Research Progress on Nitrogen Oxide Emission Control Technology for Biomass Boilers

       The fuel nitrogen content in biomass fuel is relatively low, but about 70% to 100% (by mass) of the nitrogen will eventually be converted to NOx. Moreover, the NOx emissions from burning biomass fuels such as straw are higher than those from woody fuels. In recent years, the air quality in our country has faced severe challenges, and NOx is a common atmospheric pollutant that significantly impacts residents' health, production, and daily life. Therefore, this paper reviews the current NOx combustion control technologies both domestically and internationally, summarizes the advantages and disadvantages of various technologies, discusses the bottlenecks faced by our country in NOx control technology for biomass boilers, and looks forward to future trends in this research field.

       Currently, although the main fuels in the world are traditional conventional energy sources such as coal, oil, and natural gas, the proven reserves of oil, natural gas, and coal resources that can be mined globally will be exhausted in 25, 27, and 97 years, respectively. In addition, the extraction, transportation, and use of fossil fuels can cause serious environmental pollution. With the increasing depletion of fossil energy and the worsening environmental issues, the development and utilization of clean renewable energy have attracted widespread attention worldwide. Biomass energy is the only renewable green energy that can be transported and stored, and due to its high energy conversion efficiency and environmental friendliness, it is receiving increasing attention from countries around the world. Our country is a major agricultural nation, and there is a shortage of firewood resources; therefore, mature woody biomass fuel burners from abroad are not suitable, and it is necessary to develop suitable utilization methods and combustion technologies based on our national conditions. Furthermore, in recent years, the air quality in our country has faced severe challenges. Compared to woody fuels and traditional fossil fuels, the NOx emissions from burning biomass fuels such as straw are relatively high. Therefore, traditional coal boilers are not entirely suitable for burning biomass fuels, and there is an urgent need to develop efficient biomass fuel burners to reduce NOx pollutant emissions while achieving high energy efficiency.

1 Current Research Status of Biomass Resources

1.1 Current Status of Biomass Resources in Our Country

       Biomass energy is a form of energy that uses biomass as a carrier, which converts the solar energy obtained through photosynthesis into chemical energy stored in biomass. It directly or indirectly originates from the photosynthesis of green plants and is a cheap way to store solar energy. Under the current technological level, the rural biomass resources with development value generally include crop straw, forest biomass residues, livestock manure, and energy crops. From 2001 to 2015, the total agricultural output in our country increased year by year, with the total agricultural output in 2015 being 2.24 × 10^19 J, of which the planting industry output was 1.53 × 10^19 J (68.47%), and the outputs of forestry, animal husbandry, and fishery were 4.64 × 10^18, 2.04 × 10^18, and 3.89 × 10^17 J, respectively. Among these, the resources available for agricultural biomass energy reached as high as 9.10 × 10^18 J.

       However, the quality of biomass resources in our country is relatively low, and they are highly dispersed. A large amount of biomass resources are randomly landfilled and burned, becoming waste that affects the environment. Currently, the biomass resource utilization rate is highest for agricultural by-products such as crop straw, which has been used as industrial raw materials and biofuels, totaling 4.29 × 10^18 J, accounting for 27.79% of the planting industry output. Among these, the output of crop straws such as corn, rice, and wheat accounts for 70.24% of the total straw output. However, most straw resources are still directly burned or randomly discarded, with only a very small portion being used in boilers for centralized combustion, with combustion efficiency of less than 15%.

1.2 Characteristics of Biomass Fuels

       Biomass energy is a renewable energy source with zero carbon emissions in ecological terms, and its combustion products are relatively clean. Due to the current severe air pollution and the increasing shortage of energy, the development and utilization of biomass energy have significant strategic importance for energy and environmental protection. Currently, our country has not issued a unified national standard for biomass solid molded fuels. The commonly recognized biomass solid molded fuels refer to the use of agricultural and forestry waste (such as rice husks, straw, bark, wood chips, etc.) as raw materials, which undergo a series of pretreatments (collection, drying, crushing, etc.) and are then pressed into regular, high-density shapes such as rods, blocks, or pellets using special biomass solid molding equipment. Through research and literature comparison, Table 1 summarizes the parameters of various typical biomass fuels and traditional coal in terms of industrial components, elemental composition, calorific value, etc.

Biomass fuels have a high calorific value, usually ranging from 17 to 20 MJ·kg - 1, and the volatile matter content of straw and rice husks can reach 70% to 85%, giving them excellent ignition and combustion performance as well as good coal substitution effects. In addition, the sulfur content of biomass fuels is almost zero, the nitrogen content is extremely low, and the ash content is relatively low. Therefore, the emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from biomass fuels are low, with zero carbon dioxide (CO2) emissions, minimal slag, and fly ash, and the ash can be returned to the field, thus exhibiting significant environmental protection characteristics. Although the production of biomass molded fuels requires processes such as collection, transportation, and processing, which incur certain costs, the price of biomass raw materials is low compared to current raw coal and molded coal. Therefore, biomass molded fuels still have a significant price advantage, which greatly contributes to the promotion and use of biomass energy.

