Ash-related issues during biomass combustion: Alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures

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Abstract

Biomass is available from many sources or can be mass-produced. Moreover, biomass has a high energy-generation potential, produces less toxic emissions than some other fuels, is mostly carbon neutrality, and burns easily. Biomass has been widely utilized as a raw material in thermal chemical conversion, replacing coal and oil, including power generation. Biomass firing and co-firing in pulverized coal boilers, fluidized bed boilers, and grate furnaces or stokerfed boilers have been developed around the world because of the worsening environmental problems and developing energy crisis. However, many issues hinder the efficient and clean utilization of biomass in energy applications. They include preparation, firing and co-firing, and ash-related issues during and after combustion. In particular, ash-related issues, including alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, and ash utilization, are among the most challenging problems. The current review provides a summary of knowledge and research developments concerning these ash-related issues. It also gives an in-depth analysis and discussion on the formation mechanisms, urgent requirements, and potential countermeasures including the use of additives, co-firing, leaching, and alloying.

Alkali species, particularly alkali chlorides and sulfates, cause alkali-induced slagging during biomass combustion. Thus, the mechanisms of generation, transformation, and sequestration of alkali species and the formation and growth of alkali-induced slagging, formed as an alternating overlapping multi-layered structure, are discussed in detail. For silicate melt-induced slagging (ash fusion), the evolutions of chemical composition of both the elements and minerals in the ash during combustion and existing problems in testing are overviewed. Pseudo-4D phase diagrams of (Ma2O)-MaeO-P2O5-Al2O3 and (Ma2O)-MaeO-SiO2-Al2O3 are proposed as effective tools to predict ash fusion characteristics and the properties of melt-induced slagging. Concerning agglomeration that typically occurs in fluidized bed furnaces, melt-induced and coating-induced agglomeration and coating-forming mechanisms are highlighted. Concerning corrosion, seven corrosion mechanisms associated with Cl2, gaseous, solid/deposited, and molten alkali chlorides, molten alkali sulfates and carbonates, and the sulfation/silication of alkali chlorides are comprehensively reviewed. The effects of alloying, salt state (solid, molten, or gaseous), combustion atmosphere, and temperature are also discussed systematically. For ash utilization, potential approaches to the use of fly ash, bottom ash, and biomass/coal co-fired ash as construction and agricultural materials are explored.

Several criteria or evaluation indexes are introduced for alkali-induced slagging and agglomeration, and chemical equilibrium calculation and multicomponent phase diagrams of silicate melt-induced slagging and agglomeration. Meanwhile, remedies, including the use of additives, co-firing, leaching, alloying, and the establishment of regulations, are discussed.

It is suggested that considerable attention should be focused on an understanding of the kinetics of alkali chemistry, which is essential for the transformation and sequestration of alkali species. A combination of heterogeneous chemical kinetics and multiphase equilibrium modeling is critical to estimating the speciation, saturation levels, and the presence of melt of the ash-forming matter. Further practical evaluation and improvement of the existing criterion numbers of alkali-induced slagging and agglomeration should be improved. The pseudo-4D phase diagrams of (Ma2O)-MaeO-P2O5-Al2O3 and (Ma2O)-MaeO-SiO2-Al2O3 should be constructed from the data derived from real biomass ashes rather than those of simulated ashes in order to provide the capability to predict the properties of silicate melt-induced slagging. Apart from Cr, research should be conducted to understand the effects of Si, Al, and Co, which exhibit high corrosion resistance, and heavy metals such as Zn and Pb, which may form low-melting chlorides that accelerate corrosion. Regulations, cooperation among biomass-fired power plants and other industries, potential technical research, and logistics should be strengthened to enable the extensive utilization of biomass ash. Finally, alkali-induced slagging, silicate melt-induced slagging, agglomeration, and corrosion occur concurrently, and thus, these issues should be investigated jointly rather than separately.

Introduction

Biomass as a sufficiently “green” renewable and CO2-neutral energy source has attracted worldwide attention because of the worsening energy crisis and environmental issues. By switching from coal and natural gas to biomass, the net CO2 emission per unit heating value can be reduced by 93% and 84%, respectively [1]. In 2004, the total global biomass power installed capacity reached 39 GW, and the annual energy generation achieved was approximately 200 billion kWh [2]. Data show that biomass power installed capacity in the USA has reached 10 GW [3], and a similar increase to 30 GW by 2020 is expected in China [4].

