The present invention relates to an incineration process for waste, and a device for performing the incineration process.

In particular, the invention is intended for incinerating waste or other inflammable substances in a grate incinerator, which is by far the most commonly used method for domestic waste or equivalent waste. However, the invention can also be applied in drum incinerators or fluidised bed incinerators.

It is known that for processing general waste worldwide the most commonly used method that remains as the only feasible option is incineration with energy recovery.

Traditionally the waste is incinerated in a grate incinerator in a large enclosed chamber, the floor of which consists of a "grate". This grate which bears the burning waste moves the waste from the feed side up to the discharge side of the incineration residues, i.e. bottom ashes or slags.

The incineration air is partly blown through the grate into the incinerator and partly over the grate into or over the burning waste in the incinerator. Said incineration air may or may not be heated. The incineration requires oxygen and this is supplied by air from the surroundings which chiefly consists of nitrogen (78 %) but also contains the necessary oxygen (21 %) and for the rest contains inert gases, carbon dioxide (CO ₂ ) and water vapour (1 %).

The incineration gases mostly consist of nitrogen, already present in the incineration air, but otherwise, hardly participating in the incineration process, and further of carbon dioxide (CO ₂ ), water vapour (H2O) and a residual amount of unused oxygen.

The incineration gases also contain a high number of components many of which are harmful for human health and the environment, such as acids (HCl, SO ₂ , HF, etc.), nitrogen oxides (NO _x ), carbon monoxide (CO) and unburned hydrocarbons, dioxins and furans, heavy metals and solid particles .

These components are an inevitable complication of waste incineration but there are sufficient techniques to reduce the emissions of said components to a very low level. The emission reduction, i.e. the ratio of the removed quantity of a certain component to the quantity of said component in the untreated flue gases, amounts to 99% or even more in almost all cases. Only for NO _x the emission reduction is less whereby 75% to 80% is already a very good result, which means that another 20% to 25% of the produced NO _x is emitted. In recent years, global warming has become an important social and political issue. This warming is caused mostly by the increasing concentration of CO ₂ ("greenhouse gas") in the atmosphere, but also a number of other substances such as methane. This is the result of human activities such as traffic, heating of buildings, electricity generation, industry, and the like. The flue gases of waste incineration contain approximately 9 vol% CO ₂ on average. The existing techniques to capture said CO ₂ are cumbersome and expensive.

The energy released in waste incineration is recovered as much as possible, usually in the form of steam that is used to generate electricity in a classic Rankine steam cycle and/or for heating purposes. The aim is the biggest possible efficiency, i.e. to use as much energy in a useful way as possible, in other words to waste as little energy as possible. The biggest energy loss in waste incineration is the 'flue loss', i.e. the energy leaving the flue in the flue gases.

The present invention relates to a new incineration process for waste and by extension other fuels whereby, compared with the current state of the art:

• None or hardly any CO ₂ is emitted;

• More energy can be recovered;

• The formation of NO _x is substantially lower;

• The amount of flue gases is many times less. In the current state of the art the waste incineration in a grate incinerator goes through three main processes (see also Fig. 1): 1. An incineration process in which the waste is fed to the incinerator and is mixed and incinerated together with incineration air, resulting on the one hand in hot flue gases and on the other hand in incineration residues;

2. A cooling process in which the hot flue gases are cooled in a steam boiler and the energy released during the incineration is recovered;

3. A purification process in which the cooled flue gases are purified before they are discharged into the atmosphere.

However, the mass and energy balance of this sort of known waste incineration in a grate incinerator is unfavourable. Per tonne or per 1000 kg of waste, approximately 5 tonnes of air are needed and approximately 6 tonnes of flue gases are produced. This large quantity is the result of the fact that the incineration air consists of inert nitrogen for the most part, which does not participate or hardly participates in the incineration processes, but does have to be heated and cooled again. The whole installation must therefore be sized for a flow of gas which for the most parts consists of inert nitrogen ballast.

The nitrogen in the incineration air gives rise to the formation of NO _x which, chiefly with high temperatures, is also called "thermal NO _x ". The fuel itself, i.e. the waste, also contains nitrogenous compounds which during incineration can result in NO _x which is also called "fuel NO _x ".

