BIF Boiler Regulations and Technology

Scott Drennan, Howard White, Coen Company, Inc.

Larry Hamilton, John Holiday, UCC
Paul Kute (P.E.), Jim Pollard (P.E.), Fluor Daniel, Inc.
Eduardo Angulo, Cerrey
Kent Methews, Bechtel

ABSTRACT:

This paper describes a unique design approach using plant fuel gas for the atomization of a liquid residue streams in a tangential fired boiler and meet all of the requirements listed outlined in the RCRA BIF (boiler incinerator furnace) publications. The liquid waste streams are produced from various chemical processes, some have the tendency to flash, others to polymerize when heated (thus not suitable for steam atomization). Atomization with plant gas was selected in order to reduce capital and operational costs associated with the conventional alternative of using compressed air atomization, provide a stable flame (even when water slugs are delivered), and improve burner turn down.

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. Utilize the advantages of tangential firing in order to provide an economical and reliable design alternative for the incineration and energy recovery of large quantities of liquid residue waste with low emissions.

2. Utilize gas atomization in order to provide reliable and economical alternative for the firing of thermally sensitive liquid waste streams.

INTRODUCTION:

The purpose of this discussion is to describe an economical approach for the recovery of energy from the continuous firing of liquid residue waste streams, and plant fuel gas in complete compliance’s to the RCRA BIF requirement, and current BACT guidelines for emission levels firing the plant fuel gas only.

The boilers selected for the service are equipped with the tangential fired system with the liquid residue gas atomized, an excellent choice for the firing of liquid residue waste for the following reasons:


  • The furnace acts as one large burner with a single flame ball,
  • Reduced dependence upon flame stability at the fuel injection point as a result of the large rotational flame ball created in the center of the furnace,
  • Improved operating reliability and reduced trip tendency by reducing the potential for occurrence of false trips of the burners,
  • Improved flame burning conditions by concentration of heat in the lower part of the furnace providing stable flame and improved ignition,
  • Improved fuel NOx reduction as a result of the ability to stage the flame stoichiometry and increased residence time at sub-stoichiometric conditions,
  • Increased flue gas residence time in the furnace at lower turbulence levels than in horizontally fired systems. This increases combustion efficiency and reduces emissions of NOx and unburned hydrocarbons for difficult to burn fuels.

Atomization fuel gas is selected to:


  • Reduce flashing of the burners tips due to the temperature accelerated vaporization of the liquid residue waste material (with a steam mix atomizer the liquid is heated prior to atomization in the flame box),
  • Maintain a stable flame with the off-gas supplementing the burning,
SUMMARY OF FUEL STREAMS:

The new boiler has been designed for operation with a broad range of compositions of the liquid residue streams, and plant fuel gas. The design covered the range of potential operating scenarios to include startup (after a complete shutdown), normal operation, and shutdown.

Gaseous Fuel Streams

The available gaseous fuel is as follows:

Plant Fuel Gas
This fuel is a stream primarily composed of Methane and Hydrogen with a HHV of about

850 Btu/SCF.

Liquid Fuel Streams

The three liquid residue streams are as follows:

Organic Acid Residue

This stream is primarily made up of high-molecular weight C7 and C8 organic acids with a viscosity slightly higher than water.

Design flow rate of 6,000 lb/hr with pressure at the boiler’s limit of 60 psig. The organic acid residue stream shall be split into two separately metered and flow rate controlled sub-streams, and fired from two opposite corners (designated Corners 1 and 3). Each of the two corners shall be equipped with a liquid firing gas atomized gun capable of firing 3,000 lb/hr of residue. This rate is the design flow rate of organic acid residue required to meet 50% of the boiler heat input at MCC with combined residue streams.

Plant Residue

This stream is primarily made up of high-molecular weight C6 and C7 acetates with a viscosity slightly higher than water.

Design flow rate of 3,000 lb/hr with pressure at the boiler’s limit of 60 psig. The plant residue shall be separately metered and flow controlled, and fired from one corner (designated Corner 2). The corner shall be equipped with a liquid firing gas atomized gun capable of firing 3,000 lb/hr. This rate is the design flow rate of plant residue required to meet 50% of the total boiler heat input at MCC with combined residue streams.

Vinyl Acetate

This stream is primarily made up of low- molecular weight acetaldehyde and some vinyl acetate with a viscosity slightly higher than water.

