Adoption of strategies for clean combustion of biomass in boilers

3.1.1 Description

Fouling, also known as deposits, refers to the deposition of ash on heat transfer tubes. Sulfur, chlorine and silicon are responsible for increasing the volatility of sodium and potassium (alkali compounds). These alkali compounds combine with chlorine to form alkali chlorides, which escape in the form of gas. Flue gases on cooling, deposit on heat exchanger tubes. In the presence of silicon, alkali silicates are formed; having low melting temperature, leave a sticky surface for increased deposition. The non-volatile compounds present in the fuel form ash, which consists of elements like magnesium, calcium, silicon with small amounts of sodium, potassium and aluminium (Altobelli et al. 2013). Coarse particles are found as deposits on surfaces of the heat exchanger. Potassium reacts with calcium and silicate present in ash to form a glassy structure, which is responsible for heavy deposits. Thus, fouling or deposits lead to decreased boiler efficiency and heat transfer. They also result in increased operating cost, carbon monoxide and nitrogen oxide emissions (Demirbas 2005a; Jiménez and Ballester 2007; Khan et al. 2009).

3.1.2 Solutions

Through leaching or pre-treatment of biomass with water, the ash content can be reduced (Table 2). With an increase in temperature of the water used for pre-treatment, the melting point of ash increases, which eventually improves the efficiency of the combustion process. The content of chlorine, potassium, sodium and sulfur can be reduced by 30–100%, 50–90%, 10–90% and 0–95% respectively. The concentration of silicon, titanium and aluminium can be reduced by 15% with the help of cold washing. Reduction in concentrations of the above-mentioned elements can reduce fouling (Gudka et al. 2016; Melissari 2017).

The second way of reducing fouling in wooden biomass is through the optimization of airflow rate and deposition probe temperature (Table 2). When the airflow rate and probe temperature are increased from 11 to 25 m³/h and 25–65 °C respectively, the amount of unburnt carbon can be reduced. Many non-volatile compounds like calcium, magnesium, silicon, aluminium, and titanium remain in the ash but the content of sulfur can be reduced to a great extent. The content of chlorine should be reduced in the biomass to reduce its deposition on boiler tubes (Febrero et al. 2015).

Another way for reduced fouling is the design of a hybrid system using neural networks and the Fuzzy Logic Expert System (Table 2), which is responsible for simulation, prediction, and control of soot blowing cycles. It performed thermal monitoring, fouling or deposit prediction, analysis of boiler behavior, and finding the number and timing of soot blowing cycles. The system will suggest a certain number of soot blowing cycles and the rest of them will be done automatically. This method not only reduces fouling but also helps in energy saving. On the implementation of this system, 3.5% of energy can be saved (Romeo and Gareta 2009).

The mechanism of fouling was studied and it was found that alkali sulfates play an important role in increasing the adhesion force. The reduction of adhesion force can be a solution to fouling. So, nickel alloy thermal spray coating can be done on boiler tubes, which helps in reducing adhesion force (Table 2) (Naganuma et al. 2013).

Reducing the content of potassium can also be a solution to fouling, as mentioned in Table 2. Composite additive with 20–50% blowing air, 25–65% kaolin, 20–60% MgO, and 15–30% activated alumina reduced the content of potassium and so, fouling was also reduced. With an increase in combustion temperature, the additives worked in a better way and CaO can be added to reduce the emission of HCl (Liao et al. 2015).

Chlorides are very harmful to biomass boilers and so, strategies can be adopted to reduce the content of chlorides (Table 2). Ammonium sulfate can be used as an additive for the conversion of KCl to its sulfate through which, the chloride content can be highly reduced. Combustion of biomass with peat can also be a strategy for the reduction of chlorides. Components like aluminium, sodium, and calcium present in peat ash are responsible for capturing potassium and converting its chloride into sulfate (Kassman et al. 2013).

3.2.1 Description

Agglomeration results when particles of ash form layers around particles of bed material (mostly quartz sand). It takes place due to chemical reactions, liquid phase presence, and solid-state sintering. If the melt or ash is highly viscous, it gives a glassy structure, which on cooling, does not crystallize. If the melt is less viscous, the alkali compounds melt increasing the stickiness of the particles. The alkali compounds act like bonding agents, thus, enhancing agglomeration (Llorente et al. 2006). The chemical reaction includes the reaction of potassium phosphate with either silicon or calcium, which results in silicates of potassium and calcium with a low melting point.

