A method of combusting N-containing material The present invention relates to a method of combustion of N-containing material. Greenhouse gasses such as carbon dioxide (CO ₂ ), methane (CH ₄ ) and nitrous oxide (N ₂ O) have long been known to contribute to climate change. These gasses are for example produced by human activities such as agriculture, wastewater management and combustion of N-containing biomass. In the efforts of controlling climate change, reducing emission of greenhouse gasses plays an important role. In particular, reducing emission of N ₂ O is of interest, since the impact of N ₂ O on warming the atmosphere is almost 300 times bigger than the impact of CO ₂ , and 12 times bigger than CH ₄ and about 40% of global total N ₂ O emission stems from human activities. EP0550905 describes the use of calcium based sorbents to decompose N ₂ O in flue gases from the combustion of nitrogen (N) containing fuel in a fluidized bed combustor. It is a goal of the present invention to provide a method of controlling N ₂ O released during incineration of N-containing, i.e. nitrogen containing, material. To this end, a method according to the preamble is characterized in that the method comprises: - a first stage of producing activated bed particles, the first stage comprising the steps of: - providing bed particles to a fluidized bed incinerator to form a bed of bed particles, - providing kaolin and CaCO ₃ to the bed particles of the bed, - thermally treating the provided kaolin and CaCO ₃ at a fluidized bed temperature of 600-1000 °C to obtain the activated bed particles, - a second stage of combusting an N-containing material, the second stage comprising the steps of: - providing the N-containing material, - providing the activated bed particles, - combusting the N-containing material in the presence of the activated particles. Kaolin is a clay mineral comprising aluminiumsilicate. Thermal treatment of kaolin and CaCO ₃ results in the production of a material in which at least part of the CaCO ₃ is converted into CaO. During the step of thermally treating the provided kaolin and CaCO ₃ , at least part of the formed CaO attaches to the bed particles of the bed of the fluidized bed incinerator. Thus, activated bed particles comprising CaO are produced. The term "activated bed particles" is to be understood as bed particles comprising CaO deposited thereon due to the thermal treatment of kaolin and CaCO ₃ . The fluidized bed incinerator may be a fluidized bed incinerator known in the art, such as a bubbling fluidized bed incinerator, also referred to as stationary fluidized bed incinerator, or a circulating fluidized bed incinerator. When the fluidized bed incinerator is running, the bed of bed particles is heated and fluidized, and the provided kaolin and CaCO ₃ mixes with the bed particles during the thermal treatment. The bed particles provided to the fluidized bed incinerator in the first stage may be bed particles without any CaO. However, they may also be bed particles already comprising CaO, but where the amount of CaO on the bed particles increases during thermally treating the provided kaolin and CaCO ₃ in the fluidized bed incinerator. The CaO does not form an even layer on the bed particles, but rather forms deposits. Thus, the activated bed particles have an uneven surface rather than a smooth surface, providing a larger contact area for N ₂ O and a more efficient decomposition of N ₂ O into N ₂ and O ₂ as compared to activated bed particles provided with a smooth layer of CaO. In the second stage, combustion of N-containing material results in the production of N ₂ O. The CaO present on the activated bed particles catalyses decomposition of the N ₂ O gas formed from combusting the N-containing material, resulting in a reduced N ₂ O emission from combusting the N-containing material. Reduction of the emission of N ₂ O from the combustion of the N-containing material can be inferred from the difference in N ₂ O emission when the same N-containing material is incinerated in the absence of activated bed particles, whilst further applying the same process conditions. In addition to its contribution to the deposition of CaO on the bed particles, kaolin can also bind alkali metal compositions originating from, for instance, the N-containing material, such as sodium and potassium compositions. Thus, activated bed particles comprising kaolin can bind these compositions, reducing the stickiness and agglomeration of alkali metal compositions onto the activated particles as well as onto heat exchanging surfaces, resulting in a more efficient decomposition of N ₂ O as well as an improvement of heat transfer. In the first stage, during the thermal treatment of the kaolin and CaCO ₃ , the bed particles grow in diameter and in mass due to deposition of CaO. Thus, the production of activated bed particles results in a change in mass of the bed of particles, which in turn results in a reduction of the efficiency of the fluidized bed incinerator whilst the kaolin and CaCO ₃ is being thermally treated and the activated bed particles are formed. In