Co-Gasification of High Sulfur Coal
with Coal-bed Methane to Produce
Synthesis Gas with Adjustable H2/CO
Contents for Synthesis of Value-added
This research project is the conceptive development of a process to produce synthesis gas with a flexible H2/CO ratio by the co-gasification of coal and coal-bed methane. The feasible H2/CO ratio of synthesis gas for production of chemicals is between 1 and 2. Generally the coal gasification process generates synthesis gas with an H2/CO ratio between 0.9 and 1.2, depending on property of the coal feedstock and operational modes. Reforming or partial oxidation of methane can generate synthesis gas with an H2/CO ratio between 1 and 3. Co-gasification of coal and methane is expected to generate synthesis gas with varied H2/CO ratios. However, matching and cooperation between these two reactions in kinetics and thermodynamics must be achieved for the process and quality control of synthesis gas production. The preliminary feedstock includes two Kentucky bituminous coals and simulated coal-bed methane. For comparison purposes, two other low rank coals of the United States (U.S.) are also used in tests of gasification. They are Lignite and Powder River Basin (PRB) coal, which is a Sub-bituminous coal. Steam, composed of air or oxygen, is used as an gasification agent in coal gasification and co-gasification processes of coal with methane. Catalytic performance of ashes and chars, which originally form several Kentucky coals and Lignite and PRB coal, is also evaluated. Through completion of the proposed project, answers to important questions revolving around co-gasification of coal and coal-bed methane in the following areas will be given:
1. How to control the cold gas efficiency (the sum of H2 and CO), conversion efficiency of coal and CBM and adjustability of H2/CO ratio of synthesis gas?
2. What are the main constitutes in coal ash that has a catalytic function on coal and CBM co-gasification? What kind of Kentucky coal ash has higher catalytic activity on the co-gasification process? Does the structure of coal char have impact on activity for methane reforming and partial oxidation?
3. Which is the control factor of mercury capture by solid residues of co-gasification, the active site or residue pore structure? How do you control preferable characterization of solid residues produced on mercury capture?
Other specific objectives of the proposed project also include 1) investigation of the chemistry, fundamental mechanisms, