Hydrocarbon Waste

previous Examples of Gasification of Wastes
8. Hydrocarbon Waste
8.1 Introduction

The technologies discussed previously in this book – incineration, gasification and pyrolysis – have usually been discussed with solid waste as examples. A reader will have concluded for him/herself that they can all be applied to liquid waste. When a solid is incinerated it is the gas and vapour breakdown products which burn: when a liquid is incinerated it is the evaporated liquid which burns. Not only do the same ideas apply but also the same sorts of plant, and fluidised-bed incineration of liquid wastes is quite common. Similarly, plasma gasification has found recent application to waste lubricating oil [1]([1] Song G-J., Seo Y-C., Pudasainee D., Kim I-T ‘Characteristics of gas and oil residues produced from electric arc pyrolysis of waste lubricating oil’ Waste Management – in press.). It is probably unsound to impose a distinction between solid and liquid incineration given that in either case it is gas or vapour which burns as explained above. In this chapter methods of disposal of waste hydrocarbons will be outlined. Such hydrocarbon will occur at refineries. Heavy fuel oil provides a way of making a marketable product from the heavy residual material, but such a fuel oil is never simply a ‘rag bag’ of otherwise unwanted hydrocarbons as strict specifications, for example of viscosity and cloud point, apply. Lubricants and hydraulic fluids are amongst the other products which eventually become hydrocarbon waste. Additionally to incineration and the other methods previously discussed in this book, the very important topic of re-refining of hydrocarbon waste will also feature as will cracking.

8.2 Incineration

Hydrocarbon liquids have calorific values of about 40 MJ kg^-1. As with other materials for incineration, if some return can be obtained on the heat so much the better. The table below which is followed by comments gives some examples of waste oil burning.

Reference	Details
[2] http://www.todaysure.com/projects.php	Small-scale incineration of used lubricating oil from a power station.
[3] Abegg F. ‘Small-scale waste oil incinerators’ The Northern Engineer 3 30-34 (1980)	Portable incinerators for use in Alaska.
[4] http://www.inciner8uk.co.uk/myweb2/oilboilers.htm	Oil burners in a range of sizes capable of taking from ≈ 5 to ≈ 12 litres per hour of waste oil. Applications have been to waste hydrocarbons including transmission fluid and hydraulic fluid.
[5] http://kcmshanghai.en.made-in-china.com/product/KMdQIgZEaJaU/China-Waste-Oil-Incinerator.html	An incinerator for waste oil from oil tankers, performance about 350 kW.
[7] ‘Ocean incineration: its role in managing hazardous wastes’ US Congress, Office of Technology Assessment (1986)	Mid 1980s, 8000 megatonne of tarry waste from vinyl chloride manufacture destroyed by ocean incineration annually.

The application in the first row of the table is in a remote location in the Middle East requiring selfsufficiency in such operations as oil disposal. Reference [3] gives some performance figures which will be examined below.

The information in [4] (third row of the table) can also be analysed quantitatively and this follows in the shaded area.

The burner being discussed can also use spent cooking oil. This will have a calorific value of about 37 MJkg^-1. Purely for a burner, where heat released is used as such, there is interchangeability of hydrocarbons and vegetable oils and the latter has the advantage of carbon neutrality. For use in an engine, in which heat is converted to work, there are many other factors to be considered. These include the viscosity and its temperature dependence and the cloud point. Neither factor would be at all satisfactory if an attempt was made to use heavy petroleum residue to power a compression ignition engine. Plant oils can, with due attention to quality and possible modification by esterification, be used as a fuel for compression ignition engines. A reader will be aware that this is being widely done around the world.

The entry in the following row is concerned with waste from oil tankers. This chapter is being written during the emergency in the Gulf of Mexico, where oil is leaking copiously from an exploration well. Highly serious though such incidents are, most of the oil contamination of the sea is due not to such mishaps but to operations including cleaning of oil tankers to remove the heavy material which has accumulated at the base. Given that a supertanker holds of the order of 2 million barrels of crude, such wastes summed across the supertankers of the world will be very significant indeed in quantity. The sort of combustion plant described in [5] enables such hydrocarbon waste to be incinerated. The waste will be transferred from the primary tank in which it has accumulated to a smaller one for transfer to the burner. Reference [6] describes a range of burners for such use with performances up to 1500 kW. The last row in the table is concerned with ocean incineration, which of course involves an incinerator mounted on a vessel. The vessel is taken out to sea before the incinerator is operated. Ocean incineration was proscribed by the Inter-Governmental Maritime Organisation (IMO) in 1991 [8]([8] http://www.imo.org/dynamic/mainframe.asp?topic_id=1508).

