Date: October 4, 2012
Author: Chris Koczaja
Source: Pollution Engineering
Waste to energy has become a widely used and accepted concept, but there are still a number of waste stream opportunities that are regularly overlooked. Utilizing these waste streams for energy purposes not only reduces energy purchases, but it has the added benefit of reducing disposal costs. There are transportation and tipping fees, as well as the cost to the environment, that result from disposal. Additionally, there is the expense of bringing in another form of energy. A waste-to-energy system streamlines the process and greatly reduces costs, financial and environmental.
While there are a multitude of commercially available products and technologies to convert these waste streams into usable energy, advances in commercially proven and industrial grade gasification systems offer new opportunities to pursue waste streams. This can now be accomplished not only at the 500 ton per day scale, but also down to eight to 10 tons per day, allowing users to match the equipment to the size of their waste stream rather than matching the waste stream to the size of the gasification unit.
This distributed waste-to-energy approach empowers adaptors to maximize and control their waste and energy equation, utilizing a new generation of gasifiers that cleanly convert the biomass feedstock into combustible gas that can be used for a downstream process.
Whether for a municipality, industrial or governmental site, every gasification project comes down to two basic areas of focus: feedstock and application. Once those are determined to meet acceptable technical and physical criteria, a detailed financial analysis can be completed.
The first step in implementing a successful gasification system is identifying and evaluating the waste stream, or possible biomass stream, involved. While exact specifications will vary by gasifier type and manufacturer, two of the more basic things to examine are the ash and moisture content of the feedstock. Gasifiers are flexible, but they still have specifications that need to be met to ensure low-maintenance operation, including keeping ash content low to prevent slag formation and controlling moisture content from 20 to 25 percent to maximize the conversion of feedstock. While some gasifiers can handle more moisture than others, higher moisture content consumes more thermal energy from the process. At the same time, too little feedstock moisture will not convert the fixed carbon from feedstock to gas. If the feedstock needs to be dried prior to gasification, there are waste heat opportunities that can perform this without requiring additional energy.
Biomass takes many different forms apart from the tried and true wood chip. Waste streams create a tremendous financial opportunity when captured, but also should be combined with the right technology to ensure long term results. While a gasifier’s high temperature (greater than 3,500°F in some systems) is great for breaking down material, it is also important to understand the chemistry of the feedstock going in to determine what the gas and biochar will be. Gasifier manufacturers should be consulted early in the process to help evaluate feedstock chemistry to match their respective equipment.
Feedstock can also be mixed or blended to achieve the proper properties. This makes the process more flexible and allows a user to utilize higher disposal cost waste streams.
Once feedstock is identified and evaluated, the best application for the gas must be determined. While electricity is often a first focus of projects, finding thermal applications can provide improved project financials through reduced capital costs and lower operating and maintenance costs. Supplementing an existing process with producer gas (the fuel that results from the gasification process) is another opportunity that can offer both financial benefits and increase overall system reliability and flexibility since the existing process is still in place.
Producer gas can be described as a dilute mixture of flammable gases. The flammable gas constituents are CO, H2 and CH4. The diluent gases are primarily N2, CO2 and H2O, which constitute 55 to 65 percent by volume of the total; the result is a gas with a higher heating value from 130 to 160 BTU/ft3 for air-blown gasifiers. The producer gas stream leaves the gasifier at around 1,000°F, and even in the most efficient gasifiers, contains a level of tar and particulate (mostly carbon-based) that needs to be considered when determining the application.
For thermal applications, these components can be cleanly combusted in a direct-fired thermal oxidizer creating a clean flue gas, or in a low-BTU burner and adapted to traditional thermal processes such as boilers, kilns, thermal oxidizers, dryers or just about any existing process that is currently using fuel oil, natural gas, or propane. The diluted mixture of gases also yields a lower flame temperature (approximately 2,750°F) that can reduce thermal NOX generation compared to other fuel sources. Approximately 85 to 90 percent of the feedstock energy content can be delivered to the thermal process through both the combustible and sensible heat, while the remainder goes to the biochar and radiant losses.
For electric applications, there are a few options that are commercial and vetted. Boiler steam turbines and thermal oxidizer hot-oil organic Rankine cycle generators offer robust systems that are based on the aforementioned thermal application principles to deliver clean, renewable electricity. Internal combustion engines can also be fueled with producer gas. While they offer greater system efficiency, they also require gas conditioning to remove the heat, tar, particulate and condensate from cooling the gas prior to being combusted in the engine, driving higher operating and maintenance costs.
Financials of gasification integration
Diverting waste material from landfills to a gasification process can generate significant savings for local governments and industries. The primary benefit of the process is eliminating landfill tipping fees and the expense of hauling waste by truck. Those savings alone can often justify investment in a gasification installation, providing positive cash flow and a very reasonable payback schedule.
Another way to view the benefits is through the environmental lens. Besides diverting waste from what are often overflowing landfills, the process can prevent the release of carbon while extending the life of landfills.
Analysis of a specific situation is very straightforward utilizing data to compare existing energy usage or generation with the cost of feedstock including local tipping fees, hauling, labor and funds costs. Armed with those initial figures, local managers will have the basis to make a sound decision on whether to pursue a full study for such a project.
Feedstock that has an “all-in” cost of disposal of $35 per dry ton (8,500 BTU’s per lb dry basis) can be converted into a producer gas that is negative $2 per MMBTU (combustible and sensible). Even when compared with $6 per MMBTU (delivered) of existing fuel cost, a project can realize simple payback in less than three years.
In a situation where the feedstock has an “all-in” cost of disposal of $35 per dry ton (8,500 BTU’s per lb dry basis), but needs to be mixed with 40 percent woody biomass, one could afford to pay a price of $52.5 per dry ton for the biomass and still achieve a zero dollar per MMBTU (combustible and sensible) fuel cost.
With this information in mind, there is little doubt that biomass gasification in instances where feedstock is readily available makes simple financial and environmental sense.