FGD using ammonia instead of lime or limestone can achieve higher levels of SO2 removal while eliminating liquid and solid wastes.
By Dr. Peter Lu and Dennis McLinden, Jiangnan Environmental Technology Inc.
Flue gas desulfurization (FGD) systems using lime or limestone as the chemical reagent are widely used throughout the world for SO2 emissions control at coal-fired power plants. Ammonia-based systems, however, are emerging as a viable alternative to address limitations with respect to liquid and solid waste generation and handling. Efficient Ammonia-Based Desulfurization Technology (EADS) does not generate any liquid waste streams or undesirable solid byproducts that require disposal; rather, the closed-loop process produces a salable ammonium sulfate fertilizer byproduct which can reduce more than 50 percent of the operating cost.
|Shenhua Ningxia Coal to Liquids Plant. The plant began commercial production in December 2016|
Drawbacks with Lime/Limestone
As shown in Figure 1, FGD systems employing lime/limestone forced oxidation (LSFO) include three major sub-systems:
- Reagent preparation, handling and storage
- Absorber vessel
- Waste and byproduct handling
Reagent preparation consists of conveying crushed limestone (CaCO3) from a storage silo to an agitated feed tank. The resulting limestone slurry is then pumped to the absorber vessel along with the boiler flue gas and oxidizing air. Spray nozzles deliver fine droplets of reagent that then flow countercurrent to the incoming flue gas. The SO2 in the flue gas reacts with the calcium-rich reagent to form calcium sulfite (CaSO3) and CO2. The air introduced into the absorber promotes oxidation of CaSO3 to CaSO4 (dihydrate form).
The basic LSFO reactions are:
CaCO3 + SO2 → CaSO3 + CO2 · 2H2O
The oxidized slurry collects in the bottom of the absorber and is subsequently recycled along with fresh reagent back to the spray nozzle headers. A portion of the recycle stream is withdrawn to the waste/byproduct handling system, which typically consists of hydrocyclones, drum or belt filters, and an agitated wastewater/liquor holding tank. Wastewater from the holding tank is recycled back to the limestone reagent feed tank or to a hydrocyclone where the overflow is removed as effluent.
|Typical Lime/Limestone Forced Oxidatin Wet Scrubbing Process Schematic|
Wet LSFO systems typically can achieve SO2 removal efficiencies of 95-97 percent. Reaching levels above 97.5 percent to meet emissions control requirements, however, is difficult, especially for plants using high-sulfur coals. Magnesium catalysts can be added or the limestone can be calcined to higher reactivity lime (CaO), but such modifications involve additional plant equipment and the associated labor and power costs. For example, calcining to lime requires the installation of a separate lime kiln. Also, lime is readily precipitated and this increases the potential for scale deposit formation in the scrubber.
The cost of calcination with a lime kiln can be reduced by directly injecting limestone into the boiler furnace. In this approach, lime generated in the boiler is carried with the flue gas into the scrubber. Possible problems include boiler fouling, interference with heat transfer, and lime inactivation due to overburning in the boiler. Moreover, the lime reduces the flow temperature of molten ash in coal-fired boilers, resulting in solid deposits that would otherwise not occur.
Liquid waste from the LSFO process is typically directed to stabilization ponds along with liquid waste from elsewhere in the power plant. The wet FGD liquid effluent can be saturated with sulfite and sulfate compounds and environmental considerations typically limit its release to rivers, streams or other watercourses. Also, recycling wastewater/liquor back to the scrubber can lead to the buildup of dissolved sodium, potassium, calcium, magnesium or chloride salts. These species can eventually crystallize unless sufficient bleed is provided to keep the dissolved salt concentrations below saturation. An additional problem is the slow settling rate of waste solids, which results in the need for large, high-volume stabilization ponds. In typical conditions, the settled layer in a stabilization pond can contain 50 percent or more liquid phase even after several months of storage.
The calcium sulfate recovered from the absorber recycle slurry can be high in unreacted limestone and calcium sulfite ash. These contaminants can prevent the calcium sulfate from being sold as synthetic gypsum for use in wallboard, plaster, and cement production. Unreacted limestone is the predominant impurity found in synthetic gypsum and it is also a common impurity in natural (mined) gypsum. While limestone itself does not interfere with the properties of wallboard end products, its abrasive properties present wear issues for processing equipment. Calcium sulfite is an unwanted impurity in any gypsum as its fine particle size poses scaling problems and other processing problems such as cake washing and dewatering.
