Low Pressure Off-Gas Atomization of By-Product Liquid Waste in a Fired Boiler

Scott Drennan, Howard White, Coen Company, Inc.
Paul Kuten (P.E.), Jim Pollard (P.E.), Flour Daniel, Inc.

ABSTRACT:

This paper describes a unique design approach using low pressure off-gas for the atomization of a liquid by-product waste a fired boiler. The liquid waste stream is produced from thermal cracking lighter hydrocarbons and has a tendency to polymerize at elevated temperatures, a tendency to coke in a conventional burner, and can possibly deliver water slugs to the furnace. This design provides an economical alternative for the utilization of a liquid by-product waste to generate steam and recover its energy value. In addition, atomization with low pressure gas reduces capital and operational costs associated with the conventional alternative of using compressed air or steam for atomization.

These boilers are expected to operate successfully, while meeting all emission limits and thermal performance requirements. This project demonstrated how an operating company, an equipment supplier, and an engineering contracting company can work together to develop solutions that satisfy both operational and financial objectives.

Major issues of interest to industry are as follows:

  1. Provide a reliable alternative to compressed air or steam for thermally sensitive liquid waste atomization.
  2. Provide for firing a wider range of liquid by-product waste streams in a water cooled fired furnace
  3. Utilize the advantages of low pressure gas atomization in order to provide an optimum design for energy recovery by burning liquid by-product waste with low emissions.
INTRODUCTION:

The purpose of this discussion is to describe an economical approach for the recovery of energy from the firing of a continuous liquid by-product waste stream, and multiple gas availability scenarios for available low pressure off-gas.

Atomization with low pressure off-gas is selected to:

  • Reduce plugging of the burners tips due to the temperature accelerated polymerization tendency of the waste material. (with a steam mix atomizer the liquid is heated prior to atomization in the flame box. and polymerization is more likely),
  • Maintain a stable flame with the off-gas supplementing the burning,
  • Maintain cleaner burner tips because of larger orifices sizing and the high velocity flushing action of the gas/ oil mixture.
SUMMARY OF FUEL STREAMS:

The new boilers have been designed for operation with a broad range of fuel supply alternatives. These alternatives cover the availability of fuel in a variety of operating scenarios to include startup after a complete shutdown and to cover fuel supply disruptions.

Gaseous Fuel Streams
The available gaseous fuel streams are as follows:

FUEL GRADE ETHANE:

Fuel grade ethane is a gaseous ethane rich stream from an offsite source. Fuel grade ethane is produced to a specification and the composition can vary. This stream is generally utilized as a feedstock to the Ethylene Unit. However, the stream will become the primary fuel source in event of a startup or during operation at reduced capacity.

ETHYLRNR UNIT OFF-GAS:

Ethylene unit off-gas is a gaseous stream of varying composition leftover from the ethylene production process. The stream is predominately made up of methane with varying amounts of hydrogen and propane depending upon the Ethylene unit feedstock. This gas is collected in a mix drum and the mix is consumed by the Ethylene unit furnaces and the Utility unit boilers.

SUPPLEMENTAL PROPANE:

Vaporized liquid propane is added as required to meet any of the plants energy shortfalls. This vapor propane is added to the Fuel gas mix drum in a relatively small amount.

Liquid Fuel Streams
Two liquid fuel streams are as follows:

PYROLYSIS FUEL OIL MIX (PFO):

The PFO mix material is the residue from high temperature thermal processing in a light olefin production process and is itself highly olefinic. This thermal cracked material is unstable because of the unsaturated hydrocarbons present and tends to react or polymerize in storage. The polymerization has the effect of increasing the materials molecular weight and subsequently increasing the liquid viscosity and pump ability. The polymerization accelerates at higher temperature with an approximate doubling in rate for a 10 degree C temperature rise. To minimize the adverse effects of polymerization the storage system has been designed to maintain the PFO temperature at a minimum temperature compatible with pump ability and to minimize the time the PFO material is kept in inventory. The planned storage temperature is 50 degree C with a tank system designed for a normal residence time of 2 days. The processing equipment does not allow for complete control of the product specification and this streams qualities will vary within a range of compositional parameters.

