A Review of Methodologies Used to Continuously Monitor Particulate Emissions In Wet Stacks


Industrial plant operators in Europe, US and other industrialised countries are


increasingly interested to continuously monitor stack emissions. This serves the double purpose of proving that a plant’s emissions are below agreed emission limits and also giving the plant operator feedback on the performance of arrestment plant, so that fault conditions can be quickly detected and hence emissions minimised.


Wet stacks, which are categorised as those in which there are water droplets or mist in the flue gas, are more challenging to continuously monitor than ‘dry or non condensing’ stack conditions since there are the


additional problems of overcoming interference from water droplets and the increased likelihood of instrument fouling causing errors in measurement.


However, a number of methodologies have emerged over the past 15 years which are now more widely


available to provide a choice of reliable compliance measurement or pragmatic arrestment plant control for wet stack applications.


Types of Monitoring Required


In Europe, processes falling under the Waste Incineration Directive (WID) and Large Combustion Plant Directive (LCPD) must continuously monitor particulate emissions in mg/m3


one in which the calibration of the instrument reading in mg/m3


. This type of compliance measurement is may be


relied upon within defined uncertainty limits and is defined in European Standard EN 14181. For these types of processes it is essential to use measurement techniques in which the interference effects of water droplets are negligible or eliminated.


For steel, chemical, pulp and paper and mineral processes falling under the European Integrated Pollution Prevention and Control Directive (IPPC) it is sometimes acceptable to monitor for changes in wet collector efficiency by use of a surrogate or indicative method. This type of monitoring is not required to report emissions in mg/m3 but to provide a reliable indication should the wet collector performance change adversely hence enabling emission events to be detected early.


Compliance Monitoring Techniques


There are essentially two core techniques for monitoring particulate emissions with high accuracy in wet stacks and both are extractive in nature (ie a sample is drawn continuously from the stack in a representative fashion) and passed through the analyser before return to the stack).


1) Beta Attenuation


In these instruments, a sample is collected on a filter tape for a fixed period of time (typically 15 mins). Once the sample is


Diagram of wet stack monitor


3) Heated dilution air is added to the sample to evaporate any surplus water. The dilution air volume is carefully controlled relative to the sample volume to ensure a known dilution on dust concentration. Typically this method uses a small sample volume.


Instruments using both Beta and extractive light scatter metho- dologies are accepted as methods to monitor wet stacks and there are a choice of instruments which carry QAL1 approvals (required for EN


Typical Processes with Wet Stacks


Wet stack applications are most commonly found in processes using wet scrubbing as a pollution control technique. Wet collectors are typic- ally used in processes where acid gases need to be ‘scrubbed’ as well as particulates reduced meaning they can be found in the chemical, metals, incineration, wood and power industries. While the advent of high efficiency bagfilters has re- placed wet collection in certain processes, the uptake of wet Flue Gas Desulphurisation (FGD) tech- nology in the power industry has recently seen the increase in the number of large wet stacks. In addition, wet stack conditions may also exist in processes where the stack temperature falls below dew point:


Typical applications with wet stacks are hence:


1)After FGD plant in the power industry where the stack is not fitted with reheat


2)After lime kilns in the pulp and paper industry


3)After Titanium Dioxide plant


4)Older municipal incinerators and small incineration plant 5)Metal furnace applications


6)Biomass boilers where the fuel has high moisture content


7)Drying processes (eg sugar beat and detergent dryers)


collected the tape is rotated such that the sample collected is inserted in a radioactive beta source and detector instrument. The amount of beta particles absorbed is directly proportional to the total weight of particulate. This process is repeated for a semi-continuous measurement.


Diagram of Beta Attenuation instrument 2) Extractive Light Scatter


In these instruments, the extracted stack sample is heated to evaporate any residual water droplets. Thus the sample may be analysed by a standard light scattering technique.


There are a number of heating mechanisms used by the core instrument manufacturers focused on wet stack measurement:


1) The sample pipe wall is heated over an extended length so that sufficient heat may be added to the sample to evaporate water and there is sufficient contact time between sample air and the heating surface. This was the original approach pioneered over 15 years ago.


2) The sampled gas stream passes into a chamber where it follows a spiral path around the inner heated wall. This approach ensures efficient heat transfer to the gas stream and to the water droplets which are thrown against the wall.


IET


Annual Buyers Guide 2009


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