According to most specialised literature "fuel NO _x " is the dominant form in waste incineration, particularly with moderate temperatures. The typical NO _x concentration of the flue gases at the exit of the incinerator amounts to 400 mg/Nm ³ under reference circumstances, whereas the European directives lay down a maximum of 200 mg/Nm ³ in the flue and even lower standards, to 100 mg or 70 mg, are imposed in the permit of many installations. To achieve said lower values, so-called de-NO _x -installations are needed, which exist in two variants: at high temperature without catalyst (SNCR) or at lower temperature but then with catalyst (SCR).

The foregoing shows that a logical step would be to avoid the nitrogen in the incineration air by working with pure oxygen, which is an industrial gas that is applied on a large scale in, for example, the steel industry. Although this is theoretically possible, the calculations show that the temperature in the incinerator, even at a low incineration value of the waste, increases to very high values to more than 2000°C.

This is much too high for a grate incinerator. It would in any case result in the formation of melted incineration residues and clinkers. When pure oxygen is used the theoretical flue gases only consist of water vapour, CO ₂ and a small percentage of pollutants as with classic waste incineration i.e. acids, NO _x , CO, and particles (see also Figure 2). To limit the temperature in the incinerator, a mixture of oxygen and an inert gas other than nitrogen can be used instead of pure oxygen.

An obvious option is flue gas recirculation, i.e. a part of the produced flue gas is led back to the incinerator together with the, in this case, pure oxygen. Flue gas recirculation or FGR is a known and regularly applied method, but certainly not always, in traditional waste incineration and has a favourable influence on the thermal efficiency of the installation. FGR would also limit NO _x formation.

In the concept with pure oxygen, the flue gases contain a lot of water vapour as well as pollutants. They can be removed with classic techniques of flue gas purification and flue gas condensation. In the latter the flue gases are cooled to below their saturation temperature, typically 65 °C. A lot of water is recovered in this way on the one hand, which still needs to be purified, but on the other hand a lot of heat is also recovered. This, however, concerns heat at low temperature, only suitable for heating buildings for example.

The purpose of the present invention is to provide a solution to the aforementioned and other disadvantages by providing in an incineration process, allowing the heat recovery to be increased, the CO2 emission to be prevented entirely or almost entirely and the amount of flue gas to be reduced by many factors.

To this end the present invention (see Figure 4) relates to a classic grate incinerator whereby the incineration air will be replaced by a mixture of pure oxygen and recirculated, purified and dewatered flue gas, more than half of which consists of CO ₂ .

The mass and energy balance of said incineration process according to the invention in a grate incinerator is far more favourable, and is shown in Figure 4.

1 tonne of waste requires 0.63 tonnes of pure O ₂ . The flue gases at the exit amount to less than 0.8 tonne, i.e. less than 15% of what is released with the traditional state of the art and for 90% consist of CO ₂ . It has thus become very simple to capture and store said CO ₂ ('Carbon Capture and Storage' of CCS) or to use it ('Carbon Capture and Utilisation' of CCU). However, CCS and CCU are existing concepts which demand a lot of energy and resources. In the present invention they become self-evident.

The energy balance shows that the steam cycle is considerably more efficient, in the example (in terms of figures) this is 7.5%, because the flue loss is less. Furthermore, the flue gas flow through the boiler is lower such that the steam boiler, the fans, pipes, etc. can be smaller as well, and therefore the investment cost is lower. The pure oxygen required for this process can either be purchased from third parties or be made on site with the known techniques:

* VPSA or vacuum pressure swing adsorption;

* ASU or cryogenic air separation unit;

* Electrolysis

The present invention can be used for each of said techniques. However, VPSA is less suitable because the produced oxygen gas is not pure enough, i.e. typically 90% and because the available standard production units are not big enough except for small waste incineration installations .

ASU is the classic and known way to produce gases. As 'by- product' of the production of pure oxygen a lot of nitrogen is also made of course which can be sold as industrial gas.

During electrolysis, pure water is separated into oxygen gas and hydrogen gas. This known procedure consumes a huge amount of electrical energy, far more than the waste incineration installation produces. The first goal of electrolysis is making hydrogen gas, the oxygen then being a 'by-product'.