PFO PROPERTIES:

The following Table 1 shows the typical properties of the PFO Mix material to be burned in the new boilers:

Table 1, Expected PFO Mix Characteristics

The PFO has similar viscosity characteristics to a No.4 Residual Fuel Oil and a carbon residue similar to a No.6 Residual Fuel Oil. The lower viscosity of PFO when compared to No. 6 Residual Fuel Oil alleviates some of the problems of producing proper droplet sizing with relatively cool off-gas atomization as compared to steam atomization.

The PFO has a relatively high content of soluble asphaltenes and the Conradson Carbon Residue indicates the potential for coke formation at the burner tip.

A filter is provided downstream of the pumps circulating this material to remove particles greater than 1/16 inch diameter. This will ensure the fluid can pass through the nominal 1/8 inch burner orifices. In addition, water drainage provisions will be made at the storage tank to minimize water slugging to the boiler.

DIESEL FUEL OIL:

The diesel fuel for backup use will be the Saudi Arabian domestic transportation diesel fuel specification, A-888. This material meets the following specification.

Table 2, Automotive Diesel Fuel Oil Specification, A-888

Diesel fuel firing capability has been provided in the design as a supplemental fuel for shortfalls and to provide for a “black” start capability.

BASIC DESIGN PARAMETERS:

During normal operation, 30% of the boiler load will be fired with liquid by-product waste, and 70% of the boiler load will be fired with the Ethylene Unit off-gas. However, the boilers and their auxiliaries are also designed to fire as follows:

  • 100% of the boiler load with the liquid by-product waste stream using off-gas or air atomization, or
  • 100% of the boiler load with the off-gas
  • 100% of the boiler load with at the current gas composition, or
  • any combination of these two fuels as selected by the operator.
  • also 100% of the boiler load with backup diesel fuel and air atomization, “Black Start capability”.

To achieve maximum flexibility, the boilers are equipped with four burners located to fire in a tangential arrangement. Each of the four burners are provided with one liquid by-product waste gun, one diesel gun, and one gas nozzle.

FIRING SYSTEM:

Comparison of atomization schemes
Several of the conventional methods for atomization were considered for these boilers. Included is a short summary on the options considered:

  • Mechanical atomization with a reasonable liquid pressure at the gun (100 psig) is used for installations where the turn down of the individual burners and/or a very fine atomization is not required. With available pressure of 150 psig, it is impossible to obtain the required droplet sizing to provide greater than 2:1 turndown. Therefore, to obtain the required turn-down, twice as many burners would have been required.
  • Steam atomization is the most economical and common method of atomization for the conventional diesel and #6 fuel oils (this is because the cost of the fuel gas is usually higher than that of the liquid fuels). However, waste liquids commonly have components that have the tendency to polymerize at an elevated temperature. Polymerization would lead to formation of solid carbon and increases viscosity of the liquid. The use of the high temperature steam increases the likelihood of this problem. In addition liquid waste by-products are frequently mixed with substantial quantities of carbon, coke, and metal particles which tend to form larger carbon particles and block the small orifices of the steam atomized guns.
  • Low pressure air atomization, like the gas atomization, requires less frequent cleaning of the tips. However, the compressed air has an increased capital cost requirement in an expansion project and does not provide the stable flame the combustible gas atomization provides. In addition, air atomization has the tendency to increase the conversion of fuel bound nitrogen into NOx by providing elemental oxygen at the location where fuel nitrogen evolves in the flame.
  • High pressure gas (>100psig) can be used with the conventional steam atomized guns. The higher pressure gas is not widely used because it is usually not available at the higher pressures, and because of the operating cost of compressing the high pressure gas. Furthermore, with the smaller orifices at the tip frequent cleaning of the tips (due to carbon built up) will be required.

