PROCESS AND COMBUSTER FOR CARRYING OUT OXYGEN ENRICHED COMBUSTION

Abstract: The invention relates to a process for combusting material, such as waste, fuel or the like, comprising the steps of: supplying material to be combusted into an oven, with removing parts, in particular a stoker oven, introducing a stream of primary combustion gas into the stoker oven, comprising at least 25 vol. % oxygen, combusting the material in the stoker oven at a predetermined temperature for a predetermined time, guiding the flue gas formed during combustion of the material away from the stoker oven, and recycling a predetermined amount of this flue gas, the secondary flue gas, back to the stoker oven.

Description: PROCESS AND COMBϋSTER FOR CARRYING OUT OXYGEN ENRICHED COMBUSTION The present invention relates to combusting material, in particular waste, fuel or the like. Presently waste is mainly combusted in waste combustion installations with combustion air, wherein com- plete combustion of the waste is to be ensured for as much as is possible. The PCT patent text WO 86/06151 introduces a process for combusting waste material, wherein, in order to prevent the formation of toxic organic matter and to reduce the NO-content of the flue gas, oxygen is supplied to the furnace in the place of at least a part of the combustion gas. A problem herewith however, is that at increased oxygen levels in the combustion air, the furnace temperature becomes too high. This has disadvantageous consequences for the installations. A further process for disposal of waste by combustion with oxygen is known from WO 89/03241. However this process is not suitable for ovens with moving parts, in particular stokers. The present invention has as its object, to provide an improved combustion process, in particular combustion processes in ovens with moving parts, in particular stokers. According to the present invention, a process for combusting material, such as waste, fuel or the like is provided, comprising the steps of: - supplying material to be combusted, into a stoker, - introducing a primary combustion gas stream into the stoker, comprising at least 25% volume oxygen, - combusting the material in the stoker at a pre¬ determined temperature for a predetermined time, - guiding away from the stoker the flue gas formed during combustion of the material, and - recirculating a pre-determined amount of the flue gas, or secondary flue gas, back to the stoker. With the process according to the present invention, the temperature in the stoker is controllable during combustion of material without deviation in the desired percentage of supplied oxygen being required. Using air as the combustion gas, the nitrogen pre¬ sent acts as a temperature regulator, because nitrogen behaves as an inert, non-combustible gas. Increasing the amount of oxygen in the combustion gas means that the rela- tive amount of nitrogen falls, which in turn means that the inert fraction in the combustion air falls, whereby the temperature in the stoker rises. With an oxygen level of more than 25 vol % in the combustion air, the temperature in the stoker would become too high for a controllable, safe combustion process to be achieved. Possible damage of the stoker is the direct result. Recirculation into the stoker of a part of the flue gas, formed during combustion of the material means that combustion can now be carried out with oxygen percentages in the combustion gas which are greater than 25 vol %, since the flue gas, which is also inert, replaces the inert nitrogen present in air, whereby the inert temperature regulator is provided which is needed for controllable combustion with oxygen enriched combustion air. The invention furthermore concerns a combuster for carrying out the above described process, comprising - a material supply for supplying material to be burned, into a stoker, - moving means for moving the material through the stoker, - first guiding means for guiding a primary combustion gas stream to the material, - second guiding means for guiding flue gas from the material, out of the stoker, - third guiding means for guiding a predetermined part of this flue gas, the secondary flue gas, back to the stoker, and - a filter for filtering the secondary flue gas before this is guided back to the stoker. Further details, characteristics and advantages of the present invention will become clear when referring to the following description and figures, wherein: Figure l is a schematic diagram of a stoker accor¬ ding to the present invention, Figure 2 is a schematic reproduction of a comparative combustion process model without recirculation of flue gas. Figure 3 is a comparative graph showing the temperature in the stoker (the furnace temperature) and amount of air at different oxygen percentages in the supplied primary combustion gas, Figure 4 is a schematic reproduction of a waste combustion installation model showing recirculation of the flue gas according to the present invention, Figure 5 is a graph showing recirculation of flue gas at different temperatures in the stoker (furnace