Biomass handling equipment overview

I. Dafnomilis PhD candidate Delft University of Technology Faculty of Mechanical, Maritime and Materials Engineering L. Lanphen Master student D. L. Schott Associate Professor Delft University of Technology Faculty of Mechanical, Maritime and Materials Engineering G. Lodewijks Professor Delft University of Technology Section Transport Engineering & Logistics

Biomass handling equipment overview

Keywords: biomass handling, bulk equipment, wood pellets, pyrolysis.

1. INTRODUCTION

Current bulk equipment geared towards coal, oil and iron ore is not specifically optimized for biomass handling. For the current low to medium streams of biomass in Northwestern Europe, it usually does not make sense to invest on specialized equipment. However, the market for biomass for use in energy purposes is expected to rise in the EU in order to meet the 2020 renewable energy targets [1]. A great percentage of this biomass will be imported from extra-EU source regions (especially true for countries in Northwest Europe). Consequently, the need to set up dedicated biomass handling chains is imperative in order to minimize logistics and increase throughput.

As a bulk material, biomass is generally treated similar to coal from an operational point of view. Even though terminal operations are similar from one bulk terminal to another, the handling, transportation and storing infrastructure for biomass can be unique. Kaliyan and Morey [3] state that due to its different physical and biological properties, such as bulk density, durability, moisture content and chemical activity biomass is very difficult to handle, transport, store, and utilize in its original form. Use of the wrong equipment can damage the biomass material and the equipment itself or accidents among the personnel can occur. In order to optimize the handling procedures, the equipment and techniques at the respective terminal need to cope with the specific properties of the material. As a result, most of the common problems associated with biomass, such as dust emission, fines production and self-ignition can be avoided.

The goal of this work is to create an overview of the current state of the art of biomass handling and

storage equipment and techniques used in the bulk terminal industry. Due to the sheer diversity of biomass material, focus will mainly be on several distinct forms of biomass: wood and torrefied pellets, wood chips, ethanol, biodiesel and pyrolysis oil.

2. BIOMASS CHARACTERISTICS

This section covers the biomass bulk handling characteristics that drive the design of equipment and methods of handling and storing both solid and liquid biomass. Although technical innovations (e.g. pelletization and torrefaction) have improved the physical properties of biomass, there are still many inherent problems when dealing with the material. Due to the relatively young international market, no standards could be found for the design or specifications of dedicated biomass equipment. At the moment, the published and under draft standards for solid and liquid biofuels are mainly focused on the quality of the biomass and safe handling and storing methods.

2.1 Solid biomass characteristics

In this section, the physical properties of wood chips, wood pellets and torrefied pellets are summarized and compared to coal.

Wood chips consist of larger pieces from forest residues made by cutting or chipping. A big advantage of the wood chips is their low cost, as they are mainly a waste material and do not undergo expensive treatment steps. However, they have high moisture content, low energy content and low bulk density in comparison to the other solid biomass types.

Wood pellets, consist of dried and densified sawdust, wood shavings or wood powder and are the most compact form of untreated woody biomass [4]. Wood pellets are one of the fastest growing forms of upgraded biomass in Europe, because the pelletization process is currently the most economic and energy saving way to convert biomass into a fuel with high energy density and consistent quality [2]. Wood pellets Correspondence to: Ioannis Dafnomilis MSc, PhD candidate

Department of Maritime and Transport Technology, Mekelweg 2, 2628CD Delft, The Netherlands E-mail: I.Dafnomilis@tudelft.nl

are used in many European countries for the purposes of co-firing in coal power plants in an effort to mitigate their environmental footprint, or as feedstock for residential heating.

Torrefied pellets have several advantages over wood pellets. They have increased calorific value, they are chemically stable and do not decompose, which decreases fire hazards. They do not easily absorb water, so they can be stored outdoors for short periods of time. Torrefied pellets are also more resistant regarding compression, impact and shearing forces, which reduces the formation of dust particles [5].