       Although biomass fuel is clean and renewable, with vast application potential, achieving efficient and clean combustion of biomass fuel requires specially designed burners.  Since the 1930s, many developed countries such as the United States, Japan, and Finland have gradually attached importance to and invested a lot of effort in researching biomass molding technology and wood molding fuel; after the 1980s, molding technology has matured and formed a certain scale of promotion; by the 1990s, in terms of wood pellet fuel burners, some burners in the United States, Europe, and Japan have gradually taken shape, and this technology has been widely promoted and applied in heating, heating, drying, and other fields, forming an industrial scale. In terms of China's actual situation, China is an agricultural country with rich biomass resources, and biomass energy has always been one of the main energy sources in rural areas. In China, more than 700 million tons of crop straw can be produced annually. Research shows that the direct combustion value of straw is about 55% of standard coal. Based on the calorific value, if all of it is used for combustion, it can be converted to about 400 million tons of standard coal. However, in China, there are many problems with straw utilization, such as backward utilization and significant pollution. In addition, biomass combustion abroad mainly uses wood materials, while in China, it mainly uses crop straw. Therefore, based on the national conditions of biomass combustion in China, foreign experiences and technologies cannot be directly copied.

1.3 Utilization Methods of Biomass Fuel

1.3.1 Direct Combustion Technology

       Direct combustion technology of biomass refers to burning biomass directly as fuel to utilize the heat energy generated for production and daily life. The technical requirements for direct combustion are relatively low, and the combustion method is the simplest. Research by Luo Zhongyang and others shows that since the 1980s, with the strong promotion of the Chinese government, energy-saving stoves have been widely used in rural areas. By the end of 1996, 170 million households were using energy-saving stoves, and the promotion of energy-saving stoves can reduce the country's energy consumption by several million tons of standard coal each year. However, according to Zhang Bailiang's research, old stoves not only have low thermal efficiency (usually only about 10%) and waste fuel, but also produce particulate matter, sulfur oxides, nitrogen oxides, and other pollutants during the combustion process, which can severely pollute the environment. Therefore, this traditional biomass combustion method has low combustion efficiency, wastes a lot of energy and resources, and pollutes the environment.

1.3.2 Boiler Combustion Technology

       With the gradual improvement of boiler combustion technology, it has now become an advanced biomass combustion technology.  This technology uses boilers as biomass burners, using biomass as fuel for boiler combustion, and improves the utilization efficiency of biomass by controlling the combustion conditions of the fuel in the boiler. Compared to direct combustion technology, boiler combustion technology is more suitable for centralized and large-scale utilization of biomass resources.  However, due to the complex structure of the boiler and the numerous control parameters, the requirements for using this technology are relatively high. Abroad, fluidized bed technology is a widely adopted biomass boiler combustion technology. Since it developed earlier abroad, this technology has already reached a considerable scale, and there are many mature experiences in its operation.

      Currently, there are mainly two types of biomass boilers, namely water-cooled vibrating furnaces that burn pure biomass and circulating fluidized bed boilers that co-fire biomass. Among them, the circulating fluidized bed boiler injects fuel with a particle size of 6 to 12 mm and limestone into the furnace or combustion chamber. The particles are suspended under the action of an upward flow of air (accounting for 60% to 70% of the total air volume). Secondary air enters the combustion chamber from above the boiler, promoting the complete combustion of biomass particles, with a combustion temperature of about 840 to 900°C. The water-cooled vibrating furnace throws the fuel up through periodic vibrations, jumping forward while burning, and the ash is also discharged from the end of the grate during this process.

       However, the water-cooled vibrating furnace has poor adaptability to fuel, low combustion efficiency, and high requirements for the moisture content of the fuel, making it relatively expensive. In contrast, the circulating fluidized bed boiler for co-firing biomass has lower construction costs, strong fuel adaptability, is more suitable for co-firing fuels, operates safely, has excellent environmental performance, and a wide load range. Therefore, considering the current situation of biomass energy in China, using circulating fluidized bed co-firing biomass is more suitable for China's national conditions.