In the present invention (see Figure 4) electrolysis is used to generate the required oxygen for the incineration process . The invention also relates to a device for performing the incineration process, whereby the device comprises an incinerator with a feed for the waste to be incinerated, a feed for a mixture of pure oxygen and recycled, purified and dewatered flue gas, a discharge for bottom ashes and a discharge for hot flue gases, and whereby the device also comprises an electrolysis device, with which the water that was recovered from the flue gas purification installation is separated into pure oxygen and pure hydrogen, whereby the pure oxygen is led to the incinerator after being mixed with recycled, purified and dewatered flue gases and introduced into the incinerator to maintain the incineration. The calculations show that sufficient water is recovered from the flue gases to produce the necessary oxygen . The water does have to be completely purified and demineralised. An installation of the desired size does not yet exist.

The discharge for hot flue gases in the device can be connected to the feed of a steam boiler in which the hot flue gases are cooled with water with formation of steam and are discharged as cooled flue gases, whereby the formed steam is converted into electric power in a Rankine cycle.

In the present invention the flue gases of the waste incineration rich in CO ₂ are combined with the hydrogen gas produced by electrolysis to make a useful chemical substance using existing technology. An example is the synthesis of methanol (CH ₃ OH). This is a known fuel but also a basic product for the chemical industry. When the synthesis of methanol is combined with the waste incineration installation, interesting synergies are possible, because the synthesis consumes steam but also produces energy. Such methanol synthesis installations exist on a sufficiently large scale.

With the intention of better showing the characteristics of the invention, a few preferred embodiments of an incineration process according to the invention are described hereinafter by way of an example, without any limiting nature, with reference to the accompanying drawings, wherein:

Figure 1 schematically shows a flow diagram of a traditional incineration process in a grate incinerator; figure 2 schematically shows a flow diagram of a hypothetical incineration process with pure oxygen; figure 3 schematically shows a flow diagram of an incineration process in a grate incinerator; figure 4 schematically shows a flow diagram of the incineration process according to the invention with electrolysis of water; figure 5 schematically shows a flow diagram of figure 4 with synthesis of methanol.

Figure 1 schematically shows a flow diagram 1 of a traditional incineration process in a grate incinerator 2 whereby the waste 3 is fed to the grate incinerator 2 and together with incineration air 4, i.e. ambient air is mixed and incinerated, resulting in, on the one hand, hot flue gases 5 and on the other hand, in incineration residues 6 such as bottom ashes. The hot flue gases 5 are cooled in a steam boiler 7 in which water 8 is converted into steam 9 and whereby a part of the energy is recovered that is released during the incineration. The cooled flue gases 10 are purified in a purification step 11 by means of chemicals 12, with the formation of purified flue gases 13 which are discharged into the atmosphere, and residues 14 of the flue gas purification. The figure specifies the obtained quantities starting from 1 tonne per hour (TPH) of waste 3 processing, and the supply of 4.87 tonnes ambient air 4, resulting in 214 kg bottom ashes 6, and 2.32 tonnes of steam 9 in the steam boiler 7, which recovers 492 kWh energy from the cooled flue gases 10. Said cooled flue gases 10 are purified with chemicals 12 whereby 25.1 kg residue 14 of the flue gas purification 11 is formed, and after which 5.99 tonnes of flue gas 13 are discharged into the atmosphere.

Figure 2 schematically shows a flow diagram 15 of a hypothetical but unfeasible incineration process of waste 3 in an incinerator 16, whereby the incineration air from the environment would be replaced by pure oxygen 17 (99% pure). The temperature of the incinerator 16 would hereby increase to an untenable temperature in excess of 2000 °C, resulting in a melted incinerator and incineration residues 18 and the theoretical flue gases 19 would only consist of water vapour, CO ₂ and a small percentage of pollutants such as acids, NOx, CO and particles. The total flue gas flow would be much less, i.e. 1,500 Nm3 per tonne of waste, and the thermal efficiency would be better due to far less flue loss.

