Flue gas desulphurization (FGD) systems that make use of limestone or lime as its chemical reagent are widely used globally for SO2 emissions control, especially at coal-powered power plants. However, Ammonia-based flue gas systems are slowly taking centre stage as a viable alternative to the limitations relating to solid and liquid waste generation and handling with limestone treatment.
Efficient Ammonium-Based Desulphurization System (EADS) does not produce any liquid wastes or any unpleasant solid byproducts that would need to be disposed. Instead, this closed-loop system produces ammonium sulphate fertiliser as a byproduct which helps to reduce operating cost by as much as 50% relative to other flue gas cleaning systems.
Limitations Of The Limestone/Lime System
The flue gas desulphurization systems using limestone/lime forced oxidation (LSFO) consists of 3 subsystems:
– Reagent preparation, handling and storage
– Absorber vessel
– Waste and by-product handling.
Reagent preparation involves transporting the crushing limestone from a storage silo to a feedback tank. The resultant limestone slurry is pumped to the absorber vessel together with the boiler flue gas mixed with oxidizing air. Fine droplets of the reagent are delivered by spray nozzles. The SO2 from the flue gas reacts with the reagent that is rich in calcium to form calcium sulphate (CaSO3) and carbon dioxide (CO2).
The air from the absorber oxides the CaSO3 to CaSO4 (dihydrate form as follows:
CaSO3 + SO2 → CaSO3 + CO2.2H2O
This oxidized slurry gathers at the bottom of the absorber and is recycled back to the spray nozzle headers along with fresh reagent. Some part of the recycle stream is absorbed back to the waste/by-product handling system which comprises of hydro cyclones, belt or drum filters and a wastewater holding tank. Wastewater from this holding tank goes back to the limestone reagent tank, and the overflow is removed as effluent.
Advantages of Efficient Ammonium-Based Desulphurization System (EADS)
The Efficient Ammonium-Based Desulphurization System (EADS) is a tested and proven alternative to the Limestone Forced Oxidation (LSFO) as it makes use of ammonia as the reagent for the removal of SO2 rather than limestone. The LSFO’s solid reagent milling, handling, storage and transportation are replaced in the EADS by simple storage tanks for anhydrous ammonia.
The flue gas, processing water, air and the ammonia enter the absorber which contains several levels of spray nozzles. The nozzle produces fine droplets of an ammonia-rich reagent with the flue gas as follows:
SO2 + 2NH3 + H2O → (NH4)2SO3
(NH4)2SO3 + ½ → (NH4)2SO4
The Efficient Ammonium-Based Desulphurization System (EADS) has several economic and technical benefits. These include:
– EADS has higher SO2 removal efficiency (>99%) which offers the coal-powered power plants more flexibility to combine cheaper and higher sulphur coals.
– LSFO generates only 0.7 tons of CO2 for every ton of SO2 that is removed while EADS systems produce no CO2.
– Since limestone and lime are far less reactive relative to ammonia for the removal of SO2, it, therefore, requires a higher pumping energy and consumes higher process water to achieve higher circulation rates. This leads to higher cost of operation in the LSFO systems.
– LSFO and EADS both have similar capital costs. The EADS system needs equipment for ammonium sulphate by-product processing and packing. LSFO systems do not require facilities for milling, handling and transport.
The main advantage of the EADS is the total elimination of solid and liquid wastes. The EADS is a zero liquid discharge, which completely eliminates the need for wastewater treatment. The solid ammonium sulphate produced as a byproduct can be sold as fertiliser or components of fertiliser. Also, the production of ammonium sulphate requires a converter and packaging equipment and centrifuge. These items are readily available and non-proprietary. Profits from the sale of the ammonium sulphate fertilizer can be used to defray the costs of the ammonium-based flue gas desulphurization.
Enhanced EADS In China
During the United Nations Framework Convention on Climate Change held in Paris in 2016, China announced it would drastically reduce pollution from coal-powered power plants by as much as 60%. This includes emissions of NOx and SO2 as well as carbon emissions by about 180 million metric tons over a five year period. To achieve this, her power stations would have to be upgraded with clean coal technologies like selective catalytic reduction and flue gas desulphurization.
So far, EADS technologies have been installed in about 150 power stations sulphur recovery, chemical plants and steel plants in China. In places where the EADS has been installed, SO2 removal efficiencies of 99% were recorded, and SO2 concentration went down to 17 PPM.
Also, EADS together with a patented demister on the top of the absorber vessel removes fine particulates to less than 4.72lb/MMSCF. However as this emission levels are higher than China’s Ultra low Emission Standards, an enhanced EADS version was developed by JETS Inc and has been installed in about 40 projects so far.
In conclusion, EADS is a flue gas cleaning system that helps power plants as well as industrial boilers operators to meet up with strict environmental regulations as well as providing economic advantages. EADS comes in different business models like project engineering, procurement and construction, Build-Operate-Own and Build-Operate-Transfer.