       In addition, chain grate furnaces and reciprocating grate furnaces can also be suitable for the staged combustion of biomass fuel, but both have their own advantages and disadvantages. The grate plates of chain grate furnaces can be cooled in a cycle, while reciprocating grate furnaces have more significant advantages in terms of fuel size requirements and fuel leakage compared to chain grates. Therefore, a combined grate furnace has gradually developed, which uses two different grates in the combustion equipment, for example, the front grate is an inclined reciprocating grate, increasing the volume of the combustion chamber, which is beneficial for the burnout of volatile components, and the rear grate connects to a heavy scale grate. Combined grates have better adjustment performance than single grates, allowing for more complete combustion. However, the combustion of biomass fuel in these types of boilers is still in the exploratory stage, and the technology needs further improvement and maturation.

1.3.  Current Problems of Biomass Boilers

       Currently, there are some problems with biomass boiler combustion technology. For example, to ensure that the fuel is in a fluidized state in the fluidized bed during specific combustion processes, it is necessary to strictly control the particle size of the boiler feed. To solve this problem, it is necessary to carry out preprocessing steps such as drying and crushing to homogenize the biomass fuel in terms of size, state, etc. Furthermore, when using biomass with lower density, loose structure, and poor heat storage capacity, such as rice husks and wood chips, to maintain the heat storage material required for normal combustion, it is also necessary to continuously add materials such as quartz sand to the fuel during combustion. This can lead to the production of hard-textured biomass fly ash during combustion, which can easily cause wear on the heated surfaces of the boiler. The addition of quartz sand and other additives makes further processing and utilization of ash more difficult.

       Currently, based on the direct combustion of biomass raw materials, biomass solidified molding fuel has been developed, which is formed into granular, block, or rod shapes through specialized equipment under certain temperature and pressure conditions. By preparing molded fuel, the storage and transportation capacity of biomass fuel can be improved, combustion performance can be enhanced, utilization efficiency can be increased, and the application range can be greatly expanded, making it a new type of clean and environmentally friendly biomass fuel that can partially replace coal and other fossil fuels, reducing dependence on traditional fossil energy. In China, the molded fuel prepared from straw using solidification technology can be used as industrial fuel for power generation and heating, as well as fuel for cooking and heating for rural residents.

2 Current Situation of Nitrogen Oxide Emissions in China

2.1 Mechanism of NOx Generation from Fuel Combustion

       NOx is a common atmospheric pollutant that has a significant impact on residents' health and production and daily life. Since coal is the main fuel in our country, 67% (by mass fraction) of NOx in the atmosphere comes from coal combustion emissions. The generation of NOx during fuel combustion mainly comes from three pathways: thermal NOx, fuel NOx, and prompt NOx. Thermal NOx accounts for about 20% of the NOx generated from fuel combustion and is formed by the oxidation of nitrogen in the air at high temperatures. The generation of thermal NOx is significantly influenced by temperature; studies have shown that when the combustion temperature is below 1300 °C, thermal NOx generation is almost undetectable; while when the combustion temperature exceeds 1300 °C, the amount of thermal NOx generated increases significantly. Fuel NOx refers to nitrogen compounds in the fuel that undergo pyrolysis during combustion and then oxidize to form nitrogen oxides. The generation of fuel NOx accounts for about 75% to 90% of the mass fraction of NOx produced from fuel combustion, and its generation mechanism is very complex, mainly formed by the oxidation of volatile nitrogen and coke nitrogen (Figure 1(a)). The generation of NOx is influenced not only by the type and structure of the fuel but also by combustion conditions such as concentration, temperature, and fuel composition. Prompt NOx is generated from hydrocarbon radicals (CH) in the fuel at high temperatures, which collide with N2 molecules in the air, leading to the formation of HCN compounds. The generated HCN is further oxidized to produce prompt NOx (Figure 1(b)). The amount of prompt NOx generated is very small, occurring only under low oxygen concentration conditions, and its generation process is not constrained by temperature factors, accounting for less than 5% (by mass fraction) of the total NOx generated.

       A portion of the NOx released from fuel combustion comes from the fuel nitrogen contained in the fuel itself, but compared to coal fuel, biomass fuel has a lower nitrogen content. Although the nitrogen content in biomass fuel is relatively low, studies have reported that about 70% to 100% of the fuel nitrogen will eventually be converted to NO during combustion. Our country is a major agricultural nation, producing a large amount of biomass materials each year, and biomass fuel is mainly composed of crop straw. Unlike the commonly used woody biomass materials abroad, straw biomass fuel generally has a higher mass fraction of nitrogen, such as corn straw, which can reach about 0.7%.   Therefore, controlling NOx emissions from biomass fuel combustion is a key focus of clean biomass combustion. Compared to other atmospheric pollutants, NOx is produced in the largest quantity during fuel combustion, with a mass fraction exceeding 30%. However, 70% of the NOx generated during fuel combustion comes from the direct combustion of coal. The release of volatiles, combustion of coke, and combustion of volatiles are the main processes for NOx formation. Changing the combustion form in the furnace can effectively reduce pollutant generation.