In fluidized biomass boilers, the main cause of agglomeration is the partial melting of inorganic compounds. In an agglomerated bed particle, the inner layers have a composition similar to that of the bed material and the outer layers have a composition similar to that of the fuel. Two phenomena have been studied: melt-induced agglomeration and coating induced agglomeration. In melt-induced agglomeration, the bed particles adhere to each other due to high local temperature and on the other hand, a uniform coating is present on particles of bed material. In this coating, the formation of neck starts and as this neck formation continues, a high-temperature zone forms which help in melt formation, thus, promoting agglomeration (Melissari 2017; Visser 2004; Visser et al. 2008).

According to Ohman et al. (2000), the deposition of ash on bed material is a combination of processes that include: attachment of small particles on grains of bed material followed by condensation of alkali compounds (potassium, potassium chloride, and potassium sulfate) followed by reaction of alkali compounds with the bed material surface. On deposition, each inner layer undergoes sintering and gets strengthened. Agglomeration takes place due to any of the three mechanisms. First, due to the reaction between bed material and alkali compounds, potassium silicate is formed which promotes the formation of bed layers. Second, the reaction takes place directly between gaseous potassium and bed material particles, which results in the formation of potassium silicate that initiates agglomeration. Third, adhesion of bed particles by partially melted potassium silicate and potassium-calcium/magnesium phosphates from fuel (Brus et al. 2005; Grimm et al. 2011).

It has been reported that biomass with herbaceous materials results in increased agglomeration due to the high concentration of alkali compounds. Alkali compounds like potassium, calcium, magnesium, silicon, and aluminium are highly responsible for agglomeration problems. The bottom ash captures vaporized potassium, which combines with silica present in bed material leading to agglomeration (Lin et al. 2003; Llorente et al. 2006). Biomass has a bright future and so, it is important to control the problem of agglomeration since eliminating it is not possible.

3.2.2 Solutions

The selection of bed materials can help in reducing agglomeration (Table 2). Quartz sand contains silica, potassium, and calcium and is responsible for the formation of silicate compounds with low melting points whereas olivine contains silica, calcium, and magnesium (responsible for increasing the melting temperature of potassium silicate). Results have shown that agglomeration was lesser in the case of olivine bed material in comparison to quartz sand (Brus et al. 2004; De Geyter et al. 2007; Grimm et al. 2012). Limestone of particle size 0.25–2 mm with fluidization velocity of 1.2 m/s has shown lesser agglomeration deposits than silica sand when used as bed material. In the case of limestone, calcium carbonate, calcium oxide, and calcium hydroxide escape bed material to reduce ash deposits on boiler tubes (Fernández Llorente et al. 2006). Other bed materials like alumina, ferrite oxide, and dolomite have problems related to chemical stability. Agglostop can be used as bed material because it can handle alkali related agglomeration problems (Silvennoinen 2019).

Change in reactor design can also be a solution to this problem by implementing a stirrer to break up agglomerates (which can break easily) or by passing a high-velocity gas stream or by using the concept of the hydro-beam floor in which coarse bed particles are removed repeatedly (Table 2) (Bartels et al. 2008).

Another alternative to reduce agglomeration is the usage of additives in normal silica sand (Table 2). Kaolin can reduce the concentration of potassium chloride and so, agglomeration can be reduced. But, kaolin is so expensive so, it can be replaced by ammonium sulfate, which gives the highest efficiency and is most effective in the elimination of agglomeration, after kaolin (Davidsson et al. 2008). Other additives like dolomite and alumina can also be used which do not allow reaction between potassium and silica. Ilmenite has the advantage of absorbing potassium particles from ash, thus, reducing agglomeration when used with sand bed material (Corcoran et al. 2014).

Operating parameters can be altered to control the problem of agglomeration, as mentioned in Table 2. Lower bed temperature, higher fluidization velocity, and finer bed particles are ideal for reduced agglomeration. Lower bed temperature reduces the ash-melting rate. When the velocity of the gas is high, defluidization time is more and so, breaking of agglomerates can take place easily. With finer bed particles, the defluidization time is more (Lin et al. 2003).

