CA2083248A1 - Method of generating process heat - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In a method of generating process heat, a process medium is heated in two stages to its final temperature in a process heat generator (100). Under-stoichiometric combustion is undertaken in the first stage as pre-combustion zone (107) which is located downstream of a burner device (101). Downstream of this pre-combustion zone (107), the combustion gases pass through a cooling phase which takes place by means of a heat exchanger (108). Further downstream of the heat exchanger (108), there is a reheat zone (110) and air (109) fed into this region is supplemented by cooled exhaust gas (112). This exhaust gas (112) can be introduced before the air (109) supplied or together with the latter in the region of the reheat zone (110).
A further heat exchanger (111) is located downstream of the reheat zone (110). Because the exhaust gases (115) are branched off after the further heat exchanger (111), the latter has an effect on the combustion tem-perature in the reheat zone (110). In order to avoid thermal overloading of the combustion chamber walls in the pre-combustion zone (107), the air supply (106) takes place in a counter-flow convective heat exchanger (105) which preheats the air (106) supplied to the combustion air (15) and, on the other hand, cools the combustion chamber walls. The first heat exchanger (108) extracts only a part of the heat which should have been extracted to achieve the optimum temperature range, to minimize the NOx emissions, in the reheat zone (110). This is possible because the latter combustion zone is fed with cooled exhaust gas. In this way, it becomes possible to reduce the relative heat extraction in the pre-combustion zone (107) by any required amount and this improves the economy of the installation.

(Fig. 1)

Description

Bo 5012.91 91/101 Method of generating pro~ess heat BACKGROUND OF T~E INVENTION

Field of thgLI~yy~lcn The present invention concern3 a method of ge~er-ating proces4 heat in accordance with the preamble to cIaim 1~ It also concerns a device for carrying out the method.

Di~cussion of Back~round DE-A1-37 07 773 describes a method for low-pollutant combustion which is particularly suitable for the combustion of~ fuels with bound nitrogen, for example heavy oil. In this method, fuel i8 subjected to pre~
combustion in.~a ~first com~u~tion chamber stage under suitable under-stoichiometric conditions, preferably using :pxemixing burners at very high temperatures -higher tha~ 1500:C. A reduction zone, in which the gases have to be retained for approximately 0.3 second~, directly follows the under-stoichiometric combustion zone ~o that the bonds between hydrogen atoms, on the one hand, and carbon and hydrogen atoms, on the other, are:broken down in an atmosphere lacking oxygen~ The process is carried out in an optimum man~er if th~ air iB suitably preheated before the under-stoichiometric combustion proce~s 30` that the promotion of NOX formation due to both ~CN and N~3 can be avoided as far as possible. ~hi~ reduction zone is directly followed by a zone in which just enough heat i~ taken from the combu~tion gas 80 that on transition into an over-stoichiometric reheat tage, in which the residual air is ~upplied to the combustion gas, a 2 ~ ~

suit~ble temperature window - approximately 1150 C to 1350 C - is achieved in which, as fAr as possible, no NOX i5 formed (upp~r temperature limitation) and no CO or U~C i5 "frozen in" either (lower temperature limlt). Now thi~ temperature window condition leads to the necessity of ~ very powerful extraction of heat in the under-stoichiometric region adjacent to the reduction zone, particularly when the combustion air is strongly preheated. In such a two~stage combustion installation, a co~parable amount of heat (or ind~ed more heat) may occur in the under-stoichiometric stage than occur~ in the over-stoichiometric reheat stage. Thi3 state of affairs leads to serlous disadvantages with respect to cost~
desired installation power andtor NOX emi~sion~ when existing firing in~tallations have to be converted. In this case, the existing boiler is "degraded" to a .
reheat burner of moderate output.

