INTERIM FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996 ⁽¹⁾

Project Title: GASIFICATION COPROCESSING OF ILLINOIS BASIN COAL AND RDF

ICCI Project Number: 95-1/4.1B-2

Principal Investigator: Vas Choudhry, Praxis Engineers, Inc.

Project Manager: Daniel Banerjee, ICCI

Nearly 80% of the municipal solid waste (MSW) generated nationwide consists of combustible materials. Approximately 80% of MSW is disposed of in landfills, posing environmental problems due to groundwater contamination and methane emissions. In addition, landfill disposal entails tipping fees of $30-$40/ton. The calorific value of MSW can be increased from 3,500 to 5,900 Btu/lb by removing recyclable materials and noncombustibles to produce shredded refuse-derived fuel (RDF fluff) or pelletized densified refuse-derived fuel (dRDF) which may be used for power generation. However, incineration of RDF is reported to produce emissions of mercury, hydrochloric acid gas, dioxins, and furans. In contrast, coal gasification is a high-temperature, high-pressure, partial oxidation process which converts nearly all carbonaceous materials to synthesis gas, prevents the formation of undesirable organic species, and has been demonstrated to accept municipal sludge and plastics as feeds when combined with coal.

The major project objectives were to determine whether (i) RDF fluff or dRDF can be coprocessed with coal, and (ii) use of RDF reduces fuel costs for coal gasification plants. It is anticipated that development of a method of coprocessing RDF with coal will reduce gasification plant levelized costs by significantly reducing fuel costs, thus making the combined-cycle gasification process more competitive. This, in turn, would increase the market share of Illinois Basin coals which make ideal gasification feedstocks.

Under this project, parametric studies were performed to evaluate various grinding and slurry feed preparation methods with the objective of maximizing RDF utilization. Tests confirmed that both RDF fluff and dRDF need to be subjected to primary size reduction before mixing with coal for final grinding to prepare the gasification slurry feed. Dry grinding using a stirred ball mill was identified as the preferred method for this step. The test data indicate that the power requirements for the two materials (RDF fluff and dRDF) are the same for size reduction to 20 mesh. Therefore, subsequent slurry preparation studies were performed using RDF fluff, which is the cheaper of the two materials. Tests were performed using 50/50 RDF and coal in a dry attrition mill, with coal serving as semi-autogenous media in the mill. Energy consumption levels remained the same for identical RDF fluff throughput rates in the mill both with and without coal, thus requiring no additional energy for coal size reduction. Gasifier slurry feed preparation and testing of selected coal/RDF blends in a laboratory gasifier to determine process performance are scheduled for the next quarter.

The project objective is to investigate the potential for gasification coprocessing of refuse-derived fuel (RDF) along with a high-sulfur Illinois Basin coal. The project addresses two critical issues confronting the Illinois Basin: (i) identifying methods of increasing the market share of Illinois Basin coals via gasification, and (ii) utilizing RDF which is an abundant renewable resource generated in metropolitan areas of Illinois. Municipal solid waste (MSW), which consists of household, commercial, and institutional wastes, is a major solid waste whose disposal poses long-term problems such as groundwater contamination and methane emissions. MSW, which has a calorific value of 3,500 Btu/lb, is routinely processed to recover plastics, aluminum, and other metals using standard off-the-shelf technology as part of state-mandated recycling requirements. Subsequently, other noncombustibles are removed and the particle size of the residual material is reduced by shredding to make a product called RDF fluff, which has a unit weight of 2-3 lb/ft³ and a calorific value of 5,900-6,500 Btu/lb. In order to improve its handleability, the fluff is compressed into pellets using binders. The resulting product, termed densified refuse-derived fuel (dRDF), has a unit weight of 40 lb/ft³.

RDF is combusted as a boiler fuel to generate electric power. In addition to causing boiler corrosion and slagging problems, RDF combustion has come under strict scrutiny because of concerns over the emission of mercury, hydrochloric acid gas, PCBs, PAHs, dioxins, and furans generated from the combustion of certain components in an oxidizing environment and in the presence of chlorine. Also, combustion of RDF produces ash which is typically classified as a hazardous solid waste. However, combustion of dRDF using calcium hydroxide as a binder or co-firing with low-sulfur coal may alleviate these problems.

