Quality and moisture controlled FGD-Gypsum storage

Einführung

ESI Eurosilo® offers a reliable system for storing non-free flowing bulk materials such as FGD-Gypsum in a silo. In order to eliminate block ups caused by the poor flow properties of these materials, a slotted central column was designed for uncomplicated reclaiming, regardless of storage time. Since the 1980’s, more than 60 FGD-Gypsum silos have been installed at numerous power stations around the world [1]. ESI Eurosilo® developed a custom-made retrofit silo solution for the storage of FGD-Gypsum at the MPP3 power plant in Rotterdam owned by Uniper (1).

FGD (Flue-gas desulfurization) Gypsum is a byproduct, produced by chemical scrubber emission control systems that remove the toxic sulfur dioxide from the flue gases. By meeting low moisture content conditions for the stored Gypsum, the product can be sold to third parties. In collaboration with ACO, a moisture measurement is integrated into the ESI Eurosilo® system at the Uniper power plant to continuously monitor and control the moisture content during the filling of the silo.

Basic working Principle of the ESI Eurosilo® system

The custom-made ESI Eurosilo® system at the MPP3 Uniper power plant combines minimum space requirement with large storage capacities. The entire FGD-Gypsum dewatering installation is placed on top of the storage section, thereby saving a separate dewatering building and vertical transport of dewatered Gypsum by means of a belt conveyor (3).

The FGD-Gypsum slurry with an approximate water content of 80% is pumped directly to the dewatering installation on top of the silo (1). Cyclones reduce the water content of the Gypsum slurry to around 50% during the first stage of dewatering. The resulting gypsum suspension is spread on the vacuum belt filters to achieve an end product with a moisture content between 5% and 10%. The dewatered gypsum can be sent into the silo for storage (2) or transported towards the bypass (5). The product enters the silo storage section through a hopper mounted in the centre. A short screw conveyor transfers the bulk material towards the telescopic chute. An ACO [2] moisture measurement device (4) is integrated into this screw conveyor to continuously monitor the moisture content of the ingoing Gypsum. In the telescopic chute, the product freely drops down towards the auger frame, after which it is spread in the silo by the transport augers.

During the discharge process, the augers transport the stored Gypsum towards the slotted column in the centre of the silo. The material is fed through the horizontal slots and descents freely down the column.

The FGD-Gypsum can be reclaimed into a truck or onto a belt conveyor by means of a discharge screw conveyor, located in the loadout section (3).

Reliable moisture control before and during storage

Uniper is required to guarantee a moisture content of 10% or lower for the FGD-Gypsum in order to deliver a sellable product. With the custom-made ESI Eurosilo® system at the MPP3 Uniper power plant, the desired quality is achieved by performing continuous monitoring on the ingoing flow of Gypsum with an ACO digital moisture measuring sensor (5), which is integrated into the filling screw conveyor (4). Advantage of this moisture measurement is the availability of the combined moisture values of both belt dryers.

The digital sensor supplied by ACO is based on the capacitive moisture measuring technology [3]. The sensor electronics are integrated into a robust golden housing with a resistant ceramic measuring surface.

The device identifies the moisture content of the Gypsum every 10^th of a second during the silo filling process and displays a trend of the results (6) in the control room. Based on these results, the operator can decide whether the Gypsum is stored inside the silo or not.

Calibration of the ACO moisture measurement at the Uniper MPP3 power plant has been performed by comparing a variety of lab samples with the digital output of the measurement sensor. The samples (representing a range of moisture content) were taken during the filling process and analyzed in a lab. The results have been related to the digital output of the device in order to correctly display the moisture content of the FGD-Gypsum during the filling process (6).

In order to further improve the moisture content of the Gypsum during storage, a patented drainage system is installed at silo floor level. When stored over a longer period of time, the moisture tends to migrate slowly towards the lower bulk material layers inside the silo, thereby increasing the moisture content in this region. This water is drained off via a membrane (geo textile) and a layer of granular material in which the drainage tubes are installed (7).

An optional vacuum unit can be installed to increase the effectiveness of the drainage system even further. The water of drainage system is re-used in the REA.

Key features of the custom-made Eurosilo® system with online moisture measurement at the MPP3 Uniper power plant

• No environmental pollution due to enclosed storage
• Minimal footprint for large scale storage
• Safest storage method in the market
• Maximum logistic control and flexibility
• Easy storage and reclamation of FGD-Gypsum after longer periods of storage due to the elimination of block-ups
• Minimal operating and maintenance cost
• High availability due to minimal downtime
• Reliable moisture control before storage with the online ACO moisture measurement equipment (8)
• Reliable moisture control during storage with a patented drainage system

References:

This article was created in close cooperation between ESI Eurosilo and ACO - Automation Components.

[1] ESI Eurosilo - storage without block-ups

[2] ACO - Moisture measurement - Gypsum

[3] ACO - How does a capacitive moisture measurement work?