Fuels treatment

Industrial effluent is commonly in the form of a sludge that may have a variety of constituents  

Equally important as dry volume is how the dry cake packs together in a truck for transportation. A very dry cake (e.g. 50% solids) is easier to handle than a slimier cake (e.g. 25% solids), but may in fact represent a greater truck volume, because it does not pack down so well. The actual volume reduction between the cake at 25% solids and that with 50% is only 6%, and this may require considerable extra filtration or separation effort and cost. 

Sandy sludges and those containing coarse solids can often be dewatered quite satisfactorily by simple screens. Pressure filters are then not necessary, unless there are other constituents present tending to produce a binding cake. 

Large-scale sludge fieatment plants are necessarily tailored to specific requirements, and are normally designed for continuous operation and to be automatic in use. A variety of filter-separators may be employed in the complete system. 

Effluent separators of this kind can reduce the oil content of the effluent discharge to better than 15 ppm. The units can operate by gravity flow or incorporate a low shear inlet pump depending upon the application. They can also be pressurized to retain the oil vapours. 

---

Spent Fuel Treatment. Spent fuel assembhes from nuclear power reactors are highly radioactive because they contain fission products. Relatively few options are available for the treatment of spent fuel. The tubes and the fuel matrix provide considerable containment against attack and release of nucHdes. To minimi2e the volume of spent fuel that must be shipped or disposed of, consoHdation of rods in assembhes into compact bundles of fuel rods has been successfully tested. Alternatively, intact assembhes can be encased in metal containers. 

To decide which fuel to use, a host of factors must be considered. The object is to obtain high efficiency, minimum downtime, and the total economic picture. The following are some fuel requirements that are important in designing a combustion system and any necessary fuel treatment equipment ... 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

Natural gas requires no fuel treatment however, low-Btu gas, espeeially if derived from various coal gasification processes, requires various types of cleaners for use in a gas turbine. These cycles can get very complex as indicated by a typical system, which utilizes a steam bottoming cycle to achieve high efficiency. Vaporized fuel oil gas is already cleansed of its impurities in the vaporization process. 

Figure 12-5. A typical residual fuel treatment system.

The overall costs for a VFO unit can be lower than the costs of conventional liquid fuel treatment plants. The U.S. Department of Energy conducted a survey that showed that the costs of operating a liquid fuel treatment system over a 20-year period is approximately 0.50 MMBtu... 

A fuel treatment system will effeetively eliminate eorrosion as a major problem, but the ash in the fuel plus the added magnesium does eause deposits in the turbine. Intermittent operation of 100 hours or less offers no problem, sinee the eharaeter of the deposit is sueh that most of it sheds upon refiring, and no speeial eleaning is required. However, the deposit does not reaeh a steady-state value with eontinuous operation and gradually plugs the first-stage nozzle area at a rate of between 5% and 12% per 100 hours. Thus, at present, residual oil use is limited to applieations where eontinuous operation of more than 1,000 hours is not required. 

Figure 12-11. Gas turbine fuel treatment plant investment costs. (Courtesy of General Electric Company.)...

The fuel. skid. This could contain a gas compressor if the fuel gas pressure is low and a knockout drum for any liquid contamination that the gas may have. The requirement of fuel gas pressure is that it should be operated at a minimum of 50-70 psi (3.5-4.83 Bar) above the compressor discharge pressure. The compressor and its motor drive fall under the drive level hierarchy. In the case of liquid fuels, the skid may also contain a fuel treatment plant, which would have centrifuges, electrostatic precipitators, fuel additive pumps, and other equipment. These could be directly controlled by the D-CS system, which would then report its readiness to the gas turbine controller. 

The majority of today s turbines arc fueled wth natural gas or No. 2 distillate oil. Recently there has been increased interest in the burning of nonstandard liquid fuel oils or applications where fuel treatment is desirable. Gas turbines have been engineered to accommodate a wide spectrum of fuels. Over the years, units have been equipped to burn liquid fuels, including naphtha various grades of distillate, crude oils, and residual oils and blended, coal-derived liquids. Many of these nonstandard fuels require special provisions. For example, light fuels like naphtha require modifications Co the fuel handling system to address high volatility and poor lubricity properties. 