In the gasification process, on the other hand, the high-temperature, high-pressure partial oxidation conditions provide an ideal environment for conversion of nearly all carbonaceous materials while preventing the formation of undesirable organic species. Thus, gasification does not produce the toxic emissions generated during combustion or incineration of MSW or RDF. Illinois Basin coals have been demonstrated to make excellent gasification feedstocks due to their high volatiles content. Therefore, gasification coprocessing of Illinois Basin coals with RDF offers an environmentally benign alternative to combustion of these materials.

The economic incentive for this approach relies on the revenues generated by MSW acceptance tipping fees of $30-$40/ton. The MSW can be processed to make RDF whose energy content can be used to reduce gasifier fuel costs. Coprocessing of Illinois Basin coals blended with RDF would (i) reduce IGCC plant levelized costs, thus increasing the market share of these coals, (ii) utilize a major recurring solid waste, (iii) eliminate the twofold problem of SO₂ emissions resulting from combustion of high-sulfur coals and that of toxic emissions from combustion of RDF, and (iv) relieve the burden on or eventually eliminate sanitary landfills.

In the project, the overall goal is to develop a pumpable coal/RDF feed slurry with the highest possible energy content that is acceptable to the gasifier, while maximizing the RDF component, and to evaluate it as a gasifier feed. The project goals are to:

Characterize the project coal, RDF fluff, and dRDF samples and evaluate them as starting feedstocks for preparing the gasifier slurry feed

Define the gasifier feed specifications in terms of the particle size of the RDF and coal components, and the slurry water content, viscosity, and calorific value

Develop an optimal method of preparing a coal/RDF slurry and identify a method of evaluating it as a gasifier feedstock

Conduct parametric studies in a research gasifier to investigate the impact of the RDF content in the slurry on gasifier performance and characterize the new type of gasification solid waste.

The Texaco gasification process was selected for the project because it uses a slurry feed and has been demonstrated and tested extensively over the past five years for coprocessing of coal with many waste materials including sewage sludge and plastics.

Studies were performed to evaluate various grinding and slurry feed preparation methods with the objective of maximizing RDF utilization. The test work confirmed that both RDF fluff and dRDF need to be subjected to primary size reduction prior to grinding and slurrying with coal. Dry grinding using a stirred mill was identified as the preferred mode for the initial size reduction step. The test data indicated that the power requirements for size reduction of the two materials to 20 mesh were the same. Therefore, all subsequent slurry preparation studies were done using RDF fluff, which is the cheaper of the two materials. Tests were performed using 50/50 RDF fluff and coal in a dry attrition mill, with coal serving as semi-autogenous media in the mill. Energy consumption levels remained the same for identical RDF throughput rates in the mill both with and without coal. These results indicate that the presence of the coal facilitated size reduction of the RDF fluff while requiring no additional energy to maintain the feed rate to the attrition mill.

Following this approach, large batches of RDF and coal/RDF mixtures were prepared for gasifier slurry feed preparation tests. In an effort to maintain a high solids loading in the slurry, a dispersant (Daxad) was used to improve the flowability of the slurry.

Samples of both slurry feed materials (coal and RDF fluff) have been comminuted for mixing in various proportions for gasifier slurry testing. This work will be performed as soon as the research gasifier at the Texaco facility becomes available.

An economic analysis was conducted to calculate the costs of producing a coal/RDF slurry and compare these with the costs of conventional all-coal slurries at approximately $18/ton. It is estimated that a coal/RDF slurry would cost about $8.10/ton after taking into account the credit from the MSW acceptance tipping fee. However, these figures may need to be corrected for any cost impacts due to gasifier operation.

Pages 3-13 contain proprietary information.

The remainder of this report contains proprietary information and is not available for distribution except to the sponsor(s) of this project.