Process Chemistry of Coal Utilization: Impacts of Coal Quality and Operating Conditions PDF by Stephen Niksa


Process Chemistry of Coal Utilization: Impacts of Coal Quality and Operating Conditions
By Stephen Niksa,

Process chemistry of coal utilization _ impacts of coal quality and operating conditions


Preface ix
Acronyms xv
1 Coal utilization technologies 1
1.1 Relevant domain of operating conditions 3
1.2 PCC furnaces 5
1.3 Fluidized bed technologies 10
1.4 Gasification technologies 13
1.5 Operating domain for coal conversion mechanisms 19
References 21

2 Fuel quality, thermophysical properties, and transport
coefficients 23
2.1 Coal rank 25
2.2 Coal as a binary mixture 30
2.3 Thermophysical properties of coal 37
2.4 Transport coefficients 47
2.5 Thermophysical properties of gases 51
References 52

3 Moisture release and coal drying 53
3.1 Moisture levels in coal 54
3.2 Moisture release from most coal types 55
3.3 Moisture removal from very low rank coals 57
References 71

4 Primary devolatilization behavior 73
4.1 Definitions and commercial impacts 73
4.2 The empirical basis for primary devolatilization behavior 82
4.3 Summary 133
References 138 

Further reading 141
5 Reaction mechanisms for primary devolatilization 143
5.1 Global rate expressions for primary devolatilization 146
5.2 Network depolymerization reaction mechanisms 157
5.3 Noncondensable gas compositions 193
5.4 Abridged reaction mechanisms for practical applications 201
5.5 Summary comparisons among three network models 204
5.6 Legitimate chemical reaction mechanisms 211
5.7 Particle thermal histories 212
References 217
Further reading 219
6 Quantitative interpretations of primary devolatilization
behavior 221
6.1 Quantitative model validations with data 222
6.2 Interpretations for primary devolatilization behavior 229
6.3 Indirect modeling capabilities 255
6.4 Status summary 267
References 269
Further reading 270
7 Tar decomposition 271
7.1 Commercial impacts 273
7.2 Laboratory prerequisites 275
7.3 Laboratory database on tar decomposition 278
7.4 Reaction mechanisms for tar decomposition 291
7.5 Global rates for tar decomposition and soot production 305
7.6 Status summary 316
References 318
Further reading 319
8 Volatiles reforming and volatiles combustion 321
8.1 Commercial impacts 323
8.2 Determining factors for volatiles conversion 324
8.3 Measured volatiles conversion behavior 328
8.4 Stoichiometry and thermochemistry for volatiles conversion 353
8.5 Analysis of volatiles conversion around isolated particles 356
8.6 Analysis of volatiles conversion in dense suspensions 359
References 384
Further reading 385
9 Hydropyrolysis and hydrogasification 387
9.1 Commercial impacts 390
9.2 Stages of hydropyrolysis and hydrogasification 390
9.3 Laboratory prerequisites 392
9.4 Coal conversion during hydropyrolysis and hydrogasification 397
9.5 Product distributions for hydropyrolysis and hydrogasification 413
9.6 Global rates for tar hydroconversion and oils production at
moderate temperatures 450
9.7 Summary 452
References 455
Further reading 457
Index 459

Coal is used worldwide to produce electricity, process heat, synthetic feedstocks and chemicals, metallurgical cokes for steelmaking, and specialty chemicals. The scale of these industries and the scope of their utilization technologies are among the broadest that humanity has ever devised. Whether you size it in tonnage or kilowatt-hours or monetary value, the coal enterprise is gigantic. Even while the enterprise shrinks in the developed world to contain its environmental impact, it is also expanding at a phenomenal pace across Asia, and coming to life in Africa. For as far into the future as any of us can foresee, coal utilization will keep its place among the essential technologies that can advance societies into the modern world.

During this author’s career, how we manage coal utilization technologies and perform R&D has been radically transformed. Coal characterization had been rooted in the mining industry, and pursued by analogy to rocks with optical instruments and a battery of simple, standardized tests in retorts and muffle furnaces. Two OPEC oil shocks during the 1970s prompted a complete reorganization of the coal R&D community in the United States. The commercial focus immediately shifted toward synthetic fuels and feedstocks while the government, for the first time, mobilized basic research scientists to characterize coal with the most advanced diagnostics of the day, and to unravel its conversion mechanisms at the molecular level. Indeed, most of our current understanding of coal constitution was revealed during this period.