8.3 Pyrolysis and cracking

A precise distinction between pyrolysis and cracking would be arbitrary, but it is widely known that the latter involves higher temperatures and, very often, a catalyst. In relation to liquid waste treatment the terms do seem, in the research literature and in reports and the like, to be used synonymously and accordingly will be discussed together in this text.

A study in the fairly recent literature [9]([9] Bhaskar T., Uddin M.A., Muto A., Sakata Y., Omura Y., Kimura K., Kawakami Y. ‘Recycling of waste lubricant oil into chemical feedstock or fuel oil over supported iron oxide catalysts’ Fuel 83 9-15 (2004)) is concerned with catalytic pyrolysis of used lubricating oil which, as received, contained significant amounts of soot and also gum and was therefore first treated by filtering and centrifugation. One effect of the pre-treatment was to reduce the sulphur content from 7500 p.p.m. to 1641 p.p.m. If it is desired to have a fuel for compression ignition engines amongst the products the sulphur content is very important. Reference [9] originates in Japan where, at the time the work was published, the maximum allowable sulphur content of diesel was 500 p.p.m. (this has since been reduced). The work used three pyrolysis temperatures: 200, 300 and 400°C. At any one of the three temperatures, pyrolysis of the pre-treated material for one hour over an iron/silica catalyst reduced the sulphur content at least by a factor of two. GC-MS analysis of products from pyrolysis in the presence of a catalyst showed the products to be in the very wide range of about C₅ to C₂₅. Oils pre-treated to remove sulphur as described above gave a higher proportion of lower molecular weight products – up to about C₁₀ – than oils having had no such pre-treatment. From the point of view of utilisation of the products carbon number control by catalysis clearly has the potential to improve the viability on a larger scale.

It was described in section 6.3 how biomass and coal can be co-gasified, and similarly waste oil and coal can be co-pyrolysed although such does processing does not of course have any carbon neutrality benefits. Again, an example from the research literature will be considered [10]([10] Lazaro M-J., Moliner R., Suelves I., Herod A.A., Kandiyoti R. ‘Characterisation of tars from the pyrolysis of waste lubricating oils with coal’ Fuel 80 179-194 (2001)) and the most important points brought out. Of the three classes of pyrolysis product of coal – solid, liquid and gas – one would expect that co-pyrolysing with oil would affect most strongly the liquid part. In [10] this was certainly so; pyrolysis of a particular coal at 650°C gave 15.6% by weight of liquid product, and that rose to 24.3% when at the same temperature the coal was co-pyrolysed with waste lubricant oil. Not only amounts but compositions of the liquid products differed according to whether waste lubricating oil was initially present. For example, whereas the liquid products from coal alone contained phenols in major yield, those from co-pyrolysis with oil contained phenols in much smaller yield indicating that the hydrocarbons in the oil were acting as a hydrogenating agent.

The last research report [11]([11] Demirbas A. ‘Gasoline-like fuel from waste engine oil via the catalytic process’ Energy Sources A30 1433-1441 (2008)) to be discussed in this section is focused on obtaining fuels in the gasoline boiling range from waste lubricating oil. At temperatures in the range 475-625K in the presence of an alumina catalyst a product termed by the authors of [11] ‘waste oil gasoline’ was obtained which corresponded closely to the properties of a locally obtained commercial gasoline with which it was compared. The comparison is summarised in the table below.

8.4 Gasification

There has for very many years been manufacture of a general-purpose fuel gas from low-value hydrocarbons such as heavy residue and cracking by-products. This is discussed by the present author elsewhere [12]([12] Jones J.C. ‘Hydrocarbon Process Safety: A Text for Students and Professionals’ Whittles Publishing, Caithness (2003)). This section then will be concerned not with hydrocarbons from processing but with hydrocarbons already having been used, for example as lubricants, and requiring disposal.

As has already been noted, To a good approximation any liquid hydrocarbon material can be taken to approximate in empirical formula to CH₂. Gasification with steam therefore proceeds according to:

CH₂ + H₂O → CO + 2H₂

the product gas being of calorific value about 12 MJ m^-3 and this continues in places where cheap hydrocarbon feedstock is available.

