WO2015120961A1

WO2015120961A1 - Ultra-small-scale combustion plant for biogenic solid fuels

Info

Publication number: WO2015120961A1
Application number: PCT/EP2015/000183
Authority: WO
Inventors: Mario König; Ingo Hartmann; Sebastian GÜNTHER; Mirjam Mathes; Christian Thiel
Original assignee: Dbfz Deutsches Biomasseforschungszentrum Gemeinnützige Gmbh
Priority date: 2014-02-12
Filing date: 2015-01-30
Publication date: 2015-08-20
Also published as: EP3105504B1; EP3105504A1; DE102014001785A1; EP3105504B8; DE102014001785B4

Abstract

The invention relates to a combustion apparatus loaded automatically with solid fuels, in particular for loading with biogenic solid fuels for generating heat and/or product gases, wherein the fuel is supplied via a loading device (1) to a cylindrical reactor containing a batch of fuel, over which the solid fuel is converted into gaseous reaction products on account of the effect of temperature and the supply of primary air by pyrolysis and gasification processes, said reaction products being conducted downwards through a length of pipe with devices to generate turbulence (8) and being oxidized as completely as possible in a further cylindrical reactor (7) with a greater cross section than (4) by the supply of preheated secondary air (3). A vertical, electrically operated split tube furnace (5) serves as the heat source for achieving required ignition temperatures and the formation of a bed of embers.

Description

DESCRIPTION

Smallest firing system for biogenic solid fuels

The invention relates to a mini-firing system for biogenic solid fuels, which is charged with solid fuels, in particular for charging with biogenic solid fuels for the production of heat and / or product gases.

The majority of domestic solid fuel heating systems currently available have a nominal heat output of over 5 kW.

With an increasing share of low-energy houses and energetically refurbished buildings, the installed systems often only achieve low annual utilization rates or can only be operated at part-load with low efficiency and high emissions.

In order to achieve a significant reduction in the rated power of solid fuel heating, it is necessary to develop low emission microburner systems for solid fuels with a thermal power below 5 kW, which should have improvements in the following ranges over the prior art.

Feed:

Devices with a capacity of less than 5 kW currently represent only 5% of the EU market and are fed manually only.

Manually-fed plants, especially for wood fuels, have very high emissions of carbon monoxide (CO), organic gaseous compounds (OGC) and particulates (PM) in practice.

A number of scientific studies have shown that wood-burning waste gas is one of the most important winter sources for particulate matter (PM10) and particulate matter (PM2.5). PM and its components, such as polycyclic aromatic hydrocarbons (PAH) and black carbon (BC), have a negative impact on health and the environment.

CONFIRMATION COPY From previous laboratory-scale internal investigations, it is known that automated / machine-fed systems tend to have significantly lower emissions than manually fed systems. To reduce the emissions from wood firing in the smallest power range, therefore, a mechanically charged supply of fuels is necessary. Due to the size of the pellets and the nature of the conventional dosing screws, the commercially available pellet plants are not able to realize a mechanical feed in the smallest performance range, which leads to a very discontinuous and thus also emission-rich burnup.

Furnace construction:

It is known that the combustion of biomass can be divided into two main phases: the heterogeneous conversion of solids into fuel gases (drying, pyrolysis, gasification) and the homogeneous gas phase oxidation (oxidation of fuel gases). Accordingly, the combustion plant should be divided into two zones, in which separate air streams can be supplied.

In the first phase, solid biomass is converted into gaseous reaction products by supplying primary air.

In the case of the so-called falling fire principle, the combustion gases are directed downwards through the ember bed into the second stage, the so-called oxidation or burnout zone. In the second stage, the combustible gases are oxidized as completely as possible to C0 ₂ and H ₂ 0.

The principle of optimizing the gas phase reactions with low CO and VOC emissions are known as the 3-T rule.

The basic requirements of the 3-T-Rule are: sufficiently high temperature in the gas phase oxidation (> 850 ° C), sufficient residence time of the fuel gases (> 2 s) and high turbulence for a good mixing between combustible gases and secondary air (Re> 2300) , These conditions can basically be influenced by an adequate combustion design (geometry, size) and by good insulation and flow optimization.

