LOTOXTM + AdvXTM + SBS InjectionTM: efficient combination for BREF & ZLD

18 July 2018



AECOM, ARVOS and Linde are proposing a holistic approach to meeting the BREF emissions requirements, while maintaining efficiency, and attaining zero liquid discharge (ZLD). The concept consists of a combination of three technologies: LOTOXTM (low-oxidation-temperature, low-cost NOx control using ozone injection); AdvXTM air preheater upgrade; and SBS InjectionTM (to reduce SO3 levels upstream of the air heater). By Jonas Klingspor, ARVOS; Frank Fitch, Linde Group; and Blake Stapper, AECOM


Power plants in the EU must reduce air emissions and waste water generation, while maintaining their ability to provide electricity by the most efficient and cost-effective means possible. Unfortunately, many of the technology options that are available are expensive to implement and often also result in a deterioration in net unit heat rate. What is needed is a compliance solution that can help meet the BREF limits without reducing efficiency or requiring significant capital investment.

The system proposed here consists of an upgrade of the air preheater and control of the SO3 concentration ahead of the preheater to minimal levels. Ozone is then injected upstream of the wet scrubber to oxidise the NOx to a soluble state that is captured in the scrubber slurry. The excess heat created by the air preheater upgrade is then directed to a spray dryer where the scrubber wastewater is evaporated.

The preheater upgrade employs the new AdvXTM heat recovery technology from Ljungström (a division of ARVOS). This can extract a significant amount of additional energy from existing air preheaters. The additional energy can be used in several beneficial ways including improving the boiler heat rate, evaporation of waste water and stack gas reheat.

Critical to the success of the approach described here is the ability to reduce flue gas SO3 levels to less than 10 mg/Nm3 at the air preheater inlet. AECOM’s SBS InjectionTM process is the only technology that can consistently achieve such levels. As a result, Ljungström has licensed it from AECOM for applications in Europe and in China.

By removing SO3 and lowering the flue gas temperature, a number of additional environmental benefits may be realised. These include reductions in NOx, PM, and Hg emissions, and a decrease in fresh water consumption and wastewater treatment demand. This solution also provides new operating flexibility for plants with SCR by making it possible to operate at low loads without any adverse impact on the catalyst. The SO3 control system can in addition provide mercury oxidation using AECOM’s proprietary HBS InjectionTM process which co-injects sodium bromide with the sodium carbonate solution. 

There are a number of plants that were able to comply with the IED NOx limits of 200 mg/Nm3 but are unable to achieve the BREF NOx limits with their existing equipment. These plants would benefit from a polishing technology that can achieve the required NOx reduction for a minimal capital expenditure. Linde’s LOTOXTM technology offers precisely that. Ozone is injected into the flue gas downstream of the air preheater to oxidise insoluble NOx to higher oxides that are readily soluble for capture as nitrate in the slurry of a wet flue gas desulphurisation system. The low temperature process (optimally ≤150°C) allows stable and constant control regardless of variation in load or NOx concentration.

The LOTOX process is currently in use on more than 30 oil refinery fluidised catalytic cracking units and other industrial installations, and has been installed on a 25 MW coal-fired institutional boiler, achieving NOx removal rates in excess of 90%.

Air preheater upgrade

The AdvX air preheater upgrade essentially consists of three components: installation of high efficiency and low pressure drop heating elements; provision of additional heating element surface in the air preheater baskets; and a shift in the “x-ratio”, ie, air to flue gas ratio.

In order to increase the energy capture across the air preheater, Ljungström has developed a new heating element, designated DN8TM, which exhibits superior heat transfer capabilities while providing an open profile that can accommodate additional gas capacities without an appreciable increase in pressure drop.

The required shift in “x-ratio” can be accomplished by increasing the secondary air flow rate by up to 20% (AdvX Air) or bypassing up to 20% flue gas around the air preheater (AdvX Flue Gas) see Figure 1.

In either case, the boiler efficiency is not impacted. The flue gas temperature ahead of the air preheater will stay the same while the flue gas temperature after the air heater will drop. The reduction in flue gas temperature after the air preheater will improve electrostatic precipitator performance, and a reduction in fly ash emissions should be realised.

SBS Injection

AECOM’s SBS Injection process has been proven to reliably control SO3 to minimal outlet concentrations. The technology is capable of maintaining this high performance level on plants burning high- sulphur coal and equipped with selective catalytic reduction for NOx control, where SO3 levels often exceed 100 mg/Nm3. In the SBS Injection process, sodium carbonate is injected ahead of the air heater to selectively remove SO3 from the flue gas to very low levels. The process has been applied to over 30 boilers, representing nearly 18 000 MW of generating capacity and over 10 years of commercial operating experience.

