The present invention relates to a method of scavenging alkali from flue gas; the method comprising the steps of

- introducing oxygen-comprising gas and a solid fuel comprising biomass into a combustion chamber to incinerate said solid fuel resulting in a flue gas comprising alkali, and

- introducing an additive material comprising i) clay and ii) a calcium compound into the flue gas.

It is generally known to incinerate a fuel, for example in an incinerator. Typically the heat generated is recuperated, for example to turn water into steam, which may then for example be used to produce electricity. It is also known to cool the flue gas for further treatment thereof, such as collecting of particulates or the removal of unwanted compounds prior to venting the flue gas into the atmosphere. This results in condensation of alkali-containing deposits on the internals of the incinerator, leading to various deleterious effects. By adding an additive, it is possible to reduce said adverse effects.

A method according to the preamble is known from the PhD thesis of M.P. Glazer (2007) "Alkali metals in combustion of biomass with coal". This PhD thesis discloses that the most promising additive is kaolin, an aluminosilicate clay containing hydroxyl (OH) groups.

The difference between active and non-active clay materials for the capture of alkali metals was reported to be the presence or absence of water and/or hydroxyl groups (page 23). It was reported that the presence of water and/or hydroxyl groups was essential to effectively scavenge the alkali species under combustion flue-gas conditions (page 24).

Addition of water or steam shows a fourfold increase of alkali scavenging compared to tests with no water (page 86), whilst increasing the kinetic rate of scavenging by a factor of ten (page 96).

A method according to the preamble is known from W02013093097. W02013093097 discloses a method for scavenging alkali wherein a mineral additive blend comprising a clay and a functional mineral is used. The clay is for example, kaolin, andalusite, kyanite, sillimanite, hydro-topaz, mullite, pyrophyllite, or dombassite, montmorillonite, meta-kaolin (dehydrated kaolin), beidellite, bentonite, with hydrated clays, such as kaolin, particularly preferred. The functional mineral is for example magnesium salt or calcium salt, for example, talc, dolomite, brucite and magnesium carbonate, magnesium carbonate, hydro-magnesite, vermiculite, smectite, phlogopite, clinochlore, sepiolite, attapulgite, palygorskite, calcium carbonate, calcium hydroxide, limestone, marble, chalk, dolomite, aragonitic sand, sea shells, coral, cement kiln dust, marl. Typically, the additive is introduced into a furnace, a fuel is introduced into the furnace and the two components are heated with the fuel being incinerated. As stated above, it is known in the art that water strongly enhances the capture of volatile alkali. Indeed W02013093097 discloses adding the additive as a mixture comprising at least 1% water by weight or as a slurry, and as the clay kaolin.

The object of the present invention is to reduce the operational costs of the method according to the preamble.

To this end, a method according to the preamble is characterized in that the additive is added as a powder, said additive comprising based on the weight of the additive

- at least 10% by weight meta-kaolin as the clay, and

- at least 10% by weight calcium oxide as the calcium compound; wherein an additive powder particle is an aggregate of micro-particles and a micro-particle of the additive powder particle is a micro-aggregate comprising both the meta-kaolin and the calcium oxide.

With the method according to the invention, an operational cost saving is achieved based on the active material in the additive. A typical incinerator needs 1-2 lorries with additive every week, so over 50 per year. The specific activity of the additive according to the present invention by weight is higher, which means that less material has to be transported, saving operational cost.

The combustion chamber into which the fuel is introduced is for example a fluidized bed or the chamber of a grate incinerator. The size of the fuel particles may be relatively small (e.g. in the order of millimeters or smaller) or relatively large (e.g. in the order of centimeters or larger).

The biomass is, for example, straw, refuse from industrial processes or households or mixtures thereof. The term powder or powdery material indicates material having a particle size of less than 100 pm. Between the particles there is gas (air), and the additive material can be dispersed by air.

In general, the additive material will be introduced in the flue gas where the flue gas has a temperature of at least 750°C and less than 1150°C, which conditions are typically higher than those where the condensation of alkali compounds starts to occur. In case of an incineration process involving flames, it is preferred that the additive material is injected downstream of the flames.

Typically, the flue gas contains non-gaseous material. Such non-gaseous material in the flue gas typically comprises solid or at least partially molten particles originating from the fuel. Typically, the concentration of non-gaseous material is more than 0.02% by wt. relative to the weight of the flue gas.

The method according to the invention is very suitable for the incineration of particulate waste material. Thus the particulate fuel will typically consist for more than 50%, preferably more than 75%, and even more preferably more than 90% of such material (including mixtures of household and industrial waste materials).

