Advancing Power Generation: Optimizing Coal Quality for CFB Boilers

China’s energy landscape is characterized by a reliance on coal, limited oil, and minimal gas resources. As such, coal holds a significant position within China’s energy consumption framework. Coal-fired power generation functions as a stabilizing force in China’s power sector and remains the predominant method of coal utilization. Within China’s coal reserves, a substantial portion consists of low-quality coal. The mining and processing of coal give rise to substantial quantities of washed coal, coal gangue, coal sludge, and other by-products with low calorific value. The effective integration of these low-quality coals and refractory fuels with low calorific value poses challenges when utilizing traditional pulverized coal combustion technologies.

Furthermore, the escalating issue of air pollution stemming from coal combustion demands immediate attention. Thermal power units, which constitute major consumers of coal combustion, face a pressing need to enhance efficiency and curtail pollutant emissions. The pursuit of clean and efficient coal utilization has emerged as a focal point of enduring interest among researchers. In this context, circulating fluidized bed combustion technology has garnered substantial recognition as a key facet of clean coal combustion technology.

In contrast to alternative boiler types, circulating fluidized bed combustion technology boasts an array of advantages. These encompass a broad spectrum of fuel adaptability, robust load regulation capabilities, and reduced NOx emissions. Consequently, this technology has witnessed rapid advancement in recent years, driven by its remarkable attributes.

Circulating Fluidized Bed Boiler (Cfb Boiler)

Evolution of Circulating Fluidized Bed Boiler (CFB Boiler) Technology

In 1979, Ahlstrom of Finland pioneered the development of the first international commercial Circulating Fluidized Bed Boiler (CFB Boiler), which was successfully commissioned in Pihlava. This innovation garnered significant attention due to its inherent benefits of adaptable load regulation and versatility in fuel utilization. Subsequently, the world witnessed a milestone in 1995 when the inaugural 250 MW sub-critical parameter Circulating Fluidized Bed Boiler (CFB Boiler) commenced operations in Provence, France. This event marked the commencement of the era of large-scale circulating fluidized bed technology.

Currently, the global landscape boasts over 3000 operational Circulating Fluidized Bed Boilers (CFB Boilers), reflecting the widespread adoption of this technology. Notably, China has emerged as a prominent leader in the research and development of fluidized bed combustion technology over the past half-century. According to statistical data up to the beginning of 2021, China has attained remarkable activity in this realm, with approximately 440 Circulating Fluidized Bed Boilers (CFB Boilers) exceeding a capacity of 410 t/h in active operation. This collective capacity equates to an aggregate installed capacity exceeding 83 GW, underscoring China’s significant strides in this domain.

China’s Evolution in Circulating Fluidized Bed Boiler (CFB Boiler) Technology

Since the 1960s, China has been actively engaged in the development of bubbling bed boilers, progressively formulating its distinct combustion theory and boiler design for bubbling bed applications. By the early 1980s, China had successfully installed approximately 4,000 domestically engineered bubbling bed boilers, a feat that positioned it as the global leader in installed capacity for this technology.

In the early 1980s, motivated by international advancements in circulating fluidized bed combustion technology, Chinese researchers and engineers pivoted towards the exploration of this innovative field. Concurrently, they undertook the research and development of Circulating Fluidized Bed Boilers (CFB Boilers) based on the foundational insights gained from bubbling bed research. Distinguished institutions such as Tsinghua University and the Chinese Academy of Sciences have remained at the forefront of national-level initiatives pertaining to Circulating Fluidized Bed Boiler (CFB Boiler) combustion. Their endeavors have yielded substantial contributions to the maturation of CFB Boiler design theory.

Xi’an Jiaotong University, on the other hand, has directed its focus toward comprehending the fluid dynamics within the tube’s working mass. Meanwhile, Southeast University, Zhejiang University, Huazhong University of Science and Technology, Chongqing University of Technology, and Habin University of Technology have undertaken comprehensive investigations into a broad spectrum of fundamental combustion challenges. Among these pursuits, the Chinese Academy of Sciences has emerged as a prolific developer of numerous patents in the realm of CFB boiler technology.

