PETER H. GIVEN,     Fuel Science Program Department of Materials Science and Engineering College of Earth and Mineral Sciences The Pennsylvania State University University Park, Pennsylvania

Publisher Summary

It is the geoscience of coals, that is, their geochemistry of origin and postburial geological history, that suggests that coals are a very diverse set of materials. This chapter discusses the relations between the chemistry of coal macerals and reviews the biochemistry of their precursors and the biological markers in coals that are indicative of their origins. The content of carboxyl groups in coals is commonly determined by an ion-exchange reaction with barium acetate. Values of 1–4 milliequivalents per gram have been found for lignites and lower-rank subbituminous coals—decreasing with increasing rank and essentially zero for coals of higher rank—except that HVC coals of the Rocky Mountain province may contain some carboxyl. The cations in the carboxylates of low-rank coals can give rise to calcite deposits in liquefaction reactors, can promote liquefaction, and can give rise to catalysts for the gasification of lignite chars.

I INTRODUCTION

The organic chemistry of coals has been studied with varying levels of effort for more than 100 years, yet the geological factors in coal chemistry have been almost totally ignored. It is the geoscience of coals, that is, their geochemistry of origin and postburial geological history, that is responsible for the fact that coals are a very diverse set of materials. At the same time, it is only study based on these sciences that will enable us to rationalize the diversity, to bring order out of apparent chaos. Such study can help us to understand why coals differ and to express in a systematic, intelligent manner how coals differ. It is important to understand the interrelationships between coals, whether one is engaged in a dispassionate enquiry for truth or concerned with the behavior of coals in practical use.

The science of organic geochemistry has grown prodigiously in the last 20 years, with strong support from the oil industry because of the contribution of the science to the understanding of the origins and classification of crude oils. Regrettably, there has been little support and little effort in the area of coal geochemistry. Moreover, few coal chemists have made the attempt to inform themselves about coal as an organic rock, formed by geochemical processes over geological time. For this reason, there are many papers in the literature that are unsatisfactory because the authors apparently believe they have proved something about “coal” by making experiments on one or two samples, which in any case are usually inadequately characterized.

In the first part of this article, I propose to discuss the origins of coals from a geological point of view, with the hope of showing chemists what is really meant by the concept of rank and the reasons why one would confidently predict that there should be not one but several bands of development of characteristics among coals of the United States (and elsewhere). I go on to review the relations between the chemistry of coal macerals and the biochemistry of their precursors, and also the biological markers in coals that are indicative of origins; these topics are the core of the organic geochemistry of coals. A major part of the article is then devoted to certain selected aspects of the chemistry of coals, placed, to the extent possible, in their geochemical context. The selection of topics is personal and idiosyncratic and reflects not only my own interests but also the areas I consider particularly important and/or frequently misunderstood. Some of the matters that will be discussed in this article have already received some coverage in Volume I, particularly in the article by Davidson; however, the perspective and treatment will differ in many respects. Moreover, I propose to take the liberty of expressing personal opinions on certain controversial matters. Obviously, the reader has equal liberty to disagree and have some other opinion.

There is a regrettable tendency in the coal literature for references to older work to be copied from one paper to the next, the original reference never being read or even checked. With very few exceptions, all of the...