Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Heteroatom Removal as Pretreatment of Boiler Fuels Muhammad N. Siddiquee and Arno de Klerk*
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Department of Chemical and Materials Engineering, University of Alberta, 9211 116th Street, Edmonton, Alberta T6G 1H9, Canada ABSTRACT: Heteroatom removal without the use of hydrogen was investigated for the pretreatment of heavy fuels to produce cleaner burning boiler fuels from petroleum residua and coal. This is relevant for applications, such as the production of marine bunker fuel oil, where the anticipated change in maximum sulfur specification in January 2020 is from 3.5 to 0.5 wt %. Processing challenges, such as fluidity, yield loss, and cost-effective reagents, were considered. The study drew primarily on published data; however, claims about key process steps were experimentally verified, and those results are also presented. An oxidative process that employed air as an oxidant was evaluated for oxidative liquefaction and heteroatom removal from coal and petroleum. It was found that this strategy was viable for coal conversion but not for petroleum. Oxidative coal liquefaction produced two potential boiler fuels, an oxidized partly desulfurized coal and a water-soluble coal product. This step was experimentally demonstrated. The experimental work indicated that product separation is potentially challenging. Oxidized sulfur- and nitrogen-rich material must be treated to remove sulfur and nitrogen from the bulk of the hydrocarbon mass, which would otherwise represent a substantial yield loss. This step was not demonstrated. The final processing step involved deoxygenation to improve the heating value of the cleaned boiler fuel. Catalytic deoxygenation over zinc oxide and copper oxide appeared to be promising. Copper oxide supported on a carbon catalyst was evaluated for deoxygenation. Substantial deoxygenation of a mixture of acids was achieved by conversion at 350 °C, with both ketonization and decarboxylation pathways being active for deoxygenation.
1. INTRODUCTION When boiler fuel is combusted for power generation or to provide shaft power for propulsion, most of the heteroatoms in the organic matter are converted to gaseous species. In the case of solid fuels, such as coal, some of the heteroatoms may be captured by mineral matter and leave the combustion process as ash. However, any combustion process has the potential to generate heteroatom-containing emissions from the heteroatoms that are present in the fuel. Air emission affects air quality, and generally steps are taken to limit emission of some components in the flue gas. For example, post-combustion treatment reduces particulate matter emissions, emission of species containing metals, such as Hg, and combustion products, such as SO2 and NOx.1 In this study, the high-level question was posed: is it viable to perform heteroatom removal from boiler fuels? The scope of this investigation was limited to the major heteroatoms in fuel: nitrogen, oxygen, and sulfur. These heteroatoms contribute differently to emissions during combustion. The fate of sulfur is the most straightforward; under combustion conditions, it is converted to SO x compounds. Sulfur removal from materials that are used as boiler fuels, as a recurring topic in the literature,2−5 is therefore understandable. Oxygen present as oxygenates in the fuel is incorporated into combustion products, with most of it becoming COx. However, oxygen in the fuel is only a small part of the total oxygen contained in combustion products. Most oxygen in combustion products comes from O2 added as air. The impact of oxygen that is already present in the fuel is mainly to reduce the heating value of the fuel and increase fuel consumption per unit of work performed. Similar to oxygen, nitrogen is present both in the fuel and as N2 in the air that is added to the fuel. Part of nitrogen in fuel is converted to N2 © XXXX American Chemical Society
during combustion, but at the same time, some N2 from air that is used as oxidation for combustion contributes to the formation of NOx.1 The benefit or not of removing nitrogen from the fuel is not clear. Heteroatoms can be removed at various stages during the process before they become air emissions (Figure 1). Boiler
Figure 1. Limiting air emission of heteroatoms in boiler fuels following combustion use.
fuel selection can be limited to fuels with low heteroatom content; the heteroatom content of the fuel can be decreased by pretreatment before use; heteroatoms can be captured during combustion; and heteroatoms can be removed by postcombustion treatment. Irrespective of the nature of the combustion process, fuel type, or its application, the emission of some heteroatomcontaining species is regulated, which determines the level to Special Issue: 27th International Conference on Impact of Fuel Quality on Power Production and Environment Received: November 20, 2018 Revised: January 20, 2019
A
DOI: 10.1021/acs.energyfuels.8b04044 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
reduction process. It should be added that visbreaking is the cheapest of the residue upgrading technologies on a cost per capacity basis.14 Fuel oil fulfills a market need, but from a refinery perspective, the low product value of fuel oil necessitates a low refining cost to produce fuel oil. This presents an economic challenge for any heteroatom removal process. 2.2. Potential Yield Loss. Potential yield loss affects the economics of producing boiler fuels. The percentage yield can easily be calculated from the material balance of a process (eq 1).
