In order to keep organic waste going to landfills and LFG projects successful, consider using the Steam Injection In Landfill Process to convert the organic waste inside a landfill.
Reg Renaud
Lately, organic diversion from landfills has been a hot topic in publications and on the agendas of many cities and states in the U.S. The main concern of professionals in the waste industry is what will happen to the landfill gas-to-energy (LFGTE) projects that are already in operation and future projects? Not only will LFGTE suffer from organic diversion, but the landfills have also been losing revenues from less waste crossing the tipping scales for years.
Due to aggressive recycling of nonorganic materials, landfills have been steadily losing revenues for years, but this has also raised the ratio of the amount of organic materials going into landfills. This is good for LFGTE projects, but now the state and federal governments want to divert this organic material as well. With more revenues diverted from landfills and more costly monitoring regulations and reporting, landfills are going to have difficulties meeting their long-term obligations. Landfills are required to build a trust fund to maintain the landfill in post-closure for 30 years. The percentage of funds placed in trust is based on a certain amount of revenues coming in each year and based on an increase in tonnage per year, not a drastic drop in tonnage.
LFGTE projects are also designed and financed, with offtake contracts based on a 20-year lifespan of the project. Organic waste diversion will drastically affect these contracts when the gas potential of the landfill declines due to lack of organic waste. Future LFGTE projects will be impossible to finance and offtake contracts will have to be shortened making them less attractive to off takers.
Dealing with the Transition
Currently, four states have limited bans on organic waste to landfills in place: California, Connecticut, Massachusetts and Vermont. Many other states are considering some form of organic diversion from landfills. These bans are aimed at large commercial type food waste producers such as restaurants, grocers and food processing plants. Some cities in California are initiating food separating bins for homes to be picked up at curbside.
However, it is one thing to mandate a change in waste management and another to implement and pay for it. Some cities perform feasibility and cost studies when looking to implement a change in waste disposal. It is interesting to note that when a city in California found that the cost of disposing organic waste in a landfill was on average $52.07 per ton and the cost of diverting the same waste cost about $94.00+ per ton, they still choose to go with diversion just to be PC. The public likes the idea of organic diversion because they have been told landfills are bad, but the idea is so new that they do not realize that when the current waste disposal contracts expire, their monthly bills may double or triple. Their attitude may change if they know their monthly dues will be paying for the diversion and subsidizing the landfills. Depending on how the waste is diverted, only 30 percent of the organic waste may be converted to methane and a good portion of the 70 percent remaining may end up at a landfill at $54.07 per ton.
Alternative Solutions to Keep LFGTE Projects Successful
Although organic diversion from landfills may be the current trend in waste disposal, where does that leave the organic waste already in landfills? The millions of organic tons in place are a significant potential of energy. However, gas extraction wells in landfills also remove the moisture required to make LFG and convert the organic waste. As the moisture is removed, the gas generation declines and is unpredictable. This makes designing and operating a LFGTE project difficult. The one thing that keeps the gas generation going is the addition of new moist organic waste coming into the landfill, although at a modest rate. Some landfills have to recirculate leachate in lined landfills to help keep moisture in the landfill and increase gas production. This process is only 5 percent effective in increasing gas production due to poor distribution through the waste prism—most of the liquid just channels its way to the bottom drain.1
Another alternative is to inject steam into the waste prism, which has been proven to be 100 percent effective in converting 1 ton of organic waste into 12,000 cubic feet of landfill gas at 60 percent methane and 40 percent carbon dioxide. The pilot study at the Miramar Landfill in San Diego, CA indicated a 1:1 conversion ratio of 1 cubic foot of steam was converted into 1 cubic foot of LFG. This data was placed in a committee for nine months and was verified by the California Water Quality Control Board (CWQCB) in Sacramento. Their conclusion was that the steam injection process was safe and effective enough to allow steam injection in unlined landfills as long as potable water was used. In contrast to leachate recirculation, prior to the pilot study, Miramar was recirculating 13,000 gallons of leachate per day. When this leachate was converted to steam and injected into a 1-acre test site, the steam was converted to LFG, extracted and operated a 900 Kw generator; the daily discharge was reduced to 500 gallons per day. The 500 gallons of leachate was from the daily import of waste; none of the steam returned to the bottom drain.
