### Permission to use the attached copyrighted document provided to the U.S. EPA by email dated
November 3, 2009 from J. Sloesen, Weir Minerals Netherlands b.v., to Steve Souders, U.S. EPA.GEHO® Info for sales representatives Volume 2, No. 5, May 1992



# Economical ash handling system

Ash production

Coal-fired power plants produce large quantities of fly and bottom ash. The ash contentof coal burned varies from 5 to 35%. In particular power plants in coal producing countries burn coal with a minimum of washing, hence with a large ash content. A 1000 mw power plant may produce as much as 3,000 tons of ash per day. 80 to 90% of the ash in

the coal is recovered as fly ash from the flue gas by electrostatic precipitators. 10 to 20%

remains in the bottom of the combustion

chamber as bottom ash. Some of the ash is sold (construction industry) depending on thequality, whilst the remainder has to be disposed of in the most economical way, consistent with local geographic, statutory and

environmental considerations.



### Ash disposal

The ash is generally transported from the power plant, hydraulically, pneumatically or by mechanical means (trucks, conveyors) to

form:1. Ash dumps: mostly existing of a combination of bottom ash and fly ash. 2. Natural ash dams: fly ash is pumped atlow concentrations into natural geographic areas, probably locked by a dam. 3. Artificial ash dams: using bottom ash to form dykes. 4. Backfill: used to fill stopes in underground

mines or to fill old open mine pits. The hydraulic transport of fly ash has beenlargely in the form of low concentration slurries i.e. 10-30% solids by weight. However, continued use of dilute slurry pumping has been questioned in view of:

• large quantities of contaminated water being recirculated,

• problems of controlling scale build-up and

wear, • potential for leakage/discharge of ash water from the dams and for ground water pollution,

• danger of ash dam wall failure,

• pollution of surrounding terrain by windborn dust,

• high cost of large volume dam construction,

• cost and difficulty associated with stabilizing and restoration of the site,

• use of multiple, energy-inefficient and expensively maintained centrifugal pumps for transport of large volumes of abrasivediluted slurry and contaminated return water.



### Dense ash slurry disposal

Environmental constraints on ash disposal have limited the options and have increasedcosts. The problems are substantially eliminated by preparing ash slurry at high density i.e. low water content for discharge onto a sloped disposal where the deposit stabilizes.

• Dense fly ash slurries use less water than dilute slurries. Typically, the volume flow decreases by a factor 6.

• With the natural retention of slurry water by dense ash slurry the need for an ashdam water reclaim system and return water lines can often be avoided.

• When used as mine backfill it does not add to the mine dewatering capacity and solids loading of the mine water.

• Dense fly ash slurries do not flow out to a

horizontal surface but form a shallowcone from the point of discharge. A typical slope of 2-6% will be generated in the

ash dam.

• The ash will dry out to form a fairly hard surface, unaffected by rainfall and not prone to dusting in windy conditions.

• Savings in pumping-energy can amount to 50% or more. The thickened disposal method is also applied to tailings from minerals processing plants, such as redmud, where operations are limited by water supply, environmental, or operating

cost constraints.



### Typical slurry behaviour

Dense ash slurries exhibit typical properties,

which are to be accounted for. The ratio of coarse and fine material in a slurry generally is critical for the pum pability. Experience has taught that dense fly ash slurries are capable of supporting crushed bottom ash in their naturally produced ratio. However, differentashes will produce similar rheological properties at different solids ratios.

• Relatively low settling occurs which allows variable pumping rates. Above acritical concentration no particle settlement occurs.

• Wear behaviour of abrasive particles is

less because of turbulent flow and low flow velocity.

• Scaling problems are largely eliminated.

• Dense fly ash/bottom ash slurries can be transported at low flow velocities in mild steel piping without much wear.

• Care must be exercised in that viscosityand pipe friction show exponential increase with increase in solids concentrations beyond a critical value.

• Addition of bottom ash lowers pipe friction loss at the same solids concentration by weight.

• Fly ash chemically reacts with water. Proper wetting and mixing fly ash with water is complicated.

• Positive displacement pumps are required to transport dense ash slurries.



### GEHO® technology and equipment

The preparation, pumping, pipeline transportand disposal of dense ash slurry is a challenge, which requires best of technologies and equipment. As a leader of high pressure slurry, sludge and paste pumps throughoutthe world, GEHO® PUMPS has the knowhow, experience, equipment and partnerships to handle complete power plant ash projects.Based on slurry sample testing and experience either a GEHO® crankshaft driven piston diaphragm pump or hydraulically driven piston pump is selected. The preference for diaphragm pumps is based on their high pressure capability, high efficiency and the fact that the valves only are subject to wear from the highly abrasive ash. These pumps allow slurry transport over a long distance, whilst they have a proven high up-time and low parts and maintenance needs. The ashslurry preparation plant is designed for continuous through-put and uses special equipment, instrumentation and process controls. The dry fly ash and bottom ash are unloaded from the silos and individually fed into a high density mixer by specia l-design conveyors. The mass flows are controlled to a predefined ratio. The water flow to the mixer is adjustedto control viscosity of the dense slurry mixture. An intimately mixed, homogeneous and stable slurry is obtai ned by additional agitation. The GEHO® scope of supply will vary frombasic engineering to a complete and operational preparation plant depending on individual project and customer requirements.

Weir Minerals Netherlands b.v. P.O. Box 249, NL-5900 AE Venlo Phone: +31(0)77-3895200 Fax: +31(0)77-3824844

Email: weir@weirminerals.com

Website: www.weirminerals.com

Copyright © 1992, Weir Minerals Netherlands b.v. All rights reserved.