Biomass fuel is defined as any solid organic material that can be burned and used as a source of fuel [5]. Biomass can be classified into four major categories, according to its source, i.e., woody, agricultural, waste, and excrement. Among these, woody biomass is the largest biomass energy source; it covers forest residues (e.g., dead trees, branches and tree stumps, leaves), landscaping residues (yard clippings), industrial wood residue (wood chips, sawdust, etc.), waste wood residues, and so on. Agricultural biomass contains straw and other harvesting residues from agriculture, energy plants from agriculture, residues from the food industry, and grass. Waste includes municipal solid waste, domestic wastewater, commercial waste, and construction waste. Excrement includes farm slurry/excrement and organic waste from households and industry.

Despite its well-known advantages such as high-energy-generation potential and carbon dioxide neutrality that encourage the rapid development of biomass-fired power plants, biomass combustion remains challenging for several reasons. These include problems in the pre-preparation of biofuel (referring to collection, transportation, upgrading by leaching, torrefaction and pelleting, and milling), firing and co-firing technologies (encompassing direct firing, injection co-firing, co-milling co-firing, pre-gasification co-firing, and parallel co-firing in pulverized coal (PC) boilers, FB boilers, and grate or fixed bed furnaces), and ash-related issues that occur both during and after combustion [5]. As shown in Fig. 1, ash-related problems, including alkali-induced slagging [6], [7], [11], [12], [13], [14], silicate melt-induced slagging (ash fusion) [3], [7], [9], [14], [15], [16], agglomeration [8], [14], [17], [18], [19], [20], corrosion [10], [14], [21], [22], [23], [24], and ash utilization [25], [26], are the most intractable issues. Whether in FBs or grate furnaces, high concentrations of Cl and alkali metals (K and Na) in biomass result in the rapid buildup of unmanageable deposits on the fired surfaces, particularly alkali-induced slagging on the superheater [3], [6], [7], [9], [11], [12], [13], [14], [15], [16], [27], [28] and silicate melt-induced slagging on the water wall [3], which inhibit heat transfer and reduce boiler efficiency [26], [29]. Furthermore, accumulated ash with a high Cl concentration on tube surfaces may lead to corrosion underneath the deposit [14], [22], [28]. Meanwhile, agglomeration, which originates from fused or partly fused ash, is another major challenge in biomass combustion in FBs and frequently results in defluidization and unscheduled shutdown of the entire power plant [8], [14], [17], [18], [19], [20], [28], [30]. However, the corresponding mechanisms and countermeasures for alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, and corrosion, and the utilization/disposal principles of ash residues from biomass-fired power plants remain unclear [28]. Unlike coal, biomass has various sources. For example, woody biomass is low in Si and K, yet high in Ca. Agricultural residues are high in Si and K, yet low in Ca. In contrast, animal residues are high in both P and Ca [28], [31]. Furthermore, even for a specific biomass, different planting environments [12], [32], harvest seasons [20], [33], and different parts of the biomass [34], [35] can produce distinct ash contents and compositions. This results in unpredictable and highly variable ash-related issues.

KCl and K2SO4 are the dominant alkali-containing substances that influence biomass ash-related issues [36]. While K2SO4 is a dominant species that nucleates when the gas temperature is reduced [37], KCl condenses on the K2SO4 nuclei at a low temperature [38]. Similar results were reported by Reichelt et al. [39] and Nutalapati et al. [31] from both experimental data and FACTSAGE simulation calculations. They found that K2SO4 mainly accumulates on high-temperature heating surfaces, whereas KCl accumulates on low-temperature heating surfaces. In addition, Li et al. [3] and authors [6] have indicated that both K2Ca(SO4)2 and K3Na(SO4)2 play a significant role in slagging.

A number of experimental studies concerning additives [40], [41], co-firing [42], [43], chemical pretreatment [44], [45], and alloying [46], [47], all of which change the generation and transformation processes of alkali chlorides and sulfates, have been conducted to solve ash-related problems. However, these are all possible targets because of the variability in biomass species, plant environment, combustion temperature, and atmosphere [40], [48], [49], [50], [51]; remedial methods that may be effective under one set of conditions may prove ineffective for other types of fuel and under different conditions. As an alternative, to solve these troublesome ash-related problems fundamentally and provide useful guidelines for the selection of co-firing fuels, additives, and biomass fired in utility boilers, several researchers began to focus on criterion numbers or evaluation indexes such as the alkali index (K2O + Na2O) kg⋅GJ−1 [52], (Na + K+2Mg+2Ca)/S ratio [53], (K + Na)/(Ca + Mg) ratio [54], and S/Cl ratio [55].

