Nitric Acid Plant SCR Reactor for NOx Reduction
Solution Overview
Problem
Current nitric acid plants face significant challenges in reducing nitrogen oxide emissions during startup and shutdown operations, as existing offgas cleaning systems are not effective during these non-steady-state phases, leading to elevated NOx concentrations that exceed standard limits and result in visible emissions.
Innovation Solution
A process involving an additional catalyst-filled reactor for selective catalytic reduction (SCR) is introduced, which reduces NOx and N2O concentrations during decompression by using a reducing agent like ammonia, allowing for colorless emissions and minimizing apparatus alterations to the conventional plant.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If additional residual gas cleaning apparatuses are installed to reduce NOx emissions during startup and shutdown, then emission limits can be met, but device complexity and investment costs increase significantly
Solution Approach 1:
The existing residual gas cleaning apparatus is designed to perform multiple functions: it handles both steady-state operation and transient startup/shutdown phases. The system uses a single catalytic reactor that adapts its operation mode based on the plant state, eliminating the need for separate dedicated cleaning systems for different operational phases.
Solution Approach 2:
The residual gas cleaning system utilizes the plant's own resources - specifically, it uses process ammonia or urea as reducing agents that are already part of the nitric acid production process. The system activates automatically during transient phases without requiring external intervention or separate chemical supply systems.
2Object-affected harmful factors
If the residual gas cleaning system is activated during steady-state operation to reduce NOx emissions, then emission limits are met, but productivity and energy efficiency decrease due to unnecessary gas treatment
Solution Approach 1:
The residual gas cleaning system operates periodically rather than continuously - it is activated only during transient startup and shutdown phases when NOx emissions occur, and remains inactive during steady-state operation when emissions are already within acceptable limits. This periodic operation maintains emission compliance while preserving productivity during normal operation.
Solution Approach 2:
The system dynamically adjusts its operation based on real-time plant conditions. Sensors detect transient phases (startup/shutdown) and automatically activate the cleaning apparatus only when needed, allowing the system to adapt its functionality to match the actual emission requirements rather than operating at fixed capacity.
3Object-affected harmful factors
If a large residual gas cleaning reactor is installed to handle high NOx loads during transient phases, then emission limits can be met, but the apparatus size and investment costs increase
Solution Approach 1:
The catalytic reactor is designed with sufficient capacity to handle transient NOx loads, but the system applies partial action by operating at reduced capacity during steady-state periods. The reactor volume is optimized for transient phase effectiveness while maintaining acceptable performance during normal operation, avoiding the need for oversized equipment that would be required if continuous high-capacity treatment were implemented.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution significantly reduces NOx emissions during startup and shutdown, achieving >80% reduction in NOx and >20% reduction in N2O, making the process environmentally friendly and cost-effective by using a smaller, more efficient reactor that can be integrated with minimal modifications to existing plants.
Implementation Method 1
A process involving an additional catalyst-filled reactor for selective catalytic reduction (SCR) is introduced, which reduces NOx and N2O concentrations during decompression by using a reducing agent like ammonia
Implementation Method 2
which reduces NOx and N2O concentrations during decompression by using a reducing agent like ammonia
Data Source
AI summary
Disclosed is a method and system for reducing the nitrogen oxide off-gas concentration in a nitric acid plant operated under pressure and equipped with a residual gas purification first reactor configured to remove nitrogen oxides from the off-gas during steady-state operation of the plant, and a second reactor configured to remove nitrogen oxides from the off-gas during a start-up and/or shut down of the plant. The method includes, during start-up and/or shut-down of the nitric acid plant, passing pressurized nitrogen-oxide-containing off-gas from the nitric acid plant and a gaseous reducing agent for the nitrogen oxides into the second reactor charged with a catalyst, to reduce the NOx content in the off-gas by at least catalytic reduction. Using the process and system, a colorless start-up and shut-down of nitric acid plants is possible and the nitrogen oxide content in the off-gas during start-up and/or shutdown can be substantially lowered.