2.2 Hazards of Nitrogen Oxides

       With the continuous rapid development of our economy, energy consumption has been increasing year by year, and the emissions of NOx in the atmosphere have also rapidly increased. NOx has become a major primary pollutant, mainly consisting of NO and NO2. The environmental problems caused by NOx emissions have seriously threatened the ecological environment and human health, with major hazards including: participation in the destruction of the ozone layer; formation of photochemical smog with hydrocarbons; being a major pollutant in the formation of acid rain and acid mist; causing damage to plants; and having toxic effects on human health. Therefore, it is very important to control and manage NOx in the atmosphere.

2.3 Current Status of Nitrogen Oxide Emissions from Biomass Combustion in Our Country

       The urgency of NOx pollution prevention and control is also reflected in the fact that if effective control of NOx emissions is not implemented, the significant increase in NOx emissions will offset efforts to reduce SO2. This is mainly reflected in the increase in haze days and pollution levels in economically developed regions such as Beijing-Tianjin-Hebei, the Yangtze River Delta, and the Pearl River Delta, as well as a decrease in atmospheric visibility. The type of acid rain in our country is shifting from sulfuric acid type to a mixed type of nitric and sulfuric acid. Therefore, during the 12th Five-Year Plan period, our country included NOx in total control, making NOx one of the key pollutants in the joint prevention and control planning.
       The NOx emissions of different biomass pellet fuels are listed in Table 2. By comparison, it can be seen that although the NOx emissions of woody pellet fuels are lower than those of straw pellet fuels, they have not yet fully met the current emission standards GB13271—2014 and the draft for comments on standard GB13271—200 mg·m - 3 specified for key areas, and there is still a significant distance from the new standard of 80 mg·m - 3. Vassilev et al. also compared the NOx emission patterns of woody fuels and corn straw pellets during the ignition and extinction processes. In terms of NOx components, the study found that during the entire ignition process of straw pellet fuel, the flue gas contained NO and a mass fraction of 2.6% to 6.9% of NO2, while the flue gas from woody pellet fuel only contained NO. In terms of NOx generation, the NOx emission from straw pellet fuel combustion is twice that of woody fuel.

      In 2012, the installed capacity of biomass power plants in our country (excluding bagasse power generation) reached 3.37 GW. In 2013, the biomass power generation in our country was 3.7 × 10^10 kW·h - 1, and the biomass energy consumption was 8 million tons (equivalent to nearly 4 million tons of coal). It is expected that by 2015 and 2020, the direct combustion power generation capacity of agricultural and forestry biomass can reach 4.5 GW and 7.5 GW, respectively. NOx is one of the important pollutants generated during biomass combustion power generation, and its control methods mainly include combustion control and flue gas purification. Combustion control prevents localized high temperatures by lowering the combustion temperature, usually employing multi-stage air supply, low-oxygen combustion, fluidized bed combustion, etc.; flue gas purification includes selective catalytic reduction and selective non-catalytic reduction. Unlike coal combustion, when burning biomass fuel, the combustion temperature in the furnace is generally higher than 850 °C, and nitrogen-containing products in biomass fuel can partially convert to NOx during combustion, resulting in a large amount of thermal NOx being produced. Therefore, the NOx emissions from operational biomass boilers are higher than those from coal-fired boilers. Studies have found that the NOx emission concentration from coal-fired industrial boilers without technical transformation when converted to biomass pellet fuel reaches about 330 mg·m - 3. Wang Qinchao studied the emission situation of five biomass boilers with different installed capacities, and the results showed that the NOx emission concentration ranged from 207.9 to 601.3 mg·m - 3, all exceeding the emission limit. Therefore, traditional coal-fired boilers must improve combustion methods or install post-treatment devices when converting to biomass fuel.

       Although the current "Emission Standards for Air Pollutants from Boilers" (GB13271—2014) does not specify the emission limits for NOx from biomass boilers, the NOx emission concentration from biomass boilers when burning corn straw pellets often ranges from 207.9 to 601.3 mg·m - 3. Since the emission limit in GB13271—2014 is 200 mg·m - 3, measured against this standard, the NOx emission concentration when burning corn straw pellets exceeds the limit by 0.04 to 2.01 times. The "Twelfth Five-Year Plan for Biomass Energy Development" indicates that the annual utilization of biomass pellets in China will reach 10 million tons by 2015, corresponding to the replacement of 5 million tons of standard coal (7 million tons of coal), which can reduce NOx emissions by 0.91 million tons. Therefore, the control of atmospheric NOx pollution has become urgent.

