Pellet Plants and the Pareto Principle
The accepted definition of the Pareto principle—also known as the 80–20 rule, the law of the vital few, and the principle of factor sparsity—states that, for many events, roughly 80 percent of the effects come from 20 percent of the causes.
Business management consultant Joseph M. Juran suggested the principle and named it after Italian economist Vilfredo Pareto, who observed in 1906 that 80 percent of the land in Italy was owned by 20 percent of the population; he developed the principle by observing that 20 percent of the pea pods in his garden contained 80 percent of the peas. In today’s business vernacular, it is widely accepted that 80 percent of sales come from 20 percent of clients. So what does this have to do with large-scale production of wood pellets in North America for export to Europe?
As with all systems, a typical wood pellet plant can be broken down into many different individual processes. These processes include receiving, size reduction, material transportation, drying, mixing, conditioning, pellet production, sieving/screening, cooling, pellet transportation and storage. Some plants will have fewer steps and some may have more, but in general, these are the most common individual processes of designing a plant to produce wood pellets for export from forestry feedstocks common to North America. Regardless of differences between plants, the similarities are constant.
Drying and pelletizing always receive the lion’s share—usually 80 percent or more—of attention during the design and specification phase of a new plant. However, when factoring in the cost of land, building construction, and the other equipment necessary to the pelletizing system, dryers and pellet mills represent 20 percent or less of total capital investment. About 80 percent of planning time spent on these two areas leaves a meager 20 percent of planning time to spend on everything else; therefore, the disproportionate amount of time spent evaluating and specifying dryers and pellet mills can and often does result in costly decisions once the plant is operational.
Problems and Downtime
All too often the problems and downtime experienced in a large pellet plant are not associated with the dryer and pellet mills. Commonly, it is the rest of the equipment that is responsible for most of the downtime. This is a direct result of lack of time allocated to analyzing and evaluating the cost benefit ratio of this key equipment during the plant’s design—in summary, 80 percent of time is spent designing the plant, which equals 20 percent of the investment, and 20 percent of time is spent designing the plant, which accounts for 80 percent of the downtime
Poorly designed loading and unloading systems, misapplied screening apparatus, belt conveyors where there should be chain conveyors and vice-versa, are mistakes that aren’t typically a direct result of poor engineering or bad decision making. They do tend to be, however, a result of overzealous value engineering and the desire to allocate more money toward the heart of the system: the dryer and pellet mills. These are important parts of the puzzle, but balance is essential. Applying too many resources and placing too much emphasis on just two parts of any system will produce undesirable results. The 80 percent will fail too often, negating the benefits of the 20 percent that so much time was spent researching.
As long as all the parts of the process work together in unison, turning something of little value (trees) into something of greater value (pellets) can be done on a large scale with great efficiency and continuing success. Material handling and feedstock preparation, while only one part of a complex process, is equally as important to the success of a wood pellet plant as the feedstock selection, logistics coordination, supply and demand markets and the dryer and pellet mills. They all work together toward the same goal: to generate a high-grade wood pellet that will hold up for weeks and even months while being stored and transported to customers in Europe. If careful attention is not paid to selecting the correct style, size and quality of material handling equipment that surrounds the heart of the system, then no matter how good the drying and pelletizing is, the operation will not output its maximum potential. It is only as reliable as your least reliable component.
Material handling at a large pellet plant can be broken down into many different areas, including characterization, layout, conveying and preprocessing.
Material Handling Components
Characterization of feedstock requires an understanding of its properties. Moisture, bulk density, material size and range, seasonal properties, abrasiveness and contaminants are just a few of the items that should be considered, as they all affect the equipment selection and material flow. Feedstocks and the way equipment handles them vary, so knowing the material properties and characteristics is an essential part of the specification writer’s task of configuring the correct equipment and processing methods. Just because a hammermill can offer an effective size-reduction solution for southern yellow pine does not mean that it can perform that same task on fir or spruce. One should not “copy and paste” from a previous plant’s specification.
Layout will greatly affect plant efficiency. Receiving and storage methods for the feedstock are largely determined by the results of characterization, as well as how the material is transported to the site. First-in/first-out material flow designs can protect the product from the elements, which in turn reduces drying time and saves energy. Once the feedstock has been received, the materials should be efficiently stored, and material flow should be logical and sync with the trucking and off-loading routes.
Is it important to have all the various processes close together to keep material conveying distances—and thus cost and consumed horsepower—to an absolute minimum? Most would answer yes, however, it is common, and wise, to place larger distances between various sections of the facility in an attempt to reduce the risk of a catastrophic fire that could wipe out the entire plant. Should one area have the misfortune to catch fire, there is a natural firewall inherent in the plant layout that could prevent the fire from spreading and causing greater damage.
Conveying may seem like a simple operation of getting from point A to point B, but it is quite often a continual problem area in a wood pellet plant. The answers gained during the characterization process will govern many decisions here. Distance, angle, and throughput are all aspects that need to be addressed, but conveyor type, style and maintenance needs are also key factors to consider. Will a belt conveyor suffice, or should a chain conveyor be considered over steep angles and long distances?
What is the material’s angle of repose and what cleat configuration would work best? How far can the material be transported with a single drive? In the event of a blockage, is a screw conveyor reversible so as to aid maintenance and clean out?
Many engineers look past what seems to be a relatively simple task of moving the material a short distance, but poor conveyor selection and design is often a bottleneck on what may otherwise be a very successful operation. One must be very particular and detailed in this area of design; understanding the way a feedstock reacts to differing conveying methods is critical to achieving the throughput requirements of any operation. A common practice is using belt and tubular belt designs when conveying the pellets themselves, but using a chain conveyor can offer greater value and reliability, as long as the number of transfer and impact points is kept to a minimum.
Are explosion and fire protection needed at certain transfer points? Given the attention the OSHA and the NFPA are giving to combustible dust explosions, the short answer is yes. Though it adds cost and complexity to an operation, it’s worth it. If a plant were down for many weeks due to a fire, what would be the ramifications on its future? Conveyors should be equipped with heat, smoke and flame sensors, sprinkl
ers and fire suppression equipment in accordance with local and national codes where applicable.
Preprocessing covers a very wide range of operations, but there are two main categories that require focus: size reduction and contaminant removal. Drum chippers, disc chippers and hammer mills are large, energy-hungry pieces of equipment that can make or break a plant’s success. Achieving a consistently sized and contaminant-free particle will allow the dryer and pellet mills to perform their jobs at maximum efficiency. Ease of maintenance, power consumption and risk of fire due to the heat generated during these processes should also factor into your equipment choice. These machines are no longer simply big hogs that grind and shred wood; they are specifically designed to produce a precise particle size that the pellet mills work most effectively with, which does not happen by chance.
Screening, tramp metal detection and fines-removal systems also play a large part in protecting the equipment throughout the entire plant, and help to generate a higher-grade pellet that is fit for the long transportation and storage times seen in the export market. Consistent particle size and contaminant-free feedstock are key, as they allow the heart of your system to perform at the maximum design capacity, and any shortcuts taken will be seen on the bottom line.
The Pareto Principle is proven two ways in large wood pellet plants. First in the design and engineering phase: 80 percent of planning time is most often spent on equipment that represents only 20 percent of the capital investment. Therefore, once in operation, 80 percent of downtime is the direct result of investing only 20 percent of planning time on other components that are key to the productivity and profitability of the overall system.
Author: Yuri Chocholko
North American Sales Manager of Wood, Biomass & Biofuels, Vecoplan LLC