So far, no plants are known especially for the small power range, which are designed according to the occurring low flow rates and the relatively low temperatures. The object of the invention is to provide a mini-firing system for biogenic solid fuels, ie in particular up to and including 5 kW, which overcomes the disadvantages of the prior art. In particular, this system should be operable according to the falling fire principle, be environmentally sound and have high efficiency.

The object of the invention is achieved by a method having the features according to claim 1.

Essential to the invention is that the dosage is carried out continuously.

Essential to the system according to the invention or the method according to the invention is the enabled continuous metering, for example by means of spiral dosing, of biomass, in particular small-sized biomass.

The pieces of small-sized biomass in the context of the invention should, in particular depending on the type and dimensioning of the respective furnace, at least a predetermined maximum size, such as a piece size up to 10 mm, especially in an asymmetrical piece shape whose maximum extent, not exceed.

The dependent claims 2 to 5 contain advantageous embodiments of the method according to the invention, without limiting this. The fuel is fed via a metering device 1 with a spiral conveyor in the metering tube to a primary reactor 4, in particular a cylindrical primary reactor in which there is a fuel bearing 6, on which the solid fuel is converted into gaseous reaction products due to the effect of temperature and the supply of primary air 2 by pyrolysis and gasification processes which is passed down through a pipe section with means for generating turbulence 8 and in a further cylindrical reactor, namely the secondary reactor 7, with a larger cross section than the primary reactor 4 by supplying preheated Secondary air 3 are oxidized as completely as possible.

As a heat source for the achievement of necessary ignition temperatures and the formation of an ember bed, a vertical electrically operated hinged-tube (5) can serve. To generate the necessary negative pressure for the fall fire operation, a suction device, in particular an induced draft fan (11), is used. Numerous devices for determining the temperatures 12, 13, 14 and a measuring section 10 for determining the exhaust gas composition are provided in the incinerator.

The primary reactor according to the invention is an innovation, since a conventional furnace grate is not suitable for the combustion of small-sized biomass according to the falling fire principle.

On the one hand, the problem arises that unburned material can fall through the grate and thus adversely affect the efficiency and / or emissions in a known manner. On the other hand, the openings of the grate (for example, holes or slits) can become clogged, whereby the combustion gases are no longer uniformly through the

Ember bed can flow and the combustion is at least disturbed.

In the worst case, it can also lead to a tearing or extinction of the flame.

To prevent this, a special primary reactor with a novel pyrolysis funnel was developed, through which the fuel gas can flow undisturbed down.

In particular, the primary reactor according to the invention may comprise a plate, in particular a stainless steel plate, through which one or more small metal tubes are provided, which are provided at the top with a mushroom-like cover, with a free space between the cover and the upper end of the metal tube.

Here, too, the (air) tightness of the micro-firing plant according to the invention, in particular in the combustion of biomass in two main phases, as internal investigations have shown, plays a decisive role, the was neglected in hitherto commercially available combustion plants due to the low impact at higher power / flow rates.

The geometry of the pyrolysis funnel (primary zone) and the structural design of the burner head (secondary zone) also have a major influence on the proper and low-emission use of the invention.

Different solutions for the design of the primary zone were investigated theoretically and experimentally internally.

For example, gratings and perforated plates with different mesh and

Holes and sponges made of high-alloy stainless steel or alumina tested.

While stable operating conditions could be achieved with these designs, it was not possible to realize CO-free combustion.

In addition, the pressure loss of the variants tested with supply of fuel increased very disproportionately, which can be explained on the one hand by the increasing gas viscosity with increasing temperature in the primary zone by the released heat of reaction. On the other hand, due to the thermochemical conversion of the fuel particles are smaller when passing through the openings to a strong

Decrease in the free flow cross-section of the screen, grid and perforated plates.

In addition, the shrinking fuel particles fall after reaching a certain

Size as a function of hole and mesh size through the grates and thus arrive with higher residual carbon content in the secondary zone. Thus, an accurate and targeted separation of primary and secondary zones is not safe.

