The growing trend of energy sustainability for reducing climate change increases the interest in generating renewable or green energy.  Current interests focus on wind and solar energy.  However, there is potential in renewable natural gas (RNG).  Renewable Identification Numbers (RINs) can be valuable for green energy.  For example, a dairy waste anaerobic digester generates D5 RNG that sells for more than $32/MMBtu. When you add in the city gate price (November 2022), the RNG sells at more than $38/MMBtu.  Currently, RNG production cost ranges between $7-33/MMBtu.  Farmers, RNG companies, bioreactor equipment companies, and hydrogen companies benefit from new ways to generate more RNG from existing facilities.  Let’s learn three ways to generate more RNG for more profits.

Growing Interest in Renewable Natural Gas (RNG)

What defines RNG?  For injection into a natural gas pipeline, RNG must meet both quality and minimum flow rate specifications.  The major natural gas suppliers require high methane content (> 95%) and low levels of impurities.  Purification of biogas is expensive and requires a sequence of processing steps for removing moisture, siloxanes, sulfur compounds, and CO₂.  There is some interest in using RNG as a transportation fuel.  Finally, RNG is a minor source of renewable electricity, which commands a premium.

Hydrogen Could Boost RNG Production

Hydrogen sells at $1.80/kg, which translates into $17.91/MMBtu as natural gas.  Equipment for storing hydrogen and injecting into the anaerobic digester will add to the cost, but it is still attractive compared to the current price of RNG.  The U.S. Department of Energy announced a Hydrogen Clean Shot directive to reduce the price of hydrogen to $1/kg.  Advanced Ionics announced a new steam-based approach to generating hydrogen at a lower price of $0.85/kg or less.  Natural hydrogen has a lower estimated price of $0.50-$0.75/kg.  In the United States, the first Natural Hydrogen well site in Nebraska starts in February 2023.  A successful demonstration could start a Natural Hydrogen boom similar to shale natural gas fracking.  Lower priced hydrogen makes biomethane production from biogas CO₂ more profitable.

Inflation Reduction Act of 2022 Spurs Hydrogen Innovation

The Inflation Reduction Act of 2022 includes substantial funding ($369B) for developing technologies for reducing carbon emissions by 40% by 2030.  It also includes a Hydrogen tax credit of $3/kg, which will stimulate innovation and production.

Two Limitations of RNG Production

There are two aspects of RNG production that limit the daily production rate of RNG:

1. Organic Loading Rate (OLR) limit
2. Inability to convert biogas CO₂ into methane

The OLR limit is a function of the waste quality and the difference in the fermentation and methane production rates.  Waste that is easy to degrade ferments to volatile fatty acids (VFA), such as acetic, propionic, ad butyric acids.  The conversion of these VFA to biogas (methane and CO₂) is slow with the methanogens being the rate limiting microbes.  In addition to waste quality, the anaerobic digester has a real limit with respect to solids content, since mixing at higher solids becomes difficult.  To be safe, RNG facilities operate the anaerobic digester at low solids concentration with a long retention time of two or three weeks.  Continuous feeding of the anaerobic digesters reduces spikes in VFA levels, which can cause a pH drop and reduction of RNG production.

Biogas Cleanup

After anaerobic digestion, the removal of biogas CO₂ increases the methane content of the RNG to meet specifications.  There is interest in Biomethanisation, which is the process of converting biogas CO₂ into more methane by using hydrogen gas.  Biomethanisation is a novel approach for biogas cleanup.

Overcome Both Limits With New Knowledge of How CO₂ Inhibits the Specific Growth Rate of Autotrophic Microbes

Previously, I provided a video that describes multiple business opportunities that could impact Agriculture, Environmental Remediation, Water and Wastewater Treatment, Metals Recovery, BioEnergy, and Health markets.  Autotrophic microbiology may be the last major area of untapped potential in microbiology for green energy.  Difficulty in culturing autotrophs, such as methanogens, have set back commercialization by decades.  Carbon dioxide (CO₂), the source of carbon for autotrophic cell biosynthesis, inhibits growth at elevated concentrations.

Key Insight: CO₂ Inhibits Growth of Autotrophic Microbes Including Methanogens

All autotrophic microbes share an unusual growth sensitivity to CO₂.  Elevated CO₂ concentration inhibits the growth of autotrophic microbes. Plants are known to have this growth sensitivity to elevated CO₂ concentration, so this shouldn’t surprise autotrophic microbiologists.  However the literature suggests that this isn’t the case.  A tight band of low CO₂ concentration provides optimal growth conditions for autotrophic microbes.

