FeCl3 Report

8/17/2019 FeCl3 Report

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Addition

of Iron

in ADsProposal to test if Ferric/Ferrous chloride is considered a beneficialadditive as a strategy for an odor mitigation, not to mention moreefficient digestion under anaerobic conditions.

Quasar energy group


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Table of Contents

Contents

Abstract ______________________________________________________ 1

Literature _____________________________________________________ 2

Lab testing proposal ____________________________________________ 6

Economics and Usage ___________________________________________ 8


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Pg. 01 FeCl2 /FeCl3 usage

Abstract

A demand was made recently, to evaluate the usage of FeCl2 (Ferrous chloride)

/FeCl3 (Ferric chloride) in Anaerobic digesters.

The dialogue was started after Alan Johnson, visited Lucas county Anaerobic

digester plant and the dewatering operations had little to no odor emanating

out. After conversation with Lucas county plant official it was informed that

FeCl3/FeCl2 is added to digester.

This document outlines the science, reasoning, and logic behind the usage of

Iron and It will be upto the management to decide whether it is worth

experimenting with Iron chloride. To aid the management some economics of

the process are also explained in the last section.

NYSERDA Report

states that: For

every 1 mg/l

Sulphur 5 mg/l

iron addition

results in atleast

50% reduction of

H2S.


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LiteratureTypically, biogas produced by anaerobic digesters is used to generate energy

for farm use and sale to the power grid. One of the main difficulties associated

with biogas utilization in different form is the presence of relatively high

hydrogen sulfide (H2S) concentrations in the biogas stream. Hydrogen sulfide

present in biogas corrodes engine parts in the combustion chamber, exhaust

system, and in various bearings throughout an engine. The presence of water

vapor in the biogas stream along with hydrogen sulfide exasperates this

problem by producing pure hydrogen, which accelerates cracking and blistering

of steel parts. Furthermore, combustion of biogas with hydrogen sulfide

generates sulfur dioxide which, upon reaction with water droplets, forms

sulfuric acid. Like hydrogen sulfide, sulfuric acid is also highly corrosive to

biogas handling equipment. In general, the operational hydrogen sulfide

concentration limit for most biogas utilization systems is below 800 parts per

million on a volumetric basis (ppmv) in the gas stream.

Hydrogen sulphide (H2S) is also toxic gas/Odor which is dangerous for the

personal of WWTP. When concentration of H2S in biogas produced increases

above 500 ppm risk of respiratory and central nervous system paralysis occurs

Hydrogen sulphide is also corrosive for the equipment, as quasar has

experienced in past with gas lines getting corroded. Hence H2S should be


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physically or chemically removed from biogas. Removal of hydrogen sulphide

by activated carbon, addition of iron salts or using oxidants is very expensive.

Activated Carbon costs ~$5,000/yd3.

One of the technique for removal of H2S from digester is addition of Fe. Iron

oxide could be directly added to the digesters which will result in efficiency

There are several form of iron that could be used however Fe2+ (In the the

form FeCl2) and Fe3+ (in the form of FeCl3) are most common. The chemical

reaction used to estimate the theoretical iron amount needed is as follows:

FeCl2 + H2S → FeS ↓+ 2 HCl

FeCl2 + SO4 2- → FeSO4 ↓+ 2Cl -

Based on this reaction, 1.63 mg/L of iron (Fe) is needed to react with 1 mg/L of

hydrogen sulfide.

FeCl2 also aids in digestion process. The main end-product of the sludge

anaerobic digestion process is digested sludge. Digested sludge is dark

colored suspension, which usually containing large quantities of gas bubbles. If

sludge has strong smell it means problems in digestion process.


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Following table shows experimental Sulfide and sulfate reduction in AD

digestate after addition of different concentrations of H2S 1.

1 New York State Energy Research and Development Authority (NYSERDA) Assessment of Biochemical Process Controls for Reduction of HydrogenSulfide Concentrations in Biogas from Farm Digesters Final ReportFebruary 2012


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Based on following table it can be seen that at 250 mg/l of Fe addition to the

digester approximately >50% of total Sulfide and sulfate reduction is observed.

From An initial conc of 50 PPM of sulfur.

This table could be used as reference for testing this pilot digesters in lab. This

experiment was conducted on dairy manure feedstock, hence actual

concentration required probably might be different on different kind of

feedstocks. However one thing which is noticeable in the plot, is during addition

of little Fe, Sulfide and sulfate reduce steeply before achieving a plateau which

means it became unavailable for further reaction with, and reduction of, H2S.

Same experimental results could be used for designing lab scale testing.

Depending upon availability of FeCl2 or FeCl3 dosing will be different.

However from the previous tested results, it is clear that Field testing results

showed that for 1 mg/l h2S, 5 mg/L H2S has to be added for ~50% H2S

reduction.


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Lab testing proposal

1. Measure initial Sulfur concentration in the feedstock.

2. Have two parallel digesters, one test and other control.

3. Let initial sulfur concentration be X mg/l.

4. Add Fe concentration which is 5*X= mg/L in the test digester.

5. FeCl3 addition will be higher than FeCl2 because heavier mass due to

presence of additional chlorine.

6. Observe H2S is gas, Ideally H2S should be approximately 50% ofControl reactor H2S.

7. Gather the results.


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Eg. For a lab scale digester of 25 gallon and 25 day retention time, with initial S

conc of 100 mg/l, FeCl2 required will be 0.009lbs/day and FeCl3 required willbe 0.012 Lbs/day. Digestate could be recirculated for dilution of powdered Fe

Powder.

If this experiment is successful the actual setup could look very similar, with

FeCl2 required/day around 283.607 Lbs/day, and FeCl3 required 362 lbs/day.

Intial S conc. 100.00 mg/l

FeCl2 FeCl3

0.009 lbs/day 0.012 lbs/day Digester Vol 25.00 Gal

Feed Rate 1.00 GPD

50.00 mg/l (Gas+Liq)

Add either of these

Feedstock

Intial S conc. 100.00 mg/l

FeCl2 FeCl3

283.607 lbs/day 362.883 lbs/day Digester Vol 750,000.00 Gal

Feed Rate 30,000.00 GPD

50.00 mg/l (Gas+Liq)

Add either of these

Feedstock


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Economics and Usage

Usage of this chemical could be controlled by Operator checking feedstock

sulfur content by following graphs.

I) For a typ ical 30,000 GPD of feeding r ate in a digest er , using the following

plot based on the concentration of S, Dosing rate of Iron could be established.


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II) Once Dosing rate of Fe is established and depending upon different

chemicals present, (FeCl2 or FeCl3), The dosing rate or feeding rate could be

evaluated also.

Doing this 50% of H2S reduction could be expected.


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III) Assuming it is decided FeCl2, is chosen as chemical for preference, Daily

cost of FeCl2 ($0.3/lb) usage could be evaluated from following table:


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IV) Final ly this p lot shows est imated cost o f Iron Chlor ide ($/Gallon) of

feedstock based o n di f ferent Sulphu r con centrat ions . Final H2S

con centrat ion, is expected to be 50% of the ini t ia l concentrat ion at least .

Final comments

Direct addition of ferric chloride and ferrous chloride to anaerobic digesters is

an effective method for reducing the hydrogen sulfide concentration in the

biogas produced from the digesters. Further investigation into minimizing the

effects of iron sinks/binding in the digester bulk liquid matrix, such as enhanced

delivery and digester mixing, offer the potential to further improve performance.

Formation and precipitation of iron sulfides did not result in a measurable

increase in the total solids concentration in the digester, thus there is minimal

potential for digester plugging due to this process.

Documents

FeCl3 Report