Therefore, the pyrolysis funnel according to the invention has been developed in numerous variants.

Surprisingly, it has been shown that with an opening width and an opening height of the funnel tube of 8 mm in each case with a reactor diameter of 45 mm can realize very low pressure losses in a power range of 0.5 to 5 kW. Passage of the pyrolysis zone of (still) carbonaceous solid particles by passing through the opening of the pyrolysis funnel can be effectively prevented by mounting a round and flat 16 mm diameter bell cover above the funnel tube on lands around the funnel opening of the pyrolysis funnel.

Surprisingly, it has surprisingly been shown that a slight curvature of the bell cover on the inflow side shows considerable advantages, since it makes possible a targeted but not too strong displacement of the flow line towards the reactor wall.

The fuel particles follow the flow lines and are kept away from the funnel opening.

When the fuel particles are thermochemically transferred to small ash particles with no appreciable carbon content, they are gradually transported with the gas flow down to the gas oxidation zone of the secondary zone. By a preferably sharp about 90 ° deflection in the exhaust gas block at the transition into the exhaust pipe, the ash particles are separated from the gas stream according to the principle of inertial and collected in the removable bottom of the exhaust block until removal.

The object of the invention is also achieved by a miniature firing system with the features of claim 6.

The dependent claims 7 to 12 contain advantageous embodiments of the micro-firing system according to the invention, without limiting this.

In this sense, the type and location of the use of high temperature catalysts, especially with certain active surface and species, are preferred. The use of a high-temperature catalyst allows emission-free combustion even in partial load and under transient phases, such as power change, the start and shutdown phase. In addition, pollutants such as PAH and PCDD / PCDF can be lowered well below the limit values, but also with methane, CO and soot, far below the concentration range of 5 mg / m ³ with a specially adapted high-temperature catalyst. With today's available continuous measurement technology for emission measurement technology such as FTIR or NDIR these pollutants are then below the detection limits, so that one can speak of an emission-free biomass combustion.

The catalyst consists of an alumina sponge converted to a high temperature stable catalyst with spinel-structured oxides by a novel Reactive Surface Solid Activation (RSSA) process, preferably formed from manganese oxides and other transition metal oxides. The RS SA process and the use of the catalyst as a combustion chamber-integrated system in wood furnaces has already been described in the patent application DE 10 2013 020 398.8.

The catalyst is employed in the secondary zone within the outer reaction tube and preferably has a circular cross-section of 75 mm diameter so that the gas must flow completely through the open-celled solid state structure. The pressure loss is low due to the high open porosity and is less than 10 Pa.

By means of a induced draft fan and a vacuum control, the pressure in the secondary zone upstream of the catalytic converter is kept constant in order to prevent any influence on the combustion management.

The distance of the catalyst to the burner nozzle end can be variably adjusted. By theoretical and experimental investigations, a distance of 100 mm to 180 mm to the nozzle exit has proven to be preferred.

The measurements showed an increase in temperature in the range between Nozzle outlet and catalyst by 100 to 150 K. This accelerates the oxidation rate and achieves additional emission reduction. The remaining small amounts of soot and volatile organic compounds are thus significantly reduced.

Surprisingly, it has been found by experimental investigations that two operating points can be set via the mass flow of primary air and fuel, which lead to a complete combustion - carbon monoxide-free exhaust gas - the combustion gases with sufficient admixture of secondary air with the geometry of the primary zone according to the invention (state 1: yellow Flame and state 2: blue flame).

The invention is explained in more detail below with reference to exemplary embodiments without having conclusively illustrated all possible uses of the invention.

FIG. 1 shows a possible embodiment of the miniature firing system according to the invention.

In the power range below 5 kW it is not possible with compacted solid biomass, which regularly have piece sizes over 10 mm, to enable a continuous and stationary operation of a mini-firing of biomass. The screw feeders 1 required for this purpose, for example for pellets or chaff, are not available on the market in series and would have to be produced in small batches, which would cause high costs.