which heteroatoms must be removed. These regulations are either directly aimed at emission control by placing limitations on stack emissions or indirectly as limitations on fuel quality. Large-scale stationary emitters, such as the stack emissions from boilers used in power generation, are directly regulated. Power generation facilities evolved to accept the burden of removing heteroatoms present in fuels by developing efficient post-combustion treatment.1 Small-scale mobile emitters, such as the tail pipe emissions from vehicles, are indirectly regulated. Transport fuel quality is regulated, and the burden of heteroatom removal falls on petroleum refiners.6,7 Large mobile power generators, such as boilers on ships, have come under scrutiny as emitters of SO2 in particular. The intent is to reduce the maximum allowable sulfur content in bunker fuel oil for use in open seas from 3.5 to 0.5 wt % starting on January 1, 2020.8 Ships are large enough to accommodate post-combustion treatment, but the legislative direction that was taken was to opt for regulating fuel quality. Options based on current technology that are available to refiners to reduce the sulfur content of bunker fuel oil is limited and ultimately reliant on increased hydrogen use.9 Although conventional refining technology is available to further reduce the heteroatom content of heavy petroleum fractions, the cost of doing so might be high. The challenge of desulfurization of bunker fuel oil can be restated more generally for boiler fuels that include both petroleum and coal. What are the options for removing heteroatoms present at percentage levels in heavy carbonbased fuels without the use of hydrogen? It is the purpose of this work to outline a potential pretreatment process for accomplishing heteroatom removal from bulk heteroatom-rich materials to produce a low heteroatom content boiler fuel with due consideration to cost. The development status of the different steps in the process will be discussed, and process development challenges will be outlined. The work draws primarily on published data, although key steps in the process were experimentally verified, and those results are presented.
yield = (amount of product/amount of feed) × 100
(1)
The yield is expressed differently for coal than for petroleum. Coal is sold by mass, and the yield is expressed on a mass basis. Petroleum is sold by volume, and the yield is expressed on a volumetric basis. It is therefore possible to process petroleum with no mass loss but have an increase or decrease in the yield as a result of a decrease or increase in liquid density following conversion. The gross operating profit associated with the production of boiler fuel is stated in eq 2. gross operating profit = product price(yield/100) − feed cost − operating cost
(2)
As pointed out before, for coal, the expression is on a mass basis (e.g., $/kg) and, for petroleum, the expression is on a volume basis (e.g., $/m3). What is not evident from eq 2 is how the transformation from feed to product affects the price of the product. Because the product is a boiler fuel, one can anticipate that increasing the heating value of the material will command a higher product price. One can also anticipated that, by reducing the heteroatom content (and mineral matter content), the product would command a higher price. In practice, pricing of fuel is not as straightforward and the product price does not necessarily reflect the improvement in quality in a proportional fashion. The origin of yield loss associated with coal beneficiation is different to the origin of yield loss associated with petroleum refining. Coal is not fractionated on the basis of the nature of the organic material, but yield loss is incurred during coal beneficiation, which involves size reduction and mineral matter rejection.15 Petroleum is fractionated on the basis of the boiling range and the conversion and separation processes, or processes used to produce fuel oil will change the boiling range of the product, the chemical composition of the product, and the physical properties of the product.16 Whatever technologies are selected for heteroatom removal, eq 2 should be kept in mind. 2.3. Nature of Organic Sulfur, Nitrogen, and Oxygen Compounds. Sulfur in petroleum is present as thiols, thioethers, sulfoxides, and thiophenes. In comparison to lighter boiling fractions, as the boiling fraction becomes heavier, the sulfur content increases and more than 50% of sulfur is present as thiophenes.17 Much less is known about the nature of sulfur in coal, but there appears a consensus that upward of 75% of sulfur is present as thiophenes.18,19 Mineral matter can contribute to sulfur release, for example, the transfer of sulfur from iron pyrite to organic matter.20
2. PROCESSING CHALLENGES 2.1. Fluidity. Conversion processes to remove heteroatoms from bulk heavy carbon-based fuels require the fuel to be sufficiently fluid for processing. Coal presents a direct challenge, because it is a solid. There are some options, which include coal entrainment by gas, slurry preparation, and liquefaction. The most appropriate approach depends upon the requirements of the heteroatom removal process. Keeping coal solid poses inherent mass transport limitations. However, making use of coal−liquid mixtures attracted much attention in the past as an alternative to fuel oil.10,11 More relevant to the present investigation is coal−oil mixtures, because it could be a boiler feed material that could be considered as feed for heteroatom removal. Petroleum is not a solid, but the heavier fractions of petroleum that have traditionally been used for boiler fuels is usually very viscous (near solid) at ambient conditions. There are several grades of fuel oil that are differentiated primarily on distillation range and viscosity.7,12 The viscosity of the straightrun material is often too high to be used as fuel oil, and the fluidity has to be improved by technologies such as visbreaking.13 Visbreaking is a mild thermal conversion technology. The liquid yield from visbreaking is high, and little heteroatom removal takes place during this viscosity B