temperature) as a function of the oxygen percentage in the primary combustion gas supplied, Figure 6 is a graph showing the amount of flue gas as a function of the oxygen percentage in the supplied primary combustion gas when the temperature in the stoker (furnace temperature) is 1150°C and wherein the residue oxygen percentage in the flue gas is 6.5 vol %, Figure 7 is a graph showing the volume fraction of the main components in the flue gas as a function of the oxygen percentage in the primary combustion gas supplied, wherein the residue oxygen percentage of the flue gas is 6.5 vol %, Figure 8 is a graph showing the steam debit yielded and the boiler efficiency as a function of the oxygen percentage in the primary combustion gas supplied, according to the present invention, Figure 9 is a graph showing the energy components as a function of the oxygen percentage in the primary combustion gas supplied, according to the present invention and Figure 10 shows a table, summarizing the research results. The stoker 1, figure 1, comprises a hopper 2, through which waste, for example, is supplied onto a combustion grate 3 in the stoker, wherein a furnace 4 is situated. The waste is transported through the furnace 4, for example by means of a series of downwardly inclining rollers 5 before the slag (not shown) is removed via a de- slagger 6. Primary combustion gas (shown by arrows 7) is preferably guided into the furnace 4 from beneath the rollers 5 (in a manner not shown) in order to ensure an efficient continuous combustion and also cooling of the rollers 5. The flue gas formed during combustion of the material, is lead out of the furnace 4 via a flue gas ducting 8 to a boiler (not shown) . At least one part of this flue gas, the so called secondary flue gas, shown by arrows 9, is lead back to the furnace 4 in a manner not shown, either from the flue gas ducting 8 or from the boiler, or from any other suitable part of the installation. This secondary flue gas is preferably guided by means of ventilators, not shown. Apart from controlling the temperature in the sto¬ ker, the secondary flue gas also acts to ensure mixing and complete combustion of the flue gas, resulting from the bur¬ ning waste and any unburned particles in the flue gas. A good mixing of the combustion gasses is very important for low C χ H^ and CO-emissions, gasses which are harmful to the environment, for helping to obviate the reducing gas streams which cause corrosion problems in the installations and lessening of gas stream turbulence in the stoker, whereby fly ash (solid particles which are transported along with the flue gas) can be deposited, which can lead to blockage problems. The flue gas is preferably scrubbed, before recycling takes place, in order to remove corrosion causing components such as chlorine compounds and acid components. Following recycling, further cleaning is preferably undertaken, by means of - wet scrubbing, in order to remove S0 2 , heavy metals and the like, - selective catalytic reduction, with the aid of NH 3 to reduce NO χ , - filtering by means of a not shown electro filter, or a cloth filler for example, in order to remove undesired material, such as dioxines and heavy metals, possibly present in the flue gas. Filtering of the secondary flue gas ensures that contamination, overheating and blocking of the stoker is prevented. Preferably, at least one not shown ventilator is used in order to both guide and propel the secondary flue gas back to the stoker oven. This ventilator also contributes to the provision of the correct pressure, velocity, stream and mixing conditions necessary for optimum combustion. In order to pre-warm the primary combustion gas, which leads to a quicker drying out of the waste and thereby a uniform combustion and burning out of the slag, the secondary flue gas can be guided in order to be directly mixed with the primary combustion gas. In this case, the use of steam in order to pre-warm primary combustion gas is less necessary, whereby more steam is available for energy generation. Furthermore, the secondary flue gas can be guided, in a manner not shown, along the not shown side walls of the stoker in order to cause a cooling effect and to prevent the burning of slag onto the side walls. This side wall cooling by means of the secondary flue gas, increases the life of the stoker. Research has been carried out at a waste combustion installation in order to determine the consequences of using oxygen enriched combustion gas together with recirculation of flue gas. The results have been obtained by computer simulation using as starting point for the calculations: 1) The design information of the waste combustion installation in Alkmaar, Boekeler eer, the Netherlands (per waste combustion line) , 2) A residual oxygen percentage in the flue gas, going to a flue gas treatment of 6.5 vol.