Wu [6] studied various decisive physical material properties of these three types of solid biomass fuels in comparison to coal, as well as the characteristics when the materials interact with the storage and handling equipment.

The effective angle of internal friction and the wall friction angles are quite similar for solid biomass and bituminous coal. This means that when designing a silo, the cone angle can be the same for bituminous coal as for wood chips and torrefied pellets. Wood pellets will have a bigger cone angle when the wall friction is smaller than 23° or a smaller cone angle when the wall friction is bigger than 23° compared to bituminous coal, wood chips and torrefied pellets.

The angles of repose of solid biomass and bituminous coal are more or less similar, so when designing a storage facility, a hopper or a conveyor belt, it will not alter the design [7]. An other comparison that can be made is that the energy content of the solid biomass is smaller than that of the bituminous coal, leading to much larger capacity needs during transport and storage for biomass for the same energy delivered. 2.2 Interaction of biomass pellets with handling

equipment

Pellet degardation can occur due to mechanical forces acting on the pellets, creating fines and dust [11]. Pellet abrasion and dust formation takes place along the whole supply chain of the pellets from the pellet production facility to the customer, resulting in some extreme cases in a significant part (10-20%) of fines and dust [2].

Although there are quality standards in place [4], ensuring that the pellets produced are of a high strength and abrasion resistance, this problem can not be eliminated completely.

The forces that cause damage to pellets along the handling chain can be categorized in 3 general classes:

• Compression forces, resulting from a crushing action, e.g. the crushing motion of a crane grab during loading and unloading.

• Impact forces, such as drops from great heights, peumatic conveying, transfer between transport modes etc.

• Shearing forces, causing abrasion of pellet edges and surfaces, as well as abrasion of equipment. Screw conveyors and other silo discharging equipment is an example of equipment that cause degradation of pellets due to shearing forces.

Having knowledge of what kind of forces have an impact on feedstock degradation can aid in choosing or designing equipment and methods in order to reduce those effects.

2.3 Liquid biomass characteristics

The physical properties ethanol, biodiesel and pyrolysis oil are summarized and compared to gasoline, diesel oil and heavy fuel oil in this section.

Bioethanol, is the most widely produced and transported biofuel in the world. This biofuel is obtained by fermentation of carbohydrate-rich raw materials. Bioethanol can be blended with gasoline to be used as a transportation fuel, which allowed the trading and supply chains of ethanol to be well developed. It is expected that ethanol will continue to be a significant blending component for biofuels [8].

Biodiesel has the largest share of liquid biofuel in Europe and is made by the esterification of fatty acids produced from vegetable oils, which makes it a sensitive biofuel, as the future traded volumes will be influenced by overall availability and sustainability of the feedstock [8],[9]. A benefit of biodiesel is that practically all diesel engines can run on biodiesel or blends of biodiesel.

Pyrolysis oil, also known as bio-oil or biocrude, can be used as a substitute of heavy fuel oil and is derived from plant material, like agricultural and forest waste. Pyrolysis oil is an acidic liquid with a pH of 2.2 - 3.0 compared to diesel which has a pH of 5 [10]. It is combustible but not flammable (at extremely high temperatures), ignites and burns readily when properly atomized, and once ignited burns with a stable, self-sustaining flame. Pyrolysis oil is a stable but not a homogeneous liquid, which will eventually precipitate into a viscous bottom layer if left standing for months, but it can be stirred back into the bulk with slow-speed agitation.

If we compare the physical properties of the selected liquid biomass forms to their fossil fuel equivalents, it can be seen that the density and viscosity, 2 of the most important design factors for handling and storage equipment are quite similar to their liquid equivalents [9].

3. BIOMASS HANDLING EQUIPMENT

The equipment and techniques used in (mainly port) biomass handling facilities are covered in this section. Due to a limited amount of literature on biomass handling equipment, additional information has been gained by visiting the websites of, conducting interviews and e-mail correspondences with the producers of bulk material handling equipment, as well as biomass terminal operators in the port of Rotterdam. 3.1 Solid bulk equipment

In general, 4 functions of solid bulk handling can be distinguished: transshipment, transportation, storage and reclaiming.