Therefore, compacted solid biomass, which regularly have piece sizes over 10 mm, comminuted, for example, produced with a granulator, and continuously, for example, with a volumetric screw feeder 1 with a variably adjustable screw speed via a frequency converter, fed into the primary reactor 4.

One possible embodiment for the use of comminuted solid biomass relates to the comminution of wood pellets in a granulator with a Cutting sieve insert of 4 mm sieve hole width and the subsequent separation of the particles with grain sizes greater than 1 mm and less than 4 mm.

The following metering rates as a function of the screw speed shown in FIG. 2 could be achieved with the mini-firing system according to the invention.

For example, with a screw speed of 1.15 s ^"1 and a calorific value of 16.8 MJ / kg (wood), a fuel supplied power of 2 kW (mass flow = 0.12 g / s) can be achieved with the dosing device used ,

Taking into account the fuel water content of 8% by mass and a lambda value of 1.1 results after carrying out a combustion calculation with the aid of the elemental composition of the exemplified and crushed wood pellets, a waste gas volume flow of 2.5 m ³ / h (standard state). The air requirement for complete combustion with lambda of 1.1 is 2.1 m ³ / h (standard state). Depending on the set ratio of primary and secondary combustion air flow, the chemical processes in the reaction zone can be influenced. This manifests itself in the type of flame phenomenon (yellow flame: low ratio of primary to secondary air Vp _primary / V secondary <0.25, blue flame: high ratio of primary to secondary air Vp _rimä Vsec> 1) ·

With low primary air supply, larger hydrocarbon molecules having more than two carbon atoms in the molecule are formed in the primary reactor system mainly due to the lower temperatures and the slower rate of decomposition of the wood. These hydrocarbon species react under the local air deficiency in the primary zone to higher molecular weight polycyclic hydrocarbons (PAH) to primary soot particles. These compounds having an aromatic basic structure show a yellow flame _appearance in the reaction with an _{excess of} oxygen, eg at air volume flows of _{V rimar} = 0.35 m ³ / h and V secondary = 1.75 m ³ / h.

At higher primary air supply above the pyrolysis funnel 12 takes place due to higher reaction rate and thus higher temperature fast pyrolytic Decomposition and partial oxidation of the wood components to small molecules with less than two carbon atoms, wherein the main contributions of the combustible gases are the compounds of CO, H _2, and CH. ₄ During the oxidation in the flame with excess oxygen then a blue flame appears. The PAH and soot formation can thus be effectively suppressed, for example at air volume flows of Vp _ri mär = 1.1 mVh and V secondary = 1 m7h.

The embodiment of the reactor of the inventive miniature firing system is shown in FIG. 3.

For example, lumpy biomass, eg wood pellets, but also straw, cereal waste, miscanthus and the like, are comminuted to a specific, in particular maximum particle size, eg between 1 to 10 mm, preferably between 1 to 4 mm. Subsequently, these are then supplied with a volumetric dosing device 1 with conveyor spiral in the metering tube to the preferably heated primary reactor 4 continuously. In the primary zone they are converted by pyrolytic decomposition under oxygen deficiency in fuel gas with a defined composition and then the combustible constituents with the addition of secondary air completely oxidized to CO ₂ and H ₂ O.

The feeding of the primary air is carried out vertically above the primary reactor 4 at a distance of preferably 300 mm before the fuel feed by means of a multi-hole probe 2 for uniform distribution over the pipe cross-section of the primary zone.

The gas stream absorbs the fuel particles fed with the dosing device. Since the force through the fully formed turbulent gas flow and gravity through the mass of the particles act in the same direction, the crushed particles are evenly distributed over the pipe cross-section. A turbulence of the particles, as in fluidized bed reactors, thus targeted avoided. The gas stream of primary air and the dispersed fuel particles passes through a heated, in particular by a Klapphrofen 5, heated zone and is thereby heated to above 500 ° C, so that a thermochemical reaction of the particles is started. The high molecular weight hydrocarbon molecules of the biomass (cellulose, hemicellulose and lignin) react to form fuel gas constituents. The self-adjusting axial temperature profile, the oxygen concentration, the supplied mass of fuel particles, the gas velocity and the degree of mixing and residence time of the fuel particles determines the nature of the flammable gas constituents that form.

