Small household straw in-situ drying and clean combustion device

Abstract

The application discloses a small household straw in-situ drying and clean combustion device, which is mainly composed of a biomass feeding system, a biomass combustion and heat recovery system and a flue gas treatment system. The core of the device is that the bunker of the biomass feeding system adopts a double-wall structure, and the recirculated flue gas is used to flow in the interlayer to indirectly exchange heat with the straw in the bunker to preheat and dry the straw and improve the calorific value of the straw; the biomass combustion and heat recovery system adopts a reciprocating grate and air staging air distribution technology, and the low-temperature recirculated flue gas after drying the fuel is sent back to the furnace to form a low-nitrogen combustion environment which is coordinated by the recirculated flue gas and the air staging. The application realizes in-situ drying and upgrading of the fuel, significantly improves the combustion efficiency, and significantly reduces the pollutant emission concentration, and is especially suitable for household clean energy utilization of the dispersed straw in rural areas.

Description

Technical Field

[0001] This invention relates to the field of clean utilization technology of biomass energy, specifically a small household straw in-situ drying and clean combustion device, which is specially adapted to the household processing of scattered straw resources in rural areas. It can simultaneously achieve in-situ drying of straw and low-nitrogen clean combustion, meet the basic energy needs of farmers for cooking and heating, and reduce air pollutant emissions. Background Technology

[0002] With the continuous development of agricultural production in my country, the total amount of crop straw resources generated each year is enormous, with a large portion scattered among farmers in rural areas. How to efficiently and cleanly utilize this dispersed biomass resource is key to solving rural energy problems, improving environmental quality, and promoting sustainable agricultural development. However, there are currently significant shortcomings and challenges in the treatment and utilization of straw in rural areas.

[0003] Farmers typically dispose of straw by burning it directly in the open or in traditional household stoves. These methods are primitive and inefficient, with numerous drawbacks: First, their energy utilization efficiency is extremely low, resulting in a serious waste of valuable biomass energy. Second, burning under open or uncontrolled conditions releases large amounts of pollutants, including inhalable particulate matter, carbon monoxide, unburned hydrocarbons, and high concentrations of nitrogen oxides (NOx). x Studies have shown that under typical household stove combustion conditions, the NO produced by burning straw is high. x The emission concentrations are generally no less than 300 mg / Nm³, posing a serious threat to the air quality in local and regional areas.

[0004] While existing technologies have made some progress in large-scale biomass utilization (such as biomass power plants and large industrial boilers), they are difficult to adapt to the dispersed and small-scale household scenarios in rural areas, mainly due to the following two core bottlenecks: First, the inherent physicochemical defects of straw as fuel have not been effectively addressed. Biomass straw is characterized by low energy density and high moisture content, with its net calorific value typically only between 10-15 MJ / kg. This high moisture content not only reduces the fuel's calorific value but also absorbs a large amount of latent heat of vaporization during combustion, leading to lower furnace temperatures, incomplete and unstable combustion, difficulty in improving thermal efficiency, and exacerbating pollutant emissions from incomplete combustion.

[0005] Secondly, existing straw treatment and utilization technologies mostly focus on large-scale centralized collection, transportation, and conversion. This model faces a contradiction between economic efficiency and timeliness in rural areas, particularly in the "last mile" stage. Centralized collection, storage, and transportation require the construction of collection networks, transfer stations, and large processing centers, resulting in high investment and transportation costs. Furthermore, it has poor adaptability to the seasonal production characteristics of straw, making it difficult to achieve economic viability in vast rural areas and failing to meet farmers' needs for "on-site, nearby, small-scale, convenient, and immediate" utilization. Therefore, small-scale clean combustion technologies and equipment specifically designed for dispersed straw, tailored to farmers' specific needs, and capable of achieving in-situ improvement of fuel quality represent a current technological gap and an urgent need.

[0006] Therefore, there is an urgent need in this field to develop a miniaturized device suitable for household use that can achieve on-site (in-situ) drying and upgrading of straw fuel, efficient and clean combustion, and heat recovery, so as to solve the problems of difficult, polluting, and inefficient use of scattered straw in rural areas from the source. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a small-scale household straw in-situ drying and clean combustion device. This device integrates in-situ preheating and drying of materials, low-NOx clean combustion, and deep recovery of flue gas waste heat, aiming to increase the energy density of straw fuel, improve combustion efficiency, and reduce pollutants (especially NOx). x Significant reduction in emissions.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A small-scale household straw in-situ drying and clean combustion device includes a biomass feeding system, a biomass combustion and heat recovery system, and a flue gas treatment system, characterized in that: The biomass feeding system includes a silo and a feeder. The silo has a double-walled structure, consisting of an inner shell and an outer shell, with a sealed annular flue gas channel formed between them. The interior space of the inner shell is used to store crushed biomass straw. The top of the silo is equipped with a silo cover. Medium-temperature recirculated flue gas from the flue gas treatment system is introduced into the annular flue gas channel, flowing through the outer wall of the inner shell and indirectly exchanging heat with the material to preheat and dry it. This raises the material temperature to 80-110℃, reducing its moisture content by 10-20 percentage points and increasing its lower heating value by ≥20%. The dried material is then conveyed to the biomass combustion and heat recovery system via the feeder.