Interest at the present time in gasification of waste oil is quite limited, probably because unlike the gasification of biomass it does not provide for a bonus in carbon accounting. As an example of a project into gasification of waste oil, it is reported in [13]([13] de Filippis P., Borgianni C., Paolucci M., Pochetti F. ‘Prediction of syngas quality for two-stage gasification of selected feedstocks’ Waste Management 24 633-639 (2004)) how waste oil was gasified with water and oxygen at about 800°C. Control of the H₂/CO ratio was necessary as the end use of the gas as to be chemical synthesis not burning. A gas of molar ratio H₂:CO of 1.87 was produced. When a synthesis gas rather than a fuel gas is required it is probable that the synthesis product will be methanol, made according to:

CO + 2H₂ → CH₃OH

and it is stated in [13] that a satisfactory yield of methanol requires that the H₂:CO molar ratio be at least 1.7. The value of 1.87 reported is therefore comfortably above this limit.

8.5 Re-refining

In the USA about 400000 gallons of waste motor oil are disposed of daily [14]([14] http://www.synlube.com/usedoil.htm). For this and other forms of waste hydrocarbon, re-refining is an option. A relevant point is that refining for the first time, that is refining of crude oil, does not reduce greatly the EROEI of petroleum products. A rough rule is invoked in an earlier chapter that the EROEI of a distillate is that for the crude divided by a factor of about 1.3. This augurs well for re-refining.

Examples of re-refining operations are in the table below. Re-refining is prevalent nor is it new, and the contents of the table are a small representative selection only.

Reference	Details
[15] http://www.evergreenoil.com/	Re-refining of used lubrication oil at the only such facility in the western USA. Products for eventual sale ‘light neutral base oil’, ‘mid-range neutral base oil’, ‘asphalt flux’ and ‘light end distillates’ (see below).
[16] http://nzic.org.nz/ChemProcesses/energy/7B.pdf	New Zealand: re-refining of vehicle lubricant to produce diesel and base stock. Non-volatile residue used in road building.
[17] Nakaniwa C., Graedel T.E. ‘Life cycle and matrix analysis for re-refined oil in Japan’ International Journal of Life Cycle Assessment 7 95-102 (2002)	Detailed assessment of the potential for oil re-refining in Japan.
[18] http://www.ascetriverina.com/?id=southernoilrefinerie	Wagga Wagga, Australia: a plant re-refining waste oil. Products for fuel use.
[19] http://www.scientificamerican.com/article.cfm?id=can-oil-be-recycled, [20] http://www.universallubes.com/index.php/perfproducts	Wichita KS: re-refining of used motor oil, 12 million gallons (≈ 0.3 million barrels) annually.

The terms in row 1 need some clarification. ‘Base oil’ is material suitable for lubricant use but requiring adjustment to properties including viscosity to bring it within specification for such use. This adjustment is achieved by blending. The terms ‘light’ and ‘mid-range’ refer to the density or, equivalently, the API gravity. The ‘light end distillates’ are in the diesel boiling range. Asphlatenes comprise the heaviest hydrocarbon compounds in crude oil, having molar masses up to about 1500 g. ‘Asphalt flux’ is used to dilute asphaltenes where they exist in petroleum materials, mitigating their restricting influence on flow. In considering the NZ activity (row two) it is as well to remember that that country has very few hydrocarbon resources indeed, so thrift in hydrocarbon usage is necessary. Re-refining of used lubricating oil in NZ has in fact been taking place for over half a century. In the detailed study appertaining to Japan in reference [17] the following conclusions were noted. Fuel production from re-refining is commercially viable in Japan. Specifications of products of re-refining can be controlled and made the same as the corresponding products from crude oil. On the down side, collection of small amounts of oil for rerefining itself has an energy requirement and, it is noted in [17], diesel powered vehicles for such collection would release sufficient NO_x to incur a penalty having regard to the existence of credits for such pollutants. The Wichita Kansas facility (final row) is a sizeable one, yet the amount of oil it processes in a year is utterly negligible in comparison with the amount of crude oil which the US imports from countries including Canada, Mexico and Venezuela in a single day! There are benefits from oil rerefining as described in this section, but for the practice to reduce dependence on imports would require proliferation of such facilities to an extent that is quite unrealistic.

8.6 Concluding remarks

The world in which we live is dominated by the issue of hydrocarbon availability and usage and no one factor has as strong an influence on world economics as the price of oil. When oil is burned that is the end of it: it has gone to carbon dioxide (and in so doing contributed to CO₂ levels in the atmosphere, a critically important issue in the 21st century) and water vapour. Hydrocarbon having seen non-destructive use has the same calorific value as products from newly refined crude oil and the potential for fuel use is obvious. The processes outlined in this chapter have their parts to play in times of unprecedented preoccupation with hydrocarbon supply and demand.

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