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Solvents for use in coal seams

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Published: Tuesday, 10 June 2014 21:29
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Amendments to formation water in coal seams

Gregory DiCenzo

 

Historically, coal solubility has been modified through pH changes: either to extreme acid or alkaline environment. Also contributing to desired state changes are temperature and pressure.

Though the seam can receive acid or base via injection, it is not practical to modify temp or pressure. The recent “Plasma Pulse” technology from Novas Energy would have no effect on coal solubility, although it may be a tool for resolving flow in a formation. Acid is frequently used in wells drilled into coal to clean the coal face, removing cement and obstructing coal fines from fractures.

Solvents may be used in a coal seam to raise solubility of some classes of molecules or otherwise create highly local states that might bridge polar and non-polar character, thus making, for example, coal functional groups slightly more available to agents such as bacteria acting in their vicinity.

Ethyl acetate is slightly soluble in water (08.3g/100 mL) and might raise the solubility of alkane or phenolic compounds from a coal seam. In this scenario, too, permeability may be increased via the action of precipitants being removed. It is known to be safe (at a few percent) to microbia and to increase larger coal compound availability.

Pyridine is used in analysis of coal, producing “coal extracts” – though the result may not so much be a solution as a fine dispersion. In either case, pyridine is known to swell coal and lignite ex situ. Pyridine is polar, with a moderate dialectric constant – but is aprotic and can not contribute to hydrogen bonding. Pyridine injected into a seam under pressure is unknown. Solvents such as pyridine, diesel and related compounds injected into oil wells to reduce paraffin are mixing with crude oil and do not present immiscibility problems with consequent pressure buildup. Tetrahydrofuran is similar to pyridine and has less toxic homologues.

Traditional preparations of coal extracts might use p-toluenesulphonic acid in pyridine to depolymerize coal – since coal is largely the byproduct of dehydration reactions enroute to lignite, but tending more to decarboxylation reactions during transformation to sub-bituminous coal. Indeed, de-polymerization is a common step in ex situ process; carbon monoxide is also used towards this end under pressure. However, this chemical process may not be possible in situ, in what is a largely aqueous milieu.

Effects of dialectric property change of fluid in situ is unknown; gas adsorption and desorption and the effect on permeability of sub-bituminous coal is known to affect permeability with both positive and negative correlations. Thus, in situ, addition of a miscible solvent (pyridine) may have unpredictable effects – possibly deleterious through flow resistance effects. “Safe” solvents to be injected into coal formation water would be partially water soluble solvents such as ethanol or ethyl acetate, neither of which would raise viscosity but would likely increase wettability of the carbon substrate and, at low levels, would be bio-safe. At this time, real benefits of ethyl acetate and ethanol addition to formation water are hypothetical.

Surfactants

Surfactants are diverse: ionic, zwitterionic, nonionic. Used in purely aqueous solutions, they primarily reduce surface tension or help mobilize lipid compounds in a micelle scheme. Surfactants are also used in the oil field in “polymer flooding” methods, usually to combat paraffin buildup or to recover oil in a water-bearing formation. In the latter case, interfacial tension is decreased, allowing oil globules to flow in the water channel. Alkaline polymer surfactants are used (which react with acidic components in the oil) and simple polymers are used at low rates.

Surfactants are also used in coal processes and aid in dust reduction; in situ, it is known that surfactants can mobilize coal fines after fracking – such that they can reduce permeability by clogging fractures. Thus, it is preferable that coal fines not be moved to the mobile phase, and should probably remain “oil-wet” – except that bacteria would also be excluded.

Thus, surfactants may have permeability- decreasing properties if used in situ by mobilizing particles. While helpful in oil reservoirs, they could be undesirable in coal seams.

Yet, borrowing from oil field amendments, coal seam injectate may benefit from foam and viscosifying agents which help improve penetration and sweep, thereby increasing the concentration of bio-chemo contact. Fracking, with traditional propant characteristics, will also increase penetration of bacteria in previously excluded space, thus increasing reactions sites on an area basis.

