The present invention relates to a process for incinerating solid combustible material, in particular solid combustible refuse, as described in the preamble of the first claim. In the following description with the wording refuse, solid combustible material in general is referred to as well as solid refuse material in particular.

In general, an incineration process in which the combustion of solid combustible material can be controlled on a permanent basis is highly desirable for a number of reasons. First of all, a stable combustion facilitates meeting the emission standards imposed by law for exhaust gasses, flue dust and ashes. Furthermore, by limiting temperature variations within the incinerator, energy costs for maintaining the optimal combustion conditions can be minimised. Finally, by limiting temperature variations within the incinerator also the variations in the thermal and mechanical loads to which the incinerator is subjected can be minimised, which in turn will lead to an extended lifetime of the incinerator, in particular of the feeding and combustion grate.

However, the operation of a refuse incinerator may be complicated by variations occurring in a. o. size and density of the refuse which is mostly supplied in the form of more or less dense packs, and variations in the composition of the refuse, for example its water content, which lead to variations in the calorific value of the refuse.

Variations in these parameters may largely complicate the process and its control system, in particular in case the control system aims at constant steam production output, wherein a steam controller controls the refuse combustion rate.

In a first known system aiming at constant steam production, the steam controller controls the amount of primary combustion air supplied to the incinerator, based on the steam output. The primary combustion air is responsible for the maintenance of the combustion process. This kind of system however is often burdened with the problem of an overloaded combustion grate system and incompletely burned ashes. Namely as steam output decreases, additional primary combustion air is supplied to the incinerator. This often leads to a further reduction of the combustion chamber temperature instead of an increase thereof. Cooling of the combustion chamber especially occurs in case the primary air is not capable of penetrating the refuse, for instance because the waste is too dense, or a big heap of wet refuse is formed. As the combustion rate decreases and the primary air supply is nevertheless increased, the oven cools down. Simultaneously the oxygen concentration in the flue gasses increases.

In a second known system that aims at constant steam output, the latter is controlled by controlling the amount of refuse supplied to the incinerator. Thereto, the speed of the grate supplying the refuse to the oven is varied. Such a system often entails the problem of involving an overloaded combustion grate system, especially in case the refuse is rather dense and the primary air is hardly capable of penetrating the refuse. As a result, the refuse may be incompletely burned, even when supplying a large amount of primary air.

A solution to the problem of the varying operation of a refuse incinerator is given in US-A-5.398.623. In the method disclosed in US-A-5.398.623 an attempt is made to stabilise the operation of the refuse incinerator by keeping the amount of refuse on the combustion grate system approximately constant, regardless of the calorific value or the density of the refuse. This is done by varying the speed of the combustion grate system, i. e. the speed with which the refuse is advanced through the incinerator. The amount of refuse present on the combustion grate system

is determined by monitoring the resistance exerted to the hydraulic drive mechanism which drives the combustion grate system. This resistance is measured as the hydraulic pressure in the hydraulic drive mechanism.

The method disclosed in US-A-5.398.623 has the disadvantage that the system with which the amount of refuse on the combustion grate system is monitored and controlled is one and the same system, i. e. the hydraulic drive mechanism that drives the combustion grate system. The speed of the combustion grate system is controlled by adjusting the flow rate of the hydraulic liquid in the hydraulic drive mechanism. However, by varying the flow rate of the hydraulic liquid to vary the speed of the combustion grate system, the hydraulic pressure as such in the drive system is varied and the hydraulic pressure measured will no longer correspond to the amount of refuse present on the combustion grate system. As a consequence, the method disclosed in US-A-5.398.623 cannot be used to control the amount of refuse on the combustion grate system, unless the hydraulic system is not controlled, i. e. the speed of the combustion grate system is not controlled by the hydraulic drive mechanism.

There is thus a need to find a method with which the amount of refuse supplied to the incinerator can be determined and controlled, independent of the hydraulic system that is responsible for the displacement of the combustion grate system.

It is the aim of the present invention to provide a method with which the amount of refuse supplied to the incinerator can be determined in a reliable manner and the incinerator can be operated in a stable manner.

This is achieved with the present invention with the features of the characterising part of the first claim.