%, and 3) Waste with a net heating value of 10 MJ/kg. The research concentrated on the following aspects: l) Stoker temperature (furnace temperature) 2) The amount of flue gas 3) The composition of the flue gas 4) Condensation point of the flue gas 5) Energy utilization Due to the nature of the material to be combusted and the legal requirements concerning emissions, the following criteria were taken into consideration by the research: a) A stoker needs to be able to be used for diffe- ring compositions of material to be combusted with a net heating value for example of between 6-15 MJ/kg and with differing sizes. b) The combustion gasses should reach a temperature of at least 850°C for at least two seconds, in order to ensure complete combustion; the temperature should not rise above roughly 1,300°C in order to prevent melting of the slag formed in the stoker; and the flue gas expelled into the environment should have, according to legal requirements an oxygen content of at least 6 vol.%. 1) The effect of enriching combustion gas with oxygen without recirculation of secondary flue gas, as shown in the scheme in figure 2, has been researched in order to determine the effect on furnace temperature. The results of this are shown in figure 3. Here, the furnace temperature increases from roughly l,150 β C with normal air with an oxygen content of roughly 21 vol.% to about 2,700°C with pure oxygen. The amount of air required in order to combust the waste and yet still to comply with the requirement of 6 vol.% oxygen in the expelled flue gas, fell, by this research, from roughly 90,000 m 3 per hour to about 15,000 m 3 per hour as is shown on the right hand side of the graph in figure 3, per waste combustion line in the waste combusting installation. The cause of the increasing temperature in the furnace, is the reduction in the amount of nitrogen present in the primary combustion air. As already stated, the maximum allowable furnace temperature, by waste combustion is about 1,300°C. When com¬ bustion is carried out at higher temperatures, the following problems should be taken into consideration: a) melting of slag, whereby the installation can become blocked and heat transfer hindered. b) damage to the brick work and construction of the waste burning installation can occur. For instance, dependent on the type of brick work, this can melt at a temperature of above 1,400°C and due to the higher furnace temperatures accelerated erosion and corrosion can take place. It can be concluded from figure 3 that, taking into account the maximum furnace temperature, the oxygen percentage in the primary combustion gas supplied may be, at most, roughly 25 %. Hence, combustion with oxygen enriched air is only possible to a limited extent with this system. The scheme as shown in figure 4 was subsequently developed. At the moment waste is burned with a furnace temperature of about 1,100-1,150°C. In order to compare the results of the research, with results obtained from a modern combustion installation, a computer model was set up in order to simulate the scheme from figure 4 at a furnace temperature of, amongst others, 1,150°C; the results of which are shown in figure 5. On comparing figure 5 with figure 3, it is seen that by varying the amount of recirculated flue gas, the amount of oxygen, for a given furnace temperature, can be varied in the primary combustion air to 100%. In this manner, improved combustion conditions can be realized whilst a similar furnace temperature to that of a combustion process without recirculation of secondary flue gas can be maintained. This due to the fact that this temperature can now be regulated independently from the oxygen percentage which means that at a certain temperature, differing oxygen percentages in the primary combustion gas can be set whereby the advantage is obtained that the thermal stress on the combustion installation reduces. 2) Considering the amount of flue gas, the following calculations were carried out: a) calculation of the amount of flue gas, before guiding away of the secondary flue gas; b) calculation of the amount of flue gas going to a flue gas treatment. In figure 6, the calculated flue gas amounts per waste combustion line before guiding away of the secondary flue gas is set out as a function of the oxygen percentage in the primary combustion gas supplied. From Figure 6, it can be concluded that the amount of flue gas emitted from the boiler is near enough constant. By combustion with enriched air without recirculation, less flue gas is yielded, but the furnace temperature will rise (see figure 3) . Less flue gas is also yielded with a scheme as shown in figure 4. Due to the decrease in the amount of flue gas, fewer ventilators needing only low power will be required for installations using the process according to the present invention in order to ensure optimum mixing and stream velocities of the flue gas, which leads to a significant cost and energy saving. In order to achieve the desired furnace temperature, a determined percentage of the flue gas must be recirculated. In figure 5 and 6 is to be seen that the amount of flue gas to be recirculated is greater, the higher the oxygen percentage in the primary combustion air. Accordingly, as shown in figure 5 and 6, the amount of flue