Municipal Wastewater Anaerobic Digester Design

For anaerobic digester designs, environmental engineers use a specific growth rate of methanogens of 0.30-0.38 d^-1 (Wastewater Engineering Metcalf & Eddy).  This value assumes no growth sensitivity to CO₂.  This value also assumes ideal temperature conditions, which are either mesophilic (35°C) or thermophilic (55°C).  This specific growth rate corresponds to a doubling time of 2 days.  A safety factor of 7.5 results in a solids retention time (SRT) of 15 days for a completely stirred tank reactor (CSTR) anaerobic digester.  This safety factor reduces the risk of methanogen washout and ensures a low OLR, which prevents VFA buildup.

Estimated Specific Growth Rate as Function of CO₂ Concentration

Two estimates of growth parameters for the CO₂-reducing methanogens as a function of the CO₂ concentration are shown below.  The Low and High Estimated parameters are in agreement with the conventional value at elevated CO₂ concentrations (> 40%).

However, a modest reduction in the CO₂ concentration to 10% increases the specific growth rate to 1.20-1.68 d^-1.  With the safety factor of 7.5, the SRT  for a CSTR reduces to 3.1-4.4 days.  Shorter residence time means a greater OLR or more biogas generated for a given reactor volume.  Greater increases in productivity may be possible at lower concentrations of CO₂, but OLR and reactor solids content may present new limits.

This also suggests that temperature control may not be necessary for anaerobic digestion, which would reduce design complexity and cost.  In short, CO₂ control in the anaerobic digester reduces the CAPEX and OPEX costs.  However, the control of CO₂ in the anaerobic digester will require additional investment.

Unleashing Methanogens for Greater RNG Production

Methanogens have two types of metabolism: CO₂ reducing or aceticlastic.  CO₂ reducing methanogens use hydrogen (H₂) to generate methane and make energy for cell growth.  The aceticlastic methanogens split acetic acid into methane and CO₂.  The growth rate of both types of methanogens are sensitive to the CO₂ concentration.

RNG opportunity

Anaerobic digestion of organic solids at wastewater treatment plants and dairy farms already generate Renewable Natural Gas (RNG).  Anaerobic digestion produces biogas, which is rich in both methane and CO₂.  The removal of CO₂ and impurities are necessary to meet the minimum specifications for RNG.  The estimated cost of RNG production for the anaerobic digestion of wastewater is $7-26/MMBtu.  For animal wastes the cost range is greater at $18-33/MMBtu.

Option 1 Control Anaerobic Digester CO₂ Concentration Using Conventional Biogas Conditioning Process

One option for improving the RNG production rate would be to reduce the CO₂ concentration in the headspace of the anaerobic digester using a conventional biogas conditioning process. A simple strategy uses skid-mounted equipment to control the headspace CO₂ concentration.  Increasing the OLR (feed rate) will require additional CO₂ removal capacity and biogas polishing to maintain RNG specifications.  Be sure to develop a plan with this in mind.

This approach does not take advantage of the available CO₂, but it will generate higher biogas production rates.  Operation of the anaerobic digester at a lower solids retention time (greater organic loading rate or OLR) determines the new, greater OLR limit.

Option 2 Control Anaerobic Digester CO₂ Concentration and Provide H₂ for Increasing Biomethane

The second approach builds on the first approach by providing hydrogen in the anaerobic digester.  Fast growing CO₂-reducing methanogens convert the hydrogen and biogas CO₂ into more biomethane, which increases RNG rates.

Option 3 RNGmax

A third approach realizes a higher biogas production rate for the current OLR by converting biogas CO₂ to additional RNG.  A methanogen bioreactor converts the biogas CO₂ and hydrogen to methane.  Adding the excess methanogen biomass to the anaerobic digester increases the biogas production rate and OLR limit. For maximum benefit, the RNGmax biomethane returns to the anaerobic digester to reduce the headspace CO₂ concentration for lower SRT and greater OLR.  Increasing the gas flow rate through the anaerobic digester headspace and RNGmax should allow for control of the CO₂ concentration in the anaerobic digester.  Conditioning of excess RNGmax biomethane generates RNG.  With CO₂ already removed, RNGmax requires a cheaper, simpler biogas conditioning system.

Operation at Lower CO₂ Concentration Increases Biomethane Production Rate

The discussion of the three options only considered operation at 10% CO₂ concentration.  Further reduction the CO₂ concentration increases the biomethane production rate for greater productivity and return on investment.

Four Markets for Increasing RNG Production

There are four U.S. markets for the three options presented above.  I provided 2021 figures from a database provided by Argonne National Laboratory for each market.

Agricultural Anaerobic Digesters

Agricultural anaerobic digesters is the obvious target for either Option.  The number of dairy wastewater treatment systems that include anaerobic digestion continues to grow with many farms generating RNG.  As of December 2021, there are 115 sites generating about $560M in D3 RNG (RIN Value + City Gate Value).

For Option 1, the shorter SRT allows for adding more manure waste per day.  For dairy farms that outgrew their current anaerobic digestion system, Option 1 may be attractive.  Option 3 may be of interest for anaerobic digestion systems that do not have additional available manure wastes.

