Applications/Improving NOx Process Filtration with Minor Investments
Improving NOx Process Filtration with Minor Investments
Waste to Energy (WTE), coal fire plants and most glass and steel industries face today major challenges with energy efficiency and pollution emission standards. Starting in 2010, most NOx (and Sox) emitters will have to comply with the new pollution emission regulations through 2012. Polluters “credit zone” between the east and west coasts can be shared with industries that are not yet fulfilling these regulations, as they are subject to strict review that can lead to costly penalties or plant closure. For example, newer plants that maintain their NOx emissions under the EPA standards will obtain “pollution credits” that they can sell to plants that are struggling or aren’t yet meeting the emission control obligations. This pollution credit transfer is a brilliant solution, but only for a short period of time, as the regulations will harden in 2012 and change again in 2016 and must be complied with by all emitters industries in 2018.
This is to say that not only the industries will have to comply with today’s standards, but also forthcoming future regulations while thinking ahead to ensure future emissions controls. Credits will be harder and harder to obtain or to exchange, emissions fees will rise, possibly resulting in plants shutting down. NOx emissions require constant filtration and monitoring in order to remain in line with the ever increasing EPA regulations. The efficiency of Flu Gas Treatment (FGT), including scrubber systems (wet, semi-wet, dry) combined with appropriate reagents, (i.e. Powder Activated Carbon (PAC),lime, UREA, or soda ash) are obvious health requirements. Many different filtering control and monitoring solutions are available, some more costly than others.
Below we will concentrate on cost effective NOx absorption systems, emphasizing on the storage, discharge and feed systems required for FGT and NOx absorption.
It is not surprising to find WTE and power plants using obsolete/unreliable bulk handling systems that do not and/or won’t comply with the new EPA emissions standards. It is important to understand the dry chemicals (here PAC) storage and handling issues prior to calibrating the injection system, scrubbers, etc. PAC is a particularly fine dry powder (±20lb/ft³) and has a tendency to compact while also being sensitive to moisture and temperature.
A storage silo equipped with soon to be outdated filters, detectors, bin activators and other bulk handling systems might no longer be suitable for newer EPA regulations and even dangerous while processing (causing explosions). If not correctly updated the NOx absorption could fail to comply with EPA emission requirements, not for the filtration calibration or monitoring, but simply because of unreliable discharge and feed systems. Some basic updates such as leakage control, filter efficiency, discharge system reliability, feed rate accuracy, and conveying/injection calibration can make a tremendous difference to the monitoring and filtration process control.
The first step to bulk handling system installation is the storage (silo, bins, hopper or big bag). Accurate storage calibration (volume, height, construction material, etc…) is essential to guarantee the product (PAC) storage condition. Weather conditions, storage volume, product density (compacted and aerated) and feed rate consumption are some of the calibration data that will define the storage requirements. We can define the action of bulk chemicals stored into containers in two categories: Mass flow and Funnel flow.
Mass flow applies to bulk products that easily discharge without any unloading device. This could span from plastic pellets to granules that flow like “liquid”. The definition for mass flow could be “first in, first out” as the first particles loaded into a container will be the first ones to be discharged.
On the other hand the funnel flow effect erratically discharges the product and applies in this case a first in last out phenomena. In other words the dry chemical loaded into a silo will be stabilized and compacted by its own weight. Then when discharging, only the center section of the container will empty creating a funnel effect through the center. Unfortunately, most of the dry chemical stored creates funnel flow and discharges erratically. Therefore, silo unloaders and flow promotion devices have been used over the years to remedy this issue. Three unloading methods are commonly installed: vibration, fluidization, and mechanical.
Vibration is the simplest way to prevent a funnel flow effect, impactors, vibrators and bin activators can be found in most unloading applications. However, this system might not be suitable for light density material such as PAC and hydrated lime where compaction can become an issue. Some vibrators are connected to a timer and work independently from the feed demand creating high compaction results, bridging and arching at the bottom of the silo cone resulting in the failure of product discharge. The bin activator has excellent results with some applications using an intermediate gasket placed at the bottom of the cone, thus reducing the vibration zone. However, the rubber gasket in contact with abrasive chemicals can leak over a few months, leading to an extremely costly gasket replacement while continuing to compact the product to an extent where bridging or arching persists.
Product compaction resulting from excessive silo vibrationThe opposite system to vibrators and bin activators is an air injection system or fluidization. The principle is clever and simple. By injecting fluidized air through a nozzle or air pads into the silo, the dry bulk aerates, decreasing its density and making it discharge as a fluid, thus reacting as mass flow. The main difficulties are to add a pressurized line to the silo with no moisture content. Using fluidized air charged with moisture into a silo with hydroscopic product could lead to disastrous results(clogs air pads, no discharge, product contamination…).
Rat holing effect with fluidization in PAC storage siloThe other issue is rat holing or rat hole effect. If the bulk chemical stabilizes, hence compacts and increases its density for a couple of days/week due to maintenance, holidays or any other outages, when compressed air is injected, the air will react as water and find its easiest way out creating “rat holes”. The injected air will no longer fluidize the dry chemical and instead blow out through the funnels/air tunnels created. Fluidization could also reduce the life cycle of dust filters.
Top view of a mechanical dischargerFinally, a reliable unloading solution is the mechanical discharger which doesn’t contaminate the product (moisture) nor compact it (vibration). It is generally an agitator that turns within the silo cone preventing bridging and arching. It reduces the funnel flow effect by collecting the product remaining on the side of the container to help discharging as mass flow and does not modify the product density to control efficiently the feed and discharge rate.
Now that the silo and discharge system have been reviewed, one of the most critical parts is the flow rate control and accuracy of the reagent. Mistakes have been made in the past when rotary air locks were combined with bin activator vibrators or air injection system, thus confusing the “rotary valve” as a feeder. The design of the airlock can provide certain volumes of RPM, but using aerators or vibration systems change the product density as well as the discharge rate and consistency. This results in an erratic flow rate (even with the use of a volumetric screw feeder) that cannot adjust to the NOx emissions monitoring. Therefore an efficient and reliable feed system can be installed to guarantee the appropriate metering of PAC. Mechanical dischargers, arch breakers, hoppers, and loss in weight systems are excellent options and allow easy retrofit and feed control with constant PAC density and throughput.
The use of a mechanical discharger/arch breaker does not change the density of the product (no aeration or vibrators) and does not require rotary airlocks as it is integrated within both a hopper and feeder. Furthermore, this system allows the addition of 1,2,3, up to 4 accurate volumetric screw feeders with only one mechanical arch breaker discharger, eliminating a need of bypass or diverters. In other words, an “all-in-one” system. This is an asset when the EPA standards require a backup line on all FGT processes.
Mechanical agitator C/w multiple screw feeders
The conveying system, pneumatic or mechanical and injection lances are important for injection but if consistency in discharge and feed rate is controlled, conveying and injection becomes much more reliable.
Finally, improving the storage, discharge and feed system of the FGT reagents can strongly improve the NOx emissions filtration without changing the downstream process while matching EPA standards.
For more information on FGT retrofit and NOx PAC removal, please contact Sodimate, Inc.