[0009] The biomass combustion and heat recovery system includes a furnace, an upper grate and a lower grate installed within the furnace, a graded air distribution device for supplying air to the furnace and grate, and a flue gas heat recovery unit. The furnace has a furnace door on its side wall or bottom, connected to a feeder at the front end, and an ash collection chamber at the bottom. The upper grate is a reciprocating grate controlled by a drive motor, used to receive and push material from the feeder for initial combustion, maintaining the combustion temperature in the furnace at 700-950℃. After initial combustion, the material falls onto the lower grate for further combustion and complete burning under reciprocating motion, with the final ash falling into the ash collection chamber. The graded air distribution device includes at least two independently controlled blowers and an air distribution chamber. By adjusting the airflow of each blower, air is graded and supplied to different areas of the furnace. Primary air is supplied to the upper area of ​​the upper grate, and secondary air is supplied to the lower grate area and the combustion area behind the upper grate. The ratio of primary air to secondary air is (50%-70%):(30%-50%) to suppress NO. x The generation of heat is described in the following section. The flue gas heat recovery unit comprises a lower rear smoke box, a front smoke box, and an upper rear smoke box connected in sequence, as well as a first smoke pipe bundle and a second smoke pipe bundle installed inside the boiler drum. High-temperature flue gas exits from the furnace flue outlet at the top of the furnace and flows sequentially through the lower rear smoke box, the first smoke pipe bundle, the front smoke box, the second smoke pipe bundle, and the upper rear smoke box, undergoing efficient heat exchange with the working fluid (such as water) inside the boiler drum. A safety valve is installed on the boiler drum.

[0010] The flue gas treatment system includes a main flue gas passage and a recirculating flue gas passage. The main flue gas passage is sequentially connected to the outlet of the upper rear smoke box, the dust removal water tank, the induced draft fan, and the chimney. The recirculating flue gas passage includes a recirculating flue gas inlet, a recirculating flue gas induced draft fan, a recirculating flue, a double-walled annular flue gas channel in the silo, a recirculating flue gas blower, and their connecting pipelines. The recirculating flue gas inlet is located on the main flue and is used to extract medium-temperature flue gas at 200-250℃. The flow rate of the recirculating flue gas induced draft fan is adjustable to achieve a flue gas recirculation rate of 5%-40%. This portion of the flue gas, driven by the recirculating flue gas induced draft fan, flows through the annular flue gas channel in the silo to dry the material, releasing heat and lowering its temperature, before being sent into the furnace by the recirculating flue gas blower to participate in combustion.

[0011] Furthermore, a heat exchanger partition wall can be installed inside the silo to optimize the heat exchange process and flue gas flow path.

[0012] Furthermore, the system is equipped with an observation window for monitoring the combustion status inside the furnace.

[0013] To make the technical effects of the present invention more convincing, calculations were performed on the key component "heat exchanger wall inside the silo" and the core process "flue gas recirculation preheating and drying" to quantitatively illustrate the innovation and beneficial effects of the present invention.

[0014] 1. Design and heat transfer calculation of the heat exchanger partition wall in the silo like Figure 1 As shown, a heat exchanger partition wall (1-3) is installed inside the silo. Its core function is to guide and optimize the flow and heat exchange path of the recirculated flue gas, which enters from the recirculated flue gas preheating inlet (3-4) at a temperature of approximately 220°C, within the annular channel formed by the outer shell (1-2) and the inner shell (1-4) of the silo, maximizing the heat exchange area and ensuring that the biomass fuel stored in the inner shell (1-4) is fully and uniformly preheated and dried.

[0015] Calculation basis and process: Design parameters: Assume the inner shell (1-4) of the silo is cylindrical with a diameter Din = 0.8 m and an effective storage height H = 1.5 m. The heat exchanger partition wall (1-3) inside the silo is designed as a spiral guide plate or annular baffle structure. Its function is to force the flue gas to flow in a spiral or specific deflection path along the outer wall of the inner shell (1-4) in the annular channel, thereby effectively extending the flow path and residence time of the flue gas, enhancing turbulence, and improving the convective heat transfer coefficient.

