Knocking Down the Dust

European companies that burn biomass have been managing emissions for decades. Now a common device-the electrostatic precipitator-is increasingly being used in North American biomass processing.
By Petru Sangeorzan
Biomass combustion and gasification plants have become more economically desirable due to rising energy prices. By its nature, combustion always results in a certain amount of undesirable pollutants in the flue gas. However, pollution control equipment is used to prevent pollutants from entering the atmosphere. One example of control equipment, the electrostatic precipitator, can comply with pollution control regulations while protecting the sensitive downstream components in a biomass plant.

The most important decision in designing an electrostatic precipitator for a specific application is the selection of the basic plant size. This requires a fundamental understanding of the physical and electrical processes taking place, along with an extensive data bank of relevant experiences. No single theory adequately incorporates the many process variables that have to be considered. Perhaps the best-known theory is the Deutsch Model, which is summarized in the following formula: Fractional collection efficiency= 1-e-k.

The variable "e" equals the Naperian Log Base and "k" is a constant for a particular application, which equals plate area multiplied by effective migration velocity divided by gas volume.

Compatible with Biomass
Experience shows that effective migration velocity is not a constant but rather a function of the dust and gas properties unique to each material burned in a boiler. Modifications to this basic formula may be necessary when low emissions are required.

The design criteria for an electrostatic precipitator are directly related to the process characteristics, including emission, gas volume and temperature, dust concentration and particle size, dew point, dust resistance and chemical composition of the gas, among other factors. The dust resistivity is an important factor for the dust separation. The temperature influences the amount and composition of the adsorbed substances as well as the electric conductivity of the solid body. Once these parameters are known, the design of the electrostatic precipitator can move forward.

Biomass gasification is experiencing a renaissance as a result of cogeneration. In a time of increasing fossil fuels prices, even small-scale biomass gasification plants are now economically feasible. Biomass is carbon dioxide-neutral and converts a nearly unlimited and locally available waste product into a valuable source of unlimited energy. Offsetting these favorable developments are more stringent pollution control requirements for small-scale boilers (and other combustion plants) that require the use of filtering or electrical separating systems. For biomass combustion applications, dry electrostatic precipitators are more commonly specified. Wet electrostatic precipitators are used when the waste gas includes liquid particulates, in addition to the dust.

The dry electrostatic precipitator includes self-cleaning mechanisms that remove dust in continuous operations. Electrostatic precipitators have proved successful for many years in the European woodworking industry to remove dust from flue gas produced by wood-fired boilers and dryers. The dry electrostatic precipitator is operated to reduce fly ash and dust particles as small as 0.1 microns in the waste gas. This proven dust control technology has been introduced into the North American biomass industry to meet and/or exceed most pollution control requirements.

Microprocessors allow electrostatic precipitator users to reach optimal operating conditions over a wide range of waste gas applications.

Breaking Down the Basic Functions
The basic function of the dry electrostatic precipitator is simple. Dust-laden gases are pushed or pulled through the electrostatic precipitator to remove dust and other contaminants from the flue gas before entering the environment. The dirty air flow enters the electrostatic precipitator filter and is channelled through lanes formed by the collection plates where two mechanically separated fields, arranged one behind the other, are fed by several high-voltage converters. The high voltage applied to the discharge system (70 kilovolts to 100 kilovolts) leads to negative charging of the dust particles.

The dust-laden particles in the flue gas migrate to the positively charged collecting plate and adhere to it. The dust is separated from the plates periodically by a mechanical rapping system. The separated dust falls through the electrostatic precipitator and collects in a chamber located on the bottom of the unit. This collection chamber also has a self-cleaning mechanism that removes the dust from the electrostatic precipitator.

The pressure loss across the electrostatic precipitator is only 2 to 2.5 millibar (1 millibar equals 0.0145037738 pounds per square inch). The electrostatic precipitator is able to withstand flue gas temperatures of up to 790 degrees Fahrenheit. The use of modern high-voltage converters with microprocessor controls permits optimization of operating conditions across a wide range of waste gas applications. The energy required to reach these efficiencies is extremely low, between 1.7 and 3 kilowatts per hour depending on the type and size of the electrostatic precipitator.

To continuously protect downstream components, it is important that any electrostatic precipitator or other filter provide low maintenance and high availability. Tar in the waste gas complicates the maintenance of the electrostatic precipitator, plugging in-line process equipment and hampering the operation of prime movers that use the gas (e.g., a gas engine). In this situation, it is critical that any gas-cleaning system be able to remove the tar from the waste gas.

A wet electrostatic precipitator can perform this function. The basic principle of a wet electrostatic precipitator is as follows: The process gas enters the electrostatic precipitator either horizontally or vertically. The gas is spread to a uniform flow profile across the entire filter cross-section by means of a gas distribution system. The gas flow direction through the electric field is always opposite to the direction of gravity. The process gas and the dust particles are electrically charged by means of the high voltage (75 to 135 kilovolts) applied between the corona discharge electrodes and the honeycomb-type collecting electrodes. The charged ions are produced in the corona discharge and then attach themselves to dust
particles or droplets of tar and water. These particles and droplets are negatively charged and are attracted to the positively charged electrode.

The precipitated dust and liquid flows downward (pulled by gravity) to the bottom of the electrostatic precipitator for removal. The purified gas leaves the filter through the gas outlet hood. The wet electrostatic precipitator captures tar aerosols and dust particles, thereby protecting downstream equipment from potential damage. Wet-gas cleaning has successfully been applied in electricity generation with gas engines in applications such as updraft and downdraft gasifiers, and with circulating fluidized bed gasifiers.

Unlike many forms of gas-cleaning technology, both types of electrostatic precipitators can be custom designed to achieve any required efficiency while operating at most emission levels. Burning biomass can present a special environmental challenge well suited to the use of a custom-designed electrostatic precipitator. Picking a knowledgeable electrostatic precipitator vendor with extensive experience in a wide range of industries will ensure a successful design for your facility.

Petru Sangeorzan is the national sales manager for Weis Environmental LLC. Reach him at [email protected] or (901) 531-6081.