Method for the preparation of a biomass to be fed to a biomass power plant

Abstract

Method for the preparation of a biomass to be fed to a biomass power plant, comprising the selection of biomass with a given calorific value in quantities suitable to form a total quantity of biomass having a calorific value close to the design calorific value of the steam generator boiler and sufficient to feed the boiler within a set time interval. The total amount of biomass formed does not require mixing using mechanical shovels and allows for optimisation of boiler operation, reduction of fuel consumption of operating machinery and related exhaust emissions.

Description

[0001] METHOD FOR THE PREPARATION OF A BIOMASS TO BE FED TO A BIOMASS POWER PLANT DESCRIPTION

[0002] The present invention relates to a method for the preparation of a biomass to be fed to a biomass power plant and to a method of feeding such biomass to a power plant.

[0003] Background of the invention

[0004] Biomass power plants play an important role in the production of green electricity and are a valid solution for protecting the environment. In fact, biomass is a renewable source and as such allows dependence on fossil fuels to be reduced. In general, the term “biomass” refers to any organic material derived from plants, animals, municipal organic waste and the like, which can be burned in suitable combustors to produce steam and convert thermal energy into mechanical energy in a turbine, resulting in the driving of an electrical generator.

[0005] Prior art

[0006] The amount of biomass consumed by a medium or large-scale power plant is hundreds of thousands of tons per year. For example, with reference to a 20 MWe (63 MWTh) biomass power plant, the quantity of biomass to be fed to the boiler is on average approximately 250,000 tons / year.

[0007] The management of a power plant of this type presents significant technical and logistical aspects, such as:

[0008] identification of the type of wood available at each time of the year, in particular for each week of supply planning;

[0009] identification of the areas from which to collect the biomass available in the desired period;

[0010] scheduling transport to the power plant.

[0011] Considering a weekly supply schedule, i.e., that each week the amount of biomass needed to fuel the boiler for the following week arrives at the power plant, and considering supply interruptions on holidays, it may be assumed that a 20 MWe biomass power plant receives approximately 5,000 tons of wood chips per week, transported by several dozen large trucks every day and typically unloaded in a large yard or a dedicated area within the power plant. One of the major logistical difficulties is having a fuel mix, particularly wood chips, such that the Lower Heating Value (LHV) is as close as possible to the design Lower Heating Value (LHVp) of the boiler.

[0012] The Lower Calorific Value of wood chips is calculated from the Lower Calorific Value ofanhydrous wood (estimated at 18.5 MJ / Ton) and taking into account the humidity of the wood according to the following formula (I):

[0013] LHV = ((18.5 • (100-W) - 2.44 • W) / 100) (I) where W = Wood moisture in percentage.

[0014] Typically, biomass suppliers provide various types of biomass depending on availability and on the basis of various contracts stipulated by the suppliers themselves with the owners of the forest land or the plants that produce the wood waste from which the biomass is obtained. However, supplies from the various production sites often do not meet the desired objectives. In fact, the various suppliers deliver the biomass based on its availability at the time of supply. It also happens that the supplier closest to the power plant, or the best organized supplier, supplies quantities of material sufficient to cover several days’ needs.

[0015] From the power plant's perspective, there is a fundamental need to have a sufficient quantity of biomass to feed the power plant throughout the year. Therefore, the supply also takes place with quite different types of biomass, but with the aim of creating a fuel mix with a Net Calorific Value within a range of variability deemed acceptable.

[0016] The biomass of each truck arriving at the plant is therefore checked and its moisture content is measured in order to calculate its Lower Calorific Value LHV using the formula (I) reported above.

[0017] However, obtaining a biomass with a Net Calorific Value within the acceptable range of variability compared to the boiler's design Net Calorific Value is not always guaranteed.

[0018] With reference to Fig. 1, the various loads of wood chips arrive daily transported by large trucks that enter a mixing area 10 according to the direction of the arrow X, where the wood chips are unloaded and mixed with wheeled mechanical shovels provided with buckets. The mixing of the various loads occurs according to an empirical assessment by the mechanical shovel operators, who do not always have a correct knowledge of the quality of the unloaded material. Given the empirical nature of the method, it is possible that wood chip mixtures with a Net Calorific Value outside the desired range of variability are formed, with a negative effect on the operation of the boiler and on the thermal output.