[0016] Computational Model: This process can be simplified to steady-state heat transfer through the cylindrical wall. The flue gas transfers heat to the outer wall of the inner shell (1-4) via convection, then to the inner wall surface via conduction, and finally to the biomass fuel via convection and partial radiation. The convective heat transfer coefficient h within the annular channel is crucial for calculation, and its value is related to the flue gas velocity, channel geometry, and flue gas properties (Reynolds number Re, Prandtl number Pr). The presence of the heat exchanger partition wall within the silo can significantly increase the effective flow velocity and turbulence of the flue gas, thereby increasing the convective heat transfer coefficient h. Conservatively estimated, after optimized design, the average convective heat transfer coefficient h within the annular channel can reach the range of 30-50 W / (m²·K).

[0017] Heat transfer calculation: The convective heat transfer Qc between the flue gas and the outer wall of the inner shell (1-4) within the annular channel can be estimated using Newton's law of cooling: Qc = h × Ao × ΔT. Where Ao is the effective heat transfer area of ​​the outer wall of the inner shell (1-4) (considering the expansion effect of partition wall 1-3, Ao can reach 1.5-2 times π × Din × H, i.e., approximately 6.0-8.0 m²), and ΔT is the logarithmic mean temperature difference between the flue gas and the outer wall of the inner shell. Assuming the flue gas cools from 220℃ at the inlet to approximately 150℃ at the outlet, and the average temperature of the outer wall of the inner shell is approximately 100℃, then ΔT ≈ 80 K.

[0018] Substitute into the calculation: Qc ≈ 40 W / (m²·K) × 7.0 m² × 80 K ≈ 22,400 W = 22.4 kW.

[0019] Material drying effect calculation: This heat exchange, Qc, is used to heat and dry biomass fuel. Assume the initial wet basis moisture content of the fuel is Min = 30%, the target moisture content is Mfi = 12%, and the processing capacity is mfu = 50 kg / h (approximately 0.0139 kg / s).

[0020] The heat required for water evaporation includes the latent heat of vaporization required to heat the water from its initial temperature (e.g., 20°C) to 100°C and vaporize it. The latent heat of vaporization of water is approximately 2257 kJ / kg. The total heat required to evaporate one kilogram of water is approximately 2500 kJ.

[0021] Water required to evaporate per hour: Δmwater = mfu× (Min - Mfi) / (1 - Mfi)≈ 0.0139 × (0.30-0.12) / (1-0.12) ≈ 0.00284 kg / s.

[0022] The heat power required for evaporation, Qdry, is approximately 0.00284 kg / s × 2500 kJ / kg, which is approximately 7.1 kW.

[0023] Thermal power required to heat solid fuel (specific heat capacity approximately 1.5 kJ / (kg·K)) from 20℃ to 100℃ Qsolid ≈ 0.0139 × 0.88 × 1.5 × 80 ≈ 1.47 kW.

[0024] The total heat required for material heating and drying is Qneed ≈ 7.1 + 1.47 ≈ 8.57 kW.

[0025] Conclusion: The calculated available heat exchange capacity Qc (22.4 kW) is significantly greater than the heat required for material heating and drying, Qneed (8.57 kW). This indicates that, under the stated design parameters and operating conditions, the optimized design of the heat exchanger walls within the silo ensures sufficient heat is provided to the biomass fuel within a reasonable fuel residence time, raising its temperature from ambient to 80-110°C and reducing its moisture content from 30% to below 12%, thereby increasing its lower heating value by more than 20% (verifying the description in the invention). This design effectively solves the problem of conventional silos being used only for storage and having low heat exchange efficiency.

[0026] 2. The impact of flue gas recirculation on combustion and NOx x Impact analysis of emissions As attached Figure 1As shown, a portion of flue gas at a temperature of approximately 220°C is drawn from the main flue in front of the dust removal water tank through the recirculated flue gas inlet and the recirculated flue gas induced draft fan (3-2). After being cooled by preheating fuel in the silo, the flue gas is sent to the recirculated flue gas furnace inlet (3-6) of the furnace (2-1) by the recirculated flue gas blower (3-7).

[0027] Calculation basis and process: Definitions and parameters: The flue gas recirculation rate (R) is defined as the ratio of the recirculated flue gas volumetric flow rate (Vrc) to the total flue gas volumetric flow rate (Vtotal) at the furnace outlet, i.e., R = Vrc / Vtotal. In this invention, R is adjustable, ranging from 5% to 40%.