3.2 Solid bulk transshipment

The first step when handling the solid biomass at a port terminal is to transship the solid biomass from the delivering transportation vessel, which can be a ship/barge, truck or train. This can be done via multiple options, a grab, vertical conveyor, pneumatic systems, bucket elevators or a self unloader.

When using a grab to unload the solid biomass one of the things that requires extra attention is the reduction of pellet degradation. The company Nemag [12] states that, by experience, the closed clam-shell grab design (Figure 1) reduces dust emission and breaking by 50% when used for wood pellets instead of pneumatic, continuous unloading devices.

Figure 1. Closed clam-shell grab (Nemag B.V.)

The vacuum unloader is a pneumatic transshipment system which is mainly used for ships. Janzé [5] states that pneumatic ship unloaders needs to be avoided where possible, because it causes a relatively large particle degradation due to high velocity impact during operation. However, they are sometimes preferable as they can reach a high throughput with their flexible design.

Truck and rail unloading can be done in two ways [17],[18]; the truck or wagon carrier tips its load into a reception bunker or the whole truck or wagon is tipped.

3.3 Solid bulk transportation and transfer

When the dry bulk is received, it needs to be transported to a storage area. This can be done by conveying equipment. In this section the most commonly used options for solid biomass transfer are discussed. The transfer from one conveyor to another one can be done through transfer points.

Belt conveyors are more cost effective over large distances than for example screw conveyors, because of their high throughput, relatively low power required, can be totally enclosed, which improves dust control (compared to open belt conveyors) and can be used for wood chips, torrefied- and wood pellets. However, they can be expensive to install and intermediate discharges

are problematic [13]. When choosing a belt conveyor for (wood) pellets, systems that encourage impacts or rubbing, wedging or grinding actions need to be avoided, as they can cause damage to the material [5].

Screw conveyors are commonly used in the handling of homogeneous fuels under 50 mm, like wood and torrefied pellets, but wood chips can also been handled by a (larger) screw conveyor. Screw conveyors are cheap, flexible, a closed transport system and can cover a distance up to 45m, but they require quite high power supply compared to belt conveyors [14]. Furthermore, impacts or rubbing, wedging and grinding actions needs be avoided as they can damage the pellets and wear down the equipment [5].

When handling pellets, the number of transfer points must be as low as possible to minimize the impact points and by that particle degradation and dust emission. A gentle transfer design can avoid knocking dust out of the flow [15]. Spiral or cascade loading chutes are preferred, because pellets falling from a great height in a silo will break apart [5]. Fans can create a negative pressure that directs dust into the hopper and not in the surrounding area [16].

3.4 Solid bulk storage

According to Williams et al. [13] there are 5 solid dry bulk storage types in use; silos, dome storage, flat storage, bunkers and bins. The first 3 are the most common ones used and are covered in this section. Closed storage is needed when dealing with wood chips or pellets, as high moisture contents can result in dangerous operating conditions. Enclosed storage also prevents dust from spreading. Furthermore the storage needs to be large enough to accommodate the peak throughput of biomass due to seasonal fluctuations of energy demand [11].

The silo’s loading- and unloading systems are very economical and efficient and mostly used at power plants. The construction can be made from concrete or steel. The expensive concrete storage is desirable for high throughput due to its durability, where the steel silos are more economical but not quite as durable. The pellet silos have in general two types, a tapered bottom (emptied by gravity) or a flat bottom (emptied using a circulating auger for centre feed). The maintenance and discharge time required for the flat bottom silos is usually more than the tapered [11],[13].

The flat storage buildings are an economical and efficient design and consists of high bunker style walls with a metal building or hoop type structure over the top of retaining walls. The volume for this large storage type can range from 15.000 - 100.000 m3. Loading and discharging of flat storage facilities can be fully automated, but usually will involve a labour intensive, thus expensive, step in the chain. Emptying is done mostly by a front loader either into a feed system for a boiler (power plant site) or onto trucks, vessels or rail cars for further transportation [11],[13].

