Biomass! Biomass! Biomass!
Over the past several months, this has been the rallying cry for many developers and utilities looking to assist with the national goal to expand the nation’s renewable energy portfolio. ESI is certainly well positioned to assist in these pursuits due to our number of years of experience in handling and combusting biomass. In reflecting upon those experiences and what has made some biomass conversion projects a success while others have struggled, the single biggest difference is the design and operation of the material handling systems. While problems certainly can and do exist in combustion firing technology, these technologies are fairly mature and given adequate engineering due diligence issues with these technologies can normally be avoided. Further, issues with combustion technology usually only limit operational capacity and allow operation at a reduced capacity. However, issues with the fuel delivery systems can and often do completely shutdown or severely curtail the operation of a facility. With this in mind, the ability to properly receive, size, store and reclaim biomass is critical to a successful biomass power generation system project. To this end, ESI has learned over the years to focus a great deal of attention to the proper design of a material handling system. Within the next series of articles ESI will attempt to share some of the basic design considerations that should be incorporated into a successful biomass fuel handling system.
Design Considerations- Understanding Your Material
The first and foremost consideration in designing a successful system is having a very good understanding of the fuel or material to be handled. Within the confines of the term “biomass” a great deal of extremes exist. Biomass can be woody products from trees in various forms and moistures, grasses, process waste products such as bagasse, agricultural wastes such as oat hulls as well as various other products. Each of these “biomass products” has vastly different physical properties and it is virtually impossible to build a single system (which could be afforded) that can effectively handle each of these types of streams. Even amongst the group called woody biomass a great deal of diversity exists including: tree tops, bark, wood chips, sawdust, planer shavings, construction debris, and wood pellets each with vastly different properties. Therefore, to design a system to effectively handle these products it is imperative to understand each of the potential fuels and their associated properties. In the following, we will review some of the more critical physical properties and how they affect the design of a given system.
Density
Density has a tremendous effect on the design of a handling system. Virtually every mechanical device used for conveying and storage of biomass is a volumetric device. Belt conveyor width, speed and horsepower are all directly affected by the density of the material to be conveyed. Silos or bins can only store a fixed volume of material. Densities for biomass can vary from 5-7 lbs/ft3 for dry grasses and dried wood chips to 35-45 lbs/ft3 for pelletized wood. Because most projects do not have well defined long term fuel procurement plans, it is imperative to build as much flexibility into the design of the various systems as possible. This usually means looking at ranges of the material which are reasonably possible to be handled and looking at the equipment capacities within each of the ranges. Then an evaluation of margins and associated capital costs can help to hone in on the proper compromise for the system. Wherever possible, material testing over a period of time should be performed to more closely bracket the range. It is also good practice to select different densities for different design parameters. For example, a low density should be selected for conveyor capacity, while a higher density should be used for structural design. In this way the system has built in the margins it will need for the natural ranges and variations which will exist throughout the year and life of a project.
Material Sizing
Along with density, the expected size of the material is also critical to successful operation. As mentioned previously, each of the more common methods for conveying equipment are volumetric devices. While a belt conveyor certainly has a great deal more flexibility to handle oversized material than a screw conveyor, there are still limits for system design. Typical biomass can vary in size from long stringy material to fine particle type sander dust. Knowing the expected ranges of the material enables the engineer to utilize equipment and systems which can handle a wider range of material sizes until a screening and sizing system can be employed. Also, knowing the top size and bottom size of the material will enable equipment clearances and dust control systems to be properly sized.
Tendency to Bridge
One property of biomass which is often overlooked or not considered is its tendency to bridge. Many common practices which are used to handle other bulk materials, such as coal, will simply not work on biomass due to its tendency to bridge. Because all biomass is fibrous, if given an opportunity, these fibers will interlock and form a nest. This bridge can span over several feet even with a live bottom which positively moves the material. Once a bridge forms it can be extremely difficult to clear. The addition of vibrators can often pack the material even further. There are several key areas which require consideration when trying to combat the bridging potential of biomass. First, the concept of keep it moving comes to mind. A particle in motion tends to stay in motion. This is particularly true of biomass. If you allow biomass to become idle on a conveyor, in a bin, or on a pile, you are giving that particle the chance to interlock with his neighbor and form a bridge. Second, the use of negatively sloping chutes and storage bins is critical. It is much more difficult for biomass to form a bridge in a negatively sloped hopper than in a hopper with vertical walls. Third, when changing directions with biomass you must give the material an opportunity to sort itself out before additional material is piled on top of it. This is critical when dropping from conveyor to conveyor. Finally, you should positively move the material wherever possible; relying on gravity without the aid of momentum should be avoided.
Material Moisture
The amount of moisture a material has effects almost every aspect of designing a proper handling system. Besides having an effect on the material density by adding weight, moisture can exponentially increase the stickiness of the material. Most biomass inherently has a higher percentage of moisture than other fuel products and this moisture can often vary seasonally throughout the year. Therefore, the handling system could see material which is extremely wet or even icy during winter conditions and extremely dry and dusty during summer drought conditions. To properly design a material handling system under these varying extremes can be quite challenging, but the key to success is having a thorough understanding of the possible extremes. sESI has practical knowledge of typical ranges for various materials but recommends that customer’s obtain as much data as possible about the fuel throughout the year to understand the variability of the material moisture over time.
Friability
Friability means the ability of a solid substance to be reduced to smaller pieces with little effort. ESI’s experience is that biomass in not very friable. Since biomass is fibrous, those fibers tend to hold it together and prevent it from being easily fractured. This property becomes very important when it comes to properly sizing and selecting material reduction equipment. Equipment which would do a very good job in reducing the size of rock or coal, such as pulverizers or roller mills, will not work on biomass. ESI has seen a great increase in utility customers who would like to use their existing firing equipment to “grind” biomass to allow the material to be fired in suspension. Since biomass is not friable, customers are rapidly determining that except in very small percentages, the material can’t be introduced into the existing equipment. The friability of biomass is also a major contributor in selecting the correct horsepower and air systems necessary to properly size the material.
Flammability – NFPA/NEC Class
Due in large part to the variability of the properties of biomass discussed above, the flammability of biomass is also highly variable. Fuel moisture and material sizing have the greatest effect on the flammability and the explosivity index. Extremely dry,
We hope that we have helped you understand a few of the properties biomass possesses which are critical to consider when specifying and designing a biomass handling system. One of the key factors to consider is the extreme variability of biomass. The properties of biomass can and do vary from day to day and month to month. For this reason it is critical to understand the ranges that biomass will experience and carefully consider the flexibility which should be designed into a system to enable the greatest possibility of success. In the next series of newsletter articles ESI will expand upon some specific tips and design factors which should be included in any successful fuel handling system design. In the mean time, ESI offers its expertise to assist customers considering biomass. ESI has the experience and expertise to prevent the catastrophic design issues which so often affect most biomass facilities. ESI can offer a variety of services ranging from engineering design to complete guaranteed EPC system installation. ESI will be happy to assist in any way we can. For more information please contact us at 770-427-6200.
AUTOPOST by BEDEWY VISIT GAHZLY