Microbially enhanced oil recovery (MEOR):

MEOR is a sister technology to coal solvent injections. Generally, indigenous and adapted microbes are stimulated in an oil bearing stratum, thus increasing their numbers and activity. The activity of MEOR typically is to achieve the same effect that polymer flooding achieves: decreasing the interfacial tension between oil and water phases, thus stimulating movement of oil through a formation. In some cases, it is considered that the biological activity also improves the quality of the in place oil.

In practice, limiting nutrients are provided to the microbes which are otherwise carbon heterotrophs. In some cases, microbes may be injected into a formation. MEOR can occur in wither aerobic or anaerobic settings, the latter requiring sugar amendment, typically molasses. In this case, the microbes are in place producers of otherwise desirable cellular products.


Effects of MEOR actors:

- decrease viscosity through alteration of the larger crude compounds

- surfactant production, causing movement of oil into and through the formation
- gas production: increases pressure which drives oil into moving

- acid and/or solvent production and excretion

The mere increase of biomass may cause plugging, which is usually reduced through other reservoir management methods. Otherwise, objectives for MEOR researchers are to increase bio-surfactant production and enhance the viscosity increasing effects of indigenous microbes. The latter is desirable where injecting only reaches less permeable portions of a formation, greatly reducing the expected volume of treated formation. From this perspective, rational use of MOER methods may promote solubilizing results.

In summary:

Whether stimulating indigenous bacteria to increase bio-surfactant or otherwise increase viscosity, or adding surfactants or otherwise increasing viscosity may be preferred targets for management of solvent sweep, physical parameters of the formation water would probably benefit from modifications.

Chemical parameters of the solvent profile may also benefit from modification: immiscible solvents such as pyridine or THF may be of use but at low rates to avoid over-pressuring of the formation through increased viscosity but also via effects on swelling, porosity and permeability.

More water-soluble solvents such as ethyl acetate or ethanol can be somewhat effective but are also bio-labile and short-lived. It is known that these are not harmful to the indigenous microflora, but return on investment is unknown.

pH Adjustments are temporary, and can be lost upon restoration to typical formation pH’s. Permanent pH changes are probably impractical – and may also affect the biota.


Other factors should be considered to enhancing in situ treatment of coal seams:

- increase the sweep of the injectate

- fracturing to increase contact of microbia with coal surface

- use shut-in methods rather than recirculation methods to allow for biofilm buildup and cost savings

Miscible, mutual solvents may perform acceptably in coal seams; numerous formulations are available from oilfield service providers (e.g. Weatherford) but their biostatic activity may not be known in some cases as the formulas are protected trade secrets. The intention of their use would be to create zwitterionic character in highly localized areas near coal surfaces. It is not known if this effect could occur in situ in a biogenic gas coal seam.

“Best guess” suggestion for treatment of ongoing coal processes: a mild solubility enhancer like polymer surfactant addition. A surfactant could diffuse over time; merely miscible solvents like pyridine would not move via diffusion. Surfactants coating coal fines could create bio-amenable zones for bacterial contact. And, they may create “wetted” surface of coal for both enzyme and cell contact.

Rational use of MOER techniques to change chemico-physical parameters might be too speculative at this time and also require an unknown bio-response time.

Anaerobic Digesters

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Published: Tuesday, 10 June 2014 14:51
Written by Administrator

Bioreactors: Anaerobic digesters

Gregory DiCenzo

 

Anaerobic digester diagramIntroduction

Bioreactors are reaction vessels inside which a chemical process occurs and the key reactions in that process are catalyzed by organisms. This paper will be concerned only with systems that have a significant anaerobic requirement. As operators are expected to reduce CO2 releases and even recover energy from wastewater and other carbon sources, more effort will be given managing reactors for methane production (Kraemer et al., 2012).

Reactors come in a wide range of sizes: from 0.5 L to several cubic meters. They are commonly made of stainless steel, which allows cleaning in place. Features common to all or most anaerobic reactors are 1) gastight, 2) temperature controlled, 3) ports for influent/effluent, 4) gas or pressure release, 5) pH sensing and control, 6) agitation, 7) loading access and 8) sampling port.

Reactors are operated in either a batch or continuous mode. Batch operation implies that upon loading and sealing the reactor, neither the starting material or desired product is removed until the process and its reactions have reached a predetermined set point. Continuous operation allows for continuous flow of reactants into the vessel with simultaneous output of product.