DOI: 10.1021/acs.energyfuels.8b04044 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Nitrogen is present mainly as heterocyclic nitrogencontaining compounds, pyrrolic and pyridinic, in both petroleum and coal.17−19 Some nitrogen could also be present as amines and amides. Oxygen is less prevalent in petroleum than it is in coal. In petroleum oxygen is present mainly as carboxylic acids and acid derivatives, which are concentrated in the heavier fractions.21 In coal, there is more diversity of oxygenates. Low-rank coals that are typically used for fuels contain a significant amount of oxygen in the organic matter, much of which occur as phenolic groups, carboxylic acids, and ethers.22 Oxygenates reduce the heating value of fuels, and it is partly responsible for keeping metals bound to the organic matter, for example, as metal carboxylates.23,24 Reference was made to single functional groups, but in heavy molecules, there may be multiple functional groups and a combination of heteroatoms present in a single molecule. Evidence for this is found in high-resolution mass spectrometry analyses, for example, work on vacuum residue fractions.25 Simplified thinking in terms of classifying heteroatoms “neatly” based on compound class is therefore an oversimplification when dealing with heavy oil and coal. 2.4. Heteroatom Removal by Hydrogenation. The conventional approach to heteroatom removal is to employ hydrogenation. There are several advantages to hydrogenation. Hydrogen that is incorporated into the fuel increases its heating value. The heteroatoms are removed in the form of light molecules that can be treated with standard technologies; for example, sulfur is removed as H2S; nitrogen is removed as NH3; and oxygen is removed as H2O. Hydrogenation also decreases the fuel density, partly as a result of heteroatom removal and partly as a result of saturation of aromatic structures. Hydrogen consumption depends largely upon the extent of aromatic saturation, which is often a prerequisite for heteroatom removal from heterocyclic aromatic compounds. For desulfurization of thiophenes, the hydrogen consumption depends upon the pathway by which sulfur is removed (Figure 2). Hydrodesulfurization by hydrogenolysis consumes 2 mol of H2/mol of S, i.e., 1 mol of H2 to form H2S and 1 mol of H2 to saturate carbons where sulfur was detached. Hydrodesulfurization by adjacent ring saturation prior to desulfurization consumes 5 mol of H2/mol of S, i.e., 3 mol of H2 to saturate the adjacent aromatic ring and 2 mol of H2 for hydrogenolysis to eliminate sulfur. For denitrogenation of heterocyclic nitrogen-containing compounds, ring hydrogenation prior to denitrogenation is necessary (Figure 2). The minimum hydrogen consumption for denitrogenation of pyrroles is 3 mol of H2/mol of N when pyrrole is at the edge of the molecule, i.e., 1 mol of H2 to form NH3, 1 mol of H2 to saturate one of the adjacent unsaturated bonds, and 1 mol of H2 to saturate carbons where nitrogen was attached. The minimum hydrogen consumption for denitrogenation of pyridines is 4 mol of H2/mol of N when pyridine is at the edge of the molecule, i.e., 11/2 mol of H2 to form NH3, 1 mol of H2 to saturate one of the adjacent unsaturated bonds, and 11/2 mol of H2 to saturate carbons where nitrogen was attached. If adjacent ring saturation takes place, which is necessary when heterocyclic nitrogen is not in a terminal ring, hydrogen consumption for pyrroles is 5 mol of H2/mol of N and for pyridines is 6 mol of H2/mol of N. For deoxygenation, hydrogen consumption also depends upon the nature of the functional group, but there are also other side reactions that contribute to the reaction network.
Figure 2. Hydrogen consumption illustrated by the hydrogenation of trinuclear heterocyclic compounds, from top to bottom, thiophenes, terminal ring pyrroles and pyridines, and internal pyrroles and pyridines.
For example, hydrolysis, dehydration, ketonization, and decarboxylation are possible during hydrotreating and would decrease the actual hydrogen consumption compared to pure hydrodeoxygenation. To illustrate the impact, the minimum hydrogen consumption in relation to the amount of sulfur and nitrogen removal is shown in Figure 3. This is the theoretical minimum
Figure 3. Minimum hydrogen consumption in relation to the amount of heteroatom removal based on the functional group and hydrogenation pathway shown in Figure 2.
hydrogen consumption based on the different compound classes. The consumption in standard cubic feet per barrel (scft/bbl) is based on a feed density of 1000 kg/m3 [10° American Petroleum Institute (API) gravity]. The underlying assumption in Figure 3 is that the hydrogenation is selective only for the target functional groups, which is not the case in practice. Hydrogenation C
DOI: 10.1021/acs.energyfuels.8b04044 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
challenges during fuel use than is gained by heteroatom removal, or differently put, nothing should be added that could not efficiently be removed as well.
catalysis and heteroatom removal by hydrogenation must consider sulfur, nitrogen, and oxygen together.26 Saturation of aromatics that are not associated with heteroatom removal will also take place. Material in the boiling range used for fuel oil can be hydrotreated in fixed bed reactors, or when the metal content of the feed is too high (a threshold of