gas to be treated by the flue gas treatment decreases, whereby advantages in the treatment and consumption costs can be obtained. As shown in figure 6, the primary combustion gas and secondary flue gas result in an almost constant gas stream from the stoker. The results in figure 5, were obtained for all the furnace temperature values chosen and lead to the conclusion that the amount of flue gas to the flue gas treatment is independent of the furnace temperature. A further conclusion is that the amount of flue gas going to the flue gas treatment is independent of flue gas recirculation. This yields the advantage that the furnace tempe¬ rature and the oxygen content in the flue gas, independently from one another can be set up since the furnace temperature is regulated by the amount of flue gas recirculated. 3) The changes in volume percentage of the main flue gas components is shown in figure 7. The composition of flue gas changes greatly on combustion with oxygen enriched primary combustion gas. Figure 7 shows the decrease of the volume fraction of nitrogen to zero as a result of the fact that no more nitrogen is supplied to the process, when 100% oxygen is used as the primary combustion gas. Since the percentage of nitrogen in the flue gas falls, the volume fraction of the other components in the flue gas, especially those of H 2 0 and C0 2 , increases. This yields the advantage that if the percentage of oxygen in the primary combustion gas is increased, whereby the percentage of nitrogen therein decreases, the volume percentage of damaging gasses produced, such as NO, N0 2 , N 2 0, N0 χ which are environmental polluters, decreases. 4) With an increase in the volume fraction of the flue gas components, the condensation point of each component was taken into consideration. When the temperature of the flue gas dips below the condensation point temperature of the components present therein, the components of the gas phase will go over to the fluid phase. A phase change of the components should always be avoided because of corrosion problems. From the research, it appears that the condensation point of water vapor is important. In the extreme case when the supplied primary combustion air consists of 100% oxygen, the flue gas will contain roughly 65 vol.% H 2 0. The partial pressure of the water vapor is then about 0 ,6 bar. The condensation point temperature belonging to this value is 86°C. Accordingly it has been concluded that the temperature of the flue gas should remain above the condensation point, particularly before wet scrubbing when the flue-gas comes into contact with the ventilators and so forth, unless measures are undertaken in order to fight corrosion. The chemical and physical processes of corrosion are very complex. Considering the flue gas composition; HC1/C1 2 , S0 2 , CO and salts (chlorides and sulphates) are corrosive. In the case of waste combustion, corrosion is mainly caused by chlorine. Corrosion by S0 2 proceeds much slower than chlorine corrosion. From the research into corrosion by waste combustion in coal fired plants, it appears that the ratio of HC1/C1 2 and S0 2 plays an essential role. S0 2 appears to form a layer of solvates which slows down chlorine corrosion. Starting from a maximum combustion gas oxygen enrichment (combustion gas with 100% oxygen) , 73% of the flue gasses are recirculated (see figure 5) . Without cleaning, the chlorine and S0 2 content wer6 roughly 3.7 times as high. In general, the concentration in unwanted, corrosive material increases by recirculation of the flue gas. The amount of unwanted, corrosive material delivered via the waste material remains unchanged, whilst the amount of flue gas delivered to the flue gas treatment decreases. At a recirculation percentage of 73%, the flue gas flow to be treated amounts to 27% of the amount without recirculation. The concentration in corrosive material amounts in this case to 1/0.27, which is 3.7 times the concentration in corrosive material without any recirculation. By washing only chlorine out in a wet scrubber placed in the recirculation canal, recirculation has no more effect on the chlorine concentration. The effect on the S0 2 concentration remains unchanged. The ratio C1/S0 2 thereby drops, depending on the recirculation percentage, by a factor of 3.7. The selective removal of chlorine reduces greatly the corrosion rate. Hence by carrying out an acid washing step, corrosion of the grate blocks is prevented, whilst corrosion in the furnace and boiler is slowed down. This yields the advantages of fewer process stoppages and lower maintenance costs. 