HUP0401455A2 - Method for solid wastes elimination and reutilization from fermentation process - Google Patents

Abstract

Disposal and energetic utilization of solid material produced as by-product of fermentation involves drying and incinerating the solid material in a closed process. In the closed process, humidity content of the fermentation residue is set. The fermentation residue is chopped to a defined grain size either in the same phase or separately; and then loaded into a fluidization furnace and burned in at least one cycle or at appropriate combustion parameters, and heat is gained from the gaseous combustion product or optionally in the combustion chamber for energetic utilization.

Description

What? - 1g

Process for the disposal and energetic utilization of solid material produced as a by-product of fermentation

Reporter: Dr. Pál Szentannai,

Dr. János Ősz,

Budapest

Land

Date of announcement: July 19, 2004.

The invention relates to a method for the disposal and energetic utilization of solid material produced as a by-product of fermentation, with the help of which the solid by-product produced during the post-utilization of sewage sludge can be disposed of with high efficiency and in an environmentally friendly manner.

In the well-known and increasingly widely used fermentation technology, suitable strains of bacteria produce methane-containing gas from a wide variety of organic materials. However, this process does not occur perfectly, meaning that a large amount of solid material remains in the fermentation tank (fermentor), which is both flammable and problematic to dispose of.

There are several options for disposal, depending on the source of the starting organic material. In the traditional solution, this residual soil energy can be used for replenishment either wet or after drying. However, it should be noted that this is the path whose options are becoming increasingly narrower as the composition of the material has to be subjected to more rigorous testing and as we get closer to the increasingly stringent regulations for materials permitted for use in agriculture throughout Europe. The typical sources of organic materials used as the starting point for the fermentation process are, on the one hand, wastewater treatment plants and, on the other hand, livestock farms. The extremely diverse content of harmful substances in the former does not need to be detailed, but it should be known about the latter raw material that it contains a significant amount of very dangerous biological substances: hormones, antibiotics, bacteria, dioxins, furans, etc., which, due to their high stability, do not decompose when recycled into the agricultural food chain, but rather accumulate over time. The agricultural utilization of fermentation residues is therefore becoming increasingly problematic in our country, a difficulty that is not solved by the other emerging option, landfilling. The capacity of areas that can be considered for landfilling is becoming increasingly limited due to the resistance to accepting environmentally harmful and odorous material, and the transportation to increasingly distant landfills is becoming increasingly expensive.

According to the state of the art, the solid by-products of fermentation are currently mostly utilized in agriculture. However, the disadvantage and even significant danger of this is the return of very stable toxic substances into the food chain. This process ultimately endangers the human body through food of plant and animal origin.

A narrower, specialized area of agricultural utilization is the strengthening of the soil of park grass, garden flowers and other plants, a solution known from an Austrian wastewater treatment plant. This use is described in the printed, colored publication Weitgehende Schlammbehandlung by the Austrian company ARA-Dombirn-Schwarzah GmbH (1993 Dombim, Austria).

In this process, the fermentation residue is dried using intensive drying technology, at a significant cost and energy cost, and then used as a soil conditioner in agriculture. The disadvantage of this solution is that it does not reduce the risks arising from the presence of harmful substances, but it requires practically perfect drying of the material, which means a huge energy consumption, and the dried fermentation residue offered for free to take away does not produce any economic benefit.

A process for the disposal and utilization of waste sludge from the paper industry and waste printing ink is described in Hungarian patent specification HU 211 758, during which the fermentation residue is composted and utilized in agriculture. The process is characterized by the fact that waste printing ink - preferably in an amount of 1-10 t% - is mixed with waste sludge from the paper industry containing 30-50% dry matter, and then the C/N ratio of the mixture is adjusted to a value between 1:20-1:100, preferably by adding fertilizer. The mixture is then deposited and fermented aerobically at a temperature between -5...+40 °C for 20-180 days - optionally with the use of artificial aeration - and the resulting compost is extracted after treatment.

According to the state of the art, one of the known methods for the incineration of various waste materials is the fluidized bed combustion technology, which was initially developed for coal combustion and later began to be used for the combustion of other solid fuels. The outstanding features of this boiler type include its flexibility in terms of fuel and the possibility of producing energy in a particularly environmentally friendly manner.

In fluidized bed combustion technology, the inert material (usually sand) in the combustion chamber and the solid fuel are suspended (fluidized) by air blown into the bottom of the combustion chamber, which allows for slower, lower-temperature combustion. The air velocity used fundamentally determines the behavior of the fluidized bed and thus the nature of the equipment. If a lower air velocity is used, we are talking about a fixed-bed or bubble device, while if an air velocity exceeds the minimum fluidization velocity, the air flow exiting the fluidized bed carries solid particles with it, which must be separated in a cyclone and then returned (recirculated) to the bed material. In this case, we are talking about a circulating fluidization device.

A mobile device for burning fermented sludge in a fluidized bed boiler is described in German patent specification DE 332 5078, which allows the use of sludge burning at any variable location, thus avoiding the installation of large, fixed devices. According to the solution, the components of the fluidized combustion device are built on a land or water vehicle. The description also describes the most appropriate way of arranging individual components of the combustion device on a ship as one of the possible implementations of the device. The disadvantage of the solution is that the mobile fluidized combustion device built on a vehicle can only be used for very small quantities. The referenced document does not contain detailed instructions regarding the fluidized bed combustion of fermented sludge. The mobile plant also solves the problem of disposal only periodically and does not enable economic or energetic utilization.

