Memo to: Bob Holloway, EPA OSW 

From: Lucky Benedict, EERGC Corp.

Date: May 21, 2007

Subject: Background Information and Sample Calculations for Potential
Approach to Establish DRE based Firing Rate Restrictions for ECF.

	This document provided background information for a potential approach
to establish DRE based firing rate restrictions for Emissions-comparable
fuel (ECF).  These restrictions were discussed in your recent note to
the RCRA docket.  

The approach involves:

Identifying target maximum emissions levels for two hydrocarbons
(benzene and toluene) and the 24 oxygenates listed in Table 1 to
§261.38.

Estimating destruction and removal efficiency for that compound.

Calculating a maximum ECF firing rate as a function of the concentration
of the compound in the ECF.

Target Emission Levels

	As described in the note to the docket, the target emissions levels
would be identified as:

For compounds with emissions data from oil-fired industrial boilers, the
target level would be the highest test condition average (excluding
outliers) or a 20 ug/dscm (which is identified de minimis level),
whichever is higher. 

For compounds with emissions data from only boilers that burn hazardous
waste, the target level would be the highest test condition average
(excluding outliers) or a 20 ug/dscm, whichever is higher. 

If no emissions data were available from either fuel-oil fired
industrial boilers or hazardous waste boilers, the target level would be
20 ug/dscm.

Table 1 lists the benchmark levels for each compound.

DRE versus Feedrate

	

	The DRE of a specific organic compound (contained in a liquid fuel or
waste) is dependent on numerous parameters including, but not limited
to, combustion chamber temperature, residence time of the material in
the combustion chamber, stability of the primary flame, and on adequate
mixing the fuel and/or waste and air.

	The feedrate of the target compound is also an important factor. 

	

 

NJIT Study: A recent experimental and modeling study by Brukh et al.
concluded that DREs increase with increasing feedrate of the target
compound.  The experiments were conducted in a small, well-stirred
reactor and involved the combustion of methylene 

chloride (CH2Cl2), with ethylene (C2H4) as the primary fuel, at
extremely low residence times of 5-12 milliseconds and temperatures of
1400- 1750 K (2050 -2700 °F).  Experiments were conducted at both fuel
rich and fuel lean conditions.  CH2Cl2 concentrations were low (2- 1350
ppm by volume in the main feed.)

 

	The authors modeled the combustion of methylene chloride, methyl
chloride, and benzene. They show limited experimental data for methyl
chloride and benzene from previous work.  The authors’ hypothesis is
that higher concentrations of POHC contribute additional radical
fractions and the overall result is a higher destruction efficiency.

	The modeling and experimental data support the authors hypothesis that
DRE’s increase with increasing concentration of the target
constituent.  Although the feed concentrations were low and DRE’s
achieved were low, if the data can be extrapolated to higher inlet feed
concentrations, and higher residence times typical of boilers, this
study would suggest that DREs increase with increasing composition of
the hazardous constituent.

Data from Liquid Fuel Boilers Burning Hazardous Waste: We extracted data
from the hazardous waste combustor data base for liquid fuel boilers
(LFB) that conducted DRE testing. Note that,

  --------------- (1)

where feedrate and emissions for target compound is usually reported as
a mass flow rate, for example in lb/hr.  When feedrate data were
available, or could be calculated, we used the feedrate data to
calculate a maximum theoretical emissions concentration (MTEC) for the
target compound using the formula:

 --- (2)

	

	Simultaneous DRE and MTEC data are available for approximately 200 runs
from 27 boilers for ten compounds.  These compounds are listed in Table
2 along with their incinerability (thermal stability) rank as well as
their thermal stability class.

 

	

	Figure 1 shows a plot of  penetration as function of feedrate MTEC for
the ten compounds.  While there is some scatter in the data there
appears to be a clear trend that DRE increases with feedrate.  (Note the
lines on the plot showing DRE of 99.99%, 99.995% and 99.999%).  DRE is
above five nines(99.999%) for MTECs greater than about 1x107 while MTECs
greater than 5 x 106 generally produces DRE’s above 99.995% with the
exception of two data points for hydrogen cyanide (the compound with the
highest thermal stability ranking).  The figure also shows that in
general compounds in lower thermal stability classes achieve higher
DREs.

	Figure 2 plots four of these compounds, all of which had an order of
magnitude variation in MTEC.  Best fit (power law) trend lines are shown
on the figure for each of the compounds.  Again while there is
significant scatter in the data the trendlines all indicate higher DREs
with increasing feed MTEC.

 

Firing Rate Restrictions for Boilers Burning ECF

The conditional exclusion currently being proposed for firing of ECF in
qualifying industrial boilers restricts the firing rates of ECF
depending on its constituents.

The boiler must fire at least 50% fossil fuel, on a heating value or
volume basis, whichever results in lower volume of ECF firing.

Additionally, for ECF containing more than 2% by mass of benzene, or 2%
by mass of acrolein, the firing rate of ECF will be restricted to 25% on
a volume or heat input basis whichever results in a lower volume of ECF.

                           Figure 2: Penetration versus Feed MTEC for
Compounds with a Substantial Range of Constituent Feedrates

Sample Calculation

	We made the following assumptions for the purposes of the calculations.

The primary fuel was fuel oil #4 with a heating value (HV) of 18,840
Btu/lb,  density (d ) of 7.58 lb/gal (specific gravity of 0.91), and a
composition by weight of 88.9% carbon, 10.9% hydrogen, and 0.21% sulfur.


Concentration of the specific hydrocarbon or oxygenate in fuel oil is
negligible. *

ECF consists of 25% by weight of toluene (HV=18,280 Btu/lb, d= 7.22
lb/gal, 91.3%C and 8.7% H by wt) and 75 % of non toxics having the
properties and composition of fuel oil.  