According to the method of this invention, 1) first, AP'is determined. Apro is the optimum gas pressure difference over the refuse bed on the carrier, which corresponds to an optimum

incineration process and is representative for the optimum amount of refuse on the carrier. As the refuse on the carrier constitutes a resistance which hinders the passage of the primary combustion air from a position below the carrier to the incineration zone above the carrier, AP'is representative for the optimum amount of refuse on the carrier 2) the actual gas pressure P'in the incinerator is measured at a position above the carrier 3) the actual gas pressure P9 of the primary combustion air at the position of the inlet in the incinerator, below the carrier carrying the refuse is measured 4) the difference Apr = Pg-P'is calculated. The difference Apr is proportional to the resistance sensed by the gas, when flowing from the primary air inlet through the refuse towards the incineration zone and gives an indication of the amount of refuse on the carrier 5) Api-pro = AP is calculated. AP'corresponds to the pressure difference over the carrier that corresponds to an optimum combustion process. AP is the difference between an actual pressure difference over the carrier and the optimum pressure difference over the carrier 6) at least one of the speed of the charging g the combustible material to the incineration zone or the feeding of the combustible material through the incineration zone is adjusted to minimise AP.

Making the subtraction Apr = P9-P'is in fact a first correction which minimises the influence of non-process parameters on the primary air pressure below the carrier. By this first correction the influence of non-process parameters on the adjusting of at least one of the speed of the charging of the combustible material to the incineration zone or the feeding of the combustible material through the incineration zone can be minimised.

With this first correction in particular the influence of a varying pressure in the incineration zone can be minimised. Such

pressure variations can for example be due to a varying flue gas production in the incinerator, especially with sudden changes in the physical properties or the heating value of the refuse. By the above described correction it is possible to prevent that the feeding or transporting speed of the refuse is constantly adapted to pressure fluctuations which have nothing to do with the amount of refuse on the carrier itself.

In the method of this invention, the speed of the carrier for the refuse is controlled by adjusting the hydraulic pressure of the mechanism which drives the carrier. The amount of refuse present on the carrier on the other hand is determined by measuring the gas pressure difference over the carrier in the incinerator. In that way the driving of the carrier is uncoupled from the measurement of the amount of refuse on the carrier, so that interference of both phenomena can be prevented and a reliable measurement of the amount of refuse on the carrier can be done.

This uncoupling of both mechanisms entails the advantage that the carrier can be driven in a continuous manner and operated at varying speed, and still allow a reliable measurement of the amount of refuse on the carrier.

Moreover, with the method of this invention the speed of the grate can be controlled in a continuous manner. The movement of the carrier can namely be described as a repeated alternating, slow, back and forth sliding in an approximately continuous manner, to advance the refuse over the carrier. Because the carrier can be driven in an approximately continuous manner, there is no necessity to provide dead times between the back and forth sliding of the carrier and the speed with which the carrier is displaced can be kept rather low. In that way not only a more constant steam production can be achieved, but also dust production can be reduced and sudden changes in the release of polluants in the flue gasses can be avoided, thus leading to a more stable operation of a flue gas treatment plant provided after the incinerator.

As a carrier often use is made of a combustion grate system, but other carriers generally known in the art may also be used.

In a first preferred embodiment of this invention, AP is divided by the square of the volumetric primary air flow rate v2pa (m3/s) through the carrier, as a pressure difference over a duct, i. c. a combustion grate element, is always proportional to the square of the flow through that duct. With this correction the influence of a varying primary combustion air flow rate on AP, thus on the speed of the combustion grate system can be minimised.

In a second preferred embodiment of this invention, AP/v2pa is measured at predetermined time intervals and averaged as a function of time. This is preferably done by determining and averaging Apr in a continuous manner, in particular by measuring P9 and P'and calculating the difference Apr = PI-PI in a continuous manner. In that way it can be avoided that the feeding or transporting speed of the refuse is adjusted in an unstable manner, due to quick pressure variations which are not important to the combustion process. In this way in fact a filtering of the noise on the pressure difference signal is carried out. An example of pressure variations that are not important to the incineration process as such, is in case the carrier comprises a plurality of subsequent grate elements, the dropping of an amount of refuse from one element on the next element.

Furthermore, in order to allow an optimum control of the incineration of the refuse over the entire incinerator and to ensure that the incineration process proceeds as complete as possible, the incineration zone is divided into a plurality of individual combustion zones, primary combustion air being supplied to each individual zone, the primary combustion air supply flow rate being adjusted for each individual air supply or incineration zone. Thereto, the actual gas pressure PIZ at each primary combustion air inlet device and the actual pressure Pz above the carrier in each individual incineration zone z is measured and Apr z is

calculated for each zone. However, since the pressure differences between the individual incinerating zones above the carrier for the refuse are mostly small, the values of Pz can be approximated to reasonable accuracy by a single measurement of P'in the incinerator. Preferably also the flow rate vpa is measurable and adjustable for each zone.