Aside from the experimental research on additives, co-firing, chemical pretreatment, and evaluation indexes, several researchers studied ash transformation by means of thermal-balance calculation software such as FACTSAGE, which is based on Gibbs free energy minimization [31], [56], [57], [58]. However, Bostrom et al. [48] pointed out that the ash transformation reactions may be influenced by temperature, residence time, air supply, flue gas velocity, and other factors. Because in the flame of a powder burner or PC furnace, short residence times limit the interactions and encounters between the ash and gases; the system is far from being at equilibrium. The main challenges are not only to predict local compositions and implement relevant transport equations for particles, but also to obtain a reasonable estimate of the speciation of inorganic matter, i.e., ash-forming elements, in each volume cell rather than in the pseudo-final state.

Although several studies including experiments, evaluation index analysis, and thermal-balance calculation/simulations have been conducted, ash-related problems remain unresolved, and knowledge about their causes is fragmentary. Therefore, the focus of this study is to review the progress of research and to reveal the formation mechanisms, urgent requirements, and potential countermeasures to solve the abovementioned ash-related problems. The paper consists of three main parts, divided according to the amount of research attention they have received and their importance in current practical operations in biomass power generation:

  • Part I presents the main ash-related issues, including alkali-induced slagging, silicate melt-induced slagging (ash fusion), and agglomeration, as well as countermeasures related to the use of additives, co-firing, and leaching.

  • Part II presents a discussion of the additional ash-related issues, i.e., corrosion, including various corrosion mechanisms and countermeasures or influence factors.

  • Part III focuses on biomass ash utilization as construction materials and agricultural soil ameliorants. Limitations and counterplans are also discussed.

Section snippets

Part I. Main ash-related issues during combustion: alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, and customized countermeasures including additives, co-firing, and leaching

A schematic of ash formation and transformation mechanism in biomass combustion is shown in Fig. 2. Similar to pulverized coal combustion [59], during the combustion of biomass particles and the formation of char particles, volatile organo-metallic compounds (containing Cr, V, etc.) are first released followed by devolatilization; then, partial alkali and alkali-earth elements (e.g., K, Na, and Ca) and volatile trace elements (eg., Hg, As, and Se) diffused out of the char. As the gas

Corrosion mechanisms

During combustion, the flue gas containing Cl2(g), HCl(g), NaCl(aerosol), KCl(aerosol), and others species may cause direct corrosion by accelerating the oxidation of the metal alloys. It may also influence the corrosion caused by other mechanisms such as the corrosion of superheater tubes by molten alkali salts. Alkali chlorides deposited on superheater tubes are well known to cause corrosion in boilers burning biofuels and waste, and they are also the main factor that limits the final steam

Part III. Ash utilization

With the rapid development of biomass combustion power generation, the large volume of ash by-product has become an intractable problem throughout the world. According to statistics, the worldwide annual output of biomass ash, which is comparable to that of coal ash with an annual production of 780 million tons, is approximately 476 million tons [173]. Therefore, an eco-friendly and economic solution to recycle this by-product is essential.

According to the statistics provided by the American

Conclusions

Although biomass firing and co-firing power plants have been rapidly developed around the world, these power plants continue to face various challenges that arise during combustion. This study focused on intractable ash-related issues, namely, alkali-induced slagging, silicate melt-induce slagging (ash fusion), agglomeration, corrosion, and ash utilization, as well as potential remedies, including the use of additives, co-firing, leaching, alloying, and others.

For alkali-induced slagging, which

Acknowledgements

Y Niu conceived and performed the whole research; collected and analyzed the data; and wrote the paper. H Tan and S Hui were involved in the design of Sections 2.2.1-2.2.3. The present work was supported by National Natural Science Foundation of China (51406149), the Fundamental Research Funds for the Central Universities (2014gjhz08), and China Postdoctoral Science Foundation funded project (2014T70921). The authors would like to thank Xiaolin Wei from Chinese Academy of Sciences for providing

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