Batch reactors should be sized larger to hold an economically useful amount of reactant. Continuously fed reactors can be smaller. The closed batch reactor maintains containment of all materials including the headspace atmosphere and is not dependent on influent and effluent pumps. The continuous reactor allows for monitoring and adjustment of reactants, products and inhibiting components.

Management and Operation

Management objectives may vary. In most cases, anaerobic reactors – also known as “digesters” – are operated chiefly to eliminate the carbon burden in a water phase. Thus, the main objective is to reduce the carbon load measured as “chemical oxygen demand” or “COD”.

Some waste water processes depend on aeration, that is, the intentional maintenance of dissolved oxygen (DO) in the fluid phase. In these systems, the aeration is usually accompanied by agitation and turbulence such that the organisms are best described as a “floc” which must be continuously regenerated. In the anaerobic scenario, the microorganisms or “sludge” will reside as a layer, sometimes called a “blanket” which is settled to the bottom of the reactor. Influent, which is high in COD, is pumped into the sludge for maximum contact.

The microorganisms consist of a community of diverse bacteria and Archaea and can physically associate as granules, which aids in the sedimentation, retention and overall efficiency. The granules make up the blanket and can slow solids transport while smaller dissolved sugars and volatile fatty acids (VFAs) are converted to gas and exit both the blanket and liquid phase more rapidly.

The first stage in anaerobic digestion is a hydrolysis process during which larger, complex molecules are reduced, forming smaller and more soluble products. Though this is an integral part of a wholly contained process, the progress of the first stage can be accelerated by implementing a pretreatment: which is both a physical and chemical treatment that increases surface area of the feed stock but can also decrease size while increasing solubility. The requirements for a pretreatment are minimal: an economic process carried out in a simpler containment system. Not all feed requires or can benefit from a pretreatment: only those which are composed fundamentally of larger and less soluble molecules.

Upflow anaerobic digester from Engineering Fundamentals

Most anaerobic reactors have an upward flow of influent through a bed of granules; the residence time in the granules should be sufficient to allow an economic and functionalconversion of the feedstock to biogas (although some systems might separate stages of the process into sequential reactors; i.e. emphasizing methanogenesis in the last stage reactor).

A common example is the upflow anaerobic sludge blanket (UASB) reactor which is often used for wastewater. In this scenario, the blanket is composed of granulated biofilm which contains all the bacteria and archaea necessary for hydrolyzing the feedstock and producing methane. The upward flow allows for control of the granule-feed contact and process rate, leaving a much smaller COD burden in the effluent. There can be a net gain of energy in the form of biogas depending on downstream treatment requirements.

Modifications to the UASB can includea membrane to keep solids and biomass out of the effluent or a filter material which anchors the organisms within the tank but still keep them in the process stream.


Recent case studies in anaerobic digesters

 

Gallagher and Sharvelle (Gallagher and Sharvelle, 2011) tested the efficacy of a pilot scale (114 L) anaerobic digester in a UASB configuration to treat “black water” (BW, toilet water only). In this small pilot, organic loading rate was only 0.21-0.39 kg COD m3/day with a retention time of 2.6-4.0 days. COD was reduced 72% while TSS and VSS were reduced 95%. Of interest in this BW reactor: E. coli and fecal coliforms showed greater than 90% log reduction.

Hassan and Nelson (Hassan and Nelson, 2012), in their review of dairy wastewater anaerobic digesters, recommend UASB configurations with a pretreatment to increase bioavailability and reduce obstacles to smooth operation posed by milk fat. Awareness of the high fat content and special concern for negative outcomes from spikes in vfa’s and other fatty acids require careful management of hydraulic retention times and pH/buffering management. Efficiency of biomass conversion is typical for this technology as they report 0.22 – 0.40 m3 biogas/kg COD removed.

In their review of fish processing wastewater treatment Chowdhury et al. (Chowdhury et al.) discuss the complex and potent nature of waste from fishery processing and cooking. The water contains solids, high COD and oil and other fats. The wastewater can also be relatively high in volatile amines and other nitrogenous compounds (especially NH3+ at higher pH conditions). Fish waste conversion in reactor systems such as UASBs is efficient with loading rates around 1 kg COD/m3(per day) and retention times of 11-35 days. The authors recommend a post anaerobic aerated stage to polish the effluent. They report conversion efficiencies (COD to biogas) ranging from 60-70% in various anaerobic digester formats.