3.3.1 Description

Heavy metals like arsenic, cadmium, arsenic and lead are highly toxic for the environment that are released during burning of biomass. Low quantities of copper, zinc, cobalt, selenium, nickel and manganese are required by human body for molecular and biological functions but large quantities of these heavy metals given out in the form of ash during biomass combustion pose a threat to the environment (Prashanth et al. 2015). The concentration of these heavy metals depends on the source of biomass and how far is it from the source of pollution. Heavy metals are usually found in fuel ash with particles smaller than 1 μm. Moreover, their concentration increases with the decreasing temperature of precipitation and the size of the particle (Demirbas 2008; Demirbaş 2005b). Cadmium as heavy metal is harmful to humans as it accumulates in the kidneys and causes cancer. Lead is also a toxic metal that is harmful to the brain even at low concentrations (Nawrot et al. 2006).

In the boiler, metals like cadmium, argon, zinc, and lead volatilize in the form of oxides and chlorides and deposit in the form of ash. During combustion, cadmium, and lead vapourize in the form of cadmium chloride and lead chloride respectively. Although zinc is present in fuel ash in its stable form, but the small amount of zinc gets converted to zinc chloride. Thus, sublime metal is passed on to fly-ash with the help of chlorine. Non-volatile metals such as aluminium, copper, and iron are found in coarse fly-ash because they are converted to oxide form which is very stable (Khan et al. 2009). Some heavy metals like selenium and mercury escape the biomass boiler in the form of gases from flue gas outlet. Workers may be exposed to gaseous pollutants and particulate matter generated from biomass handling, transportation, storage and ash generated from post combustion processes like cleaning of flue gases and removal of carbon dioxide. Trucker is involved in transportation of ash, fuel handling operative is involved in storage of biomass leading to exposure to off-gases from storage chamber, cleaner is involved in removal of deposits from plant, maintenance engineer is involved in maintenance of boiler leading to exposure to combustion gases and ash handling operative is involved in ash removal and its transportation to ash storage chamber (Rohr et al. 2015).

3.3.2 Solutions

The presence of heavy metals in fly-ash is a serious problem for which the following measures can be taken.

For control of heavy metals, absorption of lead and cadmium with absorbents like silicates and aluminosilicates has proved to be very effective (Table 2). The chemical reaction of heavy metals with adsorbents like alumina, silica, and bauxite can also be one of the solutions. Methods like adsorption using activated carbon, ion-exchange process, and post precipitation can also be employed to reduce heavy metal emissions to a great extent (Table 2). For the metal concentrated ash to not cause any harm to the boiler tubes, it can be disposed to a landfill (Table 2) (Khan et al. 2009). To reduce the amount of ash that is deposited to the landfill, sieve fractionation method should be preferred to reduce the amount of unburnt carbon present in the ash and followed by gasification process for removal of highly important elements that should not go to the landfill and get leached with surrounding environment (James et al. 2012).These processes basically separate out the solid unburnt and harmful soluble compounds from fly-ash.

3.4.1 Description

Emissions from biomass generally include CO, NO _x , N₂O and SO _x emissions.

CO emissions are usually a result of incomplete burning, excess air, shorter residence time, low temperature, and high concentration of carbon content in the fuel. A sudden decrease in the temperature of the flame and poor mixing of fuel with air often leads to increased CO emissions. The main reason for CO emissions is the usage of the boiler that was once used for coal combustion and has undergone no modification for the combustion of biomass. A large amount of volatile matter is present in biomass which requires longer freeboards for longer residence time, which is sometimes not provided by small-scale industries, resulting in high CO emissions. Also, heat exchangers decrease the temperature of the boiler due to which carbon monoxide cannot be fully converted to carbon dioxide. It has been reported that high moisture content, low boiler demand, the large size of fuel particle and high ash content are responsible for high CO emissions (Bignal et al. 2008; Demirbas 2005a; Khan et al. 2005, 2009).

NO_x emissions include nitrogen oxide and nitrogen dioxide, which are responsible for the formation of smog, particulate matter, and acid rain. Volatile matter consists of nitrogen-containing species (NH₃ and HCN) which oxidize to form NO _x . These emissions do not depend much on the operating conditions of the boiler but mostly depend on the nitrogen content present in the fuel, amount of available oxygen for combustion, and fuel reactivity. High moisture content and improper dosage of fuel result in high NO _x emissions. Proper regulation of oxygen also plays an important role in NO _x emissions (Khan et al. 2009; Vitázek et al. 2016).

