February 06, 2019

This new bottom ash system is designed for flexibility and loved for cost savings. Find out more in this episode of Ask the Expert.

Video Chapters:

0:00 Introduction

0:16 What purpose does a bottom ash transport system serve in a plant?

0:54 What are some of the primary factors that impact the selection of a conveyance system?

2:54 What applications and configurations was the Submerged Grind Conveyor (SGC) designed for and what sets it apart from other alternatives?

5:22 Were there any surprises in the plants where the SGC was installed?

Transcription:

Heather Muckley: Thank you for tuning in to our discussion about bottom ash conveyors.

With us today is B&W Engineer Tyler Little, an expert in ash conveyance technology from our facility in Lancaster, Ohio. Welcome, Tyler!

Tyler Little: Thank you.

HM: What purpose does a bottom ash transport system serve in a plant?

TL: Bottom ash makes up approximately 20% of the total ash produced in a modern pulverized coal unit. It is generally larger in particle size than the fly ash, which causes it to fall to the bottom of the furnace. Beneath the furnace throat is typically a water-impounded hopper. So, the ash falls into this water bath where it is quenched.

The purpose of the bottom ash transport system is then to remove the ash from this hopper, transport it away from the congested boiler area, and dewater it so that it can be disposed of as a solid.

HM: There are lots of options out there…what are some of the primary factors that impact the selection of a conveyance system?

TL: The biggest factors that impact the selection of a new conveyance system stem from the fact that (1) most of these projects are environmentally driven and must comply with new regulatory standards, and (2) every plant, at least in the US, is looking at this as a retrofit to their existing system.

On the environmental side, one of the key criteria around the new standards is that if water is used to transport the ash as a sluiced stream, all that water becomes a zero-discharge requirement. So, some of the new technologies continue to sluice the ash with transport water, but then recirculate and reuse all the water. Such an approach is often the most convenient to retrofit because the equipment in the boiler area remains unchanged and the dewatering of the ash can occur in new equipment installed remotely from the boiler area. However, it does still carry the risk of an unintentional discharge of a zero-discharge stream, not to mention the cost and complexity of recirculating millions of gallons of water a day is high.

On the other side of the spectrum are the technologies that eliminate the liability of the sluiced water stream. These systems inherently require more work in the boiler area and therefore can be a challenge to retrofit because sometimes there’s just not enough space. So, the selection of new technology is very site-specific and a number of other factors are considered such as maintenance costs, the condition of existing equipment, etc., but it seems that the plant environmental strategy and ease of retrofit are typically the biggest factors.

The selection of a new technology is very site-specific and a number of other factors are considered such as maintenance costs, the condition of existing equipment, etc., but it seems that the plant environmental strategy and ease of retrofit are typically the biggest factors.

HM: You were involved with the development of B&W’s Submerged Grind Conveyor (SGC). Can you talk about what applications and configurations it was designed for and what sets it apart from other alternatives?

TL: The idea for the SGC came from the considerations I just mentioned about the tradeoffs between environmental liability and retrofit suitability. When the CCR and ELG rules were finalized it was clear that the favored technologies would be those that did not use water for sluicing. At that time, there were some available technologies that could accomplish this, but again, the physical constraints and/or the cost to retrofit these were daunting.

The SGC captures the best of both worlds because it does not use water for sluicing, yet it is designed to fit seamlessly beneath existing bottom ash hoppers and have a high degree of flexibility in working around existing obstacles.

The SGC captures the best of both worlds because it does not use water for sluicing, yet it is designed to fit seamlessly beneath existing bottom ash hoppers and have a high degree of flexibility in working around existing obstacles.

Previous to the SGC, conversion to a non-sluicing technology required removal of the existing ash hopper, which is a costly and time-consuming job. But the SGC is designed to fit beneath the existing ash hopper and essentially just requires removal of the old sluice piping. And instead of sluice water, it uses a mechanical chain conveyor. But it’s different than a traditional bottom ash chain conveyor in that the conveyor itself is enclosed and has a very compact cross-section, on the order of 2’ x 3’. This is made possible by retaining the clinker grinder to crush any large chunks of slag that otherwise wouldn’t fit in such a small envelope.

And loading it through the grinder means that it can be oriented in any direction and not restricted to be along the furnace throat axis as is the case for traditional conveyors. It really lends itself to virtually any layout by having the capability to arrange a series of conveyors in any direction to maneuver around obstacles. And in many cases, a parallel arrangement can be used to provide 100% redundancy, which is not practical with the traditional conveyors.

HM: Any surprises in the plants where this was installed?

TL: We thought the most attractive feature would be achieving environmental compliance with a system that is minimally intrusive as a retrofit. And don’t get me wrong, that’s proven to be big selling-point. But it surprised us that the driver for our first installation was actually to reduce the ongoing costs associated with the plant’s high-pressure sluice system.

It surprised us that the driver for our first installation was actually to reduce the ongoing costs associated with the plant’s high-pressure sluice system.

The plant now has SGCs operating on two units and is forecasting that the new system will pay for itself within a few years.

HM: Wow that's fantastic. Thank you, Tyler.

Learn more about submerged grind conveyor systems

About B&W

Babcock & Wilcox is a global leader in energy and environmental technologies and services for the power, industrial and renewable markets. With headquarters in Barberton, Ohio, USA, B&W companies employ approximately 4,000 people worldwide. We have been transforming our world for over 150 years.