CZ38161U1 - A boiler furnace for burning biomass - Google Patents

Description

The technical solution falls into the area of fireplaces and biomass boilers, specifically where fuels with a higher ash content and a higher tendency to caking need to be burned. Use can also be found in larger technological units, such as in a biomass micro-cogeneration source working with an organic Rankine cycle (ORC).

Current state of the art

Combustion of biomass is traditionally carried out in units with a similar output, on the order of hundreds of kW, mainly on a fixed grate. Combustion takes place not only on the grate in a quiet filter layer, but also in the space above the fuel, where secondary air is supplied depending on the proportion of volatile combustibles released from the fuel. The grate enables the creation of a layer of burned material that has the required height and breathability.

A common arrangement of the hearth inside a boiler for burning quality biomass is such that the fuel supply, provided that the material does not stick, simultaneously serves as a driving force that moves the fuel through the grate in the combustion area and pushes the ash into the ash removal. All this under the combustion air supply. However, since the behavior of low-quality biomass does not meet the assumption of non-sticking in the burning area, inefficient combustion and clogging of the fireplace space with unwanted elements occur. From the point of view of process continuity, it is therefore necessary to ensure that the grate does not stick in the combustion area, i.e. reducing the flow of combustion air and excessive accumulation of material.

If it is necessary to burn biomass with a greater tendency to sintering, i.e. usually with a higher ash content, it is necessary to either use a more technologically advanced type of grate, such as a sliding, stepped, belt grate, etc., or to modify the fixed grate.

There is a utility model CZ 35459 Ul, the subject of protection of which is a grate hearth for burning low-quality biomass with a crank mechanism driven by an electric motor, which moves the grates in such a way that the burnt fuel is piled up into the space of the ash removal screw conveyor and at the same time the layer of burnt ash in the filter is aerated layer on the griddle. However, the crank mechanism is not suitable for this application, in addition, the issue of high-temperature corrosion of stainless steel grates is not solved here, and the technical solution aggregates large manufacturing tolerances, due to which there is a high drop of wood chips under the grate, and it also has very complex production and assembly.

Heat spraying represents a perspective technology that provides functionally effective coatings, used, among other things, in energy and combustion sources in general. These flexible, high-quality and economical technologies enable the surface properties of the components to be optimally adapted to the demanding operating conditions they are exposed to. Thermal injection technology is a known and used technology for combustion equipment, however, it has never been developed and long-term tested for organic Rankine cycle (ORC) biomass sources of low outputs.

The essence of the technical solution

The shortcomings of the existing solutions are largely eliminated by the hearth of a biomass burning boiler comprising a grate with a set of movable blades reciprocating in the direction above the grate, above which is the fuel supply to the grate, below which is the axing screw of the hearth and behind which is the ash removal shaft, according to this technical solution. Its essence is that the blades are equipped with a driving cam rotation mechanism, and the blades for digging through the burning layer are treated with heat spraying from a Ni-Cr-Mo alloy with the addition of niobium.

- 1 CZ 38161 U1

A rotary element for crushing and moving the generated ash is located behind the grate in an advantageous design at the ash removal shaft.

The mechanical grate of the biomass fireplace with heat injections, thanks to the movable blades, which are able to extend above the level of the grate, disrupts the continuous layer of sintering fuel and moves the disturbed sintering formations closer to the ash removal shaft, while ensuring the passage of the grate for primary air. The grate is driven by an electric motor with a gearbox, which is coupled to a cam mechanism and thus performs a reciprocating movement of the blades always in the same direction in any direction of rotation of the electric motor.

The grates are in one row and the mechanism is cam, which reduces the number of delicate moving parts prone to failure. The cam rotation mechanism ensures an even transmission of the power moment across the entire width of the firebox, and the blades do not stick out as in the case of the crank mechanism. The grid is undivided and removable, thus guaranteeing the uniformity of the gaps for the blades, and the grid thus increases its rigidity and precision.

The blades, which dig through the burning fuel above the level of the grate, have a suitable shape. The lower opening of the blade is in direct contact with the drive cam mechanism, the upper part enters the burning fuel. There is also a hole in the upper part for the axis of rotation of the blades. The blade is made by waterjet cutting technology from titanium-stabilized austenitic stainless steel - AISI 316Ti, and in order to increase its service life, i.e. increase its resistance to high-temperature corrosion and abrasion, it is coated with a Ni-Cr-Mo alloy with the addition of niobium, known as Inconel 625. The entire grid assembly is preferably equipped with eight blades.

The ash produced by the burning of the fuel is moved by the burning fuel towards the ash removal shaft, at the bottom of which there is an ash removal auger, which pours the ash into the dustbin. However, part of the ash will fall through the gaps between the grate and the retractable blades. To prevent this ash of a finer fraction from accumulating under the grate, the fireplace is equipped with a second ash removal auger in the under-grate space, which transports this ash through the ash plug into the ash removal shaft. The ash plug is designed as an inclined surface where the ash is accumulated in such a way as to pressure-separate the ash removal shaft from the grate area, while primary air is blown under the grate and there is an overpressure, and in the ash removal shaft there is a vacuum like in a fireplace. Both augers are driven by asynchronous electric motors with gearboxes.