N₂O oxidizes into NO in the presence of sunlight which further causes depletion of the ozone layer. Biomass fuel composition plays an important role. On the combustion of biomass, the more is the production of cyano species, the more are N₂O emissions and on the other hand, the production of NH₃ results in lower N₂O emissions. These emissions greatly depend on boiler temperature and excess air supply. At high temperature, N₂O converts into N_2, and thus, high temperature is suitable for N₂O reduction, and on the supply of excess air, there is a decrease in temperature which increases the formation of NO and N₂O. From the literature, it has been observed that biomass combustion results in lower N₂O emissions than coal and natural gas (Bai et al. 2013; Khan et al. 2009).

SO _x emissions include SO_2, SO_3, and other alkali sulfates, which react with chlorine resulting in corrosion of boiler tubes. In the boiler, at the time of flue gas cooling, the sulfates get deposited on the surfaces. It has been reported that on using biomass with coal, SO₂ emissions can be reduced because of the presence of calcium oxide and magnesium oxide that can absorb SO₂ emissions (Khan et al. 2009; Williams et al. 2012).

3.4.2 Solutions

Gaseous emissions play a major role in polluting the environment, so, the issue of release of gaseous emissions needs to be resolved. One of the solutions to reduce carbon monoxide emissions can be the design of longer freeboards which can increase the residence time of biomass in the boiler for its proper combustion (Table 2). The heat exchanger present in the freeboard should also be removed as a heat exchanger decreases the temperature which is not suitable for CO emission reduction. Air staging and boiler load should be optimized according to the carbon content in the fuel. Selective catalyst reduction is a successful method of reducing CO and NO _x emissions (Table 2). Reagent like ammonia is used which reacts with NO _x and CO in the presence of a catalyst which oxidizes CO to carbon dioxide and reduces NO _x to nitrogen. By this process, CO emissions are reduced by 70–85%, and NO _x emissions are reduced by 70–95% (Khan et al. 2008; Lamberg et al. 2011). Air staging is also an effective solution for the reduction of NO _x emissions, which prevents the insufficient supply of fuel and oxygen for proper combustion (Table 2). This inhibits the chemical reaction of nitrogen being converted to NO _x . The secondary air inlet, when placed above the fuel bed, has proved to reduce NO _x emissions significantly. Proper mixing between primary and secondary combustion zone products can reduce gaseous emissions to a great extent. The type of biomass fuel can also be changed with wood being used as biomass fuel because of its low sulfur and nitrogen content, resulting in reduced gaseous emissions (Khan et al. 2009; Liu et al. 2013; Zabetta et al. 2005).

The selective non-catalyst reduction can play a major role in the reduction of N₂O emissions with feeding urea in the post-combustion chamber and ammonia can continue as a reducing agent in selective catalyst reduction (Table 2). This hybrid technique has been reported as the most successful one for N₂O reduction. Recently, the usage of ultra-fine limestone with a particle size of 20 μm has emerged as an innovative solution for the reduction of N₂O and SO _x emissions (Table 2). The content of ultra-fine limestone needs to be optimized for nitrogenous emissions (Cai et al. 2019; Grosso et al. 2009; Mladenović et al. 2003). With the increase in particle size of limestone, the efficiency of desulphurization decreases, which results in increased SO _x emissions.

3.5.1 Description

The heating value of biomass can be defined based on high heating value (HHV) which is the overall heat content of the fuel. Low heating value can be obtained by subtracting the amount of energy required to remove the moisture content of biomass. This low heating value of biomass can be attributed to low density, high moisture, and oxygen content. Due to this, biomass experiences problems in flame stability. Coal has a higher heating value (23–28 MJ/kg) in comparison to biomass (14–21 MJ/kg). It has been reported that compounds containing carbon and hydrogen tend to increase the heating value of biomass while oxygen tends to decrease it. When compared to lignin, it has been observed that hemicellulose and cellulose present in biomass have lower heating value due to their oxidation. Ash content is also a major factor responsible for low heating value, thus, making its combustion inefficient (Demirbas 2005b; Khan et al. 2009; Saidur et al. 2011).