In summary, it can be stated that all of the solutions known today either only solve one of the issues of utilization or disposal, and do not provide a solution for the comprehensive, effective and efficient management of these two areas.

Solutions that attempt to utilize fermentation residues all do so by replenishing soil energy, but in this way highly stable toxic substances that accumulate over time are recycled back into the food chain of living things, the most important of which are hormones, antibiotics, bacteria, dioxins, and furans. The disadvantage of the known solution of primarily non-food agricultural utilization is that it does not reduce the dangers arising from the presence of harmful substances, but it requires practically perfect drying of the material, which requires a lot of energy consumption, and the dried fermentation residue offered for free to take away does not produce any economic benefit either. The efficiency of solutions that also utilize utilization is quite low and can hardly be justified economically. This part of the known solutions therefore does not solve the problem of disposing of the solid by-product of fermentation, and they have end products that pollute the environment and contain harmful substances. In addition, they do not promote the energetic utilization of this renewable energy source, and there is even a solution that consumes a lot of energy.

The known solution according to the state of the art, aimed at disposing of fermentation residue, on the other hand, does not take into account the utilizable energy content of the material and thus does not exploit the environmental, energetic and economic potential of this renewable (therefore particularly environmentally friendly) energy source.

When developing the solution according to the invention, our goal was to dispose of the substance that is also dangerous to human life, to significantly reduce the volume of the by-product that needs to be deposited, and to produce energy from this renewable energy source in such a way that the binding of harmful substances in the resulting flue gas is implemented in an integrated manner in the technology, and the resulting ash is also environmentally friendly and usable. Our goal is also to develop a combustion method that can solve the independent combustion of the fermentation residue, and as a result of which technology also produces heat that can be used energetically.

When creating the solution according to the invention, we recognized that if the disposal of the fermentation residue is carried out in a fluidized bed incinerator after the material has been properly dried and crushed, in a properly adjusted closed combustion process, then the set goal can be achieved. We also recognized that by using the appropriate combustion temperature range used in the combustion process, the nitrogen oxide released during combustion can still be kept at a sufficiently low level, while at the same time, during the combustion of the fermentation residue, the calcium and sulfur content of the material can be bound with great efficiency. This results in substances that are very slightly harmful to the environment or have a neutral effect as solid and gaseous end products of the combustion. We recognized that by using the fluidization technology according to the invention, the emission of polluting and burdening substances to the environment due to the typical components of the fermentation residue can be kept to a minimum.

The invention is therefore a method for the disposal and energetic utilization of solid material produced as a by-product of fermentation, during which the solid material produced as a fermentation residue is dried and then burned in a closed system process. The characteristic of the method is that during the process the moisture content of the fermentation residue is adjusted, then either together with this or in a separate phase - the fermentation residue is crushed to a suitable particle size, after which the prepared fermentation residue is fed into a fluidization combustion device, then it is burned there, optionally in one or more cycles while maintaining suitable combustion parameters, and then at the end of the process heat is extracted for energetic utilization from the resulting gaseous combustion product, or optionally from the firebox during the combustion process.

In a preferred embodiment of the process according to the invention, the moisture content of the solid material resulting from the fermentation residue is adjusted by drying it in one or more steps, during which the moisture content is preferably adjusted to 2-30%, optionally to 60%, but not more than 68%.

In a further preferred embodiment of the process according to the invention, the solid material resulting as fermentation residue is preferably ground or chopped in one or more steps to a particle size of 0.1-50 mm, preferably to a particle size of 4-5 mm.

In a further preferred embodiment of the method according to the invention, the combustion temperature range used during fluidization firing is 500-950 °C, preferably 800-850 °C.

In a further preferred embodiment of the process according to the invention, the solid material resulting from the fermentation residue is solar dried in one or more steps during the preparation.

In a further preferred embodiment of the method according to the invention, the extracted thermal energy is transferred to the heat-dissipating surface in a known manner, using water as a working medium, preferably with the aid of a condensation heat exchanger.

In a further preferred embodiment of the method according to the invention, an additive or supplementary fuel that aids the combustion or combustion process is added periodically or continuously to the fermentation residue to be burned during the combustion process.

In a further preferred embodiment of the method according to the invention, during the combustion process, additional combustible or non-combustible material, optionally waste and/or other fuel and/or catalyst are added periodically or continuously to the fermentation residue to be burned.

The method according to the invention is described below based on a preferred specific embodiment:

In a preferred and expedient embodiment of the process according to the invention, the solid by-product of fermentation is disposed of and utilized energetically. In order to achieve these goals, the first step is to dewater the fermentation residue. This can be done mechanically, thermally or solar, the latter being the most expedient. The moisture content of the material obtained after dehydration is less than 60%, but it is expedient to achieve a value of 30%. In order to ensure the independent combustion of the fuel, the following condition can be given as an absolute limit to the degree of dehydration:

* * * ⁼ H _o -h _y '^ΗΟΗ,Β•ΑΐΛ)Τ ^> θ ' where the variables required to calculate the characteristic quantity denoted by H _u * are the following:

H _o is the residual heat of combustion of fermentation h _v * is the difference between the specific enthalpy of water at 850 °C and the fuel inlet temperature (typically 25 °C) (typically 4.1735 MJ/kg),

Αηοη,β is the relative water vapor mass released during the combustion of the fermentation residue, λ excess air factor (typically 1.3) projected onto the fermentation residue.

hC is the specific energy required to heat the (preheated) air entering the combustion chamber to 850 °C (assuming 400 °C air preheating, hC = 0.490 MJ/kg), //Lo _T is the relative air mass per fermentation residue required for stoichiometric combustion.