Design flow rate of 3,000 lb/hr with pressure at the boiler’s limit of 60 psig. The vinyl acetate shall be separately metered and flow controlled, and fired from one corner (designated Corner 4). The corner shall be equipped with a liquid firing gas atomized gun capable of firing 3,000 lb/hr. This rate is the design flow rate of vinyl acetate required to meet 50% of the total boiler heat input at MCC with combined residue streams.

BASIC DESIGN PARAMETERS:

With a mixture of gas and liquid fuels designated as “base fuel”. The “base fuel” is when 50% of the heat input is derived from the liquid residue waste streams, and the balance from the plant fuel gas. The liquid fuel guns are gas During normal operation, the boiler is fired atomized, and fuel lances are interchangeable. The combustion controls allow the operator to select liquid residue flow rates (base loaded), and the plant fuel gas varies to control load. In addition, the controls are configured to ensure that heat input from the residue liquid streams is not more than 50% of the total heat input. The controls will reduce selected liquid residue flow rates if required. The boiler is also designed for a rated steaming capacity of 120% of MCC when firing plant gas only.

REGULATORY EMISSION STANDARDS – REGULATORY STANDARDS:

The boiler is subject to specific emission standards and monitoring, record keeping, and control requirements to comply with the BIF regulations under the RCRA statute, and with state and federal regulations imposed by the Clean Air Act Amendments (CAAA).

RCRA BIF Requirements

When firing liquid residue waste, emission standards are as follows:
Organic emissions:99.99% destruction and removal efficiency (DRE);
Carbon monoxide: £ 100 ppmvd corrected to 7% oxygen on an hourly rolling average;
Particulate matter: £ 0.08 gr/dscf corrected to 7% oxygen;
Metals: Based on screening limits and site-specific risk assessment;
HCl/Cl2: Based on screening limits and site-specific risk assessment.

The boiler design considered regulatory emission standards, as well as, the monitoring, control, and record keeping requirements. Group A parameters are required to be continuously monitored and interlocked with the automatic waste feed cutoff system with alarms. Waste feed is automatically interrupted when the following specified monitored limits are exceeded: CO concentration in the stack gas; minimum and maximum combustion chamber temperature; total waste feed rate; combustion gas flowrate or velocity; and combustion chamber pressure. There are additional parameters that are not required to be interlocked with the waste feed cutoff but need monitored on a regular or continuous basis. They are necessary to maintain operating records required to ensure operational compliance and for calculation and record keeping purposes. These parameters include: steam production rate; feed rate of metals, chloride, chlorine, and ash in the total feed stream; feed rate of each specific fuel stream; heat input of the gaseous and liquid waste fuels; liquid waste viscosity; and oxygen concentration in the stack gas. Many of the monitored parameters are limited on an hourly rolling average basis which is the arithmetic mean of the 60 most recent 1-minute average values recorded.

The requirements for permitting a boiler burning hazardous waste include conducting a trial burn and risk burn to demonstrate that the boiler will operate within the regulatory limitations and that no adverse health and welfare risks will ensue from the burning of the waste. A trial burn/risk burn plan is submitted to the appropriate agency that administers the RCRA regulations, which details the proposed boiler operation. The performance of air dispersion modeling for the trial burn/risk burn plan is necessary to assess the risks to health and welfare off-site, and which ultimately determines the feed rate limits of hazardous waste material to the boiler. The air dispersion modeling conducted under the BIF regulations primarily determines the emission rate limits of the BIF regulated metals, in addition to, hydrogen chlorides and chlorine. One aspect of the trial burn is to demonstrate that the boiler will achieve the required 99.99% destruction and removal efficiency (DRE) for organic material in the waste feed. Under specific conditions the DRE trial burn may be waived and the waiver will be applicable as long as the boiler is operated under the required conditions. These conditions include: 1) firing a minimum of 50% fossil fuel based on heat or mass input, whichever results in the greater mass feed rate of the primary fuel; 2) maintaining boiler load to not less than a minimum of 40% of the design heat input; 3) primary fuels and hazardous waste fuels must have a minimum heating value of 8,000 Btu/lb; 4) operate in conformance with the CO standard; 5) the boiler must be a watertube type that does not use a stoker or stoker-type mechanism, and; 6) hazardous waste must be fired directly into the primary fuel flame zone with an air or steam atomization firing system, mechanical, or rotary cup atomization system, with specific requirements regarding viscosity, particle size, fuel pressure, and fuel flow rate depending on the atomization system. As long as the boiler will be operated under the conditions, and if the boiler does not burn specific types of hazardous wastes composed of dioxins or derived from the listed wastes (EPA Hazardous Waste Nos. F020, F021, F022, F023, F026, or F027) it will be considered to be in conformance with the DRE standard. Before a boiler can be operated outside of the required conditions or burn the listed hazardous wastes, a DRE trial burn will need to be conducted.