Pyrolysetrichter

FIG. 3.1 shows a possible embodiment of the pyrolysis funnel 15. Burner head: secondary zone

The burner head of the micro-firing system according to the invention consists of a straight inner reaction tube 16 with 90 mm in length and 45 mm inner diameter, through which the fuel gas (reactive mixture of pyrolysis and gasification products of the biomass with air) is fed to the burner nozzle. In this reaction tube is a further chemical decomposition and optionally partial oxidation in the presence of residual oxygen of the long-chain fuel gas components to short-chain components instead.

The rate and extent of decomposition and partial oxidation also depends on the temperature profile, the oxygen concentration, the mass of fuel particles supplied, the gas velocity and the degree of mixing and the residence time, as already described for the primary zone.

The length and the supply of heat via the reaction tube play a decisive influence. In addition to the boundary conditions in the primary zone, these two parameters decisively influence the type of fuel gas composition at the nozzle outlet. The inner part of the reaction tube of the secondary zone is preferably without a sudden change in cross section in a nozzle 8 of about 56 mm in length. The inner inlet diameter is approx. 45 mm and the nozzle outlet has an inner diameter of approx. 16 mm.

Directly at the nozzle outlet of the fuel gas secondary air is injected via four cross-shaped opposed tubes with an inner diameter of 4 mm secondary air at an angle of approximately 20 ° to the vertical flow direction.

Extensive experimental investigations and theoretical calculations by the inventors of the mini-firing plant according to the invention with adjusted secondary air mass flow, an ideal mixing of low molecular weight gaseous and combustible carbon compounds was achieved with oxygen in the variable power range of 1 to 2 kW, without a flame separation or flame extinction at the nozzle exit with associated high Emissions occurs.

The nozzle outlet, the secondary air injection and the outer oxidation tube of the secondary zone 7 are particularly coordinated so that a good mixing with turbulent flow in the outer reaction tube is present.

The inner diameter of the outer reaction tube (made of glass, ceramic or stainless steel) is according to the invention about 75 mm. The length was determined from theoretical considerations to the necessary residence time on the kinetics of total oxidation of CO and methane and empirical values from experimental studies to about 1000 mm, wherein the end of the fuel gas nozzle protrudes at the burner head to about 560 mm in the outer reaction tube of the secondary zone , Thus, by adapting mass flows of fuel, primary air and secondary air, a power range of 0.5 to 2 kW within the meaning of the invention can be realized in order to realize a possible carbon monoxide-free combustion.

The choice of the material of the outer reaction tube of the invention

Smallest firing system depends on the desired heat extraction.

When using a glass tube, a large part of the heat via radiation directly to the environment are delivered, so that the erfmdungs contemporary mini-firing system can be used as a continuously operated Einzelraumfeuerung.

When using a stainless steel pipe or a tube made of SiC ceramic as an outer reaction tube, the heat can be transported to the exhaust block (tertiary zone) and passed through a water cycle to the heating network with appropriate insulation, so that a proper use of the miniature firing system according to the invention as a central heating system for low-energy houses possible is.

In the figure 3, the variant was drawn with stainless steel tube. In order to realize a visual inspection of the flame, a sight glass of quartz glass 17 with a visible cross section of one inch is used.

Exhaust block: tertiary zone

The exhaust block 9 has been constructed taking into account theoretical considerations that the heat released during combustion can be released to more than 90% of a conventional heating water circuit.

For this purpose, distributed over a circular ring and mounted in the exhaust block 9 holes for the circulation water guide have been attached.

In addition, since the exhaust block 9 has been provided with cooling fins, a proportion of about 10% of the heat is given off to the direct environment by convection and radiation.

Without flow through the Heizwasserkreislaufes the exhaust gas block is called 9 to temperatures above 150 ° C, stores heat and gives off a higher proportion of convection and radiation to the environment.