The fulfillment of this condition in the case of a given, but typical, fermentation residue tested depends practically on only two characteristics, the applied excess air factor and the air inlet temperature.

The fermentation solid by-product, which has been dehydrated to the extent specified above, must be crushed. This operation can be carried out using standard equipment (air mill, mill), but in our case it is advisable to carry it out in one unit with the dehydration, during it. This way, the initially loose material does not form lumps during the dehydration process, the subsequent grinding of which is a much more difficult and demanding task.

The resulting material can be placed in a temporary storage, a hopper, from where it can be fed into the firebox of the fluidized bed boiler with a suitably designed feeder. Since, according to our experience, the material stored in the bunker is very prone to arching, the hopper feeding device must be designed in such a way that it prevents arching. One possible solution to this is to install a mixer operating together with the feeder, covering a significant part of the hopper contents.

The firebox of a fluidized bed boiler contains an inert material, typically sand, which fills 0.5-5% of the volume of the firebox. Combustion air is blown into the firebox with this bed material from below, which causes the solid inert bed material to float, i.e. forms a fluidized layer. After preheating the fluidized bed - which can be done, for example, with the help of a gas burner - the dehydrated, chopped fermentation residue can be introduced. This can be done when the bed temperature (i.e. the firebox temperature) has reached the ignition temperature of the fermentation residue. The heating must then be continued until the operating temperature is reached, which is typically 850 °C.

At this temperature, the thermal and prompt oxidation of nitrogen in the air does not yet occur, but reactions important for sulfur fixation are already taking place.

Solid fuels, including the solid by-product of fermentation, also contain a significant amount of sulfur, which in other known combustion methods is released from the combustion chamber as extremely harmful sulfur oxides. However, the slow combustion processes of the fluidization combustion method allow these substances to bind in the combustion chamber, e.g. in the presence of calcium. A common solution in the fluidization combustion technique is therefore the addition of limestone. However, this is not necessary in the case of fermentation residue from wastewater treatment plants, because this material itself contains sufficient calcium to bind the sulfur contained in it.

In the case of a circulation fluidization device, solid particles, which contain not only inert material but also combustible material, are separated from the flue gas leaving the furnace using a cyclone and recirculated back into the furnace.

The initial particle size undergoes significant reduction during combustion. The particles leaving the cyclone are finer than the incoming dust before the cyclone, which participates in combustion (and desulfurization). The coarser fractions obviously circulate more often than the finer ones.

The flue gas leaving the furnace (typically through a cyclone) when the fermentation residue is burned contains a large amount of water vapor, even with significant preliminary dehydration, the heat of vaporization of which represents a substantial energy content. In order to utilize this, we use a condensing boiler to extract the energy released during combustion.

The solution according to the invention is described below based on the attached figures:

Figure 1 shows a possible schematic arrangement of an apparatus preferably used to implement the method according to the invention.

Figure 2 shows a flow chart of the most general procedure of the method according to the invention.

Figure 3 shows the dewatering limit curves of the drying process used in the process according to the invention.

The table in Figure 4 summarizes the characteristics of the fermentation residue examined during the experiments and its dehydrated cases to various degrees.

Figure 5 shows the values obtained during the analysis of the ash of the fermentation residue.

Figure 6 summarizes the results of the TG-MS study performed in an inert atmosphere.

Figure 7 shows the results of the TG test performed in an oxidative atmosphere.

Figure 8 shows the time evolution of the most important temperatures during the combustion of the fermentation residue and in the period immediately preceding it.

Figure 9 shows a diagram depicting the pressure difference characteristic of the dust mass in the lower part of the firebox.

Figure 10 shows the time function of the gas flows entering the firebox and the position of their associated valves.

In Figure All., we summarize some of the carbon chemical characteristics of the ash from the experimental combustion of the fermentation residue.

Figure 1 shows a possible conceptual arrangement of the apparatus preferably used for implementing the method according to the invention. The figure shows the fluidization combustion apparatus 1, which has a vertically formed, elongated combustion chamber 2 for burning the fermentation by-product. The cyclone 3 with a narrowed bottom is connected to the upper part of the combustion chamber 2 via a lateral channel. The flue gas channel 8 starting vertically is connected to the upper part of the cyclone 3, which is connected to the flue gas heat exchanger 4 containing the heat exchangers 6. The flue gas outlet 5 located on the flue gas heat exchanger 4 is the outlet opening for the gas combustion product generated as the final product of the process.