Apart from the provision of various permutations of (chemical-based) boiler water programs, it is common to find water treatment companies supplying value adding chemicals and services in other boiler plant-related areas where their expertise in applied chemical technology can deliver additional economic benefit. Such areas typically include cleaning services for boiler waterside and fireside and the provision of fuel treatments and combustion additives, dust suppressants (for coal and ash handling), acids, and cleaner products. 

Prior to shut down, switching to higher grade fuels for a week or so and increasing the use of soot blowers helps remove deposits from fireside surfaces. Additionally, fuel treatments such as combustion additives, slag modifiers, and anticaking agents may prove very useful. 

In the combustion area, heavy slag and ash may form, preventing the passage of flue gas and blinding tubes. Locations should be precisely noted to provide fireside adjustments or to implement a fuel treatment program. In coal-fired boilers, drums, tubes, and headers should be inspected for abrasion from clinker and fly ash. 

This chapter discusses the problems associated with each of the zones, the types of fuel treatments available, and benefits to be gained, and provides an outline on the type of formulations employed. 

The term cold end commonly refers to the back-end convection area of the boiler system where an economizer, air-heater, or ID fan may be found, together with the stack and (with high dust-burden flue gases) possibly a cyclone scrubber or electrostatic precipitator. Any fuel treatment applied in this area may be considered to act as a postcombustion additive. 

The ultimate answer to cold-end corrosion problems is to totally eliminate sulfur from all fuels used, although from a political or economic standpoint, this is seldom a viable option. At the boiler plant facility itself, more practical options include raising the exit gas temperatures to prevent the dew point from being reached and using selective fuel treatments. 

In any case, whether emissions are wet or dry, the presence of soot in the flue gas effectively means lost Btus. Where fuel treatments are employed, their function is to limit the supply side of flue gas emissions by improving the combustion process. 

Fuel treatments have been used for very many years as an aid to improving the combustion efficiency process. Old formulations often used saw dust, wood flour, common salt, zinc sludge, ground oyster shell, and similar crude ingredients, but could still provide a dramatic effect when thrown into a fire. The metallic salts present (sodium in salt, zinc in sludge, and calcium in shell) acted as catalysts that dramatically lowered the ignition temperature of soot deposits from around 1100 °F/590 °C to only 600 °C/315 °C the fire burned vigorously and the soot disappeared. 

Today a wide range of inorganic and organic materials are employed, and the applications have become very specific and far-reaching. These fuel treatment products are available in solid, liquid, or emulsion form and are designed to match the physical nature of the fuel. Well-designed, commonly available feed equipment allows for easy injection. Fuel additives are employed in various types of combustion processes, including ... 

There are a number of major, primary manufacturers of fuel treatments (such as McDermott-Canning, Inc. of the United States and Interplast S.A. of France), as well as many smaller regional producers. Gamlen is a very well known brand, but the group was split up some years ago and several manufacturers and suppliers around the world (including McDermott-Canning) now share the brand name. 

Apart from metals, ammonium chloride, amine, and diamine salts, and various organic polymeric dispersants and surfactants are employed in fuel treatment additive formulations. 

Because there seldom is only one specific problem associated with the fuel being used, most fuel treatments are blends of active agents, such as combustion improvers and slag modifiers. In addition, fireside problems tend to be interrelated, so that, for example, inefficient fuel combustion leads to heavy sooting in the combustion area and dry smutting from the exit gases. 

NOTE Where the fuel tank contains oily water emulsions, sludge, and particulates, the effectiveness of any fuel treatment is directly related to the ability of the additive to penetrate and attack the sludge or emulsion mass. Simply feeding the treatment to the tank does not work. Typically, some form of power-injection equipment is employed to inject the chemical under 100 to 200 psig pressure. Agitation of the entire tank contents also is required and can often be carried out using the same equipment. 

There are hundreds of fuel treatments and additive formulations in commercial use today. Following are five formulations, typical of the types of products commonly available. 

Fuel Treatment for Lower Grade Solid Fuels... 

Albanese, Vincent, M. (Nalco Fuel Tech). Evaluating the Fuel Treatment Process. Plant Engineering, USA, October 1994. 

Nalco. An Introduction to Fuel Treatment. ONDEO Nalco (Division of Suez Lyonaisse/Degrdmont), USA, 1987. 

Peter-Hoblyn, J.D. (Nalco Fuel Tech UK). Fuel Treatments for the Middle East. Private communication to C. Frayne, Aquassurance, Ltd. UK 1992. 

Fuel treatment for lower grade solid fuels 687... 

---