[0028] Effect on Combustion Temperature: Recirculated flue gas (mainly composed of N2, CO2, and H2O) is added to the combustion zone as an inert medium, not participating in chemical reactions, but absorbing heat, thus reducing the adiabatic flame temperature (Tad). Based on energy conservation and simplified calculations using a one-dimensional combustion model, flue gas recirculation of R=25% can reduce the adiabatic flame temperature of typical straw combustion by approximately 100-150℃. The reduction in adiabatic flame temperature (Tad) can significantly inhibit thermal NO production. x The formation rate of thermal NO x The generation rate is exponentially related to temperature (exp(-E / RT)).

[0029] Impact on oxygen concentration: The introduction of recirculated flue gas dilutes the oxygen concentration in the combustion zone. The original primary air oxygen concentration is 21% (volume fraction). After mixing with recirculated flue gas, the oxygen concentration of the mixture delivered to the combustion zone decreases to approximately CO2mix = 0.21 / (1 + α×R) (simplified model, where α is a correction factor considering temperature and density changes, typically slightly greater than 1). For example, when R = 25% and α ≈ 1.1, CO2mix ≈ 17.1%. This reduction in local oxygen concentration also helps suppress fuel-type NO. x and thermal NO x The generation of .

[0030] NO xQuantitative estimation of emission reduction: Combining the dual effects of air staging (introducing 60%-70% of the total air volume as primary air from under the grate to form a fuel-rich reduction zone, inhibiting the conversion of NOx precursors to NO) and flue gas recirculation (R=15%-25%), estimates can be made based on empirical formulas and existing experimental data. For biomass combustion, air staging can reduce NOx emissions by approximately 20%-30%, and flue gas recirculation (R=20%-30%) can further reduce NOx by approximately 15%-25%. The synergistic effect of both can achieve a total NOx emission reduction of ≥35%, or even higher. Therefore, the scheme described in this patent can stably reduce the NOx emission concentration from straw combustion from ≥300 mg / Nm³ using traditional methods to <200 mg / Nm³, demonstrating significant environmental benefits.

[0031] 3. System Integration and Energy Utilization Analysis like Figure 1 As shown, the energy utilization of this invention achieves three-stage cascade utilization: High-grade heat energy recovery: The high-temperature flue gas of about 850-950℃ from the outlet of the furnace (2-1) flows sequentially through the lower smoke box (2-10), the first smoke tube bundle (2-13), the front smoke box (2-11), the second smoke tube bundle (2-14), and the upper smoke box (2-12), and exchanges heat efficiently with the water in the boiler drum (2-15). This is the main heat generation stage, and most of the sensible heat in the flue gas is recovered.

[0032] Medium-grade waste heat upgrading fuel: A portion (volume flow rate percentage R) of the medium-temperature flue gas (approximately 200-250℃) exiting from the rear upper smoke box (2-12) is drawn off and used for preheating and drying the fuel in the silo. This process recovers some of the medium- and low-temperature waste heat from the flue gas and converts it into improved fuel quality (calorific value), thereby increasing the efficiency of subsequent combustion and indirectly improving energy utilization.

[0033] End-of-pipe purification and emission: After the first two stages of utilization, the remaining part of the flue gas enters the dust removal water tank (3-8) for washing and dust removal, and finally cools down to a safe range above the dew point temperature (e.g., >120℃) before being discharged through the chimney (3-11).

[0034] Simplified energy balance accounting: Assume the input thermal power of straw fuel is Qin (kW).

[0035] The primary heat exchanger (boiler drum) recovers heat power (Qboiler), which can typically reduce the flue gas temperature to about 220°C. At this stage, about 65%-75% of the fuel input heat is recovered.

[0036] The secondary heat exchange (silo drying) utilizes a portion of the heat Qdry (i.e., the 8.57 kW required for drying the aforementioned material) from the recirculated flue gas. If this heat is not utilized, it will be discharged into the dust removal system with the flue gas. Through this stage of utilization, the effective energy output of the entire system is improved, and the overall thermal efficiency can be increased by 5-10 percentage points.

[0037] Ultimately, the system's overall thermal efficiency (output effective heat / fuel input heat) can reach over 80%, far exceeding that of traditional household straw direct combustion methods (typically <60%).

[0038] Therefore, the core innovation and beneficial effects of this invention are as follows: (1) Synergistic optimization of in-situ fuel energy density enhancement and low-NOx combustion: Innovatively, waste heat drying of flue gas is deeply coupled with low-NOx combustion technology. Through a dedicated recirculated flue gas passage, medium-temperature flue gas at 200-250℃ is introduced into the double-layer wall of the silo for indirect heat exchange with straw, which can heat the material to 80-110℃, significantly reducing its moisture content by 10-20 percentage points and increasing the lower heating value by ≥20%, fundamentally solving the pain points of low energy density and unstable combustion of straw fuel. The high-calorific-value fuel after drying works synergistically with the subsequent low-NOx combustion measures to jointly improve combustion efficiency and reduce the basis for pollutant formation.