3 Nitrogen Oxide Control Technologies for Biomass Boilers

       Developed countries or regions such as the United States, the European Union, and Japan started early in NOx control efforts, and the relevant policies for NOx control are relatively mature. Currently, developed countries abroad mainly use methods such as flue gas recirculation, staged combustion, and low-NOx burner combinations to reduce NOx emissions by 30% to 70%. Although low-NOx combustion technology is currently the main NOx control technology in China, its adoption can control the NOx concentration in boiler emissions to below 200 mg·m - 3, but relying solely on this technology can no longer meet the requirements of the new standards.

3.1 Combustion Improvement Technology

       Combustion improvement technology is a technique that reduces the generation and emission of NOx by controlling combustion conditions, adjusting the temperature and air intake in the combustion zone. Compared to other nitrogen reduction technologies, low-NOx combustion technology is a relatively simple, economical, and widely applied method. The main low-NOx combustion technologies currently used include the following five types: low-NOx burners, fuel reburning technology, low excess air combustion technology, air staging combustion technology, and flue gas recirculation technology.

3.1.1 Low-NOx Burners

       The use of low-NOx burners can control NOx emissions during the fuel combustion process, while also facilitating stable ignition and complete combustion of the fuel.

       The working principle of low-NOx burners is to divide the primary combustion air into two parts: concentrated phase and dilute phase, which burn at different locations. The concentrated phase is located closer to the flame center and at a higher temperature, but due to a lower oxidation ratio, it reduces the NOx generation rate; the dilute phase is located near the water-cooled wall, where the temperature is lower and further from the flame center, although the oxidation ratio is higher, the NOx generation rate remains low, ultimately achieving the goal of reducing NOx generation and emissions.

       Low-NOx burners include several categories such as low-NOx pre-combustion burners, split flame burners, staged burners, concentrated-dilute burners, mixed promoting burners, and self-recirculating burners. The denitrification efficiency generally ranges from 30% to 60%.

3.1.2 Fuel Reburning

       Fuel reburning technology began in the 1980s and is a technique for controlling NOx within the furnace. The principle of nitrogen reduction is as follows: based on the combustion process of the fuel in the furnace, the furnace is divided into three zones along the height: the main combustion zone, the reburning zone, and the burnout zone; a strong reducing atmosphere is formed in the reburning zone through fuel staging, where NOx formed in the main combustion zone is reduced to N2 and other nitrogen-containing reducing groups (HCN and NH3); subsequently, due to incomplete combustion, the exhaust emissions can lead to environmental pollution, thus some air is supplemented in the burnout zone to form a rich oxygen combustion section, where the remaining combustible materials (CHi, CO, etc.) and nitrogen molecules are oxidized. The adoption of reburning technology can reduce the NOx emissions from coal-fired boilers to below 35% of the pre-use level.

       Fuel reburning technology has the following advantages: high denitrification efficiency, wide applicability, small boiler modification, and low operating costs, thus it has received widespread attention. Various types of reburning fuels can be used, including gases (methane, syngas, etc.), liquids (slurry, etc.), and solid fuels (coal powder, biomass, etc.). Due to the relatively low content of elements such as N and S in biomass, the widespread use of biomass can significantly reduce the generation and emission of atmospheric pollutants. Meanwhile, the components such as sodium and potassium in the ash after biomass combustion promote the reduction of NOx. Therefore, biomass fuel can be considered a good reburning fuel. However, currently, research on the denitrification characteristics of biomass reburning is relatively limited. Some studies have shown that using biomass as a reburning fuel can achieve higher NOx reduction efficiency. Adams and Harding used wood as a reburning fuel and applied reburning technology to control NOx emissions from cyclone burners. When the reburning fuel is injected in the area near the back wall of the cyclone barrel at a temperature close to 1600 °C, with a residence time of 0.3 s, the NO emissions can be reduced by nearly 60%; with the high-speed reverse injection of superheated air, the NOx reduction efficiency can reach up to about 45%. Liu and others found that using reburning technology can reduce the NO mass fraction by 50% to 60%, and there are no significant side effects on boiler operation.

3.1.3 Low Excess Air Combustion

      Low excess air combustion works on the principle of suppressing NOx generation by reducing the excess oxygen in the flue gas, so it is necessary to ensure that combustion occurs as close to the theoretical air amount as possible. This is a relatively simple method for reducing NOx emissions. However, the NOx removal efficiency of this method is only 15% to 20%, and the fuel may not burn sufficiently in this combustion environment, leading to thermal losses and reducing boiler efficiency. Additionally, due to the lower excess air, certain areas within the furnace may form a reducing atmosphere, which can easily cause fouling and corrosion of the furnace walls.