[0019] Furthermore, mixing the wood chips unloaded daily by dozens of trucks requires the continuous use of numerous mechanical shovels, which are large operating machines with internal combustion engines powered by extremely fuel-intensive diesel fuel. The continuous mixing operation of biomass therefore involves high fuel consumption and a significant impact on operating costs. Furthermore, diesel engine emissions have a significant environmental impact, both in terms of exhaust gas emissions into the environment, particularly carbon dioxide, andnoise pollution.

[0020] The biomass thus mixed is then transferred to one or more storage tanks 12, 14, 16, 18, in proximity to the feed tank 20 of the boiler (not shown).

[0021] It should also be considered that storing wood chips for a long time in high humidity can cause spontaneous combustion.

[0022] RO 133506 AO discloses a ternary solid fuel and its preparation process. The fuel consists of from 40 to 50 wt% of biological stabilized sludge, the balance being plant waste and superior coal. The fuel has a calorific power from 17500 to 20000 kJ / kg.

[0023] US 2023 / 279306 Al discloses a method for producing briquettes from a waste material comprising metals or minerals and organic waste. The calorific value of the organic fraction is such that the metal or mineral can be melted in a reactor via an autothermal reaction.

[0024] Ngunzi V. K. et al, “Optimization of economics of biomass fuel mix for boilers in tea processing through response surface methodology”; Helyon 10, (2024) e40875, disclose a cost-effective fuel mix comprising fuelwood and macadamia nutshells. The optimization study included a calorific value analysis.

[0025] There is therefore a need for a method for preparing biomass to be fed to a biomass power plant capable of eliminating or reducing the aforementioned drawbacks.

[0026] Summary of the invention.

[0027] An object of the invention is to provide a method for preparing biomass to be fed to a biomass power plant which is capable of ensuring the desired quality of the biomass, particularly with regard to the moisture content and therefore the Net Calorific Value of the wood chips.

[0028] Another object of the invention is to eliminate or substantially reduce the use of mechanical shovels for mixing wood chips, with a consequent reduction in fuel consumption and related harmful emissions into the atmosphere.

[0029] A further object of the invention is to provide a method that allows for the elimination or minimisation of the risks of spontaneous combustion of the wood chips stored in the power plant before being fed to the boiler, thus reducing storage times.

[0030] The above and other objects and advantages are achieved with a method for the preparation of a biomass in the form of virgin wood chips to be fed to the steam generator of a thermoelectric power plant, characterized by:

[0031] A. selecting a plurality of biomasses wn, from plantations in which each biomass wnhas a Lower Heating Value LHVncalculated from the moisture content (W) with the formula (I)

[0032] LHVn = ((18.5 • (100-W) - 2.44 • W) / 100) (I);where W is the wood moisture in percentage

[0033] B. defining the total quantity Qt of biomass having an average lower heating value LHVmto be fed to the steam generator in a time t, in which said average lower heating value LHVm satisfies the following relation (II) with respect to the design Lower Heating Value (LHVp) of the boiler:

[0034] 0.7 • LHVp < LHVm < 1.3 • LHVp (II)

[0035] C. calculating the quantity qnof each biomass wnhaving a heating value LHVnnecessary to form the quantity Qt of biomass having the average calorific value LHVm, according to relation (III)

[0036] Qt (LHVm) = S qn-LHVn; (III)

[0037] wherein the Lower Heating Value (LHVn) of each biomass (wn) is from 6000 to 12000 kJ / kg;

[0038] D. obtaining said quantities of biomass qnand downloading them into feed tanks of said steam generator to form said quantity Qt of biomass having heating value LHVm; wherein said quantity Qt of biomass is suitable to be fed to said steam generator. The present disclosure concerns also a method of feeding a biomass in form of virgin wood chips to the steam generator of a thermoelectric power plant, wherein said biomass is prepared according to the method characterized by features (A) - (D) above.