[0039] (2) Two-stage low-NOx combustion and adjustable parameters: It creatively combines two low-NOx combustion technologies, "staged air distribution" and "flue gas recirculation," and gives key operating parameters adjustableness. Through an independently controlled staged air distribution device, the combustion air is delivered in stages (primary air ratio 50%-70%, secondary air ratio 30%-50%), destroying NO. x A high-temperature, oxygen-rich environment is generated. Simultaneously, by adjusting the recirculated flue gas fan, a flue gas recirculation rate of 5%-40% is achieved, utilizing inert gas to dilute the oxygen concentration in the reaction zone and reduce the flame temperature. This dual-pronged approach of "air staging" and "flue gas recirculation," with adjustable parameters, allows the system to optimize operation according to different fuel characteristics and load requirements (such as continuous heating or intermittent cooking). Through synergistic action, it can reduce NOx emissions from straw combustion. x The emission concentration is reduced by ≥35% compared to traditional household combustion methods (≥300 mg / Nm³), and is stably controlled below 200 mg / Nm³.

[0040] (3) Cascaded Energy Utilization and High System Integration: The system achieves efficient three-stage utilization of flue gas heat. High-temperature flue gas (>700℃) is first used to recover high-grade heat energy in the boiler drum for heat generation; medium-temperature flue gas (200-250℃) is used to preheat and dry fuel, improving its quality; and finally, low-temperature flue gas is discharged after being purified by wet dust removal. The entire system highly integrates pretreatment, combustion, heat recovery, and pollution control into a compact device. In particular, the double-walled heat exchange structure of the silo achieves efficient gas-solid heat exchange and fuel storage in a limited space, meeting the core requirements of household scenarios for equipment miniaturization, convenient operation, and efficient energy utilization, and realizing the "on-site drying and on-site clean utilization" of scattered straw.

[0041] (4) Flexibility and wide adaptability to fuels: By adjusting the feeding rate, the proportion of staged air distribution, and the flue gas recirculation rate, this device can flexibly adapt to various straw fuels with different moisture contents (20%-35%), and can maintain efficient and stable operation under various working conditions such as high-load continuous operation (e.g., winter heating) and medium- and low-load intermittent operation (e.g., cooking). For example, during high-load continuous operation, a higher recirculation rate (e.g., 25%) and a specific air volume ratio can be used to achieve deep drying and efficient low-NOx combustion; during intermittent operation, a rapid start-up strategy and a variable recirculation rate can be adopted to balance start-up speed and fuel pretreatment effect. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the system structure of the small-scale household straw in-situ drying and clean combustion device of the present invention.

[0043] Explanation of the labels in the diagram: 1-Biomass feeding system: 1-1Bag cover, 1-2Bag shell, 1-3Bag heat exchanger partition wall, 1-4Bag shell (inner shell), 1-5Feeder.

[0044] 2-Biomass Combustion and Heat Recovery System: 2-1 Furnace, 2-2 Upper Grate, 2-3 Lower Grate, 2-4 Furnace Door, 2-5 Staged Air Distribution Blower 1, 2-6 Staged Air Distribution Blower 2, 2-7 Air Distribution Chamber, 2-8 Observation Window, 2-9 Furnace Flue Opening, 2-10 Rear Lower Smoke Box, 2-11 Front Smoke Box, 2-12 Rear Upper Smoke Box, 2-13 First Smoke Pipe Bundle, 2-14 Second Smoke Pipe Bundle, 2-15 Boiler Drum, 2-16 Safety Valve, 2-17 Ash Collection Chamber.

[0045] 3-Flue Gas Treatment System: 3-1 Recirculated Flue Gas Inlet, 3-2 Recirculated Flue Gas Exhaust Fan, 3-3 Recirculated Flue, 3-4 Recirculated Flue Gas Preheating Inlet, 3-5 Recirculated Flue Gas Preheating Outlet, 3-6 Recirculated Flue Gas Furnace Inlet, 3-7 Recirculated Flue Gas Blower, 3-8 Dust Removal Water Tank, 3-9 Flue, 3-10 Exhaust Fan, 3-11 Chimney. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, in the following embodiments, unless otherwise specified, the operations and parameter settings involved (such as recirculation rate, air volume ratio, etc.) can be adjusted and controlled through manual valves, frequency converters, or control systems.

[0047] like Figure 1 As shown, the small-scale household straw in-situ drying and clean combustion device provided by the present invention mainly consists of three parts: a biomass feeding system 1, a biomass combustion and heat recovery system 2, and a flue gas treatment system 3, with their internal processes closely connected.