A problem area for sintering fuels is the space between the movable grate and the ash drop shaft into the ash removal screw. In this area of the hearth, there is a risk of partial cooling of the ash and its clumping and sticking into larger structures, which cannot be automatically de-ashed, do not fall down the shaft and form a wall until the hearth is clogged. To eliminate these threats, the wall of the ash removal shaft is slanted and a rotary element is installed here, which crushes and actively moves the ash into the ash removal shaft during its rotation. When the ash remains for some time in the area of the rotating mechanism, which then turns and crushes any sintered structures, ash of a suitable fraction and temperature falls into the ashing screw.

Clarification of drawings

The technical solution is explained in more detail on a concrete example of execution with the help of the attached drawings, in which it represents:

Fig. 1 technical solution of a movable grate with heat sprays, including the hearth in which it is installed, in half vertical section;

Fig. 2 technical solution of a fireplace with a movable grate and both augers for ash removal, in overall 3D section; and

- 2 CZ 38161 U1 Fig. 3 detail of the interior of the hearth with a movable grate, the blades of which are engaged below the level of the fixed part of the grate.

An example of a realized technical solution

An exemplary hearth of a boiler for burning biomass contains a grate 8 with a set of movable blades 7 with a reciprocating movement in the direction above the grate 8. Above the grate 8, a fuel supply 1 is placed in the combustion area 3 in the direction 15 of fuel movement on the grate 8, under which there is an ashing screw 13 fireplace. Behind the grate 8 is the ash removal shaft 6. The blades 7 are equipped with a driving cam rotation mechanism 9. The blades 7 for digging through the burning layer are treated with heat spraying from a Ni-Cr-Mo alloy with the addition of niobium. A rotary element 11 for crushing and moving the generated ash is located behind the grate 8 at the ash removal shaft 6.

The mechanical grate 8 of the biomass fireplace with heat injections is actually used in a biomass micro-cogeneration ORC unit with a thermal output of 120 kW and an electrical output of 8 kW. This is a modified fireplace for wet biomass with a massive heavy lining, which was originally fitted with a solid grate. It consists of two floors, on the first floor there is a grate 8 and combustion takes place here, the second floor is designed as an afterburning chamber, which is then followed by a heat exchanger.

Fuel is delivered to the fire pit from the operating hopper through the rear opening through inlet 1. The sides of the lining above the level of the grate 8 are equipped with holes 4 for the supply of secondary air and also with another hole 5 for the igniter, specifically for the supply of ignition air. The hearth can be seen from the front through the hearth door 2, immediately behind it is the ash removal shaft 6 at the bottom of which is the hearth ash removal auger 13 with another direction 16 of the movement of falling ash. In the space between the shaft 6 and the grate 8, the rotary element 11 rotates, which helps to crush the resulting ash and transport it to the ash removal shaft 6.

The structure of the movable grate 8 itself consists of a fixed bed, which is attached to the metal structure of the bottom of the hearth. The entire motion system is also attached to it - the drive cam of the movement cam rotation mechanism 9. From the outside, an electric motor with a gearbox is attached to it, which rotates it. The movement of the drive cam results in an oscillating movement of the blades 7 with the axis of rotation in the axis 10 of the blades 7. The blades 7 are provided with a protective coating.

The blades 7 and the holes in the grate 8 are designed so that the burning fuel is dug up and the fall of ash under the grate 8 is minimized. However, the conducted tests revealed the necessity of removing this ash from the space under the grate, through which the primary combustion air is supplied. For this purpose, the second ash removal auger 14 is used, which pours ash through a special ash plug 12.

The control of the entire system of the grate 8 with blades 7 with sprayers is realized by a complex control system of the cogeneration unit, specifically by an industrial logic controller PLC. This is the starting of individual electric motors in set periods and sequences, which are dependent on the quality of the fuel, especially on the amount of ash. Furthermore, the control system can evaluate the failure of the movement mechanisms of the grate 8 and the screws 13, 14, using inductive sensors located on the shafts of the individual drives.

In the ORC cogeneration unit, the firebox equipped with a movable grate is followed by a special coiled evaporator, which is directly connected to the firebox. In it is vaporized ORC medium, which in this case is hexamethyldisiloxane. The entire ORC unit is located in an outdoor container.

Industrial applicability

The embodiment described in more detail above shows only one of many possible solutions that fall under utility model protection. In principle, this solution can also be used for fireplaces with larger outputs, namely

- 3 CZ 38161 U1 not only in the cogeneration ORC unit, but also in warm water or hot water boilers. This solution can be used for all fuels that have a tendency to caking, typically agrobiomass in the form of residues from agricultural plant production or low-quality wood chips with a high ash content.

Claims (2)

1. The hearth of a boiler for burning biomass, containing a grate (8) with a set of movable blades (7) with a reciprocating movement in the direction above the grate (8), above which there is a feed (1) of fuel to the grate (8), below which 5 is an auger (13) ash removal of the hearth and behind which there is a shaft (6) of ash removal, characterized in that the blades (7) are equipped with a driving cam rotation mechanism (9); and that in order to dig through the burning layer, the blades (7) are treated with heat spraying from a Ni-Cr-Mo alloy with the addition of niobium. 2. Boiler hearth according to claim 1, characterized in that a rotary element (11) for crushing and moving the generated ash is located behind the grate (8) at the ash removal shaft (6).