As coals’ molecular features were exposed, a hope rose that the engineering support for coal technologies would soon be brought to parity with recent advances in petrochemical engineering. The search for reaction mechanisms was on! I vividly remember the largest hotel conference halls in Pittsburgh and Washington, District of Columbia packed to standing room capacity for project review meetings sponsored by the US Department of Energy (DoE). Audiences reacted to the latest test data and modeling approaches with vigorous debates, because the scientific findings were being fast-tracked into technology development programs. Managers at America’s largest petrochemical, aerospace, and defense contractors perceived huge payoffs down the road, while they struggled up the steep learning curves for coal handling and processing. But by the middle of the 1980s, petroleum markets had relaxed to an agreeable equilibrium and government funding in the American coal community took a nosedive.

Other coal research communities took their turn on the leading edge. Australians made seminal contributions to char oxidation kinetics throughout the 1970s and 1980s, then introduced and rapidly developed computational fluid dynamics (CFD) simulations for coal-fired furnaces. The physicochemical architecture in these first CFD simulations remains in place today, while CFD grew into a cornerstone of engineering design throughout the world. The German community led from the middle of the 1980s through the beginning of the 1990s with support from the mining industry, making major contributions to pyrolysis and gasification mechanisms, and to testing at elevated pressures. Japanese academic researchers made major contributions in coal liquefaction, and also compiled the databases on pyrolysis, oxidation, and gasification kinetics that clearly revealed the coal quality impacts for the first time. But by the dawn of the Clinton Administration in the United States, the American community had dwindled and largely withdrawn into DoE’s National Energy Technology Laboratories.

English, German, Portuguese, and American engineers rapidly expanded applications of CFD furnace simulations through the 1990s by focusing them toward NOX emissions control with coordinated testing at laboratory and pilot scale. Through the same period, researchers in the United States, United Kingdom, and Australia largely unraveled the scientific basis for ash fouling and slagging problems in utility furnaces, and Northern Europeans described how CFBCs operate. Italians and Scandinavians brought fluidized bed processing into the mainstream. From the mid-1990s through the mid-2000s, Japan re-focused its academic research on support for CFD, to extend these capabilities to gasification and other high-pressure technologies. The Australian community shared this focus on gasification technology development via CFD over most of the same time period, and also pressed forward in the rational basis to manage boiler fouling and slagging problems.

Since then, the South Korean community worked on technology development and CFD, while Australians pioneered the characterization and utilization of brown coals, which are now poised to enter the world coal trade. The Chinese community has become the most active, by far, emphasizing basic research capabilities and incremental advances to established methods for just about every utilization technology covered in earlier decades, including liquefaction, hydropyrolysis, and other long-neglected synfuels schemes. This community is very well positioned to make larger contributions.

Japan and, more recently, Germany are committing resources toward coal flame characterization based on large eddy simulations, which may ultimately eclipse conventional CFD. But compared with their former productivity, American, Japanese, English, and most other European research communities have largely receded. This juncture is an appropriate time to ask, “What was accomplished?” The widespread implementation of modern analytical techniques, especially 13C NMR, FTIR, TGA/MS, GC/MS, and a multitude of new chromatography packages, gave a much clearer picture of coal structure and constitution, albeit in qualitative terms rather than numerical values. Massive volumes of performance data on pyrolysis, combustion, gasification, and solvation revealed the underlying coal quality impacts in the reaction kinetics. As seen throughout this book, the bulk of these observations have already been synthesized into accurate predictive capabilities for devolatilization, and into comprehensive mechanisms for combustion and gasification, even in complex syngas mixtures across a broad pressure range.

The parallel advances in coal utilization technology have been nothing short of spectacular. Several of the synfuels production schemes put forward in the last century are now operating in China at commercial scale. Emissions of NOX and SOX from the power sector have fallen dramatically with the near-universal adoption of low-NOX burners, deep air staging, SNCR, reburning, SCR, and wet FGD. Both CFBC and AFBC have thoroughly penetrated the power sector. At the same time, efficiencies for coal fired power plants grew with the adoption of ultra-critical steam cycles and entrained-flow gasifiers. And now Hg emissions are being controlled in utility gas cleaning systems with activated carbon injection, new catalyst formulations for SCR, and additives for wet FGD.


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