Gasification is flexible not only to what feedstocks it can use, but also to what end is needed from its conversion process. Clean, renewable electrical generation behind the meter and creation of Class A biosolids are possible through gasification.
By Nancy Cooper
Gasification has a long and rich history, and it is widely used in Europe. It is just now growing in popularity in the U.S. In recent years, the research has paid off with new technologies that make the process more efficient with less maintenance required.
The Global Syngas Technologies Council states that there are more than 272 operating gasification plants worldwide with 686 gasifiers. There are currently 74 plants under construction worldwide that will have a total of 238 gasifiers and produce 83 MWth. Thirty-three gasification plants are located in the U.S.1 Currently, China has the largest number of gasification plants.
When organic materials are heated in an oxygen-starved, pressurized environment, the materials cannot ignite. The heat forces the complex carbon molecules to break apart forming a mixture of gases, called producer gas or synthetic gas. This gas can be combusted to produce electricity or steamed off without harming the atmosphere. All the organic materials are not converted to gas—part of them reform into a material called biochar, a carbon-rich soil amendment (which we discuss later). This environmentally friendly process is known as gasification.
To cite a recent EPA report entitled Technology Assessment Report – Aqueous Sludge Gasification Technologies: “Gasification offers a potentially viable option compared to conventional methods for sludge disposal. Gasification is capable of providing a clean and manageable process with the possibility of net energy gains. The variability and lack of information on commercial scale systems however, makes it difficult to ensure a complete analysis and concrete conclusions on sludge gasification’s viability.”2
So, what can be done with the syngas produced in the process? Let’s examine the four options of gasification process flow below.
#1: Fluidized Bed Gasification
Some fluidized bed gasification systems are designed specifically for processing biosolids. The system will reduce the volume of biosolids by 90 percent leaving only an inert biochar that will be beneficially used. The renewable energy that is generated from the biosolid-generated syngas is then recovered and used within the system, so no fossil fuels are used during operations. It will also reduce greenhouse gases due to the reduction in trucking miles associated with conventional disposal methods.
#2: Multi-Feedstock Electrical Production
In this example, multiple feedstocks are acceptable for biomass gasification. This can result in more organic material being diverted from landfills and that will lower local carbon footprints. A perfect, clean energy storm exists for locations near wastewater treatment plants as the plant’s sludge now becomes a feedstock and not an addition to the local landfill. The wood waste from discarded pallets of local businesses and rights of ways’ street and park trimmings can be used as well. In addition, water from the plant can also be cycled through the gasification plant to be used for cooling instead of industrial fans or to power Organic Rankine Cylinder generators to produce power behind the meter. This method also eliminates costs associated with transportation of sludge disposal (see Figure 1).
The multiple feedstocks are sized and dried for best efficiency, then mixed together before entering the gasification chamber. The producer gas is sent to a thermal oxidizer where it is ignited and the energy from that is used for power generation or used to fuel a steam turbine that can power boilers, sludge dryers or kilns. Electrical generation can be used to offset current electrical utility costs or to supplement power needs. All gasification processes will have a leftover product—called biochar—that has value and can be repurposed further as a soil enhancement, carbon black production, etc.3
Examples of tested and proven feedstock include:
• Industrial or municipal sludge
• Food processing wastes
• Chips from wood waste or forest residuals
• Purpose-grown biomass crops
• Crumbed tires
• Ag waste such as corn or cotton stalks
• Processed and pelletized municipal solid waste
#3: Wood Waste to Power
A wood waste feedstock plant is ideal for industries with solid wood waste streams. Examples would be hardwood floor or cabinet manufacturing. Also, companies with increasing loads of broken wood pallets that want to eliminate landfill trips and their associated costs. With a wood feedstock only, there is no mixing of materials. The chipped wood goes directly into the gasification reactor. From there, the syngas goes into the thermal oxidizer where it is combusted and the energy used to run a generator or kiln, etc.
#4: Creating Class A Biosolids
The EPA defines Class A biosolids as biosolids that contain no detectible levels of pathogens. Class A biosolids that meet strict vector attraction reduction requirements and low levels metals contents, only have to apply for permits to ensure that these very tough standards have been met. It further states that anyone who wants to use biosolids for land application must comply with all relevant federal and state regulations. In some cases a permit may be required.4
As we have said previously about the gasification process, wood is converted into a producer gas that can, when combusted, use that energy to fuel a sludge dryer. The sludge dryer will convert the sludge into a Class A material. Also, the condensed steam generated during this process can also be sent back to the wastewater treatment company for other uses (see Figure 2, page 42).
Not all of the feedstock in any type of gasification process is used to generate energy. In some cases, as much as 85 percent of the feedstock converts to a syngas. The other 15 percent leaves the process as a carbon-rich charcoal type substance called Biochar. Biochar can be used in agriculture and in various industries. Benefits of biochar via gasification include, but are not limited to:
• As a soil amendment (farmers, golf courses, city and state parks)
• Industries readily purchase carbon black that can be made from biochar. The EPA defines carbon black as a black, powder or granular substance made by burning hydrocarbons in a limited supply of air. Uses for carbon black include automotive belts and hoses, industrial rubber products, paint, inks, plastics, ceramics and tire production.4
• Opportunities for partnerships with local industry, municipal leaders
• New revenue stream for gasification plant owners
• Markets alleviate owners from transportation disposal costs
• The gasification process takes harmful particulates out of solid waste streams
Gasification is flexible not only to what feedstocks it can use but also to what end is needed from its conversion process. Clean, renewable electrical generation behind the meter and creation of Class A biosolids are possible through gasification. The leftover biochar is gaining popularity and being further examined for new uses and possible new revenue streams. | WA
Nancy Cooper is Communcations Manager for Aries Clean Energy (Franklin, TN). For more information, call (615) 471-9299, e-mail [email protected] or visit www.ariescleanenergy.com.
2. EPA report: 600R12540 Technology Assessment Report Aqueous Sludge Gasification Technologies
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