SUMMARY OF ~E INVENTIO~
Accordingly, one object of the invention is to provide remedy in this connection and, as ~peci~ied in the claims, it is based on the object of reducing the relative heat extraction in the under-~toichiometric stage by any required amount in a method of the type mentioned at the beginning.
The e~sential advantage of the inve~tion may be ~een in the fact that only part of the heat which has to be extracted in order to achieve the optimum temper-ature window in the over-stoichiometric region is extracted in the under-~toichiometric region adjacent to the reduction zone Heat extraction in the under-stoichiometric region could al80 be avoided completely, if need be. So that the ideal temperature window can, neverthele~s, be achievedj cooled exhaust gas is supplied to the under-~toichiometric gases before the 3upply of the re~idual air or together with the re9idual air-- Both internal and external exhaust gas 2~3~

recirculation can be considered for this purpose. In this way, it become~ possible to reduce the relative heat extraction in the under-stoichiometria 3tage by almost any required amount and therefore to make the cost, QUtpUt and installation size of a retrofit installation very variable. In order to avoid thermal overloading of the combustion chamber walls in the under- toichiometric stage, in which high temperatures are present and only a small amount of heat i5 removed, the air supply and the preheating of the air (which is generally necessary) can take place in a counter-flow convective heat exchanger which preheats the combustion air supply, on the one hand, and cool~ the combustion chamber walls, on the other.
Advantageous and expedient further developments o~
the solution of the object of the invention are speci-fied in the further dependent claims.
, BRIEF DESCRIPTION OY T~E DRAWIN&S
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as t~e same becomes better understood by reference to the following detailed description when considered in connection with the accompanying draw-ing~, wherein all the elements not immediately neces-sary for understanding the in~ention are omit~ed and wherei~ the direction of flow of the media is shown by mean3 o~ arrows.

Fig. 1 shows a process heat generator, ig. 2 shows, in perspective vie~ and appropri-ately sectioned, a burner in the form of a double conLcal burner and igs. 3, 4, 5 show corresponding sections through the plane~ III (= Fig. 3), IV-IV
(= Fig. 4) and V-V (= Fig. 5), these sections only providing a diagrammatia 4 ~ J~L 8 repre entation of the double conical burner according to Fig. ~O

DESCRIPTION OF THE PREPERRED EMBODIMENTS
Referring now to the drawing~/ wherein like reference numerals and letters de~ig~ate identical or corresponding part~ throughout the several views, Fig. 1 show3 a proce ~ heat generator which con ist~
essentially of a burner device and two combustion ~tages. At the highest point in the process heat generator, a burner device 101 for liquid a~d/or gaseous fuel is provided a the heating means. Par-ticularly suitable for thi~ purpose i a premixing burner, namely of the so-called double conical ~urner type, which i~ physically described in moxe detail in the following Figures 2-5. Fundamentally, in such a burner 101, a preferably liquid fuel 12 is supplied via at least one centrally placed no~zle and a preferably gaseous fuel is supplied via further fuel nozzles which are located in the internal space of the burne.r 101 in the region of the air inlet slots. A~ ignitable~mlx-ture~ appear6 in ~the bur~er lOl~itself, the reaction zone 103 extending from this combustion into a pre-com-bustion zone 107 located downstream of the burner 101.
This pre-combu3tion zone 107 forms the reduction zone 104 of the process heat generator. Located approxi-mately at the end of the pre-combustion zone 107 are the inlets of an air duct 105 which i~ concentric with the pre-combustion zone 107 and via which primary air 106 i~ supplied to the burner 101. The air duct 105 acts as the air preheating system for the primary air 106 ~o that the burner is supplied with thermally pre-pared combustion air 15 and the reaction stage lQ4 act~
as a convective cooler. This thermal preparation of ~he primary air 106 before the under-stoichiometric combustion proces~ ensures that the proce~s is carried out in an opti~um manner because promotion of the formation of NOX due to both HCN and NH3 i8 avoided, as far as possible, by means of it. This combu~tion generally takes place under- toichiometrically and preferably, in fact, with an excass air number lambda of 007, this quantity being considered as an approximately optLmum value of a range of lambda between 0.6 and 0.8 which is suitable in practice.
Because of this lack of air and the corre~pondingly small proportion of oxygen (which is uniform over the whole volume of the pre-combustion zone 107) in the pre-combuRtion phase, no such oxides of nitrogen which are con~erved during the subsequent cooling of the combustion gases occur as i~termediate products. In this pre-combustion phase, only very hot, low-oxygen combustion gases occur and in these, the nitrogen bound in the fuel is, in the main, reduced because o the high temperature~ and the lack o~ oxygen. After this under-stoichiometric pre-combustion, the partially burnt gases first flow through a heat exchanger ~108 lof any given type) in which, fundamentally, sufficient ~eat i~ extracted from them to achieve a temperature window of 1150-1350 C, within which oxides of nitrogen can hardly form, on transition into an adjacent ~ver-stoichiometric reheat zone 110. The relationships associated with this extraction of heat concern the fact that this temperature window condition makes a very powerful extraction of heat necessary in the under-stoichiometric region, i.e. adjacent to the reduction zone 104, particularly in those caseg in which the combustion air is strongly preheated~ In a two-stage combustion system of the present type~ it i8 quite possible for the heat occurring in the under-stoichiometric stage to be comparable with or even greater than that in the over-stoichiometric stage. As already ¢onsidered above with respect to the prior art, this condition is beset with disadvantages. For this rea~on, provi~ion i~ made for the heat exchanger 108 to extract only part of the heat which would be intrinsically neaessary to achieve the postulated 32 ~