At the bottom of the cyclone 3 there is a siphon 7 with a siphon air inlet 19 and an air distributor 13, from which a sloping chute 11 leads to the bottom of the firebox 2 of the fluidization combustion device 1. At the bottom of the firebox 2 there is a primary air inlet 15 and an air distributor 12. Furthermore, the starter burner 14 extends into the lower part of the firebox 2. On the side of the firebox 2 there are also a secondary air inlet 16 and a tertiary air inlet 17, which serve to set and regulate the appropriate combustion conditions of the combustion process. Furthermore, the slag discharger 18 leads out from the lowest part of the firebox 2, which serves to remove solid material collected during combustion.

To monitor the temperature parameters of the combustion process, the TI lower combustion chamber temperature meter, T2 middle combustion chamber temperature meter, T3 upper combustion chamber temperature meter, and the T4 outlet flue gas temperature meter are located in the flue gas outlet 5 of the flue gas heat utilization unit 4 in the corresponding parts of the combustion chamber 2. To monitor the pressure parameters of the combustion process, the Pl lower combustion chamber pressure meter and P2 middle combustion chamber pressure meter are located in the corresponding parts of the combustion chamber 2.

In the case of the device shown in Figure 1, the properly dried and shredded fermentation by-product to be fired is fed into the dropping system 9 hopper. The amount fed is controlled by the feeder 10 at the bottom of the hopper 9 . From the feeder, the fermentation by-product to be fired enters the bottom of the firebox 2 of the fluidization combustion device 1 through the inclined chute 11, from where it enters the fluidization process taking place inside the firebox 2 with the help of the primary air introduced there, during which it continuously burns. During the firing process, the fed fuel continuously burns independently while maintaining the appropriate combustion parameters. The gaseous combustion product generated during the fluidization firing process and the partially unburned solid material are fed into the upper part of the cyclone 3 through the channel connected to the upper part of the firebox 2. The gaseous combustion product and the solid material discharged from the combustion chamber 2 are separated in the cyclone 3, and the gaseous combustion product is conveyed to the combustion chamber 4 by means of the flue gas channel 8 extending from above. Here, the heat of the gaseous combustion product is extracted and further utilized by means of the heat exchangers 6. The solid material separated in the cyclone 3 is conveyed to the siphon 7 at the bottom of the cyclone 3, from where it is returned to the bottom of the combustion chamber 2 through the inclined chute 11. In this way, the not yet completely burned particulate solid material is returned to the combustion process. The air distributor 13 in the siphon 7 and the siphon air supplied there assist and regulate the return flow of the solid particles.

Figure 2 shows a flow chart of the most general process of the process according to the invention. The main steps of the process according to the invention are as follows. First, the fermentation residue is dehydrated and its moisture content is adjusted, then either together with this or in a separate phase, it is crushed to the appropriate particle size. This is followed by feeding into the combustion device, then combustion in it, optionally recycling in several cycles. At the end of the process, heat utilization is performed from the resulting gaseous combustion product, but optionally this can also begin from the firebox during the combustion process. The extracted heat energy is transmitted to the heat-dissipating surface in a known manner, using water as a heat carrier.

The starting material used in the process, the solid by-product of fermentation, is first fed into the dewatering and grinding equipment, from where it is fed into the collecting chamber of the fluidization boiler feeder. The feeder feeds the dewatered, ground solid material into the high-temperature bed material in the firebox, where it begins to burn. The bed material, together with the combustible solid material, is kept in a fluidized state by the (primary) air blown in from below, which also provides part of the oxygen required for combustion. Additional air can be introduced as secondary and tertiary air at higher levels of the firebox. The flue gas produced during combustion is discharged at the top of the firebox. In the case of so-called circulating fluidization boilers that use a higher fluidization air velocity, the flue gas stream also entrains solid particles (inert dust and combustible material), which can be separated in the cyclone. The separated solid material then returns to the firebox. The flue gas, which has been (mostly) cleaned of solid matter, enters the flue gas heat recovery unit, whose task is to utilize the energy content in the flue gas by cooling it. In the solution included in the invention, a significant amount of water vapor in the flue gas is condensed here for the purpose of energetic utilization of the evaporation heat.

At the preferred combustion chamber temperature of around 850 °C, typically used in the process according to the invention, the thermal and prompt oxidation of nitrogen in the air does not yet take place, which is the determining factor in the formation of ΝΟχ in other combustion solutions; further reduction of nitrogen oxide emissions is achieved by the divided introduction of air. The use of a fluidized bed also provides the opportunity to effectively bind the sulfur released from the fuel during combustion, which is typically achieved by adding limestone. At this time, the already oxidized sulfur is converted directly or indirectly into gypsum with the added limestone, i.e. it is bound in the combustion chamber itself — instead of being released into the environment in the form of SO2 or SO3. The gypsum thus formed — after purification — can even be used for industrial purposes.

Temperatures, pressures and gas flows are measured by transmitters connected to a computer data logger. The components CO2, O2, SO2, NO, NO2, CO, THC (combustible hydrocarbons) in the flue gas are recorded by a digital analysis data logger. The overall evaluation is based on additional instruments, sampling and other information.

Figure 3 shows the dewatering limit curves of the drying process used in the process according to the invention. The diagram indicates the maximum moisture content of the fermentation residue intended for independent combustion under given conditions. For example, with an excess air factor of (1)λ=1.3 and an air preheating temperature of 0 _L ev,be ⁼ 400°C, a maximum moisture content of γΗθΗ<67.9% is allowed for independent combustion at a typical combustion chamber temperature of 850°C in fluidized bed combustion.