If the solids generated in the LSFO process are not commercially marketable as synthetic gypsum, this poses a sizeable waste disposal problem. For a 1000 MW boiler firing 1 percent sulfur coal, the amount of gypsum is approximately 550 tons (short)/day. For the same plant firing 2 percent sulfur coal, the gypsum production increases to approximately 1100 tons/day. Adding some 1000 tons/day for fly ash production, this brings the total solid waste tonnage to about 1550 tons/day for the 1 percent sulfur coal case and 2100 tons/day for the 2 percent sulfur case.
A proven technology alternative to LSFO scrubbing replaces limestone with ammonia as the reagent for SO2 removal. The solid reagent milling, storage, handling and transport components in an LSFO system are replaced by simple storage tanks for aqueous or anhydrous ammonia. Figure 2 shows a flow schematic for the EADS system provided by JET Inc.
Ammonia, flue gas, oxidizing air and process water enter an absorber containing multiple levels of spray nozzles. The nozzles generate fine droplets of ammonia-containing reagent to ensure intimate contact of reagent with incoming flue gas according to the following reactions:
(1) SO2 + 2NH3 + H2O → (NH4)2SO3
(2) (NH4)2SO3 + ½O2 → (NH4)2SO4
The SO2 in the flue gas stream reacts with ammonia in the upper half of the vessel to produce ammonium sulfite. The bottom of the absorber vessel serves as an oxidation tank where air oxidizes the ammonium sulfite to ammonium sulfate. The resulting ammonium sulfate solution is pumped back to the spray nozzle headers at multiple levels in the absorber. Prior to the scrubbed flue gas exiting the top of the absorber, it passes through a demister that coalesces any entrained liquid droplets and captures fine particulates.
The ammonia reaction with SO2 and the sulfite oxidation to sulfate achieves a high reagent utilization rate. Four pounds of ammonium sulfate are produced for every pound of ammonia consumed.
As with the LSFO process, a portion of the reagent/product recycle stream can be withdrawn to produce a commercial byproduct. In the EADS system, the takeoff product solution is pumped to a solids recovery system consisting of a hydrocyclone and centrifuge to concentrate the ammonium sulfate product prior to drying and packaging. All liquids (hydrocyclone overflow and centrifuge centrate) are directed back to a slurry tank and then re-introduced into the absorber ammonium sulfate recycle stream.
The EADS technology provides numerous technical and economic advantages, as shown in Table 1.
- EADS systems provide higher SO2 removal efficiencies (>99%), which gives coal-fired power plants more flexibility to blend cheaper, higher sulfur coals.
- Whereas LSFO systems create 0.7 tons of CO2 for every ton of SO2 removed, the EADS process produces no CO2.
- Because lime and limestone are less reactive compared to ammonia for SO2 removal, higher process water consumption and pumping energy is required to achieve high circulation rates. This results in higher operating costs for LSFO systems.
- Capital costs for EADS systems are similar to those for constructing an LSFO system. As noted above, while the EADS system requires ammonium sulfate byproduct processing and packaging equipment, the reagent preparation facilities associated with LSFO are not required for milling, handling and transport.
The most distinctive advantage of EADS is the elimination of both liquid and solid wastes. The EADS technology is a zero-liquid-discharge process, which means no wastewater treatment is required. The solid ammonium sulfate byproduct is readily marketable; ammonia sulfate is the most utilized fertilizer and fertilizer component in the world, with worldwide market growth expected through 2030. In addition, while the manufacturing of ammonium sulfate requires a centrifuge, dryer, conveyer and packaging equipment, these items are non-proprietary and commercially available. Depending on economic and market conditions, the ammonium sulfate fertilizer can offset the costs for ammonia-based flue gas desulfurization and potentially provide a substantial profit.
|Efficient Ammonia Desulfurization Process Schematic|
Enhanced EADS in China
In 2016, China became one of 194 signatories to the United Nations Framework Convention on Climate Change held in Paris (Paris Agreement). During this conference, the Chinese government announced it would cut pollution from coal-fired plants by 60% to include emissions of dust, NOx and SO2, and carbon emissions by 180 million metric tons over the next five years. This is to be accomplished by upgrading power stations with clean coal technologies such as flue gas desulfurization and selective catalytic reduction.