Figure 3 schematically shows a flow diagram 20 of an incineration process of waste 3 in a classic grate incinerator 21, whereby a mixture 22 of pure oxygen 23 and of recirculated, purified and dewatered flue gas 24 is added to the incinerator, more than half of which consists of CO ₂ . 1 tonne of waste requires 0.63 tonne of pure oxygen O ₂ whereas the flue gases at the exit 24 amount to less than 0.8 tonne or less than 15% of the quantity of flue gases produced in a traditional incineration process with incineration air from the environment.

Furthermore, 90% of the flue gases consist of CO ₂ which is better suited to capturing and storing or utilising said CO ₂ . The incineration process converts the waste 3 into hot flue gases 5 and bottom ashes 6, after which the hot flue gases are cooled in a steam boiler 7 in which water 8 is converted into steam 9. The cooled flue gases 10 are purified in a flue gas purification system 11 with condensing scrubber by adding chemicals 12, and with the formation of residues 14 of flue gases. The energy balance shows that the steam cycle is considerably more efficient (now 7.5 % more) and that the flue loss is less, such that the whole installation can be sized smaller.

Figure 4 schematically shows a flow diagram 25 of a waste incineration process for an incinerator according to the invention whereby a mixture of pure oxygen and recycled, purified and dewatered flue gas is supplied to the incinerator and whereby the pure oxygen, necessary for the incineration process, is produced locally in an electrolysis device, which separates water recovered from purifying the cooled flue gases into hydrogen (30) and oxygen (26).

Calculations show that sufficient water 28 is recovered from purifying the cooled flue gases 1.0 to produce the necessary oxygen, provided that the water is completely purified and demineralised. The electrolysis device 27 uses renewable energy 29 such as solar and/or wind energy to produce oxygen 26, whereby hydrogen 30 is also produced as a by-product, which can be sold, or in a second variant of the incineration process according to the invention can also be utilised on site.

Figure 5 shows a flow diagram 31 of a variant of the incineration process according to the invention as specified in figure 4, whereby in this case the flue gases 24 of the waste incineration rich in CO ₂ , are partly combined with the hydrogen gas H ₂ 30 that was produced via electrolysis of water 28 in an electrolysis device 29 and this in a methanol synthesis device 32, with formation of methanol CH ₃ OH, said substance itself being a known fuel but also a basic product usable for the chemical industry.

The operation of an incineration process 20 of waste 3 according to the invention in a classic grate incinerator 21, is very simple and as follows. The waste 3 is incinerated in an incinerator 21 to which a mixture of pure oxygen and recycled, purified and dewatered flue gas is supplied, whereby more than half of the supplied mixture consists of CO ₂ .

The incineration of the waste 3 leads to the formation of hot flue gases 5, and bottom ashes 6 which remain at the bottom of the incinerator. The hot flue gases 5 are cooled in a steam boiler in which water 8 is heated to steam 9 and whereby a part of the energy supplied to the flue gases is recovered. The cooled flue gases 10 are fed to a flue gas purification installation 11 which by means of chemicals 12 and a condensing scrubber purifies the flue gases 10 to purified flue gas 24, after which said purified flue gas is partly discharged into the atmosphere, but is also partly recycled as purified and dewatered flue gas 24 which consists chiefly of CO ₂ and is mixed with pure oxygen 23 (99%) and returned to the incinerator to allow incineration with oxygen in the incinerator at applicable temperatures.

In the incineration process according to the invention the pure oxygen 26 required for the incineration process is produced locally in an electrolysis device 27, which separates water 28 recovered from purifying the cooled flue gases 10 into hydrogen 30 and oxygen 26, using renewable energy 29 such as solar and/or wind energy, whereby the necessary pure oxygen 26 is supplied to the incineration process, and the formed hydrogen 30 can be sold as industrial gas, or utilised on site, by letting it react with a part of the purified and dewatered flue gas 24 in a local synthesis device where organic products are formed such as, in this case, methanol in a methanol synthesis device. The formed methanol can be used both as fuel or be utilised as raw material for the chemical industry.

It goes without saying that the reaction products from the hydrogen and the dewatered and purified flue gas 24 produced on site are not limited to methanol, but other reaction products are also possible, with or without the use of catalysts.