The diagram also shows that, for example (2) with an excess air factor of λ = 1.3 and a humidity of yhoh ⁼ 70%, the air entering the combustion chamber must have a temperature of at least 0Lev,be>580°C (preheating).

The method according to the invention, as well as the results of measurements carried out during the trial operation of the method, are described below based on the following experimental example:

During the procedure, in preparation for the experimental study, we determined the composition of the fermentation by-product used as fuel:

The carbon chemical characteristics of the fuel are discussed based on the book by Brandt (1981), which is completely identical to those set out in the DIN 1942 standard in terms of notation and methodology.

The starting point is the elemental composition of the fuel, which gives the mass ratio of each component in relation to the raw fuel. The unit of measurement is always kg/kg:

7hoh moisture content, y _A ash content, yc carbon content, yH hydrogen content, y _s sulfur content, y _N nitrogen content, yo oxygen content.

Of the above, only the first six are usually measured in the laboratory, the oxygen content must be calculated from these, since the sum of the fuel components is known:

/HÓK + /A + /C + /Η + /S + /Μ + /O = 1 , which also shows that, for example, the hydrogen in the moisture content does not count towards the separate hydrogen content denoted by yn .

In terms of preparing the measurement and further tasks, it is important to convert these characteristics to other moisture contents according to the following relationship:

_vv ¹ ~/hoh ^> ' /1 /1 , , ' ~ /hoh where i is the index for the converted component (A, C, H, S, N, O); one or two commas (' or ”) refer to the initial or converted state.

The stoichiometric combustion basic quantities of decisive importance for the calculations, and their expression based on the elemental composition of the fuel - also referred to the mass of the fuel, with the dimension kg/kg - are as follows.

Theoretical dry combustion relative air mass:

AloT = 11.5122 · _rc + 34.2974 · y _H + 4.3129 · y _s - 4.3212 · y _o .

Theoretical dry relative flue gas mass:

ZW = 12.5122-y _c +26.3604-y _H +5.3129./ _s -3.3212-y _o +l.0 y _N .

Theoretical dry relative flue gas volume in normal conditions:

E _GoT = 8.8930 · y _c + 20.9724 · y _H + 3.3190 · y _s - 2.6424 y _o + 0.7997 · .

Theoretical combustion relative water vapor mass:

Αηοη,β = 8.9370 · y _H +1.0 · y _HOH .

Another important characteristic of the fuel is its heat of combustion H _o , which is nothing more than the specific energy released during combustion (taking into account that water is a liquid combustion product). Its value can also be measured in the laboratory.

From this, the running value denoted by // _u can be calculated, which is the specific energy released during combustion - taking into account that the water combustion product is in the vapor phase:

#u -¼ ·Αηοη,β ' where according to the standard h _t = 2.4425 MJ / kg, i.e. the heat of vaporization of water at 25 °C. The running value therefore takes into account that the energy released during combustion is reduced by the heat required to evaporate the resulting water combustion product. However, it does not take into account the heat also required to superheat the resulting water vapor, as well as the heat required to heat the air. Since the combustion chamber temperature of fluidization combustion is well defined (typically 850 °C), it is possible to determine the relative amount of heat released during combustion - taking into account the heat absorbed during the superheating of the resulting water combustion product to this temperature and the heating of the combustion air. This is the characteristic that we introduced above, denoted by H _u .

It should be noted that the invention includes a condensing boiler precisely because of the high moisture content, from which the specific heat that can ultimately be extracted will be higher than H _u , and even higher than H 0 (approaching H ₀ ), but in order to ensure the sustainability of independent combustion, H _v must still be definitely positive.

The overall usable specific heat inside the condensing boiler can also be determined using the following formula:

Ahoh,b~^'^l 'AloT ’ where, assuming an exit flue gas temperature of 70 °C, hy* is the difference in the enthalpy of water at 25 °C and 70 °C: 188.17 kJ/kg, hC* is the difference in the specific enthalpy of ambient air at the point of entry into the boiler and exit as flue gas: 49.01 kJ/kg (in case of 25 °C entry and 70 °C exit).

The heat actually used to heat the working medium is less than this because it is reduced by conversion losses (e.g. heating the environment), but the flue gas loss is not reduced because it has already been taken into account in the above formula.

These characteristics are summarized in the table in Figure 4 for the fermentation residue examined during the experiments, and for its dehydrated cases to various degrees. The starting data for this figure are therefore the laboratory analysis of the carbon chemical properties of the given fermentation residue (Data: Takács, 2004 a).

It can be seen that the fermentation residue cannot be fired in its original state using the fluidization firing technology, since H _u * = -0.36 MJ / kg < 0. (Nor with other technologies, because their temperatures are usually higher than the 850 °C considered here.) Some form of dehydration is therefore absolutely necessary.

In one of the technologies used in practice, this state of the fermentation residue (70.7% moisture content) is achieved by centrifugation. However, there is a more effective technology (not to mention the very energy-intensive thermal drying), in which press dewaterers are used, which achieve a moisture content of ^hoh = 60% with negligible power consumption. In the following, we will consider this value as a reference as the result of a realistically feasible solution.