order to allow the fluidized bed incinerator to keep functioning, it is common practice to regularly remove a part of the activated bed particles from the fluidized bed incinerator and to provide additional bed particles comprising no or less CaO. Typically, the removed part of activated bed particles is disposed of as waste. The method according to the invention allows the activated bed particles from a fluidized bed incinerator in which kaoline and CaCO ₃ is thermally treated to be employed for the decomposition of N ₂ O. Methods to remove activated bed particles from the fluidized bed incinerator are known to the person skilled in the art, and include the application of a funnel that can be opened and closed using a valve or slide valve, or the application of a cooled auger preceded by a closing valve. In the first stage, the kaolin and the CaCO ₃ may be provided simultaneously in a mixture. Alternatively, kaolin may be provided first and CaCO ₃ second, or CaCO ₃ may be provided first and kaolin second. Furthermore, the activated bed particles may be removed from the fluidized bed incinerator after the first stage, and the second stage may be performed separate from the fluidized bed incinerator used in the first stage. Alternatively, the N-containing material may in the second stage be provided to the same fluidized bed incinerator used in the first stage after the first stage has been completed. The bed particles provided to the fluididized bed incinerator can have a particle diameter of between 0.5 and 2 mm, preferably between 1 and 1.7 mm and more preferably between 1.2 and 1.5 mm. Bed particles of this diameter are suitable for use as bed particles in a bed of a fluidized bed reactor. Preferably, the bed particles provided to the fluididized bed incinerator are at least substantially spherical. "Spherical" means that every axis through the center of the particle is of the same length, or in other words, that the ratio of the shortest and the longest axis through the center of the particle is 1. "Substantially spherical" means that the ratio of the shortest and the longest axis through the center of the particle is at least 0.8, preferably at least 0.85, more preferably at least 0.9, even more preferably at least 0.95. N-containing material is for example sludge from waste water treatment plants, waste from hospitals or nylon containing material such as textile or rope. In addition to a reduced emission of N ₂ O, the activated bed particles can in the second stage also act as sorbent for metals such as potassium or heavy metals, such as lead or mercury, resulting in a cleaner flue gas and less deposition of solids on heat exchanging boiler tubes. Temperatures are given as temperatures measured at atmospheric pressure of about 1 bar or about 1.013 hPa. A pressure when thermally treating the provided kaolin and CaCO ₃ in the first stage is typically less than 0.2 bar overpressure, and preferably less than 0.1 bar overpressure. The amount of bed particles provided in the first stage to the fluidized bed incinerator, N-containing material and activated bed particles varies depending on the installation used. Typical amounts are 10-40 ton of bed particles provided in the first stage to the fluidized bed incinerator, 10-20 ton of activated bed particles that is in the second stage added to the incineration of the N-containing material on a per-hour basis, at a ratio of less than 1:10 compared to the hourly throughput of the N-containing material, and 10-40 ton of N-containing material that is combusted per hour. According to a favourable embodiment, the second stage is performed in a fluidized bed incinerator. In this way, the activated bed particles produced in the first stage can serve as bed particles for a fluidized bed incinerator in which the N-containing material is combusted in the second stage. The fluidized bed incinerator used in the second stage can be the same as the fluidized bed incinerator used in the first stage, or can be a separate fluidized bed incinerator. According to an embodiment, in the first stage, the step of providing the bed of bed particles with kaolin and CaCO ₃ comprises providing paper sludge or waste paper. In this way, activated bed particles resulting from incinerating paper sludge or waste paper in a fluidized bed incinerator can be employed in the second stage. Kaolin is used as a filler and coating material in paper, while CaCO ₃ is used as a whitener in paper. Waste paper or paper sludge is commonly incinerated in a fluidized bed incinerator. Thus, incinerating paper sludge or waste paper in a fluidized bed incinerator leads to activated bed particles comprising deposits of CaO. While these would otherwise be disposed of as waste, according to an embodiment of the invention they can be used in the decomposition of N ₂ O arising from combustion of N-containing material. Preferably, the bed particles provided to the fluidized bed incinerator are of a material that is solid and incombustible at said bed temperature. According to an embodiment, the bed particles provided to the fluidized bed incinerator comprise