Municipal Wastewater Anaerobic Digesters

As of December 2021, there are 26 municipalities with Anaerobic Digesters generating about $83M in D3 RNG (RIN Value + City Gate Value).  Option 1 is not attractive to municipal wastewater treatment plants employing anaerobic digestion, since additional sewage solids are not available.  However, Fats, Oil, and Grease (FOG) and septic system waste may be attractive as a feed source provided there is adequate volumetric space available in the anaerobic digestion.  Prethickening of sewage sludge may be the key to increasing the OLR with FOG and septic system waste.  Municipalities will not reduce the SRT, if it impacts biosolids regulations.  The introduction of RNGmax (Option 3) may be of interest to many municipalities, since it integrates into the existing wastewater treatment plant with a small footprint.

How About Adding Food and Paper Wastes?

The introduction of the organic fraction of solid and food waste to anaerobic digesters generates more RNG, but it involves a complicated valuation.  The RNG generated from this organic matter garners a D5 rating, which is lower than D3.  In addition, the municipality will generate more biosolids resulting in greater transfer or disposal costs.

Landfills

As of December 2021, there are 76 landfills generating about $2.1B in D3 RNG (RIN Value + City Gate Value).  The D3 RNG has a current price of $39.19/MMBtu.  The estimated cost of RNG production for landfills is $7-19/MMBtu.  There is interest in generating RNG from landfills by upgrading biogas conditioning processes.  Bioreactor landfills with direct injection of treated leachate or steam increases biogas generation rates.  The same equipment used for injecting steam could use methane as the carrier gas instead of air or nitrogen.  This approach improves biodegradability by adding moisture and improving the local growth conditions for methanogens.  The use of methane would reduce the need for further biogas conditioning that includes nitrogen separation.

All Options Available for Landfills

Using Option 1, landfill CO₂ still requires separation.  The injection of a mixture of methane and low levels of hydrogen (Option 2) yields additional RNG from landfill CO₂.  However, Option 2 uses hydrogen to convert the biogas CO₂ into more methane for greater RNG yield.  With Option 3, a bioreactor generates RNG, which simplifies the process.  Recycling the RNG into the landfill lowers the concentration of CO2, which stimulates biogas formation.  However, Option 3 does not have the advantage of utilizing the methanogen biomass for greater RNG production rates.  Adding the methanogen biomass to the landfill (mixing with fresh wastes) is lethal to the methanogens due to their sensitivity to oxygen.  However, methanogen biomass could be sold to municipalities to improve anaerobic digestion.  A cautious strategy may start with injecting RNG into the landfill to stimulate methanogenesis by lower the CO₂ concentration.  Next, the introduction of low levels of hydrogen with the RNG would evaluate whether hydrogen stimulates greater biogas production.

Food Waste Anaerobic Digesters

Food waste anaerobic digesters are less attractive due to the lower valued D5 RNG as mentioned above.  The D5 RNG has a current price of $28.29/MMBtu.  As of December 2021, there are 13 sites generating about $109M in D5 RNG (RIN Value + City Gate Value).  Existing facilities may benefit from using add-on technologies to increase D5 RNG.  All three options are available for Food Waste Anaerobic Digesters, but the lower valued D5 RNG may reduce interest.  Option 1 is the lowest cost approach for increasing biomethane production.  Compared to Municipal Wastewater Anaerobic Digesters, Food Waste Anaerobic Digesters can reduce the SRT without regulatory issues.

How To Exploit Three Options for Increasing RNG

If there is interest in licensing the patents (7,641,796, 7,655,143, 8,545,700, 8,940,159, 8,535,534, 8268,666, 9,039,897) for these options, then contact the University of South Florida (USF) about licensing options.  The USF offers multiple options for licensing that include exclusivity, region, and market.  Any company or municipality that currently generates RNG should contact them about a non-exclusive license.  For RNG companies with multiple customer, an exclusive license for a region and market may be attractive.  Hydrogen companies should partner with RNG companies to exploit these patents.  Lower quality (and priced) hydrogen could be attractive to RNG companies.  Finally, the development of turn-key systems for RNG companies for the three options could be attractive to equipment vendors.

Contact USF for Licensing Options

To learn more about the licensing options available for these patents, contact Edita Dautovic, Agreements Specialist for the USF Technology Transfer Office.  You can reach her by phone, 813-974-8480, or email at edautovic@usf.edu.

Send Me Your Questions About RNG

Send me your questions about how to generate more RNG in anaerobic digesters and landfills to pete@glixin.com.

Other Biofuel Opportunities?

My original analysis (USPTO application US 20140011185 A1) used identical non-standard conditions to evaluate the potential for alkanogens and alcohologens.  Future blog posts will describe these opportunities.