Combustion Plant-

Benefits of Sulfuric Acid (H2SO4) Dewpoint Monitoring

Direct measurement, giving accurate and reliable monitoring of the sulfuric acid dewpoint temperature will assist with on-line control of flue gas temperatures, minimising maintenance costs and improving the total efficiency of the process.

There are 3 main areas where sulfuric acid dewpoint temperature measurement can have major benefit:

• Process Control
• Thermal Efficiency
• Emissions Control

Process Control

• Manage the use of high cost fuel additives such as MgO
• Monitor SO3 slip within an ESP to improve ash collection efficiency whilst minimising sulfuric acid aerosol emissions

Thermal Efficiency

• Maximise overall boiler efficiency
• Reduce maintenance caused by cold-end corrision in maintaining the exit gas above the dewpoint temperature

Emissions Control

• Monitor sulfuric acid aerosol emissions (condensables)
• Monitor and reduce sulfuric acid smut emissions

H2SO4 Dewpoint Temperature monitoring ensures that the optimum point is maintained, just above the acid dewpoint, where no acid is being formed; but efficiency is maximized. H2SO4 Dewpoint Temperature Monitors are available for both spot checks and continuous analysis.

Applications

• Fuel Oil
• Coal
• Diesel fuel oil
• Petcoke
• Oremulsion

Sulfuric Acid Dewpoint Temperature Monitoring Products

A portable H2SO4 dewpoint temperature monitor (Lancom 200) is designed specifically for plant requiring only periodic monitoring of the H2SO4 dewpoint temperature for boiler efficiency checks, corrosion protection and evaluation of the effectiveness of corrective measures such as fuel additives and changes in the combustion conditions.

A continuous sulfuric acid dewpoint temperature monitor (Model 440) is permanently installed in order to optimize combustion conditions by maintaining the temperature of the flue gas slightly above the sulfuric acid dewpoint temperature. It can continuously monitor the changes in boiler operation and provide a direct loop input to the boiler's own combustion control system.

Process Control- Manage the use of high cost fuel additives such as MgO
Injecting Magnesium Oxide (MgO) to minimise corrosion and improve efficiency Magnesium-based fuel additives limits SO3 production by reducing catalytic formation of SO3 from SO2. They also help to neutralize sulfuric acid formed at the cold end. Too little of these additives will allow higher levels of SO2 forming free SO3, which will increase the Sulfuric Acid Dewpoint temperature, reducing efficiency and allowing for the emission of pollutants into the atmosphere. Too much additive is unnecessary and expensive - and is ultimately emitted as a pollutant.

The H2SO4 dewpoint temperature is a primary measurement to control the use of these expensive fuel additives. Fuel additives yield sulfuric less acid formation - but in low sulfur fuel applications can increase pollutant emissions due to increased resistivity of the fly ash and poorer collection by the ESP.

Monitor SO3 slip within an ESP to improve ash collection efficiency whilst minimising sulfuric acid aerosol emissions.
Monitor SO3 slip in an ESP to improve ash collection efficiency This particular problem is mainly an issue where the fuel has a lower sulfur content (and high fly ash resistivity). The injection of SO3 into flue gas immediately prior to the precipitator lowers the resistivity of the fly ash, allowing better collection by the ESP.

Any fly ash that is not collected using this method can be clearly termed ‘Sulfuric Acid Smut Emissions’.

It is also possible to over-saturate the gas stream with injected SO3 - which will produce higher levels of free SO3 and subsequent sulfuric acid formation - with the downstream problems of cold-end corrosion, visibility etc.

The concentration of SO3 in the gas stream can be accurately determined from the sulfuric acid dewpoint temperature measurement. Acid Dewpoint Temperature (ADT) monitors display the SO3 concentration as standard.

Thermal Efficiency
In order to prevent cold end corrosion and air heater fouling, it is important to ensure that the flue gas temperature is above the sulfuric acid dewpoint temperature. However, if the temperature is too high then energy is wasted and plant efficiency is reduced. In order to operate at the optimum point, a continuous sulfuric acid dewpoint monitor (Model 440) can be employed. The output from the monitor can be used within the control loop, or by manual trim, for maintaining the optimum flue gas temperature.

The continuous sulfuric acid dewpoint monitor (Model 440) should be installed at a point where the SO3 and the water have completely combined (98% or better). This is typically in locations where the flue gas temperature is below 205 °C / 400 °F (>98% combination) or below 175 °C / 350 °F (>99% combination).

An additional consideration is that the ADT can be lower at the exit of the air heater since some sulfuric acid is lost to the air heater itself. Because of this, many operators want to protect the air heater and therefore would benefit by installing an ADM prior to the air heater. The problem with this approach is that before the air heater the flue gas temperatures are higher, and therefore all the sulfuric acid may not be formed.

A solution to this issue is to pre-test using a portable sulfuric acid dewpoint temperature monitor (Lancom 200) before and after the air heater, and at the precipitator, to establish the optimum installation point for a continuous sulfuric acid dewpoint temperature monitor (Model 440).

Emissions Control
Many countries are beginning to quantify Acid Aerosol Emissions. These are normally defined as sulfuric acid emissions in a vapor or liquid state, since the acid absorbed in the fly ash is assumed to be removed by the dust collectors.
US EPA SARA Rule 313, Toxic Release Inventory (TRI), defines emissions as the qualitative or quantitative amount of sulfuric acid mist emitted from coal fired boilers over a one-year period.

From- http://www.landinstruments.com/