China’s journey in CFB Boiler technology encapsulates a progression from pioneering bubbling bed boilers to asserting its prominence in the global CFB Boiler landscape. This transformation has been characterized by continuous innovation, collaborative research, and a concerted drive towards efficient and sustainable combustion solutions.

Evolving Phases of Circulating Fluidized Bed Combustion Technology in China

The trajectory of circulating fluidized bed combustion technology’s application and advancement in China can be delineated into distinct stages, characterized by incremental learning and refinement:

Stage 1: Learning and Accumulation (1980s-1990s)

The initial stage, spanning the 1980s and 1990s, was marked by the acquisition, accumulation, and refinement of circulating fluidized bed combustion technology. In the preceding decades, major engineering institutions and thermal research establishments in China had already embarked on research concerning bubbling fluidized bed technology. Their endeavors encompassed an array of aspects, including the fluid dynamics and combustion dynamics within the bubbling bed, heat exchange between the fluid medium and the tube bundle, and the behavior of coal briquettes within the bed.

During this phase, the successful development of a 270 t/h Circulating Fluidized Bed Boiler (CFB Boiler) by Germany’s Ruch company in the 1980s garnered the attention of Chinese scientists and engineers. Bolstered by governmental support, Chinese boiler manufacturers initiated collaborative efforts with international counterparts to either import or license relevant equipment and technology. Drawing upon their amalgamated knowledge of this newfound technology and their prior experience in bubbling bed boiler development, Chinese engineers commenced the autonomous creation of Circulating Fluidized Bed Boilers (CFB Boilers). Through several years of independent exploration, coupled with assimilation and innovation, Chinese researchers and engineers systematically evolved circulating fluidized bed theory from theoretical concepts to pragmatic engineering applications.

The result of these collective efforts culminated in the successful creation of a considerable number of Circulating Fluidized Bed Boilers (CFB Boilers) with capacities below 220 t/h. Notably, significant strides were made in securing patents for pivotal components such as separators and material return devices, underlining China’s attainment of proprietary technologies and intellectual property rights in crucial areas of this technology.

Progressive Advancements in Circulating Fluidized Bed Boiler (CFB Boiler) Combustion Technology

Another pivotal phase unfolds in the swift evolution of Circulating Fluidized Bed Boiler (CFB Boiler) combustion technology, situated at the outset of the 21st century. Post-2000, driven by the demand for expansive circulating fluidized bed applications in the electricity sector, China’s prominent boiler manufacturers undertook strategic partnerships with renowned global entities. Notably, collaborations were established with entities such as Foster Wheeler Company of the USA, EVT Company of Germany, and ABB-CE Company of the USA. These partnerships paved the way for the import and assimilation of Circulating Fluidized Bed Boiler (CFB Boiler) technology ranging from 100 to 135 MW.

Simultaneously, China’s leading boiler manufacturers secured a licensing agreement for 300 MW sub-critical circulating fluidized bed combustion technology from ALSTOM of France. This achievement laid the groundwork for the conceptualization, design, and manufacture of a 300 MW sub-critical Circulating Fluidized Bed Boiler (CFB Boiler), which was subsequently operationalized in Baima, Sichuan.

The practical operation of this Circulating Fluidized Bed Boiler (CFB Boiler) served as a catalyst for furthering fundamental research into circulating fluidized bed technology. With the accumulation of prior experiences, China’s prowess in the design, manufacturing, and operational facets of Circulating Fluidized Bed Boilers (CFB Boilers) has matured significantly. The technology’s distinct attributes, including its impressive fuel adaptability and robust emission control capabilities, have garnered widespread recognition.

This stage underscored China’s substantial strides in embracing and mastering advanced circulating fluidized bed combustion technology. The amalgamation of international partnerships, licensing agreements, and indigenous expertise has fueled China’s transformation into a formidable player within the global CFB Boiler landscape.