Agricultural waste ranges from manure to highly cellulosic biomass to fruit and vegetable process waste. Some of their unique challenges can be pretreatment requirements for the straws/grasses, and buffering for the high volatile solids feed stock that can otherwise over-acidify the reactor (Ward, et al., 2008). The requirements for a successful operation of an anaerobic digesting system include an alkaline pretreatment of straws to hydrolyze cellulose components and also to reduce the particle size. Buffering may also be required to counteract rapid spikes in vfa’s because, at pH < 6.6, methanogenic efficiency is strongly reduced. Indeed, they recommend managing of the buffering capacity rather than pH to anticipate and avoid system shocks. Yields for conversion of COD to biogas 0.14 (yard waste) to .472 (mixed food waste)m3 biogas/kg VS; 0.3 m3 biogas/kg VS (corn) to 0.19 m3 gas/kg VS (straw)

They also recommend two-stage systems that can separate the hydrolytic and acidogenic processes (pH 5.5-6.5) from the methanogenic stage (optimum pH 7.0). Also, when treating these high solid feed stocks, the microbial biomass should be fixed or anchored to a support such as is found in an anaerobic filter (AF) or use a mesh or membrane that retains granules.

Yu, et al. also promote 2 stages to separate the lower pH processes from the higher pH methanogenesis process. They treated high solid municipal waste (a “shredded paste” with high organic content) in a single reactor that maintained a stirred upper zone (which contained the floating solids) and alower zone that specialized in organic degradation. The effluent was ported to a UASB “seed” reactor which processed vfa’s to methane; most effluent was released from the UASB while some biomass was returned to the first stage (Yu, et al. 2012). The best operation (best rate of biogas production) maintained a particular recycling rate of UASB fluid which carried some methanogens but only about 0.14kg COD/day compared to a feed rate of total solids of 5 kg COD/day. The recycled feed was over-represented by vfa’s which could reduce the pH if allowed to flow at too high of a rate. This experimental system was operated primarily for data collection in order to model all processes. The relatively low rate of methane production was .022 m3 CH4/kg COD per day.

Christian et al (Christian et al) describe a large, higher temperature (33°C), high throughput anaerobic membrane bioreactor (AnMBR) used to process high strength wastewater from a food processing plant. It processes 10 to 15 kg COD/m3·d while all necessary microbial biomass is retained by the membrane. Removals are >99% of COD and BOD; effluent contains 210 and 20 mg/L of COD and BOD respectively. That effluent is treated in a separate aerated process, although the sludge from that process is returned to the AnMBR for digestion.

Anaerobic hybrid reactors (AHRs) combine a UASB reactor in the lower portion of the reactor with an AF (anaerobic filter) above. During operation, the retention time can be shortened, which increases the upflow while the AF retains the solids and biomass (Ramakrishnan and Surampalli 2013). The authors of this study compared AHR operation at two temperatures: 35°C and 55°C. The synthetic feedstock simulated coal wastewater: water from liquefaction and gasification processes. The COD was 2240 mg/L and phenolics concentration of 752 mg/L. Coal gasification wastewater is considered to be relatively recalcitrant. (Wang et al. 2011). 0.48 m3 gas/kg COD for higher temp (55)

Text Box:

Operating replicate reactors (at 33°C and 55°C) the authors compared chemical removal and methane synthesis for different retention times. At each retention time, the thermophilic reactor always outperformed the mesophilic reactor with respect to COD removal, phenolic decomposition, specific methane yield and overall operational stability.

Full scale anaerobic reactors

A large AnMBR reactor in Massachussetts USA (Christian, et al. 2011) has an inflow rate or 475 m3/day (35 g/L COD). In its current form, it consists of 4 membrane-containing reactors and produces 5700-8000 m3 biogas/day (~.16-.23 m3 biogas/kg COD. This reactor at Ken’s Foods (Massachusetts, U.S.A.) has been upgraded several times because its output was exceeding daily discharge volumes as permitted, causing the food processing plant to halt production. In addition, the effluent TSS is ~ 1 mg/L well below the 600 mg/L permit.