Primary combustion air is supplied to the incinerator through a primary combustion air supply device. The primary combustion air supply device comprises an inlet through which primary combustion air is supplied to the primary combustion air supply device and an outlet through which primary combustion air is supplied from the primary combustion air supply device to an incineration zone of the incinerator.

The flow rate of the primary combustion air in each individual zone is measured by determining -the pressure of the primary combustion air at the inlet and outlet of the primary combustion air supply device, -determining the pressure difference between the inlet and outlet, -calculating the flow rate corresponding to the measured pressure difference.

With known techniques, primary combustion air flow measurements per combustion zone are often not reliable as the air supply pipes are too short to accommodate the required instruments and as the applied instruments are often drifting since they get dirty by the dust present in the airflow. The inventor has now solved these problems by determining the primary air flow rate as follows : -the pressure of the primary combustion air is measured at the inlet and outlet of the air supply device. The pressure difference between inlet and outlet is calculated, -each air supply device has a characteristic curve from which the flow rate corresponding to the pressure difference between the inlet and outlet of the primary combustion air supply device can be determined.

As an air supply device use can be made of devices

that are generally known in the art, for example an air fan or an air supply valve. In case use is made of an air fan, if so desired, the calculation may be corrected for variations in the rotation speed of the fan. Determination of the flow rate of the primary combustion air per combustion zone is also possible when primary air is supplied through the existing technique of one single fan, from which the primary combustion air is distributed towards the individual incineration zones through gas control valves, for example butterfly or register valves. In that case the pressure difference over the control valve is measured, and a calculation is done based on the characteristic curve of the control valve instead of the characteristic curve of the fan.

The present invention also relates to a device for incinerating solid combustible material, the device comprising an incineration reactor with at least one incineration zone for combusting the combustible material, a carrier for carrying the combustible material and feeding the combustible material through the at least one incineration zone, a device for supplying combustion air below the carrier and means for adjusting the amount of combustible material in the incineration zone.

The device is characterised in that the means for adjusting the amount of combustible material in the incineration zone comprises means for (1) measuring a gas pressure P'in the incineration zone, (2) measuring the gas pressure P9 below the carrier, (3) determining Apr = pi pg, (4) comparing Apr with Appro, Appro being the pressure difference P'°-P9° that corresponds to an optimum amount of material in the incineration zone, (5) means for adjusting the speed of the carrier to minimise the difference between Apr and Air..

The carrier for the combustible material preferably comprises a plurality of individual grate elements for advancing the combustible material through the incineration zone, a primary combustion air supply device being provided below each grate element, to allow an improved control of the incineration process. The most used technique for

supplying primary combustion air to the incinerator at this moment makes use of one single air fan. From the air fan primary combustion air distribution over and along the different combustion grate elements is controlled by means of butterfly or register valves. By dividing the incineration zone in a number of individual zones and controlling the primary combustion air supply in each individual zone, an improved control of the incineration process can be achieved.

The combustible material is preferably advanced through the incineration zone by means of a carrier comprising a plurality of subsequent carrier elements, some of which are slideably mounted in forward and backward direction for transporting the refuse from a former combustion zone to a next combustion zone. As carrier elements preferably use is made of individual combustion grate elements. Each carrier element preferably comprises a first combustion grate element that is slideably mounted in forward and backward direction for transporting the refuse from a former combustion zone to a next combustion zone.

Between subsequent first combustion grate elements, preferably at least one second combustion grate element is mounted, the second combustion grate elements being mounted in such a way that they can be tumbled, preferably over a preset angle to improve theintensity of the combustion.

Next to a second combustion grate element, i. e. between a second combustion grate element and the next first combustion grate element, preferably a third stationary combustion grate element is provided.

Preferably the first and second combustion elements are individually and separately controllable.

The device further preferably comprises a burn out control device to ensure that the solid combustible material has been completely burned before it is removed from the incinerator.

The invention is further elucidated in the attached figures and description of the figures.

Figure 1 shows a cross section through a reactor for

the incineration of refuse.

Figure 2 shows a detail of a combustion grate system for use in the method of this invention.

