and wood (33%), plastic (18%), glass, inert materials and metals (8%), organic matter and screened product (41%) and has a humidity content of around 37% and a lower heating value (LHV) of approximately 2,200 kcal/kg.

After rough shredding, the MSW is deposited in piles and undergoes aerobic digestion in a flow of air for approximately 15 days and loses approximately 25% of its weight to produce a bio-dried product with a humidity content of around 19% and an LHV of approximately 3,000 kcal/kg. The bio-dried product produced in this way is screened and the iron removed, and it is ground finely, losing another 25% of the original weight, to produce a refined bio-dried product containing paper, cellulose products and wood (45%), plastic (32%), glass, inert materials and metal (4%), organic matter (19%) and with humidity of around 17% and an LHV of 3,800 kcal/kg. The acidity content of the waste in production without neutralisation is around 0.4% in weight, expressed as equivalents in HCl and therefore in soda. The stoichiometric soda quantity is therefore 4.3 kg/t of waste and 14 kg/t in 30% solution.

On account of the requirements of the incinerator during fine shredding, 30 1/t of soda solution were added.

The material pre-neutralised in this way had an increase in weight and a negligible decrease in LHV (3%) and, dispersed in water, has a pH value of 10, indicating that all the added soda was buffered by the weak organic acids present. 1 ,300 t of fuel were produced in a campaign lasting 15 days, which fuel, compacted in 20 t containers , was sent to the incinerator in Filago (BG). Example 2 The incinerator situated in Filago (BG) has a combustion capacity of up

to 100,000 t/a of industrial waste consisting of polluted soil, organic sewage with high heating value, aqueous effluent with lower heating value and other miscellaneous waste.

A certain quantity of RDF is also fed into the incinerator as an heat integrator of the mix of waste fed.

The system consists of a rotary drum furnace for the solids followed by a static chamber for the combustion of the liquids and post-combustion.

The combustion gasses traverse a boiler where approximately 40 t/h of steam are produced that are expanded in a turbine to give 9 MW of electrical power, they then pass through an electrostatic filter, are cooled in an attemperator and filtered in a bag filter before being sent to the stack.

The acidity and organic micropollutants are abated by the injection of soda in the combustor and the addition of powder lime and active carbons before the hose filter. The gasses at the stack are subjected to continuous analysis by means of the FTIR (Fourier transform infrared) technique to determine the concentration of the acids.

Moreover an analysis at the stack is periodically carried out according to the standard EN 1948 to determine the concentration of dioxins and furans discharged into the environment.

During the three years approximately following start-up of the plant the concentration of HCl was found to be lower than 5 ppm (compared to the legal 50 ppm), that of SO3 practically zero and that of the micropollutants lower than the 0.1 ng/Nm3 laid down by European Union regulations. However, during the start-up period, the nebulised soda was fed with the waste - water with low heating value also into the rotary furnace and a certain deterioration was noticed in the refractory whose duration was 30% shorter than that foreseen.

It was therefore decided to nebulise the soda as far away as possible in the static chamber and during the latest shutdown (2 years after start-up) for refractory repairing, to use more acid refractories that are more resistant to the bases and periodically coating the lining of the rotary drum with vitreous matrices.

Among the tests carried out, feeding of the active carbons was also suspended for a short period, obtaining however a peak at 2 ng/Nm3 for the micropollutants.

The RDF, prepared as described in Example 1, was therefore fed into the rotary drum in place of the 3 t/h non-neutralised RDF and at the same time gradually reducing from 90 1/h to 10 1/h the 30% soda fed into the static chamber.

The concentration of HCl was found to be lower than 0.5 ppm and that of the organic micropollutants lower than 0.1 ng/Nm3, demonstrating the efficiency of abatement of the new fuel that is the object of the present invention.

In the final phase of the test, lasting approximately two weeks, the feeding of the active carbon was stopped and the organic micropollutants remained always below the limit of 0.1 ng/Nm3. It is assumed from this that the new fuel demonstrates superior efficiency in stopping the HCl at the outset compared to neutralisation in the combustion gasses once formed, and therefore in not making it available for de novo synthesis in the boiler.