[0039] Brief description of the figures

[0040] The invention is described below with reference to the accompanying Figure 1, which is a schematic representation of a part of a biomass power plant.

[0041] Detailed description of the invention

[0042] Some terms used in the present description are defined herein:

[0043] - the term “biomass” indicates a solid fuel obtained from wood reduced to flakes or chips of variable size from a few millimetres to a few centimetres, preferably between 30 mm and 5 cm, more preferably between 50 mm and 3 cm, obtained from various types of wood, as indicated above, for example from logs and twigs, called “chips”. Therefore, in the present description the terms “biomass” and “wood chips” are used with the same meaning;

[0044] - the terms “lower heating value” and “calorific value” are used interchangeably;

[0045] The terms “boiler” and “steam generator” are used interchangeably.

[0046] Furthermore, in the present description the terms “comprising” and “containing” are used interchangeably, the meaning of which does not exclude the presence of other elements, in addition to those defined after such terms. The term “consisting of’ is therefore also includedwithin this meaning. The terms “comprising” and “containing” have a broader meaning than “consisting of’,

[0047] As is known, and as mentioned above, a biomass power plant includes a steam generator in which water is heated by the combustion of biomass and the steam thus produced is fed, at an appropriate pressure, into a turbine which drives an electrical generator.

[0048] Given the large biomass consumption required to operate a medium or large-scale power plant (e.g., 5 to 50 MWe), industrial practice requires a secure supply of biomass of adequate quality throughout the year, which is why three types of biomass are essentially used:

[0049] 1) Purposely cultivated biomass (short rotation forestry), e.g. poplar wood;

[0050] 2) Forest woody biomass, obtained from forest felling;

[0051] 3) Residual biomass from the forestry and agricultural sectors, such as branches obtained from trees whose trunks are intended for various uses, for example in construction or furniture manufacturing;

[0052] Biomass is fed to the boiler in the form of virgin wood chips or shavings, supplied directly from the plantations by one or more suppliers in the desired size for introduction into the boiler. The size is usually between 30 mm and 5 cm, preferably between 50 mm and 3 cm, with reference to the long side of the chip.

[0053] The quality of biomass is mainly linked to the moisture and ash content. Humidity, in particular, plays an important role in the economic balance, as the calorific value of the fuel depends on it. The biomass types 1) - 4) mentioned above have a moisture content ranging from 35% up to 55-65%, and therefore a calorific value which may vary significantly.

[0054] The amount of heat per unit of time required by the boiler to generate the steam used by the turbine to produce a given amount of electrical energy in the same unit of time is called Boiler Thermal Power.

[0055] The corresponding fuel flow rate with which to feed the boiler is calculated by taking into consideration the hourly thermal power in the boiler (Pboiler) divided by the Lower Heating Value of the fuel (commonly called Low Heating Value, “LHV”) expressed in kJ / kg or MJ / Ton according to the formula:

[0056] Hourly fuel flow rate = Pboiler / LHVfuel

[0057] The fuel flow rate, i.e. wood chips, can therefore be referred to the time frame selected for programming the boiler feed and expressed, for example, in tons / week.

[0058] According to the biomass preparation method of the invention, the average calorific value LHVm of the biomass to be fed to the steam generator satisfies the following relationship (II) with respect to the design Lower Calorific Value (LHVp) of the boiler:0.7 • LHVp < LHVm < 1.3 • LHVp (II)

[0059] In practice, it has been found that if the average Lower Heating Value LHVmof the biomass mixture remains within a tolerance range of approximately + / - 30% of the boiler's design Lower Heating Value, the boiler can operate efficiently and effectively.