Air Permit Requirements

In addition to the RCRA BIF emission limitations, federal and state regulatory requirements for permitting boiler air emissions are achieved with the tangentially fired boiler design. Regulated boiler combustion by-products include the criteria pollutants nitrogen oxide (NOx), carbon monoxide (CO), volatile organic compounds (VOC), particulate matter (PM), and sulfur dioxides (SO2). Federal New Source Performance Standards (NSPS) for Steam Generating Units apply to boilers that are new or modified after June 19, 1984. However, a boiler firing a combination of gaseous fuel and liquid residues are only subject to the NOx standards. NOx emissions are limited to a value determined by formula, based on calculated emission rate limits for individual fuels and the heat input of the individual fuels. State requirements for Best Available Control Technology (BACT) may be more stringent than the NSPS limit. Because regulatory agencies accept the primary objective of destroying the organics in the liquid residue fuel, there is a measure of flexibility provided in assessing the NOx BACT emission limit. The tangential fired boiler is expected to achieve a NOx emission rate less than or equal to 0.12 LB/mmBtu which meets BACT requirements and is almost 1/3 of the allowable emission rate to comply with the NSPS NOx standard. Firing the combination of gaseous fuel and liquid residues subjects the boiler to NSPS requirements for a continuous emission monitor (CEM) for NOx only, however, the BIF rules also require a CEM for CO and oxygen.

An air permit for construction of the boiler was obtained with emission limits calculated using a combination of the BIF and BACT limiting factors. VOC and PM emissions were based on the BIF limits of 99.99% DRE and 0.08 gr/dscf, respectively. NOx emissions were estimated by applying the BACT limit of 0.12 LB/mmBtu for the short-term rate and 0.10 LB/mmBtu for annual average emissions. The CO emission limit of 100 ppmv was required by both the BIF regulations and BACT requirements and was utilized for short-term emission rates. An annual average CO stack concentration of 90 ppmv was applied to estimate annual CO emissions. SO2 emission rates were based on a BACT requirement that gaseous fuels contain no more than 5.0 grains of sulfur per 100 standard cubic feet of gas. Permitted SO2 emissions also considered a minimal amount of waste oil in the liquid residue fuel containing no more than 0.3 weight percent of sulfur.

Additional requirements for permitting air emissions from a boiler burning liquid residue waste streams may include dispersion modeling of the proposed contaminants. Regulatory agencies may evaluate the potential off-plant impacts from the organics and metals in the liquid residue fuel for the protection of public health and welfare, as well as, require an evaluation of the incremental increase in ambient air concentrations of the criteria pollutants (e.g., CO, NOx) for Prevention of Significant Deterioration (PSD) permitting applicability. While the boiler design and combustion technology have a significant impact on the potential off-plant concentrations of the various contaminants, the location of the boiler within the plant boundary and stack elevation are equally important in the dispersion of the stack exhaust and subsequent off-plant concentrations. Having the capability to minimize the production of combustion byproducts and reduce the contaminants leaving the stack, as in the case of the tangentially fired boiler, facilitates the use of a lower stack and provides a degree of savings in a projects capital investment.

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 the available pressure of 150 psig, it is impossible to obtain the required droplet sizing to provide greater than 2:1 turndown. Therefore, the required turn-down would require twice as many burners,
  • Steam atomization is the most economical and common method of atomization for conventional diesel and #6 fuel oils because fuel gas is usually higher cost than conventional liquid fuels. However, steam is unsuited or thermally sensitive liquid wastes that tend to polymerize at an elevated temperatures. Polymerization leads to formation of solid carbon and increases the viscosity of the liquid. Waste streams that tend to evaporate at relatively low temperatures(blocking the flow )are also unsuited for steam atomization.. In addition liquid waste byproducts 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.
  • Air atomization requires less frequent cleaning of the tips. However, the compressed air has an increased capital and operating cost and does not provide the stable flame that 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.
  • Gas atomization provides benefits as follows: requires less frequent cleaning of the tips, provides a stable flame, and reduces conversion of fuel bound nitrogen into NOx by providing elemental oxygen at the location where fuel nitrogen evolves in the flame.