Figure 4 shows a summary of the residual carbon chemical characteristics of the fermentation under study when different moisture content conditions are reached, which was prepared based on the laboratory data using the above equations.

It can be seen that the examined fermentation residue can be stably burned in a fluidized bed boiler even with the simple press dewatering process: H _u (7hoh ⁼ 60%) = 1.03 MJ / kg > 0. The usable specific heat in the condensing fluidized bed boiler is 4.47 MJ / kg.

Examination of the composition of the ash obtained during the experimental combustion process:

First of all, from the point of view of sulfur fixation, it is important to know what the ash of the calcined solid fuel (fermentation residue) consists of. In order to reveal this, a laboratory test was performed (Data: Várhegyi et al. 2004 a). Figure 5 shows the values obtained during the analysis of the ash of the fermentation residue (calcined in the laboratory).

It is already visible here, but as a result of the fluidization experiment, the presence of iron(III) oxide was even more striking, which gives the fly ash its distinct brownish color. The study of the origin of individual components found in municipal wastewater would be worth a separate investigation. The most significant component is sand (SiO ₂ ), and the significant phosphorus content obviously comes from detergents.

Of course, the amount of calcium is the most important in terms of sulfur fixation taking place in the firebox of a fluidized bed boiler (integrated into the technology), the resulting equation of which process is

CaO + SO ₂ + ΆΟζ —> CaSO ₄ .

The stoichiometric relationship is evidently valid for the calcium/sulfur mole ratio nca/s ⁼ 1. Based on the known composition data, the specific value of this characteristic is given by /CaO, where M _Ca o and Ms are the molar masses of the two substances (56.079 mg/mol and 32.064 mg/mol, respectively). This means that the fermentation residue contains more than four times as much calcium as is necessary to bind the sulfur contained in it — assuming stoichiometric relations — and the sulfur binding capacity is further increased by the additional basic components (MgO, K ₂ O, Na ₂ O). The surplus covers the needs of imperfect processes, i.e. this fuel is well suited to fluidization combustion technology, since its integrated sulfur binding property occurs without the addition of separate limestone, based on the calcium present in the fuel itself. As a result, very low SO _x emissions can be expected without any special intervention.

Conclusions that can be drawn based on the above:

- Laboratory tests of the fermentation residue revealed, among other things, that it can be stably burned using standard dehydration technology (pressing),

- It can be calculated with a specific heat of 4.47 MJ / kg in the above dehydrated state (i.e. 60% moisture content),

- based on its calcium content, when burned in a fluidized bed boiler, a separate sulfur separator or even separate limestone dosing is not required,

- the stoichiometric basic quantities necessary for the sizing of the combustion equipment have been determined.

Further measurements were performed to examine the progress of the process according to the invention and to analyze the properties of the material being fired:

In this process, we used the so-called TG-MS test method, which consists of a combination of two sub-processes. The first of these is “TG”, which stands for “thermogravimetry”. In this process, the weight of the tested material is recorded while it is subjected to a specific temperature program. In the case of fuels, the usual temperature program is to increase the temperature, e.g. at a rate of 40 °C/min up to 850 °C, and then to keep this temperature unchanged for a few minutes after reaching this value. The composition of the gas surrounding the material is controlled, which can typically be inert (argon) or oxidizing (oxygen, air). During the course of the temperature program (e.g. due to the escape of the impurities), the tested fuel gradually loses weight, and the registered change in weight is usually called the G curve of the TG test. The (numerical) derivative of the time function recorded by the computer is of course not a problem, and the resulting curve shows even more clearly the temperature ranges that are most intensive in terms of weight loss (DTG curve).

In order to obtain even more information about the combustion and annealing process, it is worth examining the exhaust gases by sorting them according to their molecular weight. The second abbreviation in the name of the procedure refers to this, the second device connected to the TG device, “MS” stands for “mass spectrometer”. Its output signals are therefore the relative amount of substances emitted at a given temperature and with a given molecular weight.

The results of the TG-MS study performed in an inert atmosphere are summarized in the attached Figure 6. The results of the TG study performed in an oxidative atmosphere are presented in Figure 7.

Conclusions that can be drawn from the above studies:

During pre-drying at 105 °C and then rinsing, the examined fermentation residue lost 68.5% of its moisture, which corresponds well to the 70.7% moisture content obtained during the analysis of the elemental composition.

In an argon atmosphere, approximately 50% of the dry matter is lost by the time the temperature reaches 850 °C, but the largest part of the weight loss occurs in the 250-500 °C range.

The combustion of semi-coke residue (char) in a gas containing 20% O ₂ takes place practically completely in the range of 350-500 °C.

Based on these, a picture emerges of the fermentation residue as a highly flammable, low-temperature burning material, which fits particularly well with the characteristics of fluidized bed combustion technology.

The studies also showed the primary forms of the most important components of air pollution (S, N) that are released from the material. Of these, it is important that nitrogen is (practically) released only as ammonia, which, under fluidized bed combustion conditions, is practically converted only into N and water.

In the following, we present a specific firing process carried out in a fluidization pilot plant based on the method according to the invention.