The micro-firing system according to the invention can therefore be used flexibly either as a single-room firing system or as a micro-boiler plant. By the by the Exhaust block guided circulating water quantity, the proportion of heat released between Heizkrauslauf and direct environment can be defined vary.

The exhaust gas block also serves for the separation and collection of the ash particles, which are produced by the decomposition and oxidation of the biomass and converted by the defined reaction to non-oxidizable inorganic materials (silicates and other oxides and sulfates as reaction products of the inorganic ingredients). The exhaust block also has the task to record the Feuerungssensorik the miniature combustion system according to the invention.

For this purpose, a screw connection with thread M85xl .5 is provided at the lower end of the exhaust block, in which the Stutzenverschraubung the sensor is introduced. The sensor system consists of an unheated in-situ lambda probe with integrated thermocouple known to those skilled in the art.

The data of the lambda probe and the thermocouple are then processed in a control module and used for the adaptation of control variables for combustion and power control.

List of reference numbers

1 dosing device

2 primary air supply

3 Secondary air supply

4 primary reactor

5 hinged-ear furnace

6 fuel supply

7 secondary reactor

8 devices for generating turbulence

9 exhaust block

10 measuring section / device

11 induced draft fan / suction device

12 temperature measurement

13 temperature / oxygen measurement

14 temperature measurement

15 pyrolysis funnels

16 sight glass

17 Inner reaction tube

Claims

1. A method for operating a mini-firing plant with a rated power of less than 5 kW for solid biogenic fuels, this method according to the camber firing principle comprising at least the following steps:

a variable-volume metering in a primary reactor of a two-stage reactor, this reactor also has at least one secondary reactor, a heterogeneous conversion of dosed solid fuels in fuel gases in the primary reactor and a homogeneous gas phase oxidation in the secondary reactor, characterized in that the metering is carried out continuously.

2. The method according to claim 1, characterized in that the pieces of solid fuels which are metered, have a size of 1 to 10 mm, preferably from 1 to 4 mm.

3. The method according to claim 1, characterized in that in the secondary reactor, a high-temperature catalyst is present.

4. The method according to claim 1, characterized in that the heterogeneous conversion of the solid fuels in fuel gases in the primary reactor and the homogeneous gas phase oxidation in the secondary reactor takes place in an otherwise airtight mini-firing system.

5. The method according to claim 1, characterized in that the primary reactor has a pyrolysis funnel.

6. Mini-firing plant with rated power below 5 kW and falling fires for biogenic solid fuels, which at least includes: a primary reactor (4) and a secondary reactor (7), wherein the primary reactor (4) above the secondary reactor (7) and between these a fuel support (6) and a pyrolysis funnel (15) are arranged, means for generating turbulence (8) , which connects with the upper opening to the fuel support (6) and the lower opening in the secondary reactor (7) opens, in the primary reactor (4) the feed device (1) and a primary air supply (2), in the secondary reactor (7 ) opens a secondary air supply (3), an exhaust system (9), which is arranged on the secondary reactor (7) and connected to a suction device (1 1), characterized in that the charging device (1) is operable continuously.

Mini-firing plant according to claim 6, characterized in that the primary reactor (4) has a heating device, in particular a hinged-ear furnace (5).

8. miniature combustion plant according to claim 6, characterized in that the primary reactor (4) has a heating device, in particular a self-regulating glow plug.

9. mini-firing system according to claim 6, characterized in that the charging device (1) is automatically operable.

10. mini-firing system according to claim 6, characterized in that the secondary air supply (3) has a device for preheating the secondary air, in particular a heat exchanger. 1. mini-firing system according to claim 6, characterized in that it has a device for combustion and power control, which at least one device for negative pressure control, for combustion control and burn-back control, sensors, measuring devices for determining the exhaust gas composition and an electronic computing and storage unit has.

12. Mini-firing installation according to claim 6, characterized in that at least the reactors (4, 7) are airtight.

13. Use of the smallest firing system for solid biogenic fuels according to at least one of claims 6 to 12 for a range below 5 KW.

14. Use of the microburner for biogenic solid fuels according to at least one of claims 6 to 12 as a central heating system for low-energy houses and / or continuously operated individual room firing.