Schievano, et al. (2011), in a compressive review of 3 full-scale anaerobic digesters, concluded that the reactors function at near lab-scale efficiencies after calculating the “biomethane potential” (BMP) and comparing to specific gas yields.

Bio-methane and biogas productions, process performances and efficiencies achieved by the three observed full scale biogas plants during the observation period (April 2008– March 2009). (Schievano et a., 2011)

 

Parameter

Rate units

Plant A

Plant B

Plant C

TS degradation yield


%

72%

73%

63%

VS degradation yield

%

79%

79%

70%


Bio-methane yield (BMY1)

%

87%

93%

88%


Bio-methane yield (BMY2)

%

88±9

93±13

84±8


Total bio-methane production (1 year)

N m3/y

1788121

2355315

931844


Total biogas production (1 year)

N m3/y

3845808

4565892

1823650


Bio-methane production rates

N m3 CH4/day

4899±352

6453±254

2553±197


Biogas production rates


N m3 biogas/day

10536

12509

4996

Volumetric bio-methane production rate

N m3/ m3 dig d-1

0.98±0.07

1.08±0.04

1.60±0.12


Specific biogas productions on w.w. basis

N m3/Mg*

85.0±4.5

118.5±8.6

80.2±5.1


Specific biogas productions on TS basis


N m3/Mg TS

703±68

644±86

619±58

Specific bio-methane productions (SMP) on w.w. basis

N m3/Mg

39.5±4.4

61.1±8.5

41.0±4.0

Specific bio-methane productions (SMP) on TS basis

N m3/Mg TS

327±36

332±46

316±31

_____________________________________________________________________________

Plant A: municipal solid waste, ~74 kg/day; Plant B: swine manure and food process waste, 66kg/day; Plant C: swine/bovine manure plus silage, whey and rice. BMY1 = (BMPin * TSin- BMPout* TSout)/(BMPin* TSin). BMY2= SMP/BMP where BMPin = biomethane potential. *Mg = 1000 kg.

 

Conclusion

Interest in anaerobic digesters draw more interest as greater value is placed on recovering energy from the waste carbon load in a process that also reduces CO2 and methane releases. While most reactors described in the literature are UASB in configuration, AnMBRs or hybrid reactors may be better suited to the slower anaerobic process (Stuckey 2012). Upflow through a filter can shorten retention time while preserving biomass. De-nitrification may become more important as anaerobic processes become more important. Options for nitrogen removal include a separate side stream process or harnessing anaerobic ammonium oxidation activity within the reactor.

Full scale reactors reduce operating costs, decrease effluent TSS and generate biogas. In addition, sludge and solids are mitigated in this process and CO2 and odor release is minimized.

References cited

Chowdhury, P., T. Viraraghavan , A. Srinivasan. 2011. Biological treatment processes for fish processing wastewater – A review. Bioresource Technology 101:439–449.

Christian, S., S. Grant, P. McCarthy, D. Wilson and D. Mills. 2011. The First Two Years of Full-Scale Anaerobic Membrane Bioreactor (AnMBR) Operation Treating High-Strength Industrial Wastewater. Water Practice & Technology Vol 6 No 2 doi:10.2166.

Gallagher, N. and S. Sharvelle. 2011. Demonstration of Anaerobic Digestion of Black Water for Methane Capture and Use in an Office Building. Water Practice & Technology Vol 6 No 1 doi:10.2166.

Hassan, A.N. and B. K. Nelson. 2012. Anaerobic Fermentation of Dairy Food Wastewater. J. Dairy Sci. 95:6188–6203.

Kraemer J.T., Adrienne L. Menniti, Zeynep K. Erdal, Timothy A. Constantine, Bruce R. Johnson, Glen T. Daigger, and George V. Crawford. 2012. A practitioner’s perspective on the application and research needs of membrane bioreactors for municipal wastewater treatment. Bioresource Technology 122:2–10.

Ramakrishnan, A., R.Y. Surampalli. 2013. Performance and energy economics of mesophilic and thermophilic digestion in anaerobic hybrid reactor treating coal wastewater. Bioresource Technology 127: 9–17.