Advancing to Supercritical Circulating Fluidized Bed Boiler (CFB Boiler) Technology

Following the successful operation of numerous large-capacity Circulating Fluidized Bed Boilers (CFB Boilers), China embarked on a trajectory that mirrored international trends. The nation initiated research and development efforts in the realm of supercritical Circulating Fluidized Bed Boilers (CFB Boilers). In 2006, with governmental support, the development of 600 MW supercritical Circulating Fluidized Bed Boilers (CFB Boilers) was set in motion.

Distinguished Chinese boiler manufacturers, including Xi’an Institute of Thermal Engineering, Institute of Engineering Thermophysics of the Chinese Academy of Sciences, Tsinghua University, and Zhejiang University, presented their respective programs for supercritical Circulating Fluidized Bed Boilers (CFB Boilers). Through unwavering dedication, researchers across these institutions collectively orchestrated the successful deployment of China’s indigenous 600 MW Circulating Fluidized Bed Boiler (CFB Boiler) at the Sichuan Baima Power Plant in April 2013. Rigorous performance testing confirmed that the boiler’s operational performance aligned with its design specifications.

The achievement of this milestone earned international acclaim, with the International Energy Agency recognizing China’s 600 MW Circulating Fluidized Bed Boiler (CFB Boiler) as a momentous breakthrough in the advancement of circulating fluidized bed combustion technology. This accomplishment not only signifies China’s ascent to the forefront of circulating fluidized bed technology but also solidifies its global leadership position.

Buoyed by the triumph of developing an array of supercritical Circulating Fluidized Bed Boilers (CFB Boilers), China has charted a course to enhance boiler parameters and capacity further. The nation has initiated research and development endeavors in the domain of ultra-supercritical Circulating Fluidized Bed Boilers (CFB Boilers), poised to further elevate its technological prowess and drive the envelope of innovation in this field.

Circulating Fluidized Bed Boiler (Cfb Boiler)

Application and Advancements in Circulating Fluidized Bed Technology

With over four decades dedicated to the research and development of circulating fluidized bed technology, China has not only cultivated independent intellectual property rights from theoretical frameworks to practical implementations but also established a comprehensive system encompassing design, manufacturing, installation, operation, and maintenance.

In line with this enduring commitment to innovation, data sourced from China Knowledge Network and national patent databases reveal an impressive annual publication count of over 700 articles in the Circulating Fluidized Bed Boiler (CFB Boiler) field, alongside an annual tally of more than 400 authorized patents.

According to data compiled from scientific and technological resources, up to February 2021, China has successfully commissioned over 440 Circulating Fluidized Bed Boilers (CFB Boilers) exceeding 100 MW in capacity. Notably, this count includes 49 operational supercritical Circulating Fluidized Bed Boilers (CFB Boilers). Among them, three belong to the 600-650 MW supercritical class, and the remaining 46 fall within the 350 MW supercritical class. China’s attainment of the highest circulating fluidized bed unit capacity globally underscores its prominence in this domain.

Presently, China is actively engrossed in the research and development of ultra-supercritical circulating fluidized bed units with capacities of 700 MW and 1000 MW. Notably, in September 2022, a landmark achievement was realized with the formulation of the world’s first 700 MW ultra-supercritical Circulating Fluidized Bed Boiler (CFB Boiler) scheme. This innovative endeavor, based on the design of the Guangdong Guoyue Shaoguan Comprehensive Utilization of Power Generation Expansion Project, successfully navigated the evaluation conducted by an expert panel.

These remarkable strides exemplify China’s unwavering commitment to pioneering advancements in circulating fluidized bed technology, epitomizing its drive to redefine the boundaries of innovation within the global energy landscape.

Prospects for Circulating Fluidized Bed Technology

3.1 Fuel Adaptability

As the proportion of raw coal washing increases, a substantial annual volume of 600 to 800 million tons of low-quality fuels is anticipated for utilization. In this context, the Circulating Fluidized Bed Boiler (CFB Boiler) emerges as a formidable solution for mitigating the challenges posed by subpar coal quality. Its inherent advantages hold significant potential for addressing low calorific value by-products such as washed coal, gangue, and sludge – byproducts that arise during coal mining and processing. Consequently, specialized Circulating Fluidized Bed Boilers (CFB Boilers) designed for low-heat-value coal, refractory anthracite, high-moisture lignite, and high-sodium coal are pivotal for the effective utilization of these unconventional fuel sources.