3.5.2 Solutions

One of the solutions to increase the heating value can the usage of fuel with high extractive content. Wood has a high concentration of extractives which directly affects the heating value of the fuel. It has been reported that extractives play an important role in increasing the heating value, thus, making the fuel desirable for combustion (Table 2) (Demirbas and Demirbas 2009).

Torrefaction and densification techniques have proved to be advantageous in improving the heating value of the fuel (Table 2). Torrefaction is a process in which biomass, in the form of chips or crushed, is fed for pyrolysis in an inert atmosphere at 200–300 °C which results in fuel with better-grinding properties and high carbon content. The solid fuels with a heating value of 21 MJ/kg had a significant increase in their heating value (23.2 MJ/kg) after a torrefaction time of 60 min. The torrefaction time needs to be optimized as after some time, it does not affect the heating value. Moreover, this process can reduce the chlorine content of fuel up to 90% with results in reduced ash content. With an increase in torrefaction temperature, the carbon and hydrogen contents increase in a significant amount, and oxygen content is reduced. After torrefaction, often densification is done in pellet mills to increase the density of the biomass fuel, which eventually increases the heating value of the same, thus, improving the combustion properties. Hydrothermal carbonization is also a pre-treatment method that has a better effect on fuel properties than torrefaction (Table 2). The durability and density of fuels undergoing hydrothermal carbonization decrease with increasing temperature. When miscanthus was used as a fuel, this process was highly effective in reducing the concentration of alkali metals, ash content, and increasing the carbon content, resulting in the increased heating value of the fuel (Kambo and Dutta 2014; Keipi et al. 2014; Wattananoi et al. 2011).

Liquid hot water treatment has also proved to be an effective solution for increasing the heating value of the fuel (Table 2). Hot water treatment results in increased heating value with a density increase of 16–25%. An increase in density increases the forces between molecules so that more heat is required to break those forces (Runge et al. 2013).

Proximate analysis using the artificial neural network can also be used to predict the heating values of different fuels depending on their compositions and accordingly, the fuel can be changed for increased heating value (Table 2) (Uzun et al. 2017).

3.6.1 Description

The storage problem is directly linked to the moisture content present in the biomass. Biomass fuels with low moisture content (around 15–20%) can be stored without drying but biomass fuels with high moisture content (around 40–50%) must be dried before storage otherwise it leads to deterioration of biomass, fire risks and loss of useful energy. Often, biomass is left open on fields for drying which does not reduce its moisture content to an acceptable level and also leads to loss of material. The farmers working nearby may experience serious health issues due to the formation of fungus and spores. Farmers are required to prepare the land for the next crop and so, they hardly allow storage of biomass on open fields. Storage at intermediate places also results in increased transportation cost due to transportation of biomass from source to intermediate place and then from that storage place to the power plant. Moreover, due to the low density of biomass, a huge amount of material is required for a considerable amount of energy. Storage can be done at supply, intermediate, and conversion sites but supply and conversion sites are usually time-constrained for agricultural and wood residues, therefore, intermediate storage is often preferred for them. Forest residues and remain in forests for several months during which open-air drying takes place. On storage, it was noticed that internal heat generation and decay of biomass can lead to fire so, it is necessary to take measures for solving storage and transportation problems (He et al. 2014; Malladi and Sowlati 2018, 2017; Rentizelas et al. 2009).

3.6.2 Solutions

Setting up of intermediate storage facility close to the power plant can be one of the solutions to handling storage problems (Table 2). The moisture content can be reduced to an acceptable level with no formation of fungus and spores. The energy of biomass can be converted efficiently with reduced transportation costs. Usually, biomass that is collected from fields contains high amount of moisture and needs to be dried in storage facilities before using as a fuel for biomass boilers. Drying of biomass usually takes place by external drying process which involves the use of hot air as an input. So, biomass containing low moisture can be dried using external drying process and will cause no degradation of biomass during the drying process. But, biomass containing high amounts of moisture can be degraded with time so, this process is not favorable for biomass containing high moisture content due to the risk of biomass degradation (Rentizelas et al. 2009). Other solutions can be adopted for biomass with high moisture content

A terminal site can be made in between the supply and conversion sites where continuous quality up-gradation can take place which would prevent deterioration of the biomass and this terminal site can provide biomass in varied seasons (Table 2). 4–11% of moisture content can be removed from biomass at this terminal site. The model has been designed which performs calculations on the amount of biomass which is to be supplied from open-air storage to closed shed storage and the amount of biomass that is to be stored in each storage (Gautam et al. 2017).