In concert, China is implementing an action plan of Energy Saving, emission reduction, upgrading and retrofitting of coal-fired power plants for the period 2014-2020. This requires that air pollutant emissions concentrations for new coal-fired power generating units generally meet the emission standards for gas-fired boiler/power generators. Emissions of particulate matter and SO2 in the discharged flue gas will need to be lower than 1.18 lb/MMSCF, 12 PPM and, respectively.
The basic EADS process described above has been installed in more than 150 power generation, chemical, sulfur recovery, and steel plants in China, demonstrating SO2 removal efficiencies greater than 99% and SO2 concentrations in the treated flue gas down to 17 PPM. In addition, the EADS absorption process in combination with a patented demister at the top of the absorber vessel removes fine particulates (1-20μm) to levels below 4.72 lb/MMSCF. However, because these emission levels do not meet China’s Ultra-low Emissions Standards, JET Inc. developed an enhanced version of its EADS technology, which has been installed on 40 projects representing over 100 absorbers.
The technology enhancements improve the performance of the original EADS system through three mechanisms, as shown in Figure 3. Collectively, absorption efficiency enhancement, acoustic agglomeration of fine particulate, and efficient demisting comprise the Ultrasound-Enhanced SO2 and Particulate Control (USPAC) technology.
|Mechanisms Comprising Ultrasound-enhanced SO2 and Particulate Control technology.|
Enhanced SO2 absorption is achieved by optimizing spray density, liquid-gas distribution, and the oxidation process. Fine particulates in the flue gas are agglomerated with scrubbing and ultrasound mechanisms, and are removed using a high-efficiency, patented demister. Using the USPAC enhancement to the basic EADS process, SO2 and particulate matter emissions meet or exceed the Chinese Ultra-low Emissions Regulations, achieving <35 mg/Nm3 for SO2 and < 5 mg/Nm3 for total particulate matter.
In September 2013, construction started on the world’s largest coal-to-liquids (CTL) plant in the Ningxia Hui autonomous region of China. Touted as the world’s largest chemical project in both the petrochemical and coal chemical industries (total capital of US$8 billion), the plant converts 22.55 million ST/year of coal into 4.46 million ST/year of oil and 174,400 SCFM of olefin synthesis gas. Shenhua Ningxia Coal Industry Group put this plant into commercial operation in December 2016.
Central to the CTL plant is the thermal power station, which consists of 10 * 200 MW ultra-high pressure coal-fired boilers. Each boiler is paired with an air quality control system consisting of:
- Selective catalytic reduction (SCR) reactors for NOx control
- Electrostatic precipitators (ESPs) for particulate matter collection (each with two chambers and six electric fields)
- Ammonia-based flue gas desulfurization systems for SO2 removal and additional fine particulate matter removal
In 2014, the plant selected JET Inc. to supply the FGD systems. Because emissions had to conform to China’s Ultra-low Emissions Regulations, JET chose the USPAC technology. Each USPAC system is designed to treat 475,500 SCFM of flue gas with an SO2 concentration of 980PPM. Initial performance of the USPAC systems since the plant entered operation in late 2016 has met or exceeded the Ultra-low Emissions Regulations, achieving outlet SO2 concentrations in the clean flue gas of less than 12PPM (dry basis, standard conditions, 6% oxygen) and particulate matter concentrations of less than 0.29lb/MMSCF. In addition, the USPAC system has demonstrated an availability of greater than 98% with greater than 99% ammonia recovery.
In order to ensure smooth implementation of this project and supply of desulfurization absorbent, Shenhua Ningxia Industry Group invested a synthetic ammonia plant with the capacity of 165,300 short tons/year to supply desulfurization absorbent for this project and other FGD projects in the group, thereby substantially reducing the cost of anhydrous ammonia from 318 USD/ston for outsourcing to 227 USD/ston, and further reducing the OPEX of the FGD units.
Comparative Economics Using EADS
Table 2 compares the operating costs at the Shenhua Ningxia CTL plant for EADS versus LSFO. If an LSFO process had been applied to this project along with commercial sales of the byproduct gypsum, the annual operating costs would be $14,642,000. In comparison, the EADS process can essentially eliminate these costs while generating a profit of over US$500,000 from the sale of ammonium sulfate (at US$90/ST), netting total annual savings of approximately $15,000,000.
The EADS technology enables power plant and industrial boiler operators to meet strict environmental regulations while providing economic benefits. EADS is available under several business models, including engineering packages with supply of key equipment and parts, project engineering, procurement and construction, Build-Operate-Transfer and Build-Operate-Own.