temperature window, i.e. the optimum temperature window, This heat extraction can, if need be, tend to zero. So that the optimum temperature window in the reheat zone 110 can, nevertheles~, be provided~ cooled exhaust gas 112 is supplied to the under-stoichiometric combustion gases downstream of the pre-combustion zone 107 and upstream of the reheat zone 110, either indi~
vidually or together with a re~idual supply o~ air 109.
If the common residual air/exhaust ga~ supply option is selected, the under-~toichiometric ga~e~ ar~ supplied with a mixture 114 of air and exhau~t gas. The gases produced by the combustion in the reheat zone 110 are cooled by mean~ of a heat exchanger 111 which is located approximately at the termination of the zone 110 mentioned last and which is designed, in associ-ation with the first heat exchanger 108, as a two-stage heat exchanger ~as is shown very well by the conduit path); both an internal and an external exhaust gas recirculation can then be considered. The pumping of the exhaust gas 112 iB undertaken by various fans or injectors ~13. The residual exhaust gas 115 which is not:required is passed to the chimney or to a~other consumption un~t. The configuration shown permits the relative heat extraction in the under-stoichiometric 3tage; i.e. at the end of t~e pre-combustion zone 107, to be arbitrarily fixed to suit requirements so that the economy of the ~olution in general and, in particu-lar,:in the case of retrofit installations (in which the po sibilitie~ for control, by means of the fuel supply for example, may be limited) i8 maximized.
In order better to understand the construction of the burner 101, it i~ advantageous to consider, sim~ltaneou~ly with Fig. 2, the individual qections shown on it, the~e sections corresponding to Fig~. 3-5.
Furthermore, in order to make Fig. 2 easily compr~h~-n-sible, the guide plate 21a, 21b shown diagrammatically in Fig~. 3-5 axe only included as indication~ in 7 91~101 Fig. 2. In the following description of Fig. 2, reference is continually made to the other figures.
The burner 101 according to Fig. 2 consist~ ~f two hollow semi-conical partial bodie~ 1, 2 whose longitudinal axes of symmetry are radially off3et relative to one another. The offset between the respective longitudinal axes of symmetry lb, 2b relative to one another produces re~pective tangential air inlet 810t5 19, 20, with opposite inflow directions~ on both sides of the partial bodies 1, 2 ~on this point, see Figq. 3-5). The COI~U~tiO~ air 15 already mentioned in the previous figure flows through these air inlet slots 19, 20 into the internal ~pace 14 of conical shape formed by the conical partial bodies 1, 2. ~he conical shape of the partial bodies 1, 2 shown has a certain fixed angle in the flow direction.
Depending on the operational use, the partial bodies 1, 2 can of ~ourse have a progressive or degressive conical inclination in the flow direction. The two shapes last mentioned are not recorded by drawing because they can be conceived without difficulty. The two conical partial bodies 1, 2 each have a ~y~indrisal initial part la, 2a which, analogously with the partial bodie~ 1, 2, extend off~et relative to one another so that the tangential air inlet ~lots 19, 20 are continuously present over the complete length of the burner 101. These initial parts can also have a different geometrical shape and, in som~ cases, they can be omitted completely. A nozzle 3, via which a fuel 12, preferably oil, or a fuel mixture is injected into the internal:space 14 of the burner 101, is accommodated in thi~ cylindrical initial part la, 2a.
Thi~ fuel injection 4 coincides approxL~ately with the narrowest cross-~ection of the internal space 14.
further fuel supply 13, preferably using a ga4eous fuel in thi~ case, is introduced via a oonduit 8, 9 re~pectively integrated into each of the partial bodies 1, 2 and i9 mixed with the combustion air 15 by mean3 J\ .~$
8 91/1~1 of a number of nozzles 17. This mixing take place in the region of the inlet into the internal space 14 in order to achieve optimum mixing 16 due to velocity.
Mixed operation using both uels 12, 13 is, of course, po~sible via the respective mean5 Qf introducing fuel.
At the pre-combustion zone end 107, the outlet opening of the burner 101 merges into a front wall 10 in which a number of holçs lOa are provided in order, if required, to inject a certain quantity of dilution air or cooling air into the internal ~pace of the pre-combustion zone 107. The li~uid fuel 12 provided through thP nozzle 3 is injected at an acute angle into the internal space 14 of the burner 101 in such a way that the mo~t homogeneou possible conical spray pattern is produced over the complete length of the burner 101 as far as the burner outlet plane. This is only possible if the internal walls of the partial bodies 1, 2 are not wetted by the fuel injection 4 which employs, for example, an air-supported nozzle or pressure atomization. For this purpose ~ the conical liquid fuel profile 5 i~ surrounded by the tangentially flowing combustion air 15 and, if required~ by a further co~bustio~ airflow:15a introduced axially. In the axial direction~ the concentration of the injected liquid fuel 12, which can also without difficulty be a mixture, is continuously reduced by the combustion air 15 flowing into the internal space 14 of the burner lO1 through the tangential air inlet slots 19, 20. The combustion air 15 can al80 be a fuel/air mixture and is, in any event, aided in the reduction of the concentration hy the other combustion airflow 15a. In as.Rociation with the injection of t~e liquid fuel 12, the optimum homogeneous fuel concentration is achieved over the cro~-section in t~e region where the vortex collapses, i.e. in the region of the reverse flow zone 6. Ignition take~ place at the tip of the reverse flow zone 6. It is only at this position that a stable flame front 7 can occur. There i~ no danger in this ~3~