3.1.4 Air Staging

       Air staging developed in the 1950s in the United States as one of the technologies to control NOx emissions and is currently widely used.  This technology reduces the amount of air in the combustion zone, where fuel is first burned under oxygen-deficient conditions, and then additional air is supplied to further combust the unburned fuel. The adoption of air staging technology reduces the mixing amount of fuel and air in the primary combustion zone, delaying the combustion process in the main combustion zone; at the same time, due to the control of the primary air flow, the oxygen content in the primary combustion zone is low, resulting in insufficient combustion of the fuel, thereby lowering the combustion temperature. Since the fuel content in this area is still high, a reducing atmosphere is formed, in which the generation rate of NOx is lower. The air staging combustion technology can achieve a denitrification efficiency of about 30%.

       Research by Shiben Gai and others shows that increasing the excess air coefficient significantly reduces the amount of NOx generated. Guo Feiqiang and others used it to achieve fuel stratification; secondary air at the throat of the combustion chamber is mainly used to disturb the volatile matter; tertiary air located in the gasification combustion chamber is used to completely combust the residual volatile matter. After the material is added to the furnace, it first enters the pyrolysis gasification zone. Since the pyrolysis gasification zone does not directly supply air, biomass will generate semi-coke solid combustible materials and reducing flue gas (H2, CO, etc.) under oxygen-deficient conditions.  On one hand, the generated semi-coke solid enters the stratified combustion zone and is fully combusted due to the supply of air, but it produces pollutants such as NOx; on the other hand, the reducing flue gas is disturbed by the secondary air, forming a strong combustion vortex at the throat of the furnace, which can undergo a reduction reaction with the NOx generated in the stratified combustion zone.  Research also shows that when the excess air coefficient is 1.75 and the volume ratio of primary air, secondary air, and tertiary air is 7:1:2, the mass concentration of NOx emissions is the lowest, reaching 83.45 mg·m - 3, close to the new standard's expectation of 80 mg·m - 3.

3.1.5 Flue Gas Recirculation

       The principle of flue gas recirculation technology is to extract a portion of low-temperature flue gas before the air preheater of the boiler and directly send it into the furnace, or mix it with primary air/secondary air and send it into the furnace, playing a role in cooling and diluting, thereby reducing the temperature and oxygen concentration during the combustion process, thus controlling the generation rate of NOx.  However, if the flue gas recirculation rate is too high, it can also lead to increased unburned thermal losses and combustion instability, so the recirculation rate is often controlled between 10% and 20%. Zhou and others pointed out in their research on straw fixed bed combustion boilers that recycling 20% of the flue gas back into the boiler not only ensures that combustion efficiency is not affected but also reduces NO emissions.

3.2 Flue Gas Denitrification

      According to whether a denitrification catalyst is used, tail flue gas denitrification mainly has the following two types: Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR).

3.2.1 Selective Catalytic Reduction

       Selective catalytic reduction technology involves sending a reducing agent (commonly NH3) into the flue gas duct to mix with the flue gas. Under the action of a catalyst, NOx generated during combustion is reduced to N2 and H2O at low temperatures of 320 to 420 °C, thus achieving NOx reduction. Generally, the commonly used catalyst is a mixture made from FeO2, Ag2O5, and WO3.

3.2.2 Selective Non-Catalytic Reduction

       Selective non-catalytic reduction technology involves directly spraying ammonia water or urea as a reducing agent into high-temperature flue gas at 900 to 1100 °C without adding a catalyst. Under high-temperature conditions, ammonia water and urea decompose into NH3 and other by-products, and then the NOx in the flue gas undergoes a redox reaction with the decomposed NH3, reducing the NOx in the flue gas to N2 and H2O.

       The main difference between selective catalytic reduction technology and selective non-catalytic reduction technology lies in their reaction conditions, specifically in whether a catalyst is used, the type of reducing agent, and the reaction temperature.

       The advantages and disadvantages of selective catalytic reduction technology and selective non-catalytic reduction technology mainly manifest in the following aspects: Selective catalytic reduction technology has a higher denitrification efficiency, reaching 80% to 90%. The disadvantages mainly lie in the high initial investment costs and operational costs of the equipment. Moreover, due to the current inadequacy of selective catalytic reduction catalyst technology in China, there is a heavy reliance on foreign technology, but foreign patent barriers have increased the operational and maintenance costs of selective catalytic reduction technology, which to some extent limits its promotion and use.  Selective non-catalytic reduction technology does not require the use of a catalyst during the denitrification process. It reacts NOx with the reducing agent NH3 under window temperature conditions of 900 to 1100 °C. The main disadvantage of this technology is that in actual combustion processes, factors such as combustion load and fuel type can affect the temperature distribution in the furnace, which in turn affects the window temperature for selective catalytic reduction. To achieve better denitrification results, the position of ammonia injection must also be adjusted according to the window temperature, which increases the technical difficulty of operation in practical use. Currently, about 96% of power plants in China mainly use selective catalytic reduction technology, while about 4% of power plants use selective non-catalytic reduction denitrification technology.