The incinerator shown in figure 1 comprises overhead cranes for transferring solid combustible material, for example refuse 1 to a reactor feed hopper and a loading chute 3. The chute in fact functions as an air seal for the top of the incinerator, but is also provided for distributing the refuse 1 to a refuse supply device 4 with which the refuse is supplied to a carrier 5 for transporting the combustible material through the incinceration zone where it is combusted. The carrier 5 can be any carrier known to those skilled in the art, but preferably comprises a combustion grate system 5. The combustion grate system is further provided for drying the combustible material, igniting and burning it in the gasification and combustion zone. To support the combustion, primary combustion air is supplied to the incinerator through a primary combustion air supply device 9, which is preferably located below the combustion grate system. The primary combustion air supply device 9 may for example comprise an air supply fan or valve or any other primary combustion air supply device known in the art. The device further preferably comprises a burn out control device to ensure that the solid combustible material is completely burnt out before it leaves the incinerator.

As can be seen from figure 2, the combustion grate system 5 used in the incinerator of this invention preferably comprises a plurality of combustion grate elements (11-16), the degree of combustion of the combustible material increasing from a former to a subsequent combustion grate element. The combustion grate elements 11-16 further function as a means for transporting the combustible material 1 from the feed hopper 3 to a former to a next combustion grate element, and finally to an ash discharge 6.

In a preferred embodiment, an air supply device is provided below each grate element 11-16 to provide an improved control

of the combustion process. As an air supply device preferably use is made of a valve or a fan, but other air supply devices known in the art may also be used. Each air supply device comprises an inlet through which primary combustion air is supplied and an outlet through which the primary combustion air leaves the air supply device towards the incinerator. Means are provided for determining the air flow rate at the outlet of the primary air supply device. This can be done by actually measuring the pressure at the inlet and outlet of the primary combustion air supply device and determining the corresponding flow from the characteristic curve of the air supply device.

The combustion grate system 5 preferably comprises a plurality individual grate elements, preferably a plurality of subsequent sliding tiles, 11-16, with which the layer of the combustible material is displaced over the combustion grate. The sliding movement of the tiles is preferably a slow, continuous movement, so as to avoid dust generation in the incinerator and increase the life time of the incinerator. Besides this, when continuously moving the tiles 11-16, a virtually continuous steam production and consequently a virtually continuous electricity production can be ensured. The sliding tiles 11-16 determine the thickness of the layer of the combustible material, the residence time of the combustible material in each combustion zone and the combustion quality.

The combustion grate system 5 further preferably comprises a plurality of tumbling tiles (17-20), which disentangle and aerate the refuse. This is important for drying and ignition of the refuse, to activate the combustion where and if necessary and to obtain a complete burn-out of the ashes. This combination of horizontal throughput action and vertical aeration action (tumbling) allows the incinerator to adapt to short and long term fluctuations in the composition of the refuse. Thereby preferably, the throughput (sliding) and aeration (tumbling) can be controlled for each individual zone (combustion grate element). In addition to this an independent control of the two motions, i. e. the sliding and

tumbling is highly desirable. Of course, the tumbling action is stopped automatically because of the increased risk of dust production when disentangling and more intense aeration of the refuse is not necessary.

Upon combustion of the refuse, flue gases are generated, which are mostly withdrawn from the incinerator through a fan 17. A refuse incinerating reactor is often coupled to a waste heat boiler 8, wherein the thermal energy contained in the flue gases is converted into steam. This steam can in turn be used for electricity production purposes, combustion air preheating, industrial processes, hot water supply etc.

In the method of this invention, first the optimum amount of combustible material to be present in the incineration zone is determined. Solid combustible material is fed to the incineration reactor and transported to and through the incineration zone at a transport speed by means of the carrier. The amount of combustible material in the incineration zone correspond to -measuring an over-all gas pressure P'in the incineration zone, -measuring a primary gas pressure below the carrier for the combustible material P9, -determining a pressure difference over the carrier OPr= P'-P9, -determining Apro which is the pressure difference over the carrier that corresponds to the optimum amount of combustible material in the incineration zone, -calculating the difference AP between Appro and AP°.

To improve the incineration process the speed of the charging of the combustible material to the incineration zone or the feeding of the combustible material through the incineration zone is adjusted to minimise AP. Preferably the speed of the feeding of the combustible material is adjusted to AP/vZpa, wherein vpa is the flow rate of the primary combustion air. AP/v2pa is measured at predetermined time intervals and averaged as a function of time or AP/v2pa is filtered.

The primary air flow rate in each individual combustion zone is measured by determining a pressure of the primary air pressure at the inlet and outlet of the air supply device, determining the pressure difference between the inlet and outlet, calculating the flow rate corresponding to the measured pressure difference.