The oxygen-comprising gas is typically air.

Obtaining the desired additive composition including the amount of meta-kaolin will typically involve the dehydratation of hydroxyl groups present in the clay used as starting material by means of a controlled heat treatment of the additive prior to its application in the incineration process, as is further detailed below. Dehydrated additive, as can be obtained by such a pre-heat-dehydratation step, was found to result in an increased efficiency of the additive when applied in the incineration process for the scavenging of alkali components. This finding is surprisingly opposite to the teachings of M.P. Glazer and of W02013093097, where the presence of water and hydroxyl groups in the additives described were indicated to substantially increase the alkali scavenging efficiency.

The additive comprises based on the weight of the additive material preferably at least 20% by weight meta-kaolin as the clay, and more preferably at least 30% by weight.

The additive comprises based on the weight of the additive material at least 20% by weight calcium oxide as the calcium compound, and more preferably at least 30% by weight. The weight ratio between CaO and meta-kaolin will typically be in the range from 1:10 to 3:1.

As indicated above, meta-kaolin is made mention of in W02013093097, but not in a composition according to the present invention, wherein each additive particle is an agglomerate of smaller particles, and a smaller particle in itself is yet another agglomerate of calcium oxide and meta-kaolin. For the sake of clarity, in the present application the additive particle is referred to as an aggregate particle and the smaller particle as a micro-aggregate.

Without wishing to be bound by any particular theory, it is believed that the additive in accordance with the present invention allows for a cycle of water (generated by the combustion of biomass) within the additive particle, which is made possible by the minute distances that the water has to travel in a micro-aggregate particle. In such a micro-aggregate particle the calcium oxide is in very close proximity to the meta-kaolin. Water is recycled within the micro-aggregate particle between the calcium and meta-kaolin compounds present in such micro-aggregate particle, allowing the additive to be effective despite a low water or hydroxyl group content of the additive introduced. Water in the flue gas is derived from the biomass and/or the combustion thereof. The amount of biomass in the fuel is for example at least 5 wt.%, typically at least 10 wt.% and preferably at least 15 wt.% of the total amount of fuel.

According to a favourable embodiment, the additive powder to be introduced is an additive obtained by heat-treatment at a temperature of at least 750°C.

Such an additive has a reduced amount of bound water, having very suitable characteristics for scavenging alkali.

According to a favourable embodiment, the heat treatment is performed at a temperature of less than 900 °C, preferably less than 850 °C, and most preferably less than 800 °C, for a time period of less than 5 minutes, preferably less than 2 minutes, and most preferably less than 1 minute.

The additive obtained in such a manner is very suitable for the method of scavenging alkali from flue gas. In case of heat treatment in the above range, at higher temperatures a shorter time period is preferred.

According to a favourable embodiment, for an additive containing calcium carbonate in the additive powder particle, the weight ratio between calcium carbonate and calcium oxide is less than 1, preferably less than 0.5 and most preferably less than 0.1.

Thus the activity of the additive material based on weight is improved and less additive material has to be transported.

According to a favourable embodiment, the free water content of the additive material is less than 0.9 wt./wt. %, preferably less than 0.45 wt./wt. %.

Such an additive material can be introduced with little risk of clogging a nozzle or conduit towards the nozzle used for introducing the additive material.

The free water content of the additive material as it is to be understood in the present application may be measured by a before and after measurement, wherein the additive material is kept at 400°C until a constant weight is reached.

According to a favourable embodiment, the bound water content of the clay of the additive material is less than 0.9 wt./wt. %, preferably less than 0.45 wt./wt. %.

Thus a better saving in operational cost can be attained.

The loss of hydroxyl groups from kaolin, that is the loss of bound water as it is to be understood in the present application, may be determined using a before and after measurement, wherein the additive material is kept at a temperature of 600°C until a constant weight is reached.

According to a favourable embodiment, the powdery additive is injected pneumatically.

This was found to reduce the risk of clogging of a conduit and/or nozzle used for introducing the additive material into the flue gas at conditions where the alkali is volatile, thus scavenging the alkali prior to condensation/deposition, protecting the heat exchanger or other equipment susceptible to an adverse effect of alkali.

According to a favourable embodiment, the additive material is obtained by combusting paper waste material.

A suitable method is disclosed in EP0796230. Advantageously, the freeboard of the combustion apparatus is controlled to a temperature of less than 1150°C to avoid breakdown of the meta-kaolin. The freeboard temperature is not necessarily restricted to less than 850°C as is necessary to obtain a hydraulic material; wherein lifting that restriction lowers the complexity of producing of the additive material.