3.2 Rapid Load Flexibility

The abundance of blazing bed material within the chamber of the Circulating Fluidized Bed Boiler (CFB Boiler) engenders stable coal combustion, consequently conferring robust low-load operation capabilities. This attribute positions it as an ideal candidate for deep peaking applications. Advancing this technology demands exploration of novel theories and processes, thereby endowing Circulating Fluidized Bed units with elevated load-shifting speeds and an extensive range of load regulation capabilities. Furthermore, coupling Circulating Fluidized Bed power generation with energy storage technology holds the promise of creating efficient and scalable energy storage solutions. This synergy could enhance the unit’s deep peaking potential while bolstering the consumption of renewable energy.

3.3 Optimizing Ash Utilization

Circulating Fluidized Bed Boilers (CFB Boilers) boast the versatility to combust a diverse array of fuels, resulting in a varied ash composition. Currently, ash utilization technologies remain in a nascent stage, leading to suboptimal ash utilization rates. Addressing this, comprehensive research on ash characteristics is imperative. Elevating the quality of ash and enhancing its economic value represents a crucial avenue. This entails promoting the utilization of desulfurization ash from Circulating Fluidized Bed Boilers (CFB Boilers) in applications such as alkaline soil improvement, composite fertilizers, road base materials, gypsum, and sintered bricks.

Moreover, the incorporation of dry jigging sorting schemes yields notable benefits, significantly reducing raw coal ash content. The corresponding reduction in sulfur content, approximately 50%, compared to pre-sorting levels is a noteworthy accomplishment. The outcomes of these tests affirm the viability of sorting higher quality concentrates to attain ash content below the target of 44%.

In sum, the future trajectory of Circulating Fluidized Bed technology hinges upon the strategic exploitation of its fuel adaptability, optimization of load flexibility, and enhanced utilization of ash. These pathways collectively contribute to the technology’s sustainable growth and its pivotal role in transforming energy landscapes.

Conclusion: Enhancing Coal Quality for Improved Power Generation

The Brazilian Candiota coal, characterized by its high ash content (>50%), sulfur content between 1% and 3%, and total moisture content ranging from 12% to 16%, is acknowledged as a subpar power coal. To address these challenges, coal beneficiation processes must be employed to curtail ash and sulfur content while bolstering the calorific value.

Experience gleaned from the preliminary application of the Medici thermal power plant underscores the significance of coal quality control. The direct combustion of raw coal without prior sorting and treatment leads to wear and tear across critical systems, including the coal conveyor, pulverization, combustion, boiler heating surface, and flue gas treatment. This operational instability adversely impacts economic performance and the plant’s overall stability. Therefore, the adoption of appropriate coal selection processes and equipment is pivotal to regulate coal combustion quality.

Pilot studies conducted by the Federal University of Southern Rhine State and the RWTH Aachen University offer substantial data backing the choice of the sorting process at the Candiota Mine. This pragmatic insight eases the implementation of coal separation projects.

Notably, the raw coal’s high void ratio poses challenges to wet sorting methods, yielding unsatisfactory results. Wet gravity flotation techniques also generate significant waste liquid and coal sludge, prompting environmental concerns and necessitating environmental safeguards during production. Consequently, for the Candiota Coal Mine, the employment of the dry jigging coal separation process emerges as a more viable choice.

Determining the output rate of refined coal post-sorting, coal quality indices, equipment output, and other pertinent parameters necessitates rigorous economic analysis and project demonstration during the procurement phase of coal beneficiation initiatives. Subsequently, discussions with coal customers are vital to ascertain the optimal strategy moving forward. In essence, elevating coal quality through judicious beneficiation emerges as a cornerstone in fostering efficient and sustainable power generation processes.

[Source] SHAO Zhong-ming, YUAN Sheng-ming, GUO Xin-ru, WANG Fang, Development and Application Research of Circulating Fluidized Bed Boiler(CFB Boiler) Technology, Power System Engineering 2023[7]:31-39

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