POWER October 2016 - 25

TOP PLANTS
1. Building on past accomplishments. One of the retired coal ash landfills at Mátra
has been used as the location for a 16-MW solar farm, the largest in Hungary. Courtesy: Mátra
Power Plant Co. Ltd.
POWER POINTS
Winning Attributes
Improved efficiency and reliability
while reducing environmental
impacts with continual upgrades
since 1969
Reduced water consumption
and problems with fugitive dust
with novel ash-handling system
and transitioned to zero liquid
discharge
Reduced emissions footprint by
selling process steam, adding
10% renewable biomass fuel,
and adding a 16-MW solar
facility-the largest in Hungary
1.5 million tons of ash. Pumping that much
slurry also meant a substantial parasitic power
drain and increased operating costs.
Worse, while much of the water was recovered
and reused, as the ash dried, the landfill
experienced severe fugitive dust problems,
leading to continual complaints from nearby
villages and farms. Dust-suppression measures
using water spraying were not successful
in sufficiently reducing fugitive dust and
further increased water usage.
In the mid-1990s, the decision was made
to change how the plant handled its coal ash
and move to an approach that would reduce
both water consumption and fugitive dust. The
plant managers opted to go with a Circumix
dense slurry system (DSS) supplied by Enexio
Hungary (then GEA EGI Contracting Engineering).
(For details of the process, see " Solid
Coal Ash-Handling System Avoids Problems
Associated with Wet and Dry Systems " in the
August 2014 issue or at powermag.com.)
Unlike lean slurry, DSS technology relies
on pozzolanic reactions between the reactive
components of fly ash and water. The slurry
forms a hard, cement-like substance after the
ash is placed in a landfill, but it is still pumpable
before the reactions complete. Water and
ash are combined in a 1:1 ratio, greatly reducing
water requirements.
As with cement used for construction,
the key to DSS is proper mixing and careful
control of the water-slurry ratio. When properly
mixed, the slurry has high homogeneity,
which ensures ideal physical characteristics
(high strength and low hydraulic conductivity)
for the final solidified product.
In the Circumix system, the mixer receives
the fly ash and combines it with sufficient
October 2016 | POWER
water for a 1:1 ratio. Rather than using mechanical
agitation, it relies on hydrodynamic
mixing inside the mixer vessel using centrifugal
pumps. Once combined, the dense slurry
is pumped to the landfill for disposal. In addition
to fly ash, other solid residuals are mixed
into the slurry, including bottom ash, coarse
ash from the economizer and air heaters,
and gypsum from the FGD system. (See the
slideshow associated with the online version
of this article for more photos of the process.)
When Mátra decided to move to a DSS for
ash handling, the approach was still a novel
technology, and regulatory authorities required
a series of feasibility studies to ensure
it would work effectively, including a small
pilot system to test the technology in action.
Permission to proceed was given in 1995.
Mátra's DSS was supplied and constructed by
Enexio, with the slurry recirculation and transport
pumps supplied by Weir Minerals (then
Warman). The new system was installed in
parallel alongside the existing lean slurry system
to minimize impact on plant operations.
The new DSS landfill was also built on top of
an old drained lean slurry pit, meaning that the
new dense slurry, once solidified, would serve
as a hard cap on the old landfill.
Solid Footing
In operation, the DSS requires roughly 20%
of the water and power needs of the old lean
slurry system. Make-up water for the DSS is
supplied by various wastewater streams from
the power plant and coal mine, such as the
FGD system, industrial wastewater, blowdown
from the auxiliary wet cooling towers,
and coal mine runoff.
Of the water used, about 66% is bound up
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with the solidified ash, 10% evaporates, and
the remaining 24% drains out as leachate.
Because the ash remains behind, the leachate
is relatively clean and can be recycled as
makeup water for the FGD system, supplying
about 90% of that system's needs and
creating a zero liquid discharge posture for
the plant as a whole. In 2015, Mátra used just
3 million m3 of water to process 2.1 million
tons of ash. The system is a closed loop, and
no water escapes into the environment (beyond
what evaporates). Monitoring wells
installed around the landfills have confirmed
that no groundwater contamination has occurred
since the DSS was implemented.
The DSS approach has other advantages.
Because the ash is solidified, fugitive dust
has been eliminated and no water spraying
is necessary. The compressive strength of the
solidified ash allows for tiered disposal, increasing
space efficiency of the landfill. As
each layer solidifies, some of the solid ash
around the edges is excavated and used to
build the dikes for the next tier above. Each
tier is about 3 m to 4 m high, eventually forming
a pyramid-like structure that is covered
with earth and landscaped as the tiers rise.
The first landfill reached maximum height a
few years ago and was used as the site for the
plant's new solar farm (Figure 1).
Since the previous round of upgrades,
Mátra has continued its tradition of improvement.
The latest overhaul, performed by
Alstom, is in the process of renovating and
modernizing the steam turbines for two of
the 200-MW blocks, including several of the
turbine subsystems. The work on Unit 4 was
completed this year, and Unit 5 will be upgraded
during an outage in 2017. ■
-Thomas W. Overton, JD is a POWER
associate editor.
25

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POWER October 2016

Table of Contents for the Digital Edition of POWER October 2016