Although feed millers cannot control the price of fossil fuels, they can dictate the amount of fuel used within the feed milling process in order to generate steam for the various aspects of the mill that require it. Fossil fuels remain a significant cost and will rise and fall in line with overall energy costs.
Several parts of the milling process should be examined when seeking to achieve savings in operational costs, and steam generation and use is one of them.
Good quality water, correctly treated for hardness, is imperative to avoid the pitfalls of scale building up within your boilers. Scale is the result of the buildup of magnesium and calcium ions in the water. It is important to remember that the natural hardness of water can vary dramatically during the year, depending on the level of the water table in the area supplying the mill.
After heavy rains in areas where the table is low, you can expect an increase in calcium levels as limestone and chalk lands are washed clean. The first sign of scale building up is usually an increase in stack temperatures as the scale acts as an insulator and prevents full transfer of heat through boiler tubes and furnace walls. This can also be associated with poor balance of air to fuel, but checking stack temperatures and fuel combustion levels in the stack on a regular basis as part of your routine boiler maintenance program will identify shortfalls in both combustion efficiency and water treatment programs.
As part of burner maintenance programs, you should check injectors and nozzles for carbon buildups that will distort oil flow or disfigure flame patterns. A simple cleaning of such parts during each shift will avoid expensive waste of fuel at the burn point.
Balancing the size of the boiler to the task at hand is imperative at the outset of any installation, and it is also important to ensure that the output of the boiler matches the demand for steam. In some mills, boilers are installed to handle a small pellet mill and a couple of storage tanks. But soon the plant output is increased, several more storage tanks are added and the boiler starts to struggle to raise sufficient steam for the demand. When this happens, you start to see poor fuel efficiency and droplets of water torn from the boiler water surface, creating acidic steam that corrodes the steam traps and piping as well as doing severe damage to pellet mill dies and pelleting efficiency. Balancing output to demand is absolutely imperative if any kind of efficiency is to be achieved.
Boiler water feed temperature is also important, and use of returns from condensate lines back through the hot well tank is key to maintaining a regular water temperature. There will obviously be low temperatures when processing commences, and care should be taken not to return condensate direct from tanks where fats and molasses are stored just in case steam coils within tanks become corroded. It won’t be the first time that fat has found its way from a storage tank right into the heart of a boiler due to a pin-prick hole in trace heating pipes.
Blowdown arrangements are necessary to maintain impurities at low levels, and times of blowdown should be controlled with automatic valves. If the blowdown periods are too short or infrequent, boiler impurities start to escalate. If the periods are too long or frequent, valuable steam is wasted. Striking the balance is bespoke to each and every plant and only experience and close monitoring will prove what the correct settings should be to optimize performance.
As with all forms of heat, steam generation and passage to the milling processes requires a substantial degree of thermal insulation to ensure maximum efficiency. While a bare section of piping may be valued by the mill operatives in the winter months, it does nothing for bottom-line performance and should be avoided at all costs. Insulation deteriorates over time and will require regular monitoring, particularly in areas where it is removed for access to valves and wheels as part of other routine maintenance operations.
Good traps, regular tests, effective water separators and a good length of valve train prior to the pellet mill are all part of a sensible and efficient installation. Choosing the correct boiler pressure and balancing the pressure at the boiler with the design pressure at the point of consumption is a key skill that should not be underestimated.
Different formulations using a variety of different ingredients will require the adoption of different pressures at the pellet mill. Generally, the more fibrous the material being processed the lower the pressure requirement. But the volume of steam required usually increases as the pressure decreases. Some poultry diets that are mainly made up of cereals and soy proteins can be processed with very high pressure and small volumes. Milk-based diets, usually adopted for baby piglet diets, require very little steam, if any, and it can be quite easy to choke a die and caramelize the milk if close attention is not paid to steam settings throughout the period of processing such diets.
FINDING THE RIGHT BALANCE
The balance of water addition and steam consumption is crucial, and close attention needs to be paid to the final moisture content of the feed being produced. While steam is used as a lubricant during pelleting to ease the material through the pellet mill dies, if the correct die specification is not chosen, the steam can be quickly evaporated through friction as material passes through the dies. The lubricating effect is then lost and power consumption at the press rises disproportionately. Choosing the correct die thickness for the materials being processed and balancing die choice with steam pressure and volume are all part of a feed miller’s art, and these skills should be respected. They should also be learned quickly by the new employee if he seeks to become a good feed miller.
The often forgotten aspect of steam generation is the volume of steam that is used for both trace heating and also for the flash-raising of fat temperatures immediately prior to fat enrobing or coating plants. Fats will not break down if they are flash-heated very briefly, but they will be absorbed much more readily at higher temperatures. Striking the balance between fat temperatures and pellet temperatures at the point the two combine at the enrobing plant can have a significant impact on fossil fuel consumption and the resultant quality of the finished feed produced.
Balancing the boiler performance correctly and making optimum use of the available fuel is quite difficult. Monitoring the amount of fuel consumed and the pounds of steam produced for a given diet, and gaining some idea of what steam remains unused and returned as condensate is the subject of constant appraisal. Formulations that are difficult to pellet, need compression to hold the pellet together and are not responsive to steam need to be identified and treated differently from those formulae that can be easily pelleted, do not break down and generate excessive fines that need to be reprocessed.
The choice of pellet mill conditioner will have a significant influence on steam use since turbulator conditioners have a very different usage pattern than the kettle-type conditioners, as dwell times, action of moving parts and the resultant rate of steam absorption will vary tremendously. Further studies of conditioning principles will deliver cost savings by way of better understanding the utilization of steam in any given installation.
Finally, the use of automatic pellet mill controllers, while desirable for maintaining regular pellet mill performance results, does not generally enhance the efficient use of steam and reduce steam generation costs. It is more important that the mill operator pay close attention to variances on a daily basis and to the statistical analysis of both pellet mill performance and fossil fuel usage. This will pay dividends for the mill owner in the long run.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J B Bradshaw Ltd. He may be contacted at firstname.lastname@example.org?.