A model has been made to reduce the risk of fire when the amount of biomass exceeds the limit (Table 2). When storage is more than the prescribed limit, the model applies extra handling and personnel charges so that proper care is given to biomass, resulting in reduced risk of fire (Shabani and Sowlati 2013).

To reduce transportation costs, mixed transport can be used which may include transportation from the supply site to the intermediate site through trains, and then final transportation from the intermediate site to the conversions site can be through trucks (Table 2). Pipelines can also be used for long-distance transportation of biomass mixed with carrying fluid like oil or water but it results in the reduced heating value of biomass (Flodén and Williamsson 2016; Malladi and Sowlati 2018).

Pre-processing of biomass is another solution for efficient storage and transportation because pre-processing of agricultural and forest residues results in increased density which reduces storage and transportation costs (Table 2). Pre-processing includes the conversion of agricultural residues into bales, forest residues undergo chipping and grinding. The pre-processing of biomass at intermediate storage locations has proved to be highly efficient (Flisberg et al. 2015).

3.7.1 Description

Corrosion is a serious problem due to which rupture and thinning of boiler tubes takes place resulting in an unwanted shutdown of boilers. Superheater, reheater, economizer, and air preheater are often at the risk of corrosion (Kawahara 2016b). In comparison to fossil fuels, biomass contains higher amounts of alkali metals, ash content and moisture. On the combustion of biomass, alkali metals are often vaporized in the form of salts containing sulfur, silicon, potassium, and chlorine, which are highly corrosive (Jiamin and Zhansong 2009). With boilers operating at a temperature of 750 °C, these salts get deposited on boiler tubes. The tubes used in boilers are such that they form a protective oxide layer on them at high temperatures. On the deposition of these salts, the oxide layer dissolves, thus, leaving the boiler tubes unprotected. When moisture present in biomass reacts with unprotected surfaces, it leads to high-temperature corrosion which degrades the surfaces, resulting in combustion inefficiency and ineffectiveness (Coleman et al. 2008; Demirbas 2005a; Fauchais and Vardelle 2012; Mahajan and Chhibber 2019; Oksa et al. 2014a; Yuan and Wang 2010).

3.7.2 Solutions

From the literature, mainly two solutions have come up to control corrosion. First is the use of superalloys as a material for boiler tubes (Table 2). The second solution is the application of thermal coatings, by different thermal spray techniques, on the existing material of boiler tubes (Kalivodova et al. 2005; Sidhu et al. 2006; Zhang et al. 2020) (Table 2). Another solution for corrosion reduction can be the implementation of techniques, for reducing chlorine content present in biomass, which includes pre-treatment, adsorption, and absorption using various chemicals as discussed above. Table 3 shows the chemical composition of alloys used in this literature.

Failure usually takes place due to residual stresses and intergranular corrosion so, Gao et al. (Gao et al. 2012) developed a novel stainless steel Super 304H with high-temperature strength and excellent corrosion resistance. Boiler tubes made with this material have proved to be corrosion resistant. Superalloy C22 when tested as superheater material, in 80 wt% KCl + 20 wt% K₂SO₄ for 218 h, has shown best corrosion resistance due to stable nickel, iron and chromium oxides present on it, which were responsible for corrosion-resistant behavior (Luo et al. 2012). On comparing the corrosion resistance of Superni 718 and Superni 76, it was observed that Superni 76 showed better corrosion resistance due to the presence of chromium oxide, ferrous oxide and NiO. This superalloy, when tested at 900 °C in Na₂SO₄ + 60% V₂O₅, gave the best corrosion resistance out of Superni 718 and Superfer 800H due to the presence of protective oxides. Thus, Superni 76 can be used as boiler tube material (Gupta et al. 2012; Kamal et al. 2010). Ni₃Al based alloys have also shown good corrosion resistant properties with IC21 having the best corrosion resistance out of IC32 and IC6. Ni₃Al based IC21 is favorable for its usage in the salt environment of the boiler, with the addition of yttrium and chromium which are responsible for improving the corrosion resistance of IC21 alloy. Nickel-based N07263 has shown good corrosion resistance in the presence of salts due to its dense, adherent and continuous layer on surfaces (Cho et al. 2012; Zhao et al. 2014). When superalloys like Superni 75, Superni 600, Superni 718 and superfer 800 were placed in a boiler environment at 540 °C for 1000 h, maximum weight loss was observed for Superni 600 and minimum weight loss was observed for Superni 718. Fly-ash particles often deposit on superalloy surfaces, forming oxides which further prevent penetration of other particles, thus, forming a protective layer against corrosive species (Mishra et al. 2014).