LOTOX NOx polishing technology

The LOTOX process is based on the excellent solubility of higher order nitrogen oxides. Coal-fired utility boilers produce NOx that is made up of NO and NO2, which are relatively insoluble in aqueous streams. The injection of ozone at low temperature oxidises the NOx to N2O5, which is highly soluble. The flue gas moisture and the liquid in the FGD system easily and quickly converts N2O5 to nitric acid (HNO3) based on the following reactions:

NO + O3 à NO2 + O2

2NO2 + O3 à N2O5 + O2

N2O5 + H2O à 2HNO3

The rapid reaction rate of the first two reactions makes ozone highly selective for treatment of NOx in the presence of other compounds such as CO and sulphur oxides (SOx). This results in high ozone utilisation efficiency for NOx removal with no oxidation of CO or SOx in the design retention time. Any unreacted ozone in the flue gas is minimal and is readily scavenged by the sulphite that is present in the scrubber. The conversion of the N2O5 into the aqueous phase is rapid and irreversible, allowing for near-complete removal of NOx.

Ozone is produced onsite by passing oxygen through a conventional ozone generator. Oxygen is stored as a liquid, or can be generated onsite. Ozone generators are used for a variety of applications, including drinking water, waste water, pulp bleaching, and swimming pool water treatment. In LOTOX applications, the amount of ozone production capacity is determined by the amount of NOx present in the flue gas and the targeted removal efficiency. During operation, the ozone is generated “on demand” based on the incoming NOx level, so that no onsite ozone storage is required.

The LOTOX system utilises the existing duct to form a reaction zone where ozone is delivered into the flue gas stream via an array of injection lances. The injection grid is designed using computational fluid dynamics to provide uniform distribution of the ozone and adequate mixing to effect a rapid reaction. Typically, the injection location is selected to allow a short residence time prior to entering the scrubber.

The LOTOX process utilises some parasitic power for the ozone generation (and to produce the oxygen, where applicable). However, since the ozone requirement is directly proportional to the mass of NOx that is being removed, the operating costs can be managed by minimising the inlet NOx to the system. Therefore, when used as a polishing control system, it provides a cost-effective alternative for achieving emission reduction rates that can exceed those of SCR. The LOTOX system has been successfully commercialised over a range of industrial combustion and chemical NOx-generating applications. The photos above show LOTOX on a fluidised catalytic cracking unit (left) and on a 25MW coal-fired institutional boiler (right).

Benefits

The additional heat energy from an air preheater upgrade can be used to eliminate the waste water treatment system, utilising the AdvX Flue Gas option (depicted in Figure 1). In this case, some of the flue gas bypassing the air heater is introduced into a spray dryer to evaporate the slurry. This not only achieves ZLD, it removes the need to treat the nitrates that will build up in the slurry due to the NOx capture process associated with the LOTOX system. This approach avoids the operating and maintenance costs of waste water treatment as well as future requirements to upgrade the system to meet new more stringent emission limits. An overview of AdvX in waste water evaporation mode is shown in Figure 2.

Improved performance

In summary, the implementation of an AdvX air preheater upgrade in combination with SBS Injection and LOTOX provides numerous benefits including:

  • From SBS Injection: reduction of SO3 in the SCR (if present), ahead of the air preheater, and ultra-low stack emissions provides substantial benefits and a completely clear plume. 

Corrosion: sulphuric acid condensation and subsequent corrosion in vessels and ductwork is avoided reducing maintenance costs.

Hg: mercury oxidation and removal across ESP and wet FGD systems is enhanced leading to ultra-low Hg emissions. Hg control may require use of the complementary HBS Injection system.

Air preheater: condensation of sulphuric acid, ammonium bisulphate and subsequent fouling of the air preheater is avoided. Reduced soot blowing will increase heating element life. Additional energy can safely be extracted.

Water consumption: decreased due to reduction of flue gas chloride concentration, and decreased purge frequency.

  • From the AdvX air preheater upgrade:

ESP: reduced temperature across the ESP will substantially reduce fly ash emissions.

Efficiency: extracting additional heat across the air preheater will improve plant efficiency.

Water consumption: decreased due to lower air preheater outlet temperature.

Zero liquid discharge.

  • From LOTOX:

NOx: BREF compliance at minimal capital cost.

Increased Hg oxidation.

Stable and reliable operation with dirty and fluctuating flue gas streams.  

Lotox
Lotox Figure 1. AdvX Air and AdvX Flue Gas
Lotox Figure 2. AdvX Flue Gas system in conjunction with waste water evaporation. The result: zero liquid discharge
Lotox


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