a compound comprising at least one of silicon, calcium, magnesium, aluminium, and iron. The bed particles comprise for example limestone, dolomite, aluminiumoxide, bauxite, iron oxide or iron ore. Preferably, the particles comprise silicate. According to an embodiment, the provided bed particles comprise sand particles. Sand is a common material for beds of fluidized bed incinerators. River sand particles have a suitable diameter for use as bed particles in the bed of a fluidized bed incinerator. Preferably, in the first stage, the step of thermally treating the provided kaolin and CaCO ₃ is performed for a duration between 2 and 192 hours. A relatively long duration of treatment results in activated bed particles comprising more CaO as compared to shorter durations of treatment. Due to the higher amount CaO, the activated bed particles contribute to a more efficient decomposing of N ₂ O in the second stage. Preferably the step of thermally treating the provided kaolin and CaCO ₃ is performed for a duration of between 6 and 168 hours, more preferably between 12 and 120 hours, more preferably between 24 and 96 hours. The person skilled in the art will be able to select a suitable duration depending on the diameter of the bed particles provided to the fuididzed bed incinerator and the desired diameter of the activated bed particles. The progress of the deposition of CaO onto the bed particles can be monitored by the increase of the diameter of the bed particles during thermal treatment of the provided kaolin and CaCO ₃ . Alternatively, the increase of bed mass can be monitored by measurement of the increase in pressure drop between an air distributor plate underneath the bed and a splash zone (where the flue gas leaves the bed) above the bed. According to an embodiment, in the second stage, at most 5 mass% CaO is present based on the total mass of N-containg material and activated bed particles. Preferably, the second stage is carried out in a bubbling fluidized bed incinerator, or a circulating fluidized bed incinerator. A bubbling fluidized bed incinerator is also referred to as a stationary fluidized bed incinerator. Preferably, in the second stage, the mass ratio of activated bed particles to N-containing material is at least 1:20, preferably at least 1:15 and more preferably at least 1:10. In this way, N ₂ O is more efficiently decomposed. Preferably, the fluidized bed temperature in the first stage is 650-950°C, preferably 700-900 °C, and more preferably 750-850 °C. In this way, CaO is formed more efficiently on the bed particles. According to an embodiment, between the first and second stage, the activated particles are provided with a catalyst. Suitable catalysts are for example redox catalysts, such as catalysts comprising Fe ²⁺ /Fe ³⁺ , Cr ⁴⁺ /Cr ⁶⁺ , V ⁴⁺ /V ^2.5 +, Cu ⁺ /Cu ²⁺ , for example FeO/Fe ₂ O ₃ , CrO ₂ /CrO ₃ , VO2/V ₂ O ₅ , Cu ₂ O/CuO. Finally, the present invention relates to use of particles comprising CaO deposited thereon, wherein the particles are used in a process of combustion of N-containing material. Preferably, the particles comprise at least 5 mass% CaO deposited thereon. According to an embodiment, the particles comprising CaO deposited thereon are obtained by - providing bed particles to a fluidized bed incinerator to form a bed of particles, - providing kaolin and CaCO ₃ to the bed particles of the bed, and - thermally treating the provided kaolin and CaCO ₃ at a fluidized bed temperature of 600-1000 °C to obtain activated particles comprising CaO deposited thereon. According to an embodiment, the process of combustion of N-containing material comprises the following steps: - providing the N-containing material, - providing the activated particles comprising CaO deposited thereon, - combusting the N-containing material in the presence of the activated particles. The invention will now be illustrated with reference to the example section below. In an exemplary embodiment of the invention, paper sludge comprising kaolin and CaCO ₃ is introduced in a first fluidized bed incinerator comprising a bed of river sand particles having a diameter of about 1.5 mm. The bed of particles is then heated to a temperature of 800 °C at atmospheric pressure for 24 hours in order to incinerate the paper sludge. This results in the formation of deposits of material comprising CaO on the bed particles of the bed of the fluidized bed incinerator. Thus, activated bed particles comprising CaO and having a diameter of 2-4 mm are obtained. After removal from the first fluidized bed incinerator, the activated bed particles are introduced in a second fluidized bed incinerator. Then sludge from a waste water treatment plant comprising N-containing material is introduced in the bubbling bed incinerator, and combusted in the presence of the activated bed particles. Combustion of the N-containing material results in the formation of N ₂ O. The CaO present on the activated bed particles acts as a catalyst for the decomposition of N ₂ O into N ₂ and O ₂ , thus reducing the emission of N ₂ O from the combustion of the N-containing material.