Coatings on superalloys and other substrates have come up as an economical solution for controlling high-temperature corrosion (Kaur et al. 2008). Usually, the cost is very high on changing the material of boiler tubes and so, coatings are mostly preferred. On detonation spray of 75Ni25Cr on 347HSS, the coating remained intact to the substrate with less metal loss in comparison to that of the bare sample. Cr₂O_3, K₂CrO₄ and NiCr₂O₄ oxides were responsible for corrosion resistance while iron oxides failed to serve the purpose because of their porous nature (Gada et al. 2019). The application of chromium oxide coating (plasma-sprayed) on SA213-T22 alloy has helped in preventing cracks, spalling, scales and weight gain during corrosion at 850 °C. Apart from chromium oxide, Cr₃C₂–25NiCr (HVOF sprayed) has also reduced corrosion of base material by the formation of protective oxides with a 25% decrease in corrosion after application of the coating. The addition of molybdenum and aluminium to nickel-based HVOF coatings enhanced the high-temperature corrosion resistance of the substrate. The coating of NiCr on ASTM-SA210 Grade A1 alloy gives the best corrosion resistance while WC-Co coating gives the worst corrosion resistance. Chromium rich oxide scale due to Cr₃C₂–NiCr coating has contributed to corrosion reduction on SAE-347H steel at 700 °C (Kaur et al. 2009, 2012; Oksa et al. 2014a; Singh et al. 2006, 2017).

Coatings can also be applied to superalloys to enhance their corrosion-resistant behavior. Superni 76 has already proved to increase the corrosion resistance of boiler steel but the application of NiCrAlY coating can further improve the corrosion resistance of superalloy by forming a continuous layer of aluminium oxide (Rana et al. 2014). The corrosion resistance of Superfer 800 can be increased by the application of either Ni–5Al or NiCrAl coatings through HVOF spraying technique, which leads to the formation of aluminium and chromium oxides, preventing further penetration of corrosive species (Mahesh et al. 2008a,b). For improving corrosion resistance of Cr₃C₂–25NiCr coated Superni 600, ceria oxide can be added to Cr₃C₂–25NiCr in an optimum weight percentage, leading to the formation of CeO₂ and CeS, which are responsible for reducing porous scale and weight loss in bare Superni 600 (Kamal et al. 2008; Mudgal et al. 2014a). On addition of ceria oxide, along with CeO₂ and CeS, the formation of Cr₂O₃, NiCr₂O₄, NiS and Ni was also observed which provided high temperature corrosion resistance to Superni 600. Table 4 summarizes the performance of various alloys, in which the “Performance” column shows comparison of alloys in decreasing order of their corrosion resistance.

Application of coatings enhances the corrosion resistance of alloys which can be observed from Table 5. HVOF thermal spraying technique of a coating with high chromium content has been preferred for substrates because of the good adherence and high-density properties achieved by HVOF process. Also, chromium coating on reaction with salts forms chromium oxide which plays a key role in improving the corrosion resistance of the substrate. However, it has been observed that the type of biomass being burnt and thermal cycles taking place inside the biomass boiler also play a primary role in the determination of the coating required. Therefore, for the wood-fired plant with less number of thermal cycles, Ni and Ni₂Al₃ coatings are preferred, which on the other hand, show poor behavior in fire straw plants with high thermal cycles (Oksa et al. 2013; Wu et al. 2018). Thus, the coating to be used depends on the material of the boiler tube, number of thermal cycles, coating technique and type of biomass being burnt.