3.3 Advanced Reburning

       Advanced reburning technology combines fuel reburning with selective non-catalytic reduction denitrification technology, which is currently a highly promising NOx control technology. By combining with selective non-catalytic reduction denitrification technology, it can further reduce NOx emissions based on fuel reburning, making it a more thorough NOx reduction technology. The principle of advanced reburning is to inject a reducing agent into the reburning zone or burnout zone to further reduce NOx generation. The key to advanced reburning technology is the synergistic effect of fuel reburning and selective non-catalytic reduction in two stages, broadening the reaction temperature window and reducing the impact of a narrow temperature window on selective non-denitrification efficiency.

       Han and others chose advanced reburning technology to reduce NOx emissions during biomass boiler combustion. First, simply optimizing operating conditions can reduce NOx by 54% to 67%; subsequently, further injecting ammonia water, urea, sodium carbonate, and other reducing agents can collaboratively reduce NOx emissions, achieving a NOx removal rate of 85% to 92%, with very significant denitrification effects.

Research Progress on NOx Control Technology of Biomass Boilers

      Currently, the NOx control technology for biomass boilers is mainly divided into two categories: in-furnace denitrification and flue gas tail denitrification.

       In-furnace denitrification refers to low-nitrogen combustion technology. For biomass boilers, commonly used low-nitrogen combustion technologies include flue gas recirculation technology. There are two processes for flue gas recirculation: (1) The flue gas after the induced draft fan is directly introduced into the primary air fan inlet. This scheme does not require modifications to the primary air fan, and the recirculated flue gas does not need a draft fan, saving electricity and energy, and the transformation is simple. The NOx mass fraction can decrease by 20% to 40%. (2) The flue gas after the induced draft fan is directly introduced into the primary air chamber and secondary air chamber of the furnace. This scheme requires the primary air fan to operate at reduced load, and the recirculated flue gas also needs to be equipped with a high-temperature draft fan, with wind pressure comparable to that of the primary air fan. This scheme increases operational electricity consumption, and the transformation is relatively complex, with the nitrogen oxide mass fraction potentially decreasing by 25% to 50%. Flue gas recirculation technology may also lead to an increase in the concentration of sulfur dioxide pollutants and moisture content in the flue gas, but it reduces the total flue gas emissions.

       Flue gas tail denitrification includes selective catalytic reduction and selective non-catalytic reduction. Selective catalytic reduction denitrification technology is the most widely used; however, due to the characteristics of biomass fuel (low calorific value, high potassium content), the flue gas temperature at the tail of biomass boilers is low (280 °C), with high moisture content, a large proportion of fine fly ash particles, and high potassium metal content, which easily leads to poisoning, deactivation, blockage, and abrasion of conventional catalyst layers, resulting in excessive NOx emissions. Currently, the longest continuous operation time for foreign catalysts in denitrification is three months. Therefore, if selective catalytic reduction denitrification technology is adopted, it is essential to address the issues of catalyst poisoning, deactivation, blockage, and wear. The selective catalytic reduction denitrification system consists of a flue gas system, ammonia preparation and spraying system, catalyst and ash blowing system.   The flue gas system includes catalyst temperature, pressure, etc.; the ammonia preparation and spraying system includes dilution air, cooling air, ammonia water, compressed air, etc.; the catalyst and ash blowing system includes acoustic blowers, compressed air, solenoid valves, etc. The selective catalytic reduction catalyst consists of two layers, arranged between the economizer and air preheater. The selective catalytic reduction denitrification system uses a 20% ammonia water solution as a reducing agent. Practice shows that the selective catalytic reduction denitrification system can control NOx emission levels below 50 mg·m - 3, with a denitrification efficiency exceeding 80%, and the ammonia escape mass concentration is far below the design requirement of 2.3 mg·m - 3, meeting ultra-low emission requirements. Jiaxing Xinjia Aisi Thermal Power Co., Ltd. has implemented measures in a biomass circulating fluidized bed boiler with a gas production capacity of 130 t per hour: using plate catalysts, and setting up a self-developed large particle fly ash collector 1.5 m upstream of the catalyst layer, effectively solving the problem of catalyst layer blockage; appropriately increasing the flow rate within the catalyst layer to reduce the adsorption probability of fine fly ash particles, extending the chemical activity life of the catalyst. Selective non-catalytic reduction denitrification technology can effectively remove NOx from conventional coal-fired circulating fluidized bed boilers, but it does not achieve the expected effect on biomass boilers with a gas production capacity of 130 t per hour. A large number of experiments were conducted on the position and number of spray guns, and it was found that due to the uneven temperature field in the biomass boiler compared to coal-fired boilers, high-temperature and low-temperature areas are often interspersed, causing the injected reducing agent (ammonia water) to not be in the optimal reaction temperature range, resulting in the coexistence of NOx reduction reactions and ammonia oxidation reactions, with similar reaction rates. Under the conditions of a furnace temperature of 800 °C and an NH3/NO2 volume ratio of nearly 2, the highest efficiency of NOx removal only reached 15%. Therefore, selective non-catalytic reduction technology cannot be used as a denitrification technology for fully burning biomass boilers. The pH value of the desulfurization tower also has a certain impact on NOx removal. Under the conditions of an oxidant volume fraction of 0.1% and a pH value of 6, the highest efficiency of NOx removal is 31%. Therefore, selective non-catalytic reduction technology cannot be used as a denitrification technology for fully burning biomass boilers.