case of blow-back sf the fl~me into the inner part of the burner 101, as can alway~, potentially, be the case in known premixing systems and against which remedy i sought in such cases by means of complicated flame holders. If the combustion air 15, 15a is preheated, accelerated overall evaporation of the fuel takes place before the point at the outlet from the burner 101 is reached at which the ignition o the mixture takes place. ~he preparation of the combustion airflows 15, 15a can be extended by the a~mixture of recirculated exhaust gas. In the arrangement of the cone angle and the width of the tangential air inlet slots 19, 20 of the conical partial bodies 1, 2, narrow lLmits have to be maintained in order to achieve the combustion airflow field desired for flame stabilization, with its rever~e flow zone 6 in the region of the burner outlet.
In general, it may be stated that a change in the width of the air inlet slot~ 19,- 20 leads to a displacement of the reverse flow zone 6: the displacement is downstream when the width of the air inlet slots 19, 20 is reduced. In this connection, it should be noted that the position of the reverse flow zone 6, once fixed, is intrinsically stable because the swirl rate increas~s in the flow direction in the region of the burner 101. A~ already indicated, the axial velocity can be changed by an appropriate upply of axial combu~tion airflow lSa. The design of the burner is outstandingly suitable for modifying the tangential air inlet slot~ 19/ 20 to suit requirements o that a relatively large operational band wi~th can be achieved without changing the design length o the burner 101.
The geometrical configuration of the guide plates ~la, 21b may be seen from Figs. 3-5. They have to ful~
fil flow inlet functions by extending, a~ a function of their length, the respective end of the conical partial bodies 1, 2 in the incident flow direction of the com-bustion air 15 into the internal space 14 of the burner 101. The ducting of the combu~tion air 15 into the 2 ~
91~101 internal space 14 of the burner 101 can be optimized by opening or closing the guide plates 21a, 21b ahout the center of rotation 23 placed in the region of the tan-gential air inlet slots l9, 20. This is particularly neces~ary when the original gap size of the tangential air inlet slots 19, 20 i9 modified. The burner 101 can also, of course, be operated without guide plates 21a, 2lb or other auxiliary means can be provided for this purpose.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims,: the invention may be practiced otherwise than as specifical.ly described herein.

