GCI TECH NOTES ©
In the early days of waste fuel use, it was easy to avoid fuels with particulates. Then, all you needed to do was strain the stuff out. There usually was not very much of it and it was still easy and relatively inexpensive to dispose of these solids. Over time, the generators reduced waste volumes and users competed for a smaller volume of liquid waste which was, unsurprisingly, higher in solid content.
These solids created problems. The settling out of solids in tanks was the first problem. This was handled by installing tank mixers. At first, a three horse power mixer was simply slapped onto a 20,000 gallon tank and considered adequate. Now, however, mixer selection and tank design are interdependent and the horse power is nearly twice that once thought sufficient. Some hazardous waste blenders have 20+ horse power mixers on 20,000 gallon tanks to handle high viscosity/high solids fuel.
But, keeping the solids suspended in the tank meant more solids sent down the supply lines to the burner. This created more problems to solve: pipelines plugged, control valves controlled poorly or plugged altogether and burner nozzles (patterned on fuel oil nozzles) plugged. To make matters worse, the solids frequently were abrasive. A variety of grinders were installed to reduce the particulate size. These grinders were selected from existing industries. At first, it was in-line grinder pumps and homogenizers from the paint and chemical industries. This equipment was not rugged enough to handle the variety of solids, and they would self-destruct within weeks. Eventually, the waste fuel industry settled on specially modified "Muffin Monsters". "Muffin Monsters" were designed to grind up the solids in sewage. Unlike the paint industry or chemical industry, the sewage treatment industry has virtually no control over what they have to manage. If someone can flush it down a toilet, it could end up in the suction of a sewage pump. Paper, rags, wood, wire, plastics, ceramics, etc. would all end up in the sewage. Hence the "Muffin Monster" with its torque sensing reversing electronic controls and its high strength, high hardness cutters worked well. Modifications were required to handle solvents that attack certain elastomers, and to achieve higher fluid pressure ratings.
"Muffin Monsters" are not enough, however. Particulates can re-agglomerate in the storage tanks due to physical or chemical processes and particles which can pass through a set of 9mm x 9mm cutters in a "Muffin Monster" when turned a few degrees will not pass through a 9mm hole. After trying the grinder pumps and homogenizers again, the waste fuel industry has settled on Moyno-Pipeliners to provide fluid with reasonably uniform particle size. The Pipeliner has a hardened steel plate perforated with sized holes (generally 4.5, 6 or 9mm). A cutter blade sweeps by the inlet side of the holes cutting or shattering any trapped particles. Rubbery materials still a problem for these devices as well as anything that can wedge in the holes. Generally, the Pipeliner performs well on a recirculated stream or as a final particle sizing device in the supply line to the burner.
Dealing with the plugging of pipes due to particulates requires achieving a turbulent flow rate in the fluid as it is pumped to the burner. Generally, this means achieving a Reynolds number in excess of 2,000. (GCI provides a Reynolds number calculator) However, achieving a turbulent flow rate frequently requires larger pumps with more horse power and increased care in pipeline sizing and design. Reducing flow obstructions such as minimizing the number of elbows and tees and using long radius elbows when possible will reduce friction losses and erosion.
Particulates have always been a problem to control trains. The control valve acts as an obstruction and particulates can quickly plug the valve opening. This loss in flow results in the automatic control valve opening all the way, the plug then breaks and the fluid surges into the burner at flow rates much higher than desired.
The industry has dealt with this in two ways. A vee-port ball valve is used as a control valve. The vee-port ball is specifically designed to control fluids that contain particulates.The second way is to control the pressure of the fluid being fed to the control valve so that the desired flow can be achieved at a larger control valve opening.This reduces the likelihood that the particulates will plug the valve. This can be accomplished by using a pressure control valve on the return side of the fuel supply loop or by utilizing an SCR control to decrease the speed of the fuel supply pump.
Another problem caused by particulates in the control train is the plugging of the flow meter. Most waste fuel systems utilize a Coriolis mass flow meter. Earlier models of these instruments utilized multiple u-tubes, many with tubes less than 9mm in diameter. Now, these are available in a single straight tube design with tubes near one inch in diameter.
Waste fuel burner assemblies were, at first, modified fuel oil burner nozzles. There are very few of these any more. The vast majority are custom nozzle assemblies fabricated on-site and are designs that incorporate an open-ended waste fuel lance with external air atomization.
The ultimate particulate size reduction system is a ball mill. Ball mills have been used in the mining industry for decades to reduce rocks to a fine powder. This is how cement companies prepare the raw feed and reduce the product clinker into that fine grey powder. Such systems when modified to handle hazardous wastes are expensive and require considerable expertise in design and operating technique to ensure safe operation. There are a few companies who have developed this technique of size reduction for hazardous waste fuels. These companies understandably have been very hesitant to provide any details on their processes.
If you are interested in additional information on particulate size reduction or particulate management, please feel free to contact us or visit our web site library for additional articles on the use of hazardous waste fuels and their managment.