4 Difficulties in NOx Control of Biomass Boilers

4.1 Insufficient Reserve of NOx Control Technology

       Currently, China's control of NOx is still in the pilot and initial stages, and the control technology is not yet fully mature. Moreover, since China's biomass resources are mainly agricultural by-products such as straw, while foreign countries have long relied on woody biomass and its molded fuels, the mature experiences and technologies from abroad cannot be fully applied in China. It is particularly important to develop biomass NOx control technology suitable for China's national conditions.   At this stage, China mainly adopts low NOx combustion methods to reduce NOx emissions, with a control efficiency of about 30% to 50%. Currently, although some people have also adopted end-of-pipe treatment technologies to achieve denitrification, the operational effects and economic benefits have not yet reached an ideal level.

4.2 High Cost of NOx Control

       Currently, there are some difficulties in the transformation of boiler NOx control. For example, the furnace of industrial boilers is relatively small, and if low NOx combustion technology is to be implemented, there are difficulties in the transformation, and the cost of reducing NOx emissions is too high. The current cost of desulfurization technology is about 800 yuan per ton, while denitrification technology costs nearly 2000 yuan per ton. Pollution control for coal-fired boilers will significantly increase the compliance costs, so it is necessary to provide certain compensation through preferential electricity pricing policies. The "Environmental Protection Electricity Price and Operation Supervision Measures for Coal-fired Power Generation Units" clearly stipulates that coal-fired power generation units must install post-treatment facilities such as desulfurization, denitrification, and dust removal. The electricity generated must implement environmental protection electricity price policies, including surcharges for desulfurization, denitrification, and dust removal, with specific compensation standards being a surcharge of 1.5 cents per kWh for desulfurization, 1 cent for denitrification, and 0.2 cents for dust removal.

       However, due to differences in fuel, combustion conditions, and other factors, the flue gas denitrification technology and equipment for coal-fired boilers cannot be directly applied to biomass boilers. Currently, there is no mature technology available for denitrification of biomass boilers. Developing denitrification technology for biomass boilers, implementing low NOx combustion transformations, and installing denitrification devices will inevitably increase environmental protection costs, significantly raising the economic costs of biomass boilers and limiting their development to some extent.

5 Development Trends and Prospects

(1) Considering the biomass fuel itself, simply burning straw (rice, wheat, etc.) molded fuels usually has a high ash content and a lower calorific value compared to woody materials. Mixing agricultural by-products such as walnut shells and peanut shells into the raw materials to prepare mixed molded fuels can compensate for these shortcomings, becoming a new development trend in the preparation of biomass molded fuels.

(2) Considering the combustion equipment for biomass fuels, currently, the mature commercial burners are quite suitable for the combustion of wood biomass fuels, while their combustion capability for straw biomass fuels is relatively poor, leading to easy coking and slagging, making it difficult to maximize the thermal efficiency of the fuel. Moreover, even with the installation of post-treatment devices, the NOx emissions are hard to meet national standards, which has become a significant technical challenge. Therefore, how to precisely control the temperature changes during the combustion process, achieve real-time measurement of NOx generated during combustion, and effectively control the NOx emissions from biomass burners will be one of the future development directions.

(3) There is an urgent need for mature NOx control technology for biomass fuel combustion emissions. Future research should start from the mechanism of NOx generation, focusing on optimizing fuel properties and conducting precise combustion process control.

     Research on the entire combustion process control and the development of efficient and low-cost flue gas denitrification post-treatment technology, exploring designs and solutions to improve denitrification efficiency and reduce environmental protection costs, while addressing China's energy predicament and achieving the goal of clean production.