Preparing the fuel for the experiment:

Approximately 150 kg of the fermentation residue was shoveled into bags in the wastewater treatment plant and transported in the trunk of a car to the hall that also housed the experimental equipment. Due to the limitations of the experimental equipment (mandatory cyclone cooling, small dosing capacity), extensive drying of the material became necessary. This was carried out in a room where an average air temperature of 28 °C prevailed with adequate air movement. Here, 10 kg of fuel was spread out at a time. Successful drying in these quantitative units could be achieved in 5-7 days, when its moisture content decreased from the initial 70.7% to 27.9%. Incidentally, this method of drying also raises the possibility of low-temperature dehydration in real equipment, e.g. in a solar solution, the energetic impact and limits of which are shown in Figures 3-4.

During the drying process, the fermentation residue not only underwent a significant weight loss, but also formed into rather large lumps, with a diameter exceeding 4-8 cm. Due to the dimensions of the feeder, but also for the sake of good combustion, it was necessary to grind the dried fermentation residue, which resulted in a powdery material that could be fed into the furnace. The grain size of this fuel was quite coarse, since most of the material was over 4 mm.

Combustion conditions

It is also worth examining the conditions in the furnace and other parts of the equipment, since all results, including the composition of the flue gas and the resulting ash, are the result of internal processes.

Figure 8 shows the time evolution of the most important temperatures during the fermentation residue firing (and in the period immediately preceding it). Due to the forced cooling of the cyclone and the narrow capacity of the feeder, the furnace temperature of 850 °C, which is considered typical for fluidization firing, could not be achieved; during the experiment, this temperature was around 650 °C.

Figure 8 shows the most important characteristic temperatures measured during the first experimental combustion of fermentation residue in a fluidized bed boiler. The figure shows the conditions prevailing in the combustion chamber. The technological placement of the measurement points is done at the appropriate location of the equipment according to Figure 1. After the heating phase, at To+22’, we switched to the addition of fermentation residue; the indicated times are the times of taking the wort samples.

The diagram depicting the pressure difference between the measuring points Pl and P2 of the firebox is shown in Figure 9. This pressure difference is characteristic of the dust mass and its concentration in the lower part of the firebox, i.e. (approximately) in the dense phase.

The diagram shows an increasing trend, sometimes with breaks, but always present, valid for the entire previous measurement period, which is caused by the constant increase in the amount of bed material due to the continuous accumulation of the significant ash content of the solid fuel.

The following diagrams (Figure 10) show the gas flows that significantly influence the degree of fluidization, the completeness of combustion, and the distribution of dust in the furnace (mentioned earlier), and the time function of the position of the valves used to adjust them. The total air (yLev) and primary air (yPrim) valves can be moved manually (by operator intervention issued on the computer), while the flue gas valve (yFG) is the intervention organ for the automatic control of the fire pressure.

Analysis of the ash produced during the experiment:

At the end of the periods characteristic of each phase of the fluidization firing experiment, we took samples of the ash. The significant iron(III) oxide content of the residual ash from the fermentation was clearly visible in the brownish color of the samples.

The extracted slag samples were examined in two laboratory analyses, by loss on ignition and carbonate carbon dioxide analysis (Data: Takács, 2004 b), and by sieving. Some of the carbon chemical characteristics of the ashes from the experimental combustion of the fermentation residue are summarized in the figure below. (All data are in mass percentage of the original sample.)

The ash analysis clearly shows that the fermentation residue in the experiment was practically completely burned out. The moisture content of all three ash samples is negligible, and the amount of material that can be removed by heating contains practically only carbonate carbon dioxide. This means that the ash produced during the fluidization firing experiment, carried out under otherwise unfavorable conditions, at a furnace temperature of 650 °C, contains (practically) neither organic residues nor combustible losses.

The particle size distribution of the ashes confirms the previous statement regarding perfect combustion, as there is no trace of the coarse graininess characteristic of the ash from the fermentation residue when added to the firebox. In addition, it should be noted that the characteristic, pungent odor of this fuel and its tendency to curdle completely disappeared during combustion.

One of the advantages of fluidized bed combustion technology is that the resulting ash (as a result of the sulfur fixation process taking place in the furnace) can be used as a raw material for cement production in the construction industry. However, this is sometimes limited to certain areas of use (e.g., the construction of utility tunnels, mine walls) by the detectable radioactivity of the ash, which obviously originates from the coal used as the starting material. However, when measuring the ash of the fermentation residue, we found that it does not show any radioactivity, meaning that this ash can be used without restrictions in the construction industry — even for residential buildings.

Conclusions drawn from the combustion experiment:

We succeeded (to our knowledge, for the first time) in experimental combustion of fermentation residue from a modern wastewater treatment plant in a semi-industrial fluidized-bed combustion plant. However, this required modifications to the plant, the most important of which, necessary for stable combustion, was carried out, but several other (less important) modifications were not possible within the framework of this work. Adapting to this, we increased the heat input in several ways during the experiments, with electric heating, continuous use of a gas burner and preliminary drying of the fuel. In addition to all this, we were able to achieve relatively stable operation at a combustion chamber temperature of 650 °C. This was due to the technical limitations of the experimental plant.

Despite this unfavorable condition, very good (low) SO2 and unburned organic matter concentrations were detected in the flue gas. However, the measured CO and NO _x emissions were very high, which is completely understandable based on the forced deviation from industrial conditions. To experimentally eliminate this and to confirm the favorable results, the indicated modifications and further experiments are necessary.