[0048] The core operating logic of this invention's device lies in the following: before entering the furnace, the wet straw is preheated and dried in a specially designed hopper by medium-temperature flue gas (200-250℃) from the system itself, thereby improving the combustion quality of the fuel (increasing the lower heating value and reducing the moisture content); in the furnace, a low-NOx combustion technology combining air staging and flue gas recirculation is adopted to ensure efficient and clean combustion; the high-temperature flue gas generated by combustion first deeply recovers heat in the boiler drum, and part of the waste heat it carries is used to preheat the fuel, while the remainder is discharged after purification, realizing the cascade utilization of energy. The following examples of two typical operating conditions will provide a detailed description of the operating mode, including specific operating parameters (such as recirculation rate, staging air distribution strategy, fuel moisture content, and changes in calorific value).

[0049] Background: This system provides daily heating and domestic hot water supply for a farmer in northern China during winter. It requires continuous 24-hour operation and has a high heat load demand. The fuel used is corn stalks harvested that year, with a relatively high initial wet basis moisture content (received basis), approximately 30-35%.

[0050] System operating parameters and steps: The user fills the inner shell (1-4) of the hopper with crushed corn stalks with an initial wet basis moisture content of approximately 32%. Its initial lower heating value (LHV) is approximately 11.5 MJ / kg. Close the hopper cover (1-1).

[0051] Start the main induced draft fan (3-10) to establish negative pressure in the flue. Start the staged air distribution blowers 1 (2-5) and 2 (2-6). Set the staged air distribution strategy as follows: primary air (supplied by blower 2-5 through the upper nozzle of upper grate 2-2) accounts for 60% of the total air volume, and secondary air (supplied by blower 2-6 through the lower grate 2-3 area) accounts for 40%, aiming to establish a fuel-rich reduction zone in the lower part of the furnace and suppress NO. x generate.

[0052] Manually ignite the ignition material at the front end of the grate on the furnace (2-1) to start combustion.

[0053] After the system has been running for about 20 minutes, and the flue gas temperature at the outlet of the upper smoke box (2-12) has stabilized at about 220℃, start the recirculated flue gas induced draft fan (3-2) and the recirculated flue gas blower (3-7). Set the flue gas recirculation rate (the ratio of recirculated flue gas volume to main flue gas volume) to 25%.

[0054] Flue gas at approximately 220°C is drawn out from the inlet (3-1) on the main flue (3-9) and fed into the annular channel of the double-walled silo (between 1-2 and 1-4) via the recirculation flue (3-3) and inlet (3-4). The flue gas flows within the channel for approximately 3-5 minutes, undergoing thorough indirect heat exchange with the straw inside the inner shell (1-4). After this preheating and drying process, the straw is heated to 95±5°C, its wet basis moisture content drops below 12%, and its lower heating value (LHV) increases to approximately 13.8 MJ / kg, representing a 20% increase in calorific value.

[0055] The feeders (1-5) are automatically controlled according to the indoor temperature, and feed the dried high-calorific-value straw into the furnace at a relatively fast speed (e.g., once every 15-20 minutes).

[0056] Inside the furnace, pre-dried fuel is mixed with primary air and ignited and combusts violently on the reciprocating upper grate (2-2). Recirculated flue gas (approximately 150°C) from (3-6) is mixed with secondary air and then fed into the upper part of the combustion zone. This configuration achieves the coupling of "air staging" and "flue gas recirculation": staging reduces the oxygen concentration and temperature in the main combustion zone, while recirculated flue gas further dilutes the oxygen partial pressure in the combustion zone and reduces the peak flame temperature. Together, these factors significantly suppress thermal NOx. x The generation process is as follows. In this mode, the furnace temperature is stabilized at approximately 850-900℃.

[0057] High-temperature flue gas flows sequentially through the furnace flue inlet (2-9), the lower rear smoke box (2-10), the first smoke tube bundle (2-13), the front smoke box (2-11), the second smoke tube bundle (2-14), and the upper rear smoke box (2-12), transferring most of the heat to the water in the boiler drum (2-15).

[0058] After heat exchange, the flue gas (temperature approximately 200℃) enters the main flue (3-9). Approximately 25% of the flue gas volume (i.e., a recirculation rate of 25%) is drawn out by the recirculated flue gas induced draft fan (3-2) for drying the silo. The remaining approximately 75% of the flue gas volume enters the dust removal water tank (3-8) for water bath dust removal and cooling, and is finally discharged by the main induced draft fan (3-10) through the chimney (3-11) at a flue gas temperature of approximately 120℃.

