August 1, 2017
Biomass Engineering and Equipment (BE&E) of Columbus, Indiana, US, has provided material handling solutions for two advanced waste-to-energy gasification plant in Tennessee for Aries Clean Energy (formerly PHG Energy).
By Tim Brown
Biomass Engineering and Equipment (BE&E) of Columbus, Indiana, US, has provided material handling solutions for two advanced waste-to-energy gasification plant in Tennessee for Aries Clean Energy (formerly PHG Energy). The Smart Conveyor and Smart Container products from BE&E have proven to be the right choice for the reliability and efficiency called for by these plants.
In October 2013, a waste-to-energy gasification plant was commissioned in Covington, Tennessee. One of the mail goals of the project was to help the community divert thousands of tonnes of wood waste and sludge from the landfill each year and greatly reduce the annual cost for tipping fees at the landfill.
The city was awarded a $250,000 (€222,000) Clean Energy Grant from the Tennessee Department of Environment and Conservation. In October 2016, a similar waste-to-energy gasification plant was commissioned in Lebanon, Tennessee.
The system in Lebanon utilizes a 64-ton (58 tonne) per day gasifier. Wood and tyres are cut to between 1” and 3” (2.5 cm – 7.5 cm) in size and wood chips are mixed with sludge from the water treatment process before gasification. Syngas is produced from the gasification process which is combusted in the industrial thermal oxidizer. Thermal energy is transferred to heat water which drives three organic rankine cycle (ORC) generators that produce 420kW of electricity. The electricity produced powers the water treatment facility saving the city thousands of dollars in utility costs each year.
The site of the project is next to Lebanon’s wastewater treatment facility. Wood is sourced and prepped locally. It is estimated that more than 8,000 tons (7,257 tonnes) of wood waste and sludge is now being diverted annually from the landfill. More than 400 tons (363 tonnes) of tyres will also be gasified each year.,
This is an example of a great project that saves the city thousands of dollars annually in landfill fees and utility payments. Other projected benefits include the reduction of carbon emissions by 2,500 tons (2,268 tonnes) per year, while 8% of the input material results in biochar which is recyclable.
The key to a good bioenergy project is a material handling system that performs. For the Covington facility, wood chipping and screening start the process. The chips are then conveyed into a covered storage container that is fed with a levelling conveyor. The push/pull wedge floor system then feeds a metering bin which meters the flow rate of the chips into the gasifier feed conveyor. This part of the project is a key component in the overall process. If the conveyors in the Covington project fail to deliver the prescribed flow rate of chips to the gasifier the entire system fails to operate at peak performance, or possibly it will fail to operate completely. Unscheduled shutdowns or poor performance are both costly and avoidable.
Eliminating weak links
When wood processing plant managers are asked to identify the weak link in their operations the answer is often conveyors. Why do sawmills, pellet mills, biomass power plants and other wood processing facilities have major issues with conveyors? Every biomass project offers its own unique issues. Fuels are difficult to refine and consistency can be an issue. Wood fuels can be especially challenging. In many instances, the wrong conveyor is chosen for a given application.
Conveyors are often selected to connect major pieces of a project together as an afterthought. After the major pieces have been selected the owners, engineers and project managers may be looking for areas to save money on the project. This is a mistake that is repeated over and over again. The failure to identify and invest in the correct conveyor for each application can cost millions in lost production, excessive maintenance and in some cases total replacement of conveyors that fail at start-up. Awarding the conveyor package of a multi-million dollar project to the lowest bidder can be problematic and create a weak link between the storage system and the fuel processing system.
The right conveyor
All conveyors are not created equally. Belt conveyors, screw conveyors, single drag chains, dual drag chains, and pneumatic systems all have their strengths and weaknesses.
Belt conveyors, while great for large volumes, are quite messy and leak large amounts of material at the head and the tail. Limited angles to reach high elevations force long runs and a maze of switchbacks to get material to the desired height. Open belts will leak dust and there are safety concerns from open pinch points.
Screw conveyors, while perfect for precise material metering are often impractical. High friction within the screw requires high horsepower to move material and screws are limited to short runs in straight lines.
Typical drag chain conveyors require high horsepower due to the chain running directly on the floor of the conveyor frame. This wears out the chain, the floors, and the paddles. Chain wear is high and replacement costs can also be very expensive.
How does it work?
The Smart Conveyor is best described as a twin chain drag conveyor. The system works by pulling material through the conveyor using paddles attached to paddle frames. Paddle frames are connected to tabs that are welded onto the chain. The system is a modular bolt together construction utilizing straight sections and curve sections. Steep inclines (up to 75 degrees) are possible with curve sections that are designed for wear strip changes from outside the conveyor. The twin chains are totally supported in wear materials outside the material path allowing the chain to glide through the conveyor with very little friction. Low horsepower is required to operate the conveyor due to the low friction loads.
The paddles do not touch the floor or walls which also helps with friction loads. The system is extremely quiet since there is no metal to metal contact within the conveyor except for the chains rotating around the sprocket. The chains and paddles are enclosed in a dust tight frame that eliminates the large messes found at the heads and tails of other conveyor systems. The system is also very safe due to no openings, drive belts or pinch points. Strength of the conveyor frame is another benefit when compared to other systems. Each side panel has ten bends in the steel to create the channel for the chain which results in incredible strength for each section.
The project at Covington required a long run from the chipping operation, across a creek, and then rise to the sludge mixing point. This was achieved with one conveyor and one gear motor. Other systems would have required at least two conveyors and two gear motors. The conveyor travels horizontally along the ground for approximately 100’ (30m) then angels upward for another 25’ (7l8m) to meet up with the mixing port.
At the Lebanon facility, the receiving station for the gasifier feedstock is much closer to the gasifier than it is in Covington. This requires the conveyors carrying the feedstock to rise at a 75 degree angle and deliver material to the top of the gasifier more than fifty feet in the air. This would be impossible with a belt conveyor or conventional drag chain.
It is critically important to choose the right conveyor for each application. The initial savings realized by selecting the least expensive option will quickly disappear if an expensive chair must be replaced every year, or if the conveyor is unable to deliver the required capacities for the project. PHG Energy selected the Smart Conveyor from BE&E to satisfy several of their project needs. Considerations included: enclosed and dust tight, ability to rise at a steep incline due to space constraints, robust design with long life cycle, capable of long runs, low maintenance, and ease of maintenance. A project is only as strong as its weakest link, and so it is vital to choose the right conveyor that fits the long-term goals for the project.
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This article was written by Tim Brown, business development director at Biomass Engineering & Equipment. Visit www.biomassengineeringequipment.com