For high capacity storage a costly dome storage can be used, which is usually constructed using concrete. A dome storage structure needs an reclaiming system to reclaim the stored material [13].

3.5 Solid bulk reclaiming systems

After storage, the solid biomass needs to be reclaimed for further transport to another location within the port (preferred with the first-in, first-out principle), like the loadout system [5]. A requirement of the reclaiming system is that it should be adequate for enclosed storage and enclosed transport systems. Wu [9] has listed several types of reclaimers (also summed below) that can be used for enclosed storage practices, but these reclaiming systems are not specially designed for the selected biomass. However, there are several types of reclaimers that are designed (or adjusted) to handle biomass. In this section the walking floor-reclaimers, flat bottom silo systems and screw reclaimers are discussed.

The walking floor is designed for difficult bulk solids, like wood chips and wood (-and torrefied) pellets and used for square and rectangular bunkers or silo’s (Figure 2). The hydraulic cylinders, attached at one side of the push floor, move a series of parallel pusher frames back and forth and by that displace a layer of material from the storage onto a transport equipment.

Figure 2. Walking floor reclaimer (KEITH Mfg. Co.)

Other advantages of this system is that it works on the first-in, first-out principle, has low power use and maintenance costs and can be placed in the economic flat storage buildings.

To discharge materials from a flat bottom silo, a back and forth going sliding frame driven by powerful hydraulic cylinders can be used. This type of reclaimer can break any bridges formed in the silo and promotes first-in, first-out bulk material flow into one or more screw conveyors.

There are two types of screw reclaimers/tube feeders; the linear and circular one. The linear screw reclaimer has the advantage that it automatically blends the material and has variable input and output rates. This type of reclaimers and the tube feeders can be used in rectangular bunkers.

A tube feeder consists of a screw conveyor inside a protected rotating tube. The tube allows the material to fall on the screw conveyor, eliminates the external static material pressure on the screw and deliver a uniform feed to the screw. This type of handling is much gentler and consumes as little as 25% of the power needed for traditional exposed screw reclaimers [17]. Furthermore, compared to the screw reclaimers, the wear on the mechanical parts is reduced, as the required starting torque and possible wood chips degradation reduces [18].

An overview is made of the most commonly used handling equipment for solid biomass in Figure 3.

Figure 3. Overview of solid biomass handling equipment

3.6 Liquid bulk equipment

The handling functions for liquid fuels of any kind have a continuous nature, and as such the handling chain for liquid biomass products is quite similar to

other bulk liquid fuels. Limited amount of information could be found on the subject of liquid biomass handling equipment in literature.

Information was also collected with an interview and e-mail correspondence with J.W. Bots from the company VTTI, which specializes in handling different

kinds of liquid fuels. Bots [19] states that on several VTTI-terminals the installations for the storage of biomass are identical to those of liquid fossil fuels. 3.7 Liquid bulk transshipment

Most of the ports that handle liquid fuels offer cargo-handling equipment for loading and unloading of tankers. This type of cargo-handling equipment can be adapted to the cargo, but that depends on the average volume of trade of a certain type of cargo in the respective port. Pumps are used for the unloading (and loading) of the liquid bulk and are usually located onshore.

Additionally chemical tankers usually have pumping equipment installed on board as well [22]. There are different kinds of pumps, depending on the liquid properties (e.g. viscosity) and use, such as operational conditions (e.g. temperature, pressure), flow rate etc. These pumps can have a heat barrier for temperatures up to 400°C. A critical part of these pumps are the seal systems and according to Bots [19] the best available technology of this moment is the double mechanical seals with a fluid barrier and leak detection systems.