High Pressure Gas Atomization Benefits:


  • Cleaning will be reduced by 1/5 of the frequency required for the standard steam atomized guns. The relatively large orifices are less likely to coke, while the sweeping action of the gas/oil mixture cleans and cools the cap which tends to minimize carbon built-up.
  • High water content(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 gas 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 increases the destruction efficiency in a given furnace(when compared with the standard atomization methods).


Figure 1, MV Atomizer Detail


  • Atomization with a combustible gas provides for a safer ignition. The controls will be configured to admit the liquid by-product waste only when a flame of atomized gas is confirmed.
  • Oil guns using high gas pressure atomization are smaller than those using low pressure gas atomization which makes the guns easier to handle. They also require the conventional pressure differential method for controls instead of the flow ratio control for the low pressure gas atomization guns.
  • Reduces conversion of fuel bound nitrogen into NOx by limiting elemental oxygen at the location where fuel nitrogen evolves in the flame.

The oil guns are the Coen MV model which are modified steam atomized guns. For this project, relatively low pressure gas is used for atomization (75 psig) with a liquid pressure of 50 psig. The liquids fired have a maximum viscosity of 100 SSU. Atomization with this gun is accomplished in two stages: in the first stage, gas and oil is mixed in multiple venturies. In the second stage, the gas oil mixture is re-mixed and passes through orifices in a specially drilled cap at a high velocity. The orifice placement on the cap provides 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 gas pressure atomized MV gun. Figure 1: MV Atomizer Detail Tangential firing The main features of a well designed combustion chamber are to provide: · sufficient time, · temperature, and · turbulence in the presence of adequate oxygen content to achieve the targeted destruction efficiency of the materials being burned. Both front and tangentially fired boilers were proposed for the service. The tangential fired Combustion Engineering VU-60 boilers (furnished by Cerrey) were selected for the service, see Figure 2 below.


Figure 2, Tangential Fired Combustion Engineering VU-60 Boiler


With the tangentially fired furnace the fuel and air are projected from the corner wind boxes along a line tangent to a small circle, lying in a horizontal plane at the center of the furnace. Intensive mixing occurs where these streams meet. A large-scale, rotational motion similar to a cyclone is created in the center which spreads out and fills the furnace volume [Singer, 1991]. The burning process occurs in the furnace volume in the form of a “fireball” (Figure 3 is an artist view of the firing pattern in the furnace). When the “fireball” condition is achieved, the furnace essentially becomes one large burner and the burner management system (BMS) ignores a “no flame” signal from an individual scanner. With false “no flame” signals common for waste streams, this system reduces the occurrences for nuisance trips of the individual burners and results in a more robust combustion situation than with wall fired burners on a similar furnace.


Figure 3, C-E Tangential Firing


Another advantage to the tangential firing is that the burners are configured along the furnace corner where space will not limit the number of guns, which can be stacked vertically. This arrangement makes it easy to accommodate the firing of multiple waste streams simultaneously. From the stand-point of destruction efficiency, and stable combustion process, concentration of the wind boxes in the lower part of the furnace provides more heat for ignition as well as longer flue gas travel. Finally, the boilers proposed with the tangential firing were offered with 25% lower level of guaranteed NOx emissions.

Control System

The high pressure gas atomization requires a similar approach for the controls as is used for the steam or air atomization, which is to maintain pressure differential between the atomizing gas and the liquid residue. The pressures of the gas and liquid required for optimum combustion process are controlled providing the proper spray characteristics. This requirement will not allow variation of the off-gas flow rates through the atomizer at a constant liquid residue flow rate. The implication is that too much gas atomizer flow rate can produce too fine a spray with the potential for combustion instabilities such as rumble. Conversely, too little 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 gas pressure and liquid residue pressure for the given liquid flow rate.

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 mmBtu/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 (PFO). See Figure 4 below:


Figure 4, 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.

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
  • Gas 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

REFERENCES:

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