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Basic Nitrogen

Nitrogen in liquid fuels (renewable fuels are included here) may be classified arbitrarily as basic and non-basic (Table B-1). The basic nitrogen compounds, which are composed mainly of pyridine-type homologs and occur throughout the boiling ranges of organic compounds, which are usually of the pyrrole, indole, and carbazole types, have a decided tendency to exist in the higher boiling fractions and crude oil residua and in liquids from alternative energy sources such as biomass and organic waste.

Organic nitrogen-containing compounds have many important roles in nature (hence, also in biomass) and display an enormous structural diversity, in which the nitrogen atoms can form part of simple functional groups or complex heterocyclic systems. The basic nitrogen compounds also have varying degrees of substitution and oxidation. The most noteworthy of these naturally occurring molecules are proteins, and most vitamins and hormones. Furthermore, nitrogen compounds in liquids produced from a variety of sources (including biomass and organic waste) can have a deleterious effect on any catalysts used to upgrade the liquid.

Basic nitrogen compounds with a relatively low molecular weight can be extracted with dilute mineral acids; equally strong bases of higher molecular weight remain unextracted because of unfavorable partitioning between the oil and aqueous phases. A method has been developed in which the nitrogen compounds are classified as basic or non-basic, depending on whether they can be titrated with perchloric acid in a 50:50 solution of glacial acetic acid and benzene.

Table B-1 Non-basic and basic nitrogen types.

Nitrogen compounds extractable with dilute mineral acids from renewable fuels typically consist of alkyl pyridine derivatives, alkyl quinoline derivatives, and alkyl isoquinoline derivatives carrying alkyl substituents, as well as pyridine derivatives in which the substituent was a cyclopentyl or cyclohexyl group. The compounds that cannot be extracted with dilute mineral acids contain the greater part of the nitrogen in crude oil and are generally of the carbazole, indole, and pyrrole types.

Thus, the presence of the various types of nitrogen compounds in any feedstock must be monitored because they can critically affect aquatic ecosystems, especially their high levels can cause eutrophication under certain conditions. Ammonia, organic species, nitrate derivatives (-NO₃), and nitrite derivatives (-NO₂) can be present in wastewaters. In general, ammonia and organic species are the most common in raw water. Ammonia is the initial product of the decay of nitrogenous organic wastes, its presence being an indication of such wastes. Typically, the nitrate content in waters is low but can rise to appreciable levels in wastewater from agricultural activities as a result of soil fertilization. Also, nitrite derivatives are present in low concentration since these are rapidly oxidized to nitrates, while nitrite derivatives can be found in wastewaters from the microbiological reduction of nitrate or the oxidation of ammonia.