Benefits of Slag Utilization
Some of the perceived advantages of using the slag by-product, instead of the current practice of landfilling are:
The interest in using slag is largely driven by the avoided landfill cost1. Avoided landfill costs can be significantly different for utilities with and without captive landfills. For slag producers with captive landfill, use of any by-product results in 100% savings of operating costs but only partial savings of the capital of the landfill. On the other hand, utilities without captive landfills have zero capital cost but high operational costs. Thus, any material not sent to the landfill results in much higher cost savings for IGCC plants without captive landfills, than those with captive landfills. Coal utilization byproduct (CUB) landfilling costs (capital and operating) in the State of Ohio, for example, ranged in the past from about $3 to $35 per ton for plants with and without captive landfills. CUB producers with captive landfills had low landfill costs (approximately $3 to $15 per ton). However, CUB generators without captive landfills generally had much higher landfilling costs (about $10 to $35 per ton) due to high tipping fees and longer haulage distance.
Another benefit associated with CUB utilization can be quantified as the intrinsic value of land not needed for disposal purposes. It is obvious that almost any tract of land will have a lesser environmental quality if it is used as a disposal site rather than left in its natural state. The mere operation of a large disposal site over a long period of time increases the potential for accidental environmental damage due to loss of vegetation, surface runoff, airborne dust from trucks, etc. It is, therefore, assumed that the environmental benefit of diverting CUBs from disposal sites takes the form of a value assigned to each acre of landfill space “avoided.” This benefit accrues to any use of CUB, assuming that there is no additional environmental disturbance at the utilization site merely to accommodate the CUBs2.
Barriers to Slag Utilization
The principal barriers to gasification slag utilization can be classified into three main categories: 1) institutional, 2) regulatory, and 3) legal. The institutional barriers include restrictions on use of CUBs through requirements, standards, specifications, policies, procedures, or attitudes of organizations and agencies involved in CUB use or disposal. This can also include economic, marketing, environmental, public perception, and technical barriers. Some examples are local material transport requirements, opposition from established raw material marketers, unknown long-term effects on products made from slag, and product durability concerns.
Regulatory barriers include federal, state, and local legislation and permitting requirements. Most states currently do not have specific regulations addressing the use of CUBs, and requests for specific uses are handled on a case-by-case basis or under generic state recycling laws or regulations.
Legal barriers include contract, patent, liability and some regulatory issues. Critical to overcoming the barriers and creating successful IGCC slag uses will be demonstrating that such practices are technically safe, environmentally sound, socially beneficial, and commercially competitive. Improved specifications, fact sheets, design manuals, and testing procedures need to be developed and widely distributed in collaboration with government and university researchers and standard-setting organizations.
Efforts to educate regulators, policy-makers, engineering consultants, potential end-users, and the general public are very important. The educational efforts should focus on neutralizing the association of the term “waste” with IGCC by-products, and should emphasize their environmental safety (non-toxicity) and their potential uses, benefits and drawbacks. The public in particular should be made aware of the environmental costs of landfilling and the environmental and social benefits resulting from reclamation and other efforts using IGCC byproducts.
Current Experience with Gasification Slag Utilization
In addition to the successful assessment of Wabash slag to produce lightweight aggregate, the Polk plant has successfully processed slag for use in cement production. In order to meet the required slag specification, the size of the fines handling system was doubled, and additional slag handling equipment was installed to deal with unconverted carbon in the fines. As a result, Polk produced 2000 tons of slag in the summer of 2001 that was used by the cement industry at lower cost than Class I landfill disposal. Some further process modifications, accomplished during an outage in the fall of 2001, have enabled the plant to better separate unconverted carbon and produce slag that is more consistently suitable for the cement industry3. Not only does the slag meet specifications, but also the unconverted carbon can be recycled back to the plant or used elsewhere. However, a negative impact of the improved slag generation capability is that the plant must operate at reduced load. Load reduction is necessary, because more oxygen is needed to gasify the fines, but the oxygen plant, specifically the main air compressor, cannot supply the required capacity. In order to eliminate the load restrictions and ensure Polk’s long-term viability, another source of air for the oxygen plant is needed. This problem is specific to the Polk plant and not inherent in IGCC technology; i.e. a new design would not have this problem.
Note that this particular problem with complete carbon conversion does not generally apply to all gasifiers. For example, the BGL gasifier has not demonstrated this problem due to the nature of its slag removal system.
Solid Waste/Byproducts of Gasification