According to a favourable embodiment, the weight ratio between CaO and meta-kaolin in the additive material is in a range of 1:4 to

2:1.

Such a powdery additive has improved reactivity, allowing less additive material to be transported.

The invention will now be illustrated with reference to the example section below.

EXAMPLE SECTION

In the powder of aggregated micro-particles, the distance between the calcium and meta-kaolin compounds in the additive is minute. This is achieved by producing the additive from a dispersed phase wherein kaolin is present and a dissolved phase wherein a calcium oxide precursor, typically calcium hydroxide, is present. The kaolin is finely dispersed with a typical particulate size of less than 50 pm, preferably less than 20 pm, even more preferred less than 10 pm. The calcium precursor typically is dissolved in the liquid of the dispersed phase containing the kaolin. If this is not the case, the dispersed and dissolved phases are mixed together prior to the further treatment described below. The dissolved calcium is made to precipitate onto the kaolin, which can be done by leading C0 ₂ gas through the suspension, causing calcium carbonate to precipitate. The thus obtained suspension contains an intimate mixture of kaolin, and calcium carbonate. The suspension is then filtered to obtain a starting material for a heat treatment to obtain the additive material as will be described below. After the filtration, the starting material contains micro-aggregates of particles, wherein each particle contains both the kaolin and the calcium oxide precursor.

A similar starting material to obtain the additive through heat treatment, containing a similarly intimate mixture of kaolin and calcium carbonate can be obtained from waste paper and/or residues that stem from the recycling of waste paper. In this case, the multitude of paper recycling and paper production processes involving repeated dissolving, dispersion, and drying, has provided for a similar intimacy between the kaolin and calcium precursor as obtained from the precipitation method described in the previous section.

The heat treatment of the starting material (which is carried out prior to and separate from the actual application of the resulting additive in the incineration process according to the present invention), sees on steps of i) evaporation of excess physical water (thermal drying), ii) conversion of kaolin into meta-kaolin under the release of bound water from the dehydratation (elimination of OH groups) present in the kaolin, seriously reducing the number of OH groups present in the kaolin (dehydration); and iii) conversion of at least part of the calcium carbonate into calcium oxide under the release of carbon dioxide (calcination). Advantageously, care is taken to prevent unwanted temperatures and residence times wherein meta-kaolin is converted into less wanted minerals like mullite, and to prevent reaction between (meta)kaolin and calcium into less wanted minerals like gehlenite - which processes occur at temperatures of more than 800°C, and residence times at such temperature of several minutes.

Typically, the heat treatment is carried out by exposure of the starting material to a controlled temperature of less than 900 °C, preferably less than 850 °C, most preferred less than 800 °C, for a limited time period of less than 5 minutes, preferably less than 2 minutes, most preferred less than 1 minute.

Although the controlled heat pre-treatment adds an additional step in producing the additive material prior to its application in the incineration process, the obtained increased reactivity more than offsets this apparent disadvantage. Moreover, the method results in a more stable and predictable composition of the additive as compared to efforts wherein the heat treatment is incorporated in the incineration process. The heat treatment thus furthermore does not place additional demands on operations of the incineration process.

As an example, the following heat treatment of the starting material was found to result in the additive described in the invention:

Starting material obtained from kaolin and calcium carbonate containing paper waste was thermally treated in a fluidized bed installation having a freeboard in the presence of oxygen-comprising gas. The fluidized bed is operated at a first temperature and the temperature of the freeboard is at a second temperature, wherein at least one of the first temperature and the second temperature is at least 750°C, whilst the residence time at temperatures of more than 860°C is kept at less than 10 seconds. This method resulted in a conversion of more than 90% of the kaolin into meta-kaolin, whilst the conversion of calcium carbonate into calcium oxide was observed to be over 40%. Micrographs of the thus obtained additive material showed very fine particles with dimensions of well below 10 pm, and even sub-pm that were present as individual particles and as porous aggregates wherein the particles were connected by means of small necks, that appeared to have formed during the thermal treatment.