In general, low pressure gas atomization is ideal for an installation for which low pressure gas (defined as a 25-35 psig gas that is no longer valuable for processing) is available. It is an ideal medium because it provides stable flame, quick ignition, does not heat the liquid stream and requires minimum cleaning, with the required turndown ratio.

Low pressure off-gas atomization
The low pressure gas atomizing gun was selected for the following reasons:

  • Cleaning will be reduced by between 1/5 to 1/10 of the frequency required for the standard steam atomized guns. Relatively large orifices reduces less likely to coke, while sweeping action of the gas/ oil mixture cleans and cools the cap which tends to minimize carbon built-up. Because liquid by-product waste streams are generally more susceptible to plugging, reduced frequency of cleaning of these guns will make a significant improvement of operation.
  • High water content condition (in the forms of slugs) is overcome due to the extremely stable nature of combustible gases emitted from the same orifice as the liquid.
  • Providing combustion fuel at the base of the flame produces more rapid ignition, and accelerates the rate of the combustion of the liquid. These features increases retention time at a higher flame temperature, which will increase the destruction efficiency in a given furnace (when compared with the standard atomization methods).
  • An additional advantage to the atomization with a combustible gas is a safer ignition. The controls will be configured to admit the liquid by-product waste only when a flame of atomized gas is confirmed. Control logic will be used to prevent this requirement when firing with the backup air atomized case.
  • For improved system dependability, when off-gas is not available, the liquid will be atomized with plant air supplied at the same pressure.

The oil guns are the Coen MA low pressure (25 psig) gas atomized, high pressure (125 psig) liquids with maximum viscosity of 100 SSU. Atomization with this gun is accomplished in two stages. In the first stage, liquid is atomized mechanically in a mixing chamber where the liquid is also mixed with the off-gas. In the second stage, the two-component mixture passes through orifices in a specially drilled cap at a high velocity. The orifices are located on the cap providing excellent droplet disbursement of oil particles in the size range of 20 to 200 mm into the furnace. The orifices are specially drilled in order to provide the flame shape and turn down requirements for each oil type and furnace configuration. Figure 1, shown below, is a cross section of a typical low pressure atomized MA gun showing the liquid waste pressure atomized through a simplex tip, impacted with the swirling off-gas, and exiting through the plain-orifice cap. Primary atomization of the liquid occurs at the simplex tip and secondary atomization will occur as a result of the shear through the cap with the off-gas [Drennan, 1997].


Figure 1, MA Atomizer Detail

Control System

As stated in the description for the guns, the low pressure off-gas atomization requires a different approach for the controls. The quantity and pressures of off-gas required for proper combustion is solely dependent upon providing the proper PFO spray characteristics. This requirement will not allow variation of the off-gas flow rates through the atomizer at a constant PFO flow rate. The implication is that too much off-gas atomizer flow rate can produce too fine a spray with the potential for combustion instabilities such as rumble. Conversely, too little off-gas atomizer flow rate can result in a coarse a spray leading to increased particulate emissions and excessive opacity. The proper flow rate of the atomization media is controlled as a specific off-gas pressure and PFO pressure for the given PFO flow rate. In order to provide a complete range of operating flexibility, the system is equipped with air atomization back-up system, and air purge for the liquid guns.

Combustion Test
With the boilers a critical part of the plant, a combustion test firing of PFO atomized with the low pressure gas was conducted. The atomizer was test fired in a package boiler simulation facility at a firing rate of 20 MBtu/hr using natural gas atomization at low pressures with the heaviest component that makes up the bulk of Pyrolysis Fuel Oil (PFO), called Heavy Pyrolysis Oil (HPO).
See Figure 2 below:

Figure 2, Horizontal Package Boiler Combustion Test Facility

The test facility was instrumented to provide measurements of excess air, CO, NOx, air register pressure drop, flame length, atomizing pressures of PFO and off-gas, atomizing off-gas to PFO mass ratio and flow rates.