5) During the research into the consequences of combustion with enriched air on the energy utilization, the energy balance of the waste combustion installation in Alkmaar Boekelemeer, the Netherlands was taken, where the gross electric efficiency is 26%, wherein the efficiency is calculated by: MW electricity generated Gross electric efficiency = MW of waste With respect to the calculations concerning the introduction of the application of oxygen enriched combustion gas on the energy utilization, two variations were looked at. Firstly the consequences for energy utilization with a new installation. Secondly consequences for energy utilization with an already existing installation. For a new installation, designed in order to take into consideration the advantages of the present invention, more energy for steam generation will be available (see figure 8 and figure 10) which yields an increase in the gross electric efficiency of the installation of roughly 26 to 30%. The generated electric power per waste combustion line will increase from 13.8 to roughly 15 MW, as shown in figures 9 and 10. By both new and existing installations, the thermal energy loss via the chimney will reduce from about 6.2 to 2.1 MW on using oxygen enriched primary combustion gas. The explanation for this reduction is the reduction in the amount of flue gas going to the flue gas treatment. The flue gas losses have been calculated with the aid of the formula M*CP*ΔT. The Cp-value of the flue gas, concentration values, will indeed increase, but this increase has less effect on the chimney losses than the reduction in the amount of flue gas. Another advantage of the process according to the present invention is that the furnace temperature can be easily programmed, so that the quality of the slag is impro¬ ved. The reasons herefor are the following: 1) the volatile heavy metals present in the fuel, will, at a higher furnace temperature, for a great deal vaporize and therefore less often appear in the slag, whereby less extraction of this matter is needed, and 2) the organic material in the fuel will, by an increase in the furnace temperature, for a greater part be combusted. This yields an advantageous effect on the effluent behavior of especially copper. This effect is attributed to the fact that copper forms soluble complexes with uncombusted organic material. A summary of the research results is shown in table 10. It will be clear that the process according to the present invention is applicable to all combustion processes carried out in installations with moving parts for example by both energy generating processes and waste combusting processes. As such, the present invention is not limited by the above description, but is rather determined by the scope of the following claims.

Claims: CLAIMS 1. Process for combusting material, such as waste, fuel or the like, comprising the steps of: - supplying material to be combusted into an oven, with moving parts, in particular a stoker oven, - introducing a stream of primary combustion gas into the stoker oven, comprising at least 25 vol.% oxygen, - combusting the material in the stoker oven at a pre-determined temperature for a pre-determined time, - guiding the flue gas formed during combustion of the material away from the stoker oven, and - recycling a pre-determined amount of this flue gas, the secondary flue gas, back to the stoker oven. 2. Process according to claim 1, wherein the combustion temperature is at least roughly 850°C and at most roughly 1750°C. 3. Process according to claim 2, wherein the combustion temperature is at least roughly 1000°C and at most roughly 1300°C. 4. Process according to claim 3, wherein the combustion temperature is roughly 1150°C. 5. Process according to any the preceding claims, wherein the amount of secondary flue gas recycled back to the stoker oven, is between roughly 5 and 80%. 6. Process according to claim 5, wherein the amount of secondary flue gas recycled back to the stoker oven, is between 20 and 70%. 7. Process according to any of the previous claims, wherein the flue gas is kept above a temperature of roughly 86°C, before wet scrubbing. 8. Process according to any previous claim, wherein the flue gas is scrubbed before recycling. 9. Process according to claim 8, wherein the secondary flue gas is filtered before being recycled back to the stoker oven. 10. Combuster for the combustion of material such as waste, fuel or the like comprising: - a material supply for supplying material to be combusted into a stoker oven, - first guiding means for guiding a stream of primary combustion gas to the material, - second guiding means for guiding flue gas from the material away out of the stoker oven, - third guiding means for guiding a pre-determined amount of these flue gas, the secondary flue gas, back to the stoker oven and - a filter for filtering the secondary flue gas before these are guided back to the stoker oven. 11. Combuster according to claim 9, wherein the material supply is a hopper, wherein the moving means are formed by a series of rollers, wherein the first, second and third guiding means are formed by ventilators and wherein the filter is an electro filter. 12. Use of a combuster according to claims 9 and 10, for carrying out a process according to claims 1-8.

Inventors: OP T ENDE HARMANNUS CHRISTOFFE, OP 'T ENDE, HARMANNUS, CHRISTOFFER
Applicants: KEMA NV, OP T ENDE HARMANNUS CHRISTOFFE, N.V. KEMA, OP 'T ENDE, HARMANNUS, CHRISTOFFER
Application year: 1995
Main classification: F23G5
IPC codes: F23G5, F23L7