Claims (10)

1. A method of generating process heat in which a process medium is heated in two stage to its final temperature by combustion products of one or more fos-sil fuels in a process heat generator, an under-stoichiometric combustion zone being effective in a first stage downstream of a burner device, the combus-tion gases being cooled downstream of this under-stoichiometric combustion and combustion taking place with the addition of a quantity of air in a reheat zone downstream of the cooling phase of the combustion gases, wherein downstream of the under-stoichiometric combustion zone, the combustion gases are subjected to heat extraction which is found to be less than the pro-portion necessary to achieve a temperature range for minimized NOx emission in the subsequent reheat zone and wherein cooled exhaust gas is supplied before or together with the addition of the air quantity in order to achieve this temperature range in the reheat zone.

2. The method as claimed in claim 1, wherein the temperature range is between 1150 and 1350 °C.

3. The method as claimed in claim 1, wherein the combustion air for the burner device is thermally prepared.

4. A device for carrying out the method of claim 1, wherein the device consists of at least one burner, wherein the under-stoichiometric combustion zone is adjacent to and downstream of the burner, wherein a heat exchanger is placed in the combustion region of this combustion zone, wherein a reheat zone follows downstream of the heat exchanger, wherein a further heat exchanger is located at the termination of the reheat zone and wherein an exhaust gas recirculation conduit branches off downstream of the further heat exchanger and ends in the region of the reheat zone before or together with an air supply conduit.

5. The device as claimed in claim 4, wherein the burner consists in the flow direction of at least two hollow, conical partial bodies which are positioned one upon the other and whose longitudinal axes of symmetry extend radially offset relative to one another, wherein the offset longitudinal axes of symmetry create tangen-tial inlet slots with opposite flow directions for a combustion air flow, wherein at least one nozzle is placed in the hollow conical space formed by the coni-cal partial bodies, the injection of fuel by this noz-zle being located centrally relative to the longitudi-nal axe of symmetry, extending offset relative to one another, of the conical partial bodies.

6. The burner as claimed in claim 5, wherein a further combustion air flow can be supplied axially into the internal space of the burner via the nozzle.

7. The burner as claimed in claim 5, wherein further nozzles for injecting a further fuel are present in the region of the tangential inlet slots.

8. The burner as claimed in claim 5, wherein the partial bodies widen conically at a fixed angle in the flow direction.

9. The burner as claimed in Claim 5, wherein the partial bodies have an increasing conical inclination in the flow direction.

10. The burner as claimed in claim 5, wherein the partial bodies have a decreasing conical inclination in the flow direction.