The combustion characteristics of the gas atomized HPO were found to be satisfactory in:


  • atomization quality
  • excess air operation
  • opacity and particulate emissions
  • flame shaping
  • turn down
  • atomizing gas consumption

These results were then compared to cases where steam or air atomization was used. The steam atomization tests proved to produce very coarse droplet size distribution resulting in high levels of opacity. The air atomization tests proved to be inferior to the gas atomized test. An alternate benefit of the gas atomization was demonstrated where the atomizing gas remained stable when the liquid flow was eliminated. Additionally the test allowed for the development of an optimal burner cap to minimize potential carbon build-up.

Particulate emissions are highly sensitive to the local combustion conditions of droplet diameter, excess air, temperature, and residence time. It is impossible to simulate all of these conditions in the proposed tangentially fired system with the horizontally fired boiler simulator. Instead, the estimated droplet size, gas temperature and available oxygen path history for the droplet through the subject boiler, and fuel characteristics such as Conradson Carbon and distillation curve were used with well developed computer models to predict the particulate emissions from the off-gas atomized PFO in the subject boiler.

The data collected was used to adjust the atomizer geometry and operating conditions of PFO and off-gas pressures to achieve the desired combustion characteristics.

The particulate results showed that guaranteed emission levels are easily met with low pressure gas atomization in the subject boiler. Higher particulate emissions were found for air atomization, but still meet the specified guaranteed values.

The NOx and CO results showed that guaranteed emission levels are easily met with low pressure gas atomization in the subject boiler. Projected values for the NOx are approximately the same as the original projected values (65 mg/MJ), the CO emissions level are well below earlier projected values (50 mg/MJ).

POTENTIAL FOR APPLICATION:

There are two major categories of potential applications for the gas atomization of liquids

  • Gas atomization of conventional heavy fuel oils (such as #6 oil), and
  • as atomization of the liquid waste fuels.

For many years availability of high pressure fuel gas was limited, and it’s cost significantly higher than competing fuels (such as fuel oil and coal) and in general gas atomization of liquid fuels was not economically feasible. Now that high pressure gas is more available, and it’s cost is in line with other competing fuels, the improved thermal efficiency, and lower emissions could justify conversion of existing steam or air atomized liquid fuels to gas atomization. Also with increased quantities of liquid waste and more stringent environmental regulations the marketing and/or disposal of these streams is more difficult for Petrochemical and chemical plants.

Gas atomization of the liquid waste streams provides economical, easy to operate disposal and energy recovery of the liquid waste streams. In addition, low pressure gas atomized guns enables using a normally waste off gas for atomization (typically available at lower pressures).

It is anticipated that the number of operators using this technique will grow both for domestic and foreign locations.

CONCLUSIONS:

The following conclusions can be drawn from the results obtained in the present study:

  • Natural gas atomized PFO is a viable combustion technique providing excellent atomization, satisfactory flame shaping capability, good turn down characteristics, and reasonable opacity at low excess air levels.
  • The natural gas atomized HPO produces excellent combustion characteristics when operated at the reduced gas consumption of 25%.
  • Natural gas atomization of the HPO did not pose any problems with cap fouling or internal atomizer passage plugging.
  • The atomizing gas remained stable once the HPO liquid flow was eliminated. This raises the potential to fire gas through the atomizers when no PFO liquid is available.

Low pressure natural gas atomization appears to be a superior atomization technique when compared with steam or air atomization. In order to improve the ability to predict emissions and combustion performances, data from operating units will be collected.

REFERENCES:
  1. Singer, J.G. editor, Combustion: Fossil Power, Combustion Engineering publisher, Fourth Edition, pp. 8-26 to 8-30, 1991.