[0060] Therefore, each biomass, designated as wn, must have a lower heating value LHVnsuch that, supplied in the quantity qnin a given time interval t, it forms a mixture having an average heating value LHVmthat satisfies the above relation (II) with respect to the design lower heating value (LHVp) of the boiler:

[0061] 0.7 • LHVp < LHVm < 1.3 • LHVp (II)

[0062] The total quantity Qt of biomass having the average calorific value LHVm, which is fed to the boiler in time t satisfies the relationship:

[0063] Qt (LHVm) = S qn-LHVn; (III)

[0064] The method for preparing biomass to be fed to a biomass power plant according to the present invention is now described in detail with reference to each of the features A - E, which are not to be understood as necessarily successive steps of the method, and also with reference to Fig.

[0065] 1.

[0066] A

[0067] Feature A consists in selecting a plurality of biomasses wn, where each biomass wnhas a lower calorific value LHVnsuitable for forming a mixture having an average calorific value LHVm compliant with what is specified in the subsequent steps of the method.

[0068] As mentioned, biomass suppliers of the biomass in form of virgin wood chips coming from the plantations - who may be different from the owners of the forest land or the plants where the woody materials from which the biomass is produced originate - provide various types of biomass depending on local and time availability, also taking into account the fact that biomass production sites are preferably close to the power plant, in order to facilitate transportation thereof.

[0069] Given the fundamental need to have a sufficient quantity of biomass to power the power plant throughout the year, the plant operator agrees to supply itself with quite different types of biomass with the aim of creating a fuel mix with a Net Calorific Value within a range deemed acceptable for fuelling the boiler.

[0070] The biomass, already in the form of wood chips, is transported by large trucks that arrive at the power plant, where the moisture content of the biomass is measured in order to calculate its Lower Calorific Value (LHV) using formula (I) and verify its correspondence with the purchase order.LHVn = ((18.5 • (100-W) - 2.44 • W) / 100) (I);

[0071] where W is the wood moisture in percentage.

[0072] According to one aspect of the method, the plurality wnof biomasses preferably comprises 2 to 6 biomasses (wi - we).

[0073] According to another aspect of the method, the lower calorific value LVHnof each biomass is preferably between 6000 and 12000 kJ / kg.

[0074] B

[0075] This feature consists in defining the total quantity Qt of biomass having an average calorific value LHVm to be fed to the steam generator in a time t, for example in a week.

[0076] Weekly programming allows for the accumulation in the power plant of all the biomass needed to power the boiler in the following week. This allows stocking up even taking into account holidays or any days on which biomass is not supplied according to the pre-established schedule. Weekly scheduling allows for any missed deliveries to be made up for on a later day. It is therefore possible to prepare a biomass whose average calorific value LHVmsatisfies the relation (II) with respect to the design Lower Calorific Value (LHVp) of the boiler:

[0077] 0.7 • LHVp < LHVm < 1.3 • LHVp (II)

[0078] As mentioned above, it has been found that when the average Lower Heating Value LHVmof the biomass mixture remains within a tolerance range of approximately + / - 30% of the boiler's design Lower Heating Value, the boiler can operate efficiently and effectively.

[0079] C

[0080] This feature consists in calculating the quantity qnof each biomass wnhaving a heating value LHVn necessary to form the quantity Qt of biomass having the average calorific value LHVm, according to relation (III)

[0081] Qt (LHVm) = S qn*LHVn; (III)

[0082] This relationship can be satisfied by different quantities qn of biomass with different calorific value LHVn, provided that the condition imposed by the relationship (II) is respected.

[0083] 0.7 • LHVp < LHVm < 1.3 • LHVp (II)

[0084] In essence, the biomass preparation method involves each supplier supplying a biomass wnwith a calorific value LHVnand in a quantity qnsuch that the sum of these biomasses wnforms the total quantity Qt of biomass required to feed the boiler in the week following the preparation of the biomass itself, with the average calorific value of this biomass satisfying relation (II), i.e. not varying by more than + / - 30% with respect to the design lower calorific value LHVp of the boiler.

[0085] DAccording to this feature, the quantities of biomass qnare discharged directly into the boiler feed tanks and form the total quantity Qt of biomass having calorific value LHVmrequired to feed the boiler in time t.