Determination of conversion of kaolin to metakaolin

To determine that the kaolin is converted to a satifsfactory degree into metakaolin, Thermogravimetric characterization (TGA) can be applied, wherein the weight reduction due to the dehydratation of the kaolin-hydroxyl groups is measured. This method is well known to someone skilled in the art, and is for instance described in "T.M. Zewdie et. al.; Fabrication and Characterization of Metakaolin Based Flat Sheet Membrane for Membrane Distillation; Advances of Science and Technology; 7th EAI International Conference 2019; Springer Nature Switzerland 2020; page 657". The weight loss typically occurs at temperatures between 400 and 600°C. When calcium carbonate is present, care has to be taken not to confuse the weight loss due to the calcination of the calcium carbonate for that due to the dehydratation of the kaolin, as both weight losses occur at similar temperatures, when measured in nitrogen. The weight loss caused by calcination of the calcium carbonate can however be suppressed (moved to a higher temperature) by carrying out the measurement in a C0 ₂ atmosphere. Alternatively, the evolving gases occurring during weight loss can be analysed, after which the amount of bound water loss caused by loss of hydroxyl groups in the kaolin between 400 and 600°C can be used to calculate the amount of kaolin dehydratation. Application of the above method, revealed that the conversion of kaolin into metakaolin, using the preparation method described above was more than 90%. Typically the loss of bound water due to the loss of hydroxyl groups in kaolin found between 400 and 600°C is less than 1.1 wt.%, and preferably less than 0.45 wt.% by weight of the additive material.

EXAMPLE TO DEMONSTRATE ALKALI SCAVENGING BY THE ADDITIVE

To demonstrate the beneficial effects of various additives on boiler tube materials, the following set-up was applied: A small boat-shaped container made from 15Mo3 steel (a typical boiler-tube material) of approximately 2.4 cm in length and 0.4 cm in height, that was open and exposed to the atmosphere at the top, was used as described below The container was filled with a mixture of 1 gram of pure sodium chloride and/or 0.4 gram of additive material. The solid materials placed in the container were fine powders with a typical particulate size of 5-20 micrometers. Prior to their placement in the container, materials were manually mixed The container and its contents was placed in a quartz-glass tube, that was surrounded by an electrically heated furnace Simulated flue gas containing 6% 0 ₂ ,1% S0 ₂ ,73% N ₂ ,and 20% H20 was led through the glass tube at a rate of 1500 ml/min. The gas was thus contacted with the container and its contents The furnace was heated to 600 degrees centigrade and kept at this value to expose the container and its contents to this temperature and the simulated flue gas for a time period of 4 hours After removal of the container from the quartz-tube, the contents were removed from the container. The container was then carefully wiped, rinsed with water, and dried to the ambient atmosphere, The material that was collected from the container after its exposure to 600 degrees centigrade and the simulated flue gas was washed with water to remove dissolvable salts like the non-scavenged sodium chloride. The material that was not dissolved in the rinsing water, was dried and further characterised by means of electron microscopy and Energy Dispersive X-Ray characterisation (EDX), which methods are considered to be known to someone skilled in the art. The ratio of non-dissolvable Na: A1 was used to evaluate the alkali scavenging capacity of the applied additives. After being used in the above experiment, each container was weighed. The weight was then compared to the initial weight of the container prior to the experiment. Since corrosion results in the transfer of part of the material of the container to the corroding material, which in this case was the sodium chloride, the measured weight loss of the container is used to indicate the degree of sodium chloride induced corrosion. The observed weight loss was normalized to the maximum weight loss measured, which occurred as expected in the experiment wherein pure sodium chloride was applied without the application of alkali scavenging additives.

The following experiments were carried out

A Container plus 1 gram of NaCl, no further additives

B Empty container

C Container plus 1 gram of NaCl and 0.4 gram of kaolin

D Container plus 1 gram of NaCl and 0.4 gram of meta-kaolin/calcium oxide additive

The following Na scavenging results were obtained from the experiments:

A no Na scavenging measured; no A1 added; ration of Na : A1 undetermined

B no Na applied in test, thus no Na scavenging measured

C Na:Al (wt/wt) 0.38

D Na:Al (wt/wt) 0.78

These results demonstrate the improved capacity of the additive of the patent application as compared to pure kaolin which until now is seen as the best alkali scavenger known from literature

The following corrosion rate results were obtained from the experiments:

A Weight loss of container due to corrosion 100% (reference as described above)

B Weight loss of container due to corrosion 30%

C Weight loss of container due to corrosion 30%

D Weight loss of container due to corrosion 30%

These results demonstrate: The serious unwanted effect of alkali chloride on the corrosion of boiler tube material as can be seen from comparison of results A and B. · The significant reduction of alkali induced corrosion to reach levels of non-alkali induced corrosion by means of kaolin, in line with the known state of the art (Glazer) as can be seen from comparison of results A, B and C. The same reduction of alkali induced corrosion by the meta-kaolin/calcium oxide additive as can be seen from comparison of results A, B and D.