During the experiment, the ash generated from the fermentation residue was (practically) perfectly burned, showed no unpleasant properties or radioactivity at all, and therefore can be used in cement production.

Summary and results of the experiences gained during the material testing and pilot operation of the method according to the invention:

- In the framework of the research work, the fermentation residue, which has been thoroughly examined in many aspects, can be considered typical, it is produced in large and increasing quantities, because it comes from a wastewater treatment plant whose technology is modern, and the further spread of this technology is already visible.

- This fermentation residue is not classified as hazardous waste, but due to its odor, organic matter content, and repulsive macroscopic appearance, its agricultural use and disposal are increasingly costly and even almost insurmountable.

- This material can be burned independently even when commercial dewatering technology (e.g. pressing) is used.

- Due to the approximately 60% moisture content remaining after pressing, its energetic utilization in a condensing boiler is recommended. In this case, its usable specific heat content is 4.47 MJ/kg.

- The fuel is particularly well suited to fluidization combustion technology because

- ignites at low temperatures and burns easily;

- it makes good use of the integrated sulfur fixation property of the fluidization firing technology, because its own basic sulfur content ensures the fixation of its moderate sulfur content, i.e. the sulfur fixation function of this firing method is implemented - without the need to separately provide for limestone dosing;

- the resulting ash (as a raw material for cement) can be used in the construction industry, even for residential buildings, because it does not show any radioactivity at all.

- The fermentation residue tested (due to its high surface moisture) is very prone to arching, which is very unfavorable for dosing from the bottom of the hopper. Therefore, it is advisable to install a mixing structure to be built into the hopper in the feeder we recommend. In the case of energy utilization in a fluidized-fired condensing boiler, the heat transfer surfaces of the flue gas outlet can be made of a material with lower corrosion resistance (i.e. cheaper) due to the minimal SO2 content of the flue gas.

- Further experiments are needed to confirm some measured results (e.g. the surprisingly low SO2 emissions) and to determine optimal settings and sizing data.

Advantages of the solution according to the invention:

- Its application allows the disposal of harmful components of solid matter produced as a by-product of fermentation in the most efficient, thermal way.

- The fermentation residue is used as a renewable fuel.

- The sulfur content is bound in the furnace without the use of separate additives and additional equipment, in an integrated manner in the technology and by utilizing the calcium present in the fermentation residue.

- Due to the relatively low temperatures used, harmful nitrogen oxide emissions are negligible.

- The condensing boiler type generates energy in an extremely efficient manner, even with relatively low levels of dehydration.

- The resulting ash does not contain radioactive isotopes, therefore it can be used without restrictions in the construction industry.

In the process according to the invention, a fluidized bed boiler is used, because the outstanding properties of this boiler type include its flexibility in terms of fuel and the possibility of producing energy in a particularly environmentally friendly manner. Typically, we recommend installing a hot water boiler rather than a steam boiler, which is justified by its relatively small size.

The disposal process provided by the process is better, more efficient, more economical than other processes, and can therefore be applied more widely. It is energy-saving because its maintenance does not require separate combustion, external energy sources, and can even be used for energy production. It is also very beneficial from an environmental perspective because it practically completely eliminates the substances in the fermentation by-product that otherwise pollute the environment.

Claims (8)

PATENT CLAIMS: 1. Process for the disposal and energetic utilization of solid material produced as a by-product of fermentation, during which the solid material produced as a fermentation residue is dried and then burned in a closed-system process, characterized in that during the process the moisture content of the fermentation residue is adjusted, then - either together with this or in a separate phase - the fermentation residue is crushed to a suitable particle size, after which the prepared fermentation residue is fed into a fluidization combustion device, then it is burned in it, optionally in one or more cycles while maintaining suitable combustion parameters, and then at the end of the process heat is extracted for energetic utilization from the resulting gaseous combustion product - or optionally from the combustion chamber during the combustion process. 2. The method according to claim 1, characterized in that the moisture content of the solid material resulting from the fermentation residue is adjusted by drying it in one or more steps, during which the moisture content is preferably adjusted to 2-30%, optionally to 60%, but at most to 68%. 3. The method according to claim 1 or 2, characterized in that the solid material resulting as fermentation residue is crushed to a particle size of 0.1-50 mm, preferably to a particle size of 4-5 mm, in one or more steps, preferably by grinding or cutting. 4. The method according to any one of claims 1-3, characterized in that the combustion temperature range used during fluidization firing is 500-950 °C, preferably 800-850 °C. 5. The process according to any one of claims 1-4, characterized in that the solid material resulting from the fermentation residue is solar dried in one or more steps during the preparation. 6. The method according to any one of claims 1-5, characterized in that the extracted thermal energy is transferred to the heat-dissipating surface in a known manner, using water as a working medium, preferably with the aid of a condensation heat exchanger. 7. The method according to any one of claims 1-6, characterized in that during the firing process, an additive or supplementary fuel that aids the firing or combustion process is periodically or continuously added to the fermentation residue to be fired. 8. The method according to any one of claims 1-7, characterized in that during the firing process, additional combustible or non-combustible material, optionally waste and/or other fuel and/or catalyst are added periodically or continuously to the fermentation residue to be fired.