[0059] Operational Results: Under this high-load continuous operating condition, the system's heat output remained stable. Calculations show that the boiler's thermal efficiency exceeded 82%. Due to thorough fuel drying (moisture content reduced from 32% to <12%), combustion was stable and complete. Furthermore, the adoption of coupled low-NOx technology resulted in low NOx levels in the flue gas. x The emission concentration is stable at 160-180 mg / Nm³, which is more than 40% lower than that of traditional household combustion (≥300 mg / Nm³).

[0060] Operating conditions: During spring and autumn, the system operates for approximately 1.5-2 hours each in the morning, noon, and evening for cooking. The fuel is a mixture of wheat straw and weeds, with an initial uneven moisture content, ranging from approximately 20-28%.

[0061] System operating parameters and steps: About 30-40 minutes before each cooking session, the user adds the mixed fuel with an average wet base moisture content of about 25% to the hopper, with an initial LHV of about 12.2 MJ / kg.

[0062] The startup process is the same as in Example 1, but the tiered air distribution strategy is adjusted to adapt to lower loads: the proportion of primary air (supply of 2-5) is increased to 70%, and the proportion of secondary air (supply of 2-6) is reduced to 30% to maintain a thicker fuel layer and a stable firebed.

[0063] Start the main exhaust fan (3-10) and the staged air distribution blower, and set the air volume to "low level".

[0064] Due to the intermittent operation of the system, the flue gas temperature in the main flue rises rapidly during the initial startup phase. When the flue gas temperature at the outlet of the upper flue gas box (2-12) reaches 200℃, the recirculated flue gas induced draft fan (3-2) and blower (3-7) are immediately started. The flue gas recirculation rate is set at 15% during this stage.

[0065] The recirculated flue gas at approximately 200°C rapidly preheats the fuel in the hopper via convection. Because the fuel's residence time in the hopper is relatively short during cooking (approximately 30 minutes), the fuel is quickly heated to 70-80°C, reducing its moisture content to approximately 18% and increasing its lower heating value to approximately 13.4 MJ / kg, an increase of about 10%. This effectively solves the problems of smoke and unstable combustion during the initial ignition of wet fuel.

[0066] The feeder (1-5) manually or intermittently automatically controls the feeding amount according to the cooking fire power requirements, and the furnace temperature is maintained at about 750℃.

[0067] After cooking, turn off the feeder. Once the remaining fuel on the grate has burned out, first turn off the staged air distribution blower. Continue running the main induced draft fan (3-10) and the recirculated flue gas induced draft fan (3-2) for 10-15 minutes. During this time, the flue gas recirculation rate can be manually or automatically increased to 30-40%, utilizing the system's waste heat and a higher proportion of recirculated hot flue gas to deeply and slowly penetrate and dry the remaining fuel in the silo prepared for the next meal. This process further reduces the fuel's moisture content, creating better conditions for the next startup. After this stage, turn off all fans.

[0068] Operational Results: Under this intermittent operating condition, the system demonstrated excellent responsiveness and adaptability. By adjusting the recirculation rate (15% during operation and 30-40% during the drying period after shutdown) and airflow ratio, rapid start-up and stable cooking were achieved. The fuel was effectively preheated in a short time (moisture content decreased from 25% to 18%), and combined with low-NOx combustion technology, the cooking process produced no black smoke and low NOx emissions. x The emission concentration was below 200 mg / Nm³. Post-shutdown waste heat drying further optimized fuel quality, demonstrating the system's potential for intelligent and refined operation.

[0069] The above embodiments detail how, under different load demands, by adjusting key parameters such as the flue gas recirculation rate (e.g., 25% for continuous high loads, 15% for intermittent low loads supplemented with high-proportion drying after shutdown) and optimizing the staged air distribution strategy (adjusting the ratio of primary and secondary air to adapt to different combustion intensities), the effective control of fuel moisture content (e.g., drying from 32% to <12%, or rapidly drying from 25% to 18%) and calorific value (increase by 10%-20%) is achieved, ultimately reaching the goal of high-efficiency and clean combustion. This verifies the significant advantages of the device of the present invention in terms of adjustable parameters and strong adaptability to operating conditions. The purpose of the present invention is to overcome the shortcomings of the prior art and provide a small-scale household straw in-situ drying and clean combustion device. This device integrates in-situ preheating and drying of materials, low-NOx clean combustion, and deep recovery of flue gas waste heat, aiming to improve the energy density of straw fuel, increase combustion efficiency, and reduce pollutants (especially NO). x Significant reduction in emissions.