[0086] It should be noted that the trucks enter the power plant according to the arrow Y and unload the quantities qnof biomass into one or more storage tanks 12, 14, 16, 18 so as to directly form the total quantity Qt of biomass having calorific value LHVm, according to the relation (III)

[0087] Qt (LHVm) = S qn-LHVn; (III)

[0088] without the need for any mixing with mechanical shovels. In fact, the total quantity Qt is formed simply by superimposing the quantities qnunloaded directly from each individual truck into one or more storage tanks 12, 14, 16, 18. This progressive unloading of the different biomasses wnallows the formation of a sufficiently mixed and homogeneous biomass for feeding to the boiler. Furthermore, the subsequent transport from the storage tanks 12, 14, 16, 18 to the boiler feed tank 20, and the transfer from this tank 20 to the boiler involves further mixing and homogenization of the biomass, as described below.

[0089] The quantity Qt of biomass that has been transferred to the feed tank 20 from the storage tanks 12, 14, 16, 18 is suitable to be fed to the steam generator.

[0090] The present disclosure concerns also a method of feeding a biomass in the form of virgin wood chips prepared according to the features (A) - (D) above to the steam generator of a thermoelectric power plant. The method comprises transferring the total quantity Qt of biomass from the storage tanks 12, 14, 16, 18 to the steam generator by means of moving grates, which also perform further mixing of the wood chips.

[0091] In conclusion, the method of the invention allows the logistics office of the power plant to authorize the cutting and supply of only certain quantities qnof biomass wnwith a certain calorific value LHVn(related to the moisture content W of the wood) from certain production sites, based on the type of biomass and the distance from the power plant, in order to directly obtain a mixture with the desired features. This mixture is formed by accumulation directly in the storage tanks without preliminary mixing treatments, thus improving the entire supply chain and the quality of the fuel introduced into the boiler.

[0092] The method therefore allows the boiler to be fed with biomass with a calorific value LHVm that satisfies relation (II), i.e. it does not vary by more than + / - 30% compared to the design lower calorific value LHVp of the boiler. This ensures better efficiency and energy efficiency of the boiler.

[0093] The supply of biomass for the plant is managed on the basis of the thermoelectric power plant's production schedule in terms of MWh / week.EXAMPLE

[0094] The following example, given for illustrative and non-limiting purposes, concerns the preparation of biomass in the form of wood chips for the weekly feeding of the steam generator of a 20 MWe power plant.

[0095] The supply of biomass is managed on the basis of the thermoelectric power plant's production schedule in terms of tons / week. The required amount of biomass is divided among the various suppliers by agreeing on a certain number of daily transports from previously authorised production sites.

[0096] The steam generator was fed with 4500 tons / week of biomass with a calorific value of 8500 kJ / Kg (+ / - 30%) for a weekly total of 38250 MJ.

[0097] Four types of biomass (from wl to w4) were selected from suppliers capable of supplying a certain quantity qnof biomass of various types with calorific value LHVnvarying from 6800 to 11200 kJ / Kg, depending on the type of biomass supplied.

[0098] The total amount to be fed per week was Qn = 38250 MJ.

[0099] The average design calorific value (8500 Kj / kg) = 4500 ton.

[0100] The biomass specifications were as follows:

[0101] Supply Type 1 : Biomass wi with humidity 47.5% and LHVi = 8500 kJ / kg Supply Type 2: Biomass W2 with humidity 45% and LHV2 = 9077 kJ / kg Supply Type 3 : Biomass W3 with humidity 55% and LHV3 = 6800 kJ / kg Supply Type 4: Biomass W4 with humidity 35% and LHV4 = 11200 kJ / kg The weekly supply was:

[0102] Biomass wi = 1000 tons with LHVi = 8500 MJ

[0103] Biomass W2 = 950 tons with LHV2 = 8623 MJ

[0104] Biomass W3 = 2000 tons with LHV3 = 13600 MJ

[0105] Biomass W4 = 700 tons with LHV4 = 7840 MJ

[0106] The total weekly quantity Qt supplied was 38563 MJ, according to relation (III):

[0107] Qt (LHv) = X qn*LHvn

[0108] The biomass was discharged directly into storage tanks 12, 14, 16, 18, which were then fed into the boiler feed tanks 20.