Marine loading arms are an other solution to (un)load ships. The marine loading arms have a diameter size ranging from 10 - 60cm and are made from carbon steel, low temperature carbon or stainless steel [20]. Operating temperatures can range from -196 °C to +250°C. Critical parts of a loading arm are the swivel joints, which make it possible to manoeuvre the lifting arm without product leakage.

3.8 Liquid bulk transportation

After unloading, the liquid bulk needs to be transported to the storage tank by pipelines [22]. These pipelines can be equipped with insulation and/or heating elements depending on whether the liquid needs it. The pipelines that will transport pyrolysis oil require physical enhancements and should be made from Stainless 304, -316, HDPE, EPDM, PVC or Teflon, because of the acidic and corrosive nature of pyrolysis oil [22]. Besides the heating of biodiesel to decrease its

viscosity, the carbon- or stainless steel pipelines for biodiesel and ethanol do not need to be adjusted [19]. 3.9 Liquid bulk storage

Liquid fuels are stored in tanks. These storage tanks are built according to existing standards, (e.g. PGS29 in the Netherlands) because liquid biomass is considered a dangerous good. The material, coating and design specifications of the tank need to be matched to the stored product [19]. The most common material for these tanks is welded steel or concrete [13]. Tanks need to be inspected frequently for leakage, moisture and particulates and adhere to strict environmental legislation.

When storing ethanol, an alcohol-like and flammable substance, it is advised to store it in double wall steel fuel storage tanks. Ethanol can corrode aluminium as well as certain grades of plastic, zinc, rubber and other soft metals. Another disadvantage of ethanol is that it readily mixes with water, which makes moisture control important to ensure the ethanol is of a dispensable quality [21].

Pure biodiesel can be transported and stored in equipment designed for diesel fuel without any problems, but zinc and copper leech into biodiesel, causing impurities. Procedures need to be taken in storage and handling to prevent the temperature of biodiesel from dropping below its cloud point [22]. Water promotes slime and bacteria growth and fuel degradation in biodiesel and must be avoided [21].

Pyrolysis oils is not an easy type of liquid biomass to handle [22]: If left standing for long periods, pyrolysis oil will eventually precipitate because it is not a homogeneous liquid, though it can be stirred back into the bulk with a slow-speed agitation. It is recommended to store pyrolysis oil insulated, with a frequent stir around 10°C, with a pour point of below zero. Just as the pipelines, the storage tanks need to be made from Stainless 304, -316, HDPE, EPDM, PVC or Teflon, due to the pH of 2-3, acidic and corrosive nature of pyrolysis oil.

An overview of all the commonly used handling equipment for liquid biomass is given in Figure 4.

4. CONCLUSION

The main goal of this work is to offer an expert view into solid and liquid biomass handling and storage for the types of biomass included in the scope.

When dealing with solid biomass, care must be given in order to prevent the degradation of the material due to handling forces acting on it. Equipment and handling techniques are geared towards minimizing dust and fine production, containment of dust and particulates and ensuring a dry environment throughout the chain.

When selecting liquid bulk equipment, it is important to investigate if the properties of the material are suitable for that type of equipment. Types of liquid bulk may possibly affect certain equipment materials or the other way around, where the equipment material affects the quality of liquid bulk. In any case, the right type of equipment material needs to be chosen. Biodiesel needs to be stored at a certain temperature and therefore tanks and pipes needs to be insulated and heated and ethanol kept free from moisture. Pyrolysis oil is the most troublesome liquid form of biomass, as it requires regular stirring, insulation and specific coatings in storage and transporting equipment.

Some future research pathways are: identifying with greater accuracy the compression, shearing and impact forces inflicted on solid biomass during handling and storage in order to locate in which part of the chain to focus to avoid associated problems; research into safe and efficient handling of liquid biomass, especially pyrolysis oil, compared to current bulk liquid fuels; Improving the design of currently used equipment and handling chains and developing new equipment/techniques for more efficient handling and storage.

ACKNOWLEDGMENT

The work performed is part of the BioLogiK NL project, supported by the Top Sector Energy consortium and the Ministry of Economic Affairs of the Netherlands.

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