 A DRE of 99.99% is achieved for the combustion of toluene.

	The calculation is performed for a primary fuel mass feedrate of 100
lb/hr.  Thus the thermal feedrate of the primary fuel is given by:

Thermal Feed Primary Fuel = 100 lb/hr*18,840 Btu/lb= 1.884 MM Btu/hr--
(3)

	Using assumptions 1-3 above, we can calculate the properties and
composition of ECF.

HV ECF = 0.25*18,280+0.75*18,840=18,700 Btu/lb – (4)

  -- (5)

Composition ECF = 89.5% C, 10.3% H, 0.16% S --- (6)

	The process involves selecting a firing rate for ECF, calculating the
concentration of the toxic substance in the stack gas, and repeating the
calculation if concentration is different from the target levels listed
in Table 1.  

	We initially assume that the ECF firing rate (FR) is 10% by heat input.
 Therefore, the thermal feedrate of ECF can be calculated from the
thermal federate of the primary fuel in (3)

  ---- (7)

	Now that we have the thermal feedrate of ECF, we simply need to divide
by the ECF heating value to get the ECF mass feedrate which is 11.18
lb/hr. And the ECF FR by volume is:

 

The composite fuel composition is calculated using:

 

	This results in a composite fuel composition: 88.96% C, 10.84% H, and
0.21% S.

Assuming the composite fuel undergoes complete combustion in air:

  

 

 

	The amount of O2 required for complete combustion at a stoichiometric
ratio of 1.0 is given by

Oxygen requirement = Moles CO2 + ½*Moles H2O + Moles SO2 – Moles O2
in fuel

                                   = 8.284 + (6.059/2) + 0.0067 = 10.126
lbmol/hr 

Assuming, combustion air consists of 79% N2 and 21% O2 by volume,  

		N2 from comb air = 3.76 * 10.126 = 42.562 lbmol/hr

Total molar flowrate (dry) = Moles CO2 + Moles SO2 + Moles N2  = 8.284 +
0.0067 + 42.562 = 50.853 lbmol/hr

	Assuming standard conditions of 25oC and 1 atmosphere pressure, the
stack gas flowrate can be calculated as

Stack gas flowrate = 50.853 lbmol/hr * 386.7 dscf/lbmol * hr / 60min =
327.7 dscfm

	From 25% Toluene of 11.18 lb/hr ECF fed in, and 99.99% DRE is achieved,
the mass emission of Toluene is calculated:

Toluene emissions = (1 – DRE) * Feed =(1 – 99.99%) * (11.18*25%) =
0.000279 lb/hr T

	The mass emissions concentration of toluene is given by:

 

	

	Since toluene concentration is greater than the  target of 120 ug/dscm,
 we revise the initial assumption for  firing rate of of ECF,  and
iterate to get to the target toluene concentration.  The calculated
allowable ECF firing rate limit would be 7.8% (by heating value) or 8%
(by volume).

Results

	The maximum firing rate as a function of concentration of the
hydrocarbon/oxygenate in ECF for DRE of 99.99%, 99.995%, and 99.999% are
displayed in Table 3.  The table shows, within each DRE, the firing rate
limits as the concentration of the compound in ECF is varied.  The table
also shows estimated stack gas concentration and MTECs for each
calculation.  Note that the calculated stack gas concentration of the
compound is not always the target level in Table 1, because the firing
rate would be “capped” at either 25% (for ECF containing greater
than 2% benzene or acrolein) or at 50%.

	The shaded cells on Table 3 are firing rate limits that would apply if
we make the following assumptions based on the DRE versus MTEC plots
shown earlier;

For MTEC  > 1.0E7 ug/dscm, DRE= 99.999% is achievable

For 5.0E6 < MTEC < 1.0E7, DRE= 99.995% is achievable

For MTEC < 5.0 E6, DRE = 99.99% for compounds in thermal stability class
1 or 2 (benzene, toluene, and methyl methacrylate), and DRE=99.995% for
others.

This approach would result in the firing rate limit first decreasing and
then increasing as the concentration of the compound in ECF increases. 
This is due to our unrealistic assumption that the DRE is a discrete
(step) function of MTEC as opposed to a continuous function.

 Note from Bob Holloway to Docket ID No. EPA-HQ-RCRA-2005-0017 dated
April 25, 2007. 

 This level is equivalent to approximately 0.01 ppmv total hydrocarbons
(on a propane basis) for the 26 hydrocarbons and oxygenates.

 Brukh, R., R. Baret, and S. Mitra, New Jersey Institute of Technology,
“The Effect of Waste Concentration on Destruction Efficiency During
Incineration,” Environmental Engineering Science, Vol. 23, No. 2,
2006.

 Note for several cases DRE was reported without emissions or feedrate
data.  If emissions were reported the feedrate was calculated from
equation 1.

 Penetration is the fraction of the constituent remaining in the stack
gas and is equal to the ratio of emissions over feed or 1-DRE/100.

 Properties and composition of fuel oil based on average values found
in,  Stultz & Kitto, eds, “Steam: It’s Generation and Use,” 40th
Edition, 1992, p 8-15.

 Properties (specific gravity, molecular weight, heating value) of the
hydrocarbon or oxygenate constituents are from table 2-1 of the draft
TSD for the proposed ECF conditional exclusion.

 The ultimate stack gas concentration calculated is independent of the
assumed mass feed rate of primary fuel.

 The low concentration was typically selected to be slightly greater
than the specification limit (or detection limit if the specification is
nondetect) that apply to comparable fuels as listed in Table 1 of
§261.38

Table 2: Thermal Stability Ranking for Compounds with DRE Data

Table 1: Target Emissions Levels for Feedrate Restrictions.