[0109] The transport was carried out using 28-ton dump trucks.

[0110] The following were therefore used:

[0111] 36 trucks to transport biomass wi

[0112] 34 trucks to transport biomass W2

[0113] 75 trucks to transport biomass W325 trucks to transport biomass W2

[0114] The biomass transported by each truck was accumulated one on top of the other in the storage tanks 12, 14, 16, 18, respecting the proportion expressed in relation (III), thus forming the mixture Qt ready to be transferred to the feed tank of the boiler 20. From this tank 20 the biomass was fed to the boiler by means of the moving grate system 22, 24 to the boiler, whose operation and energy yield were optimal for the whole week.

[0115] The method has therefore allowed for a significant improvement in fuel quality and therefore in the efficiency of the power plant, with a reduction in the costs of using the wheel loader and a lower environmental impact, thanks to the reduction in noise and exhaust emissions.

Claims

CLAIMS1. Method for the preparation of a biomass in the form of virgin wood chips to be fed to the steam generator of a thermoelectric power plant, characterized by:A. selecting a plurality of biomasses (wn) from plantations, in which each biomass has a Lower Heating Value (LHVn) calculated from the moisture content (W) with the formula (I);LHVn = ((18.5 • (100-W) - 2.44 • W) / 100) (I);where W is the wood moisture in percentage;B. defining the total quantity (Qt) of biomass having an average Lower Heating Value (LHVm) to be fed to the steam generator in a time t, in which said average Lower Heating Value (LHVm) satisfies the following relation (II) with respect to the design Lower Heating Value (LHVp) of the boiler:0.7 • LHVp < LHVm < 1.3 • LHVp (II)C. calculating the quantity (qn) of each biomass (wn) having a Lower Heating Value (LHVn) necessary to form the quantity Qt of biomass having the average calorific value (LHVm), according to relation (III)Qt (LHVm) = S qn-LHVn; (III)wherein the Lower Heating Value (LHVn) of each biomass (wn) is from 6000 to 12000 kJ / kg;D. obtaining said quantities of biomass (qn) and downloading them into feed tanks of said steam generator to form said total quantity (Qt) of biomass having average Lower Heating Value (LHVm); wherein said total quantity (Qt) of biomass is suitable to be fed to said steam generator.

2. Method according to claim 1, characterized in that said plurality of biomasses (wn) comprises from 2 to 6 biomasses.

3. Method according to claim 1 or 2, characterized in that said plurality of biomasses (wn) comprises 4 biomasses.

4. Method according to one or more of the preceding claims, characterised in that the moisture content of said biomass is from 35% to 65% .

5. Method according to one or more of the preceding claims, characterised in that said quantities (qn) of biomass (wn) are withdrawn from said plantations, transported directly to said thermoelectric power plant are downloaded into one or more storage tanks (12,14, 16, 18) so as to directly form the total quantity (Qt) of biomass having average Lower Heating Value (LHVm), according to said relation (III).

6. Method according to claim 5, characterized in that said quantities (qn) of biomass (wn) are downloaded into one or more of said storage tanks (12, 14, 16, 18) without mixing operations, such that said total quantity (Qt) of biomass having average Lower Heating Value (LHVm) is formed by accumulation of said quantities (qn) of biomass (wn) in said storage tanks (12, 14, 16, 18).

7. Method according to one or more of claims 5 - 6, characterized in that the total quantity (Qt) of biomass is transferred from said storage tanks (12, 14, 16, 18) to one or more boiler feed tanks (20) and is suitable to be transferred from said feed tanks (20) to the boiler by means of a moving grid device, whereby said total quantity (Qt) of biomass is further mixed.

8. Method of feeding a biomass in form of virgin wood chips to the steam generator of a thermoelectric power plant, wherein said biomass is prepared according to the method of one or more of claims 1-7.

9. Method according to claim 8, characterized in that said total quantity (Qt) of biomass is transferred from said feed tanks (20) to said boiler by means of said moving grid device.

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