NOx ABATEMENT BY PROVEN TECHNOLOGIES TO REDUCE POLLUTION CAUSED DURING COMBUSTION TO MEET CURRENT REGULATORY NORMS

Abstract

The power sector is currently seeking the cost-effective minimization of NOx emissions. This demanding challenge is being met by Yara through advanced technologies successfully applied in European and other power plants.

The advanced technologies reduce nitrogen oxides for any operating scenario with NOx abatement solutions such as Advanced Selective Non-Catalytic Reduction (SNCR) or reduced-size configurations of Selective Catalytic Reduction (SCR). The Indian Environment Protection Amendment rules establish new NOx limits for thermal power plants (TPP) depending on the year they were installed. Selection of the most cost-effective solutions to meet the new standards will be critical. The variety of characteristics of the Indian coal plants (unit capacity, burner layout, coal composition, boiler age, cycling operation, etc.) requires specific analysis of the plants and unique approaches different from the conventional ones.

Introduction

Increasingly stringent worldwide environmental legislation is requiring the coal power sector to install high-efficiency abatement technologies to comply with the pollutant emission limits imposed by the applicable national standards.

The Indian Environment Protection Amendment Rules establish new environmental limits for thermal power plants (TPP) depending on the year they were installed. Table 1 summarizes the values indicated by the Ministry of Environment.

In particular, nitrogen oxide (NOx) reduction can be performed through the application of different strategies. Secondary measures of abatement control are those systems that reduce the NOx content after the formation in the combustion using a chemical reagent. These are SNCR (selective non-catalytic reduction) and SCR (selective catalytic reduction) systems.

• SNCR is the most appropriate and widely used technology to reach the reduction efficiencies of up to 50% required in many processes
• SCR is the catalyst technology used to reach the high reduction efficiencies of up to 99% required in most countries

Technical Approach

1. SNCR technology

In order to minimize NOx emissions with a cost-effective approach, YARA has developed a non-catalytic abatement technology that is an effective complement to SCR systems.

In general, the maximum potential of non-catalytic secondary measures, such as SNCR, has been shown in small industrial boilers. However, the abatement efficiency achieved in larger coal-fired power units has been lower, mainly due to their size and the difficulties in identifying optimum temperature windows with variable operating scenarios.

The following are the key performance parameters in the SNCR process:

Temperature – O2 and CO concentrations define the operating temperature window where the abatement reactions occur at high-efficiency rates.

NOx profiles – NOx profiles are produced in the combustion zone based on the specific design of the boilers. However, these profiles are non-uniform and not constant over time due to the dynamics of the combustion process. Precise knowledge of the NOx content in the injection zones is needed to achieve optimized injection of the correct reagent quantity per injection port.

The approach designed by YARA is intended to fit these features by achieving a highly flexible reagent injection in the most suitable furnace areas. The availability of advanced in-furnace monitoring capabilities provides a temperature profile and composition (NOx, O2 and CO) upstream of the injection zone to assure optimized injection profiles based on the individual control of reagent injection in each lance. This optimized injection approach brings two direct benefits: higher NOx reduction rates compared with conventional applications and effective control of ammonia slip.

2. SCR technology

The abatement of NOx (NO and NO2) is achieved by the use of a “selective catalytic reduction (SCR) system.” This is a dry flue gas treatment process, that uses ammonia (NH3) as a reducing agent and a catalyst.

The selective catalytic reduction of NOx is performed on a catalyst (substrate material TiO2 as anatase and transition metals such as V, W and/or Mo as active sites) with ammonia as a reducing agent, forming harmless reaction products according to the following reaction mechanisms:

NO2 + NO + 2 NH3 ↔ 2 N2 + 3 H2O 4 NO + 4 NH3 + O2 ↔ 4 N2 + 6 H2O 2 NO2 + 4 NH3 + O2 ↔ 3 N2 + 6 H2O

Ammonia (NH3) is injected into the flue gas and reacts with NOx on the SCR catalyst, resulting in nitrogen (N2) and water (H2O).

In this case, aqueous ammonia (24%) is used as a reduction agent. Few side reactions occur under certain conditions.

One of these is the oxidation of SO2 into SO3, the so-called SO2 conversion:

2SO2 + O2 ↔ 2SO3

This reaction has to be minimized by optimal catalyst design to avoid the formation of ammonia-sulphate compounds in the catalyst and downstream of the SCR system.

• Industrial Environmental Solutions

Emission Standards Notified in India in 2020

• NOx Reduction Technology

SNCR system

Removal of nitrogen oxides (NOx) using selective non-catalytic reduction (SNCR) technology.

YARA SNCR low NOx system

SCR system

Removal of nitrogen oxides (NOx) using selective catalytic reduction (SCR) technology.

SCR catalytic reduction method

The SCR is the best-developed and most widely used method for removing NOx from flue gases.

• It consists of the reduction of NO and NO2 to N2 under the influence of ammonia on the catalyst layer
• The general reactions are:

4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O

NO₂ + 4 NH₃ + O₂ → 3 N₂ + 6 H₂O

• The optimal temperature range for the reaction is 315 – 400 °C
• Typically, ammonia is added in the form of ammonia water in an amount less than stoichiometric (0.8 – 1.0 mol/mol NOx)

NOx Care SCR System

SCR Design Criteria

• highest NOx removal efficiency
• lowest possible NH3-slip
• lowest possible pressure drops
• highest possible availability
• highest possible flexibility across the whole load range
• lowest possible impact on the existing system
• optimum solution invest vs operating costs

The Yara Hybrid System

Anhydrous Ammonia Storage & Handling System

• ammonia unloading compressor skids
• compressors-2 N0’s min. 1100 LPM, reciprocating oil-free type
• PESO/CCOE guidelines restrict EFCV shutdown after 570 LPM
• compressor requirement is for 1100 LPM, which is possible with a 2-bay transfer

A Short Introduction to Thermal Oxidizers

A direct-fired thermal oxidizer destroys volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), that are created through chemical processes found in industrial exhaust streams. The chemical process of thermal oxidation is quite simple; the exhaust stream temperature is raised to between 850 to 1200°C, a temperature at which the chemical bonds that hold the molecules together, are broken. It is the most widely used abatement technology for industrial processes that produce exhaust streams with high levels of pollutants and can achieve over 99% hydrocarbon destruction rate.

Author

Gaurav Samkaria

Business Manager

Yara Environmental Technology Pvt Ltd