Claims

1. A small-scale household straw in-situ drying and clean combustion device, comprising a biomass feeding system (1), a biomass combustion and heat recovery system (2), and a flue gas treatment system (3), characterized in that: The biomass feeding system (1) includes a silo and a feeder (1-5); the silo adopts a double-wall structure, including an inner shell (1-4) and an outer shell (1-2), and a closed annular flue gas channel is formed between the inner shell (1-4) and the outer shell (1-2); the internal space of the inner shell (1-4) is used to store biomass straw materials, and the annular flue gas channel is connected to the recirculated flue gas passage of the flue gas treatment system (3) to introduce medium-temperature recirculated flue gas to indirectly exchange heat with the material, thereby achieving preheating and drying of the material; The biomass combustion and heat recovery system (2) includes a furnace (2-1), an upper grate (2-2) and a lower grate (2-3) disposed in the furnace (2-1), a graded air distribution device for supplying air to the furnace, and a flue gas heat recovery unit; the graded air distribution device is configured to deliver air into different areas of the furnace (2-1) in stages; the flue gas heat recovery unit includes a boiler drum (2-15) and a bundle of flue pipes disposed inside it for recovering heat from high-temperature flue gas; The flue gas treatment system (3) includes a main flue gas passage and a recirculated flue gas passage; the main flue gas passage is used to purify the flue gas after combustion and lead it to the chimney (3-11) for discharge; the recirculated flue gas passage includes a recirculated flue gas inlet (3-1), a recirculated flue gas induced draft fan (3-2), an annular flue gas passage of the silo, a recirculated flue gas blower (3-7), and a recirculated flue gas furnace inlet (3-6) leading to the furnace (2-1), which is used to lead part of the flue gas back to the silo to dry the material and then send it back to the furnace (2-1).

2. The small-scale household straw in-situ drying and clean combustion device according to claim 1, characterized in that: The annular flue gas passage of the silo is provided with a heat exchanger partition wall (1-3), which is a spiral guide plate or annular baffle structure, used to guide and optimize the flow path of the recirculated flue gas in the annular passage and enhance heat exchange.

3. The small-scale household straw in-situ drying and clean combustion device according to claim 1, characterized in that: The flow rate of the recirculating flue gas induced draft fan (3-2) is adjustable to achieve a flue gas recirculation rate of 5%-40%.

4. The small-scale household straw in-situ drying and clean combustion device according to claim 1, characterized in that: The graded air distribution device includes at least two independently controlled blowers (2-5, 2-6) and an air distribution chamber (2-7). By adjusting the air volume of each blower, the primary air and secondary air are supplied in a ratio, wherein the primary air volume accounts for 50%-70% of the total air volume and the secondary air volume accounts for 30%-50% of the total air volume.

5. A small-scale household straw in-situ drying and clean combustion device according to claim 4, characterized in that: The primary air is mainly supplied to the lower grate (2-3) area, and the secondary air is mainly supplied to the furnace area above the upper grate (2-2).

6. The small-scale household straw in-situ drying and clean combustion device according to claim 1, characterized in that: The upper grate (2-2) is a reciprocating grate that can be controlled by a drive motor.

7. A small-scale household straw in-situ drying and clean combustion device according to claim 1, characterized in that: The flue gas heat recovery unit includes a first flue gas tube bundle (2-13) and a second flue gas tube bundle (2-14). The flue gas heat recovery unit also includes a rear lower smoke box (2-10), a front smoke box (2-11), and a rear upper smoke box (2-12) connected in sequence. After the high-temperature flue gas is discharged from the furnace flue outlet (2-9), it flows in sequence through the rear lower smoke box (2-10), the first flue gas tube bundle (2-13), the front smoke box (2-11), the second flue gas tube bundle (2-14), and the rear upper smoke box (2-12).

8. A small-scale household straw in-situ drying and clean combustion device according to claim 1, characterized in that: The recirculated flue gas inlet (3-1) is located on the main flue (3-9) after the outlet of the flue gas heat recovery unit and before the dust removal water tank (3-8), and is used to extract medium-temperature flue gas at 200-250℃.

9. A combustion control method based on the device according to any one of claims 1-8, characterized in that, Includes the following steps: Based on the operating load requirements, the flue gas recirculation rate, the staged air distribution ratio, and the feeding rate are set. After the system starts, once the flue gas temperature reaches the set value, the recirculated flue gas passage is activated to preheat and dry the biomass fuel in the silo using the recirculated flue gas. The dried fuel is then fed into the furnace and undergoes low-NOx combustion in conjunction with the staged air distribution and the recirculated flue gas. The high-temperature flue gas generated by combustion first recovers high-grade heat energy through the flue gas heat recovery unit, and then part of the flue gas is drawn back for fuel drying, while the remaining flue gas is purified before being discharged.

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