Schievano, A. G. D’Imporzano, V. Orzi, F. Adani. 2011. On-field study of anaerobic digestion full-scale plants (Part II): New approaches in monitoring and evaluating process efficiency. Bioresource Technology 102:8814–8819.

Stuckey, D. 2012. Recent developments in anaerobic membrane reactors. Bioresource Technology 122:137–148.

Wang, W., Wencheng Ma, Hongjun Han, Huiqiang Li, Min Yuan. 2011. Thermophilic anaerobic digestion of Lurgi coal gasification wastewater in a UASB reactor. Bioresource Technology 102:2441–2447.

Ward, A.J., Hobbs, P.J., Holliman, P.J. andD.L. Jones. 2008. Optimisation of the anaerobic digestion of agricultural resources. Bioresource Technology 99:7928–7940.

Yu, L.,Q. Zhao, J. Ma, C. Frear, S. Chen. 2012. Experimental and modeling study of a two-stage pilot scale high solid anaerobic digester system. Bioresource Technology 124:8–17.

Landfill Biogas

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Published: Thursday, 09 January 2014 04:23
Written by Administrator

 

Landfill Methane Opportunities

Gregory DiCenzo

Landfills are areas that are filled, sometimes with rubble for renovation but, in the context of this document, with municipal waste material. How that waste is managed differs widely, from simple burial, to temporary holding for later recycling but, in the last 40 years, it is being managed more for biogas recovery.

Landfill biogas is a mixture of gasses, usually 40-60% CH4 and most of the balance being CO2[1]. The value of biogas is in the methane content, which is a product of microbial processes acting on the organic matter in the landfill waste.

When landfill gas was recognized as a problem (explosion hazard) methods were considered for remediation and flaring was the most common option chosen. Later, then, methane emissions to the atmosphere from landfills were considered an environmental hazard and landfills over 2.75 million tons were considered worthy of monitoring, if not ameliorating. The hazardous biogas migration can enter 1) atmosphere, 2) groundwater, and/or 3) close-by structures.

The anatomy of a landfill is simple: layering of a day’s waste often with compaction and a cover of soil or other material. In areas where preventing hydrologic contamination is paramount, rubber liners can define the bottom of the landfill to block leachate from leaving the system.

When managing a landfill for gas production, control and reuse of the leachate is critical for operation and lining is, therefore, required. The system is managed to be anaerobic as H2S is often elevated in landfill biogas. However, the landfill is not closed until it is retired; in fact, mass is predicted to be biologically removed at some rate allowing for increased additions, necessitating a non-sealed structure until formal shut-in.

Water is required for methanogenesis to start-up; indeed, in a more temperate climate, methanogenesis will begin within a year or two; in an arid climate such as the desert Southwest 5 years is a better expectation.[2] 

In addition to moisture, nutrients, type of waste material, etc also determine the rate of gas generation[3]:

 “The value of [methane rate constant] is a function of (1) waste moisture content, (2) availability of nutrients for methane-generating bacteria, (3) pH, and (4) temperature.”

Disposition of methane from managed landfills is most commonly via flaring, thus converting it to CO2. Flaring is used because it is the simplest method. One alternative to flaring is to provide a biological, oxidizing cover to the landfill when gas collection is inefficient and emissions containment is a top priority (see Appendix 1). Another alternative to flaring, if conservation is desired, is to upgrade the gas, compress it, and sell it to a pipeline.

“Landfill Gas to Electricity” (LFGTE) is yet another option and implies burning gas on-site to drive turbines and generate electricity; co-generation of heat may be part of the design. Microturbines to combust the low BTU biogas are already prevalent in the anaerobic digester biogas market and can be employed with landfill biogas generation systems.

Landfill gas capacity currently entails 621 operational sites in the U.S. [1] which produce 1978 MW and 311 mmscfd. An additional 450 candidate landfills suitable for landfill gas production have been identified which could produce an additional 805 MW electric and 470 mmscfd.

Infrastructure of a landfill gas operation is complex and can cost several millions of dollars per site. Figure 1 depicts a generalized system.

 

 

 

[1] From “Wikipedia” ” Landfill gas also contains varying amounts of nitrogen and oxygen gas, water vapour, hydrogen sulphide, and other contaminants. Most of these other contaminants are known as "non-methane organic compounds" or NMOCs.”