As organic diversion increases and gas production decreases, the steam process will enhance gas production to allow the LFTGE project to continue in a more controlled operation. It is sometimes assumed in the industry that steam injection will shorten the life of the gas field, but it is actually the opposite. Most LFGTE projects in the past without the threat of diversion usually come up short on the 20 year lifespan of the project due to lack of LFG. This is not due to lack of organics, but the lack of moisture in the waste prism from gas extraction. When gas production bottoms out in 10 to 15 years and when as many extractions wells have been installed as possible, the power plants or scrubbers are removed. At this time, at least 50 percent of the organic material that was put in the landfill is still undigested. The steam will convert this last 50 percent over the next 10 to 20 years or more and keep the LFGTE project going. Steam injection can produce 27 times more LFG then naturally produced LFG. The most important factor of steam injection is the recovery of air space in an active landfill. This can be worth $100 million to a landfill owner with the benefit of refilling the same space that was paid for before, over and over again.
As stated before, organic diversion will have a negative effect in the financial stability of landfills due to diminished tipping fees. However, by installing a Material Recovery Facility (MRF) at the landfill site and a Steam Injected Biomass Reactor (SIBR), this can be averted. This would allow the tipping fees to continue for the landfill, the organic material to go into the SIBR and the inorganic material to go into the landfill. The gas generated by the SIBR would go to the LFGTE project. With steam injection in the landfill and an SIBR operating, an existing 1 to 3 Mw project could be increased to 15 to 20 Mw if the grid could handle the power, or the gas could be used for other fuels such as CNG, LNG, methanol and ethanol.
Time and Cost
There are some limiting factors that may complicate installing a SIBR at a landfill site. One is that it requires about 2 acres of land, plus what is needed for the MRF; the landfill can take curbside foodwaste sorting, which would eliminate the need for an MRF and the cost. Landfills in rural areas that take in green waste and agricultural wastes could have enough to feed a SIBR; however, in the northern parts of the country this feedstock is seasonal, but foodwaste is not. Where wet digesters cannot digest green waste, the SIBR can digest it.
Currently, Digestion Rate Tests (DRT) are being conducted on various feedstocks at a Universal Waste System site in Perris, CA. The small 20-ton SIBR has demonstrated that it can digest cow manure, food waste and, most importantly, green waste (palm tree waste proving to have the slowest digestion rate). The DRT is a batch type test, which means it is filled with feedstock closed-up, purged of air and the steam process begins. The full size 500 ton per day unit has two hoppers that purge the air, steam prep the feedstock and then convey it to the reactor continuously to produce 6,000,000 cubic feet of biogas per day. This unit will also be tolerant of small inorganic material that may enter the waste stream since it will be able to auger discharge indigestible waste from the bottom of the reactor without interrupting the process.
Wet anaerobic digesters like the one in Perris, CA cost about $30 million. About 500 tons of foodwaste per day is converted to 229 tons and produces 3,000 cubic feet of biogas per organic ton. The remaining 271 tons will have to be removed from the digester and sold as a compost or disposed of. The cost of the 500 ton SIBR is $2.5 million and can produce 6,000,000 cubic feet per day of biogas at 60 percent methane.
A professor from the University of Scotland was asked at a London conference why the SIBR could achieve such high gas yields from the steam process compared to wet digesters. Her reply was that “it doesn’t throw out the baby with the bathwater.” Meaning that there is not any water to go sour and having to be changed every 21 days, so the bacteria stays in the steam reactor and continues to grow with new waste coming in every day.
Keeping a Balance
The best and cheapest alternative to organic diversion would be to keep the organic waste going to landfills and using the Steam Injection In Landfill Process to convert the organic waste inside a landfill. It only costs about $600,000 to install the steam injection process into 5 acres of the landfill and it will produce the same 6,000,000 cubic feet of LFG per day or 15 Mw of power for about 1.2 years and then another 5-acre plot will have to be installed. Many cities build transfer stations in strategic locations to minimize the amount of packer trucks going to outlying landfills. The same logic should be used for an SIBR. The organic waste in a given area further away from the landfill could be used to produce CNG and fuel the packer trucks hauling the waste and sell gas to the public as well.
If every landfill in the U.S. used the steam injection process, we would be the first generation to control our own, our parents and our grandparents MSW in our lifetime and not leave it for the next generation to deal with. | WA
Reg Renaud is president of STI Engineering, Inc. (Silverado, CA) and has been involved in the geotechnical and environmental industry for 37 years. He has been developing in-situ technology for landfill investigations and the Landfill Gas Profiling Method since 1984. He holds patents for Steam Injection in landfills and for Steam Injected Biomass Reactors, a more advanced anaerobic digester. Reg can be reached at (714) 649-4422 or visit www.airspacerecovery.com.
Note
- SWANA Proceedings – 6th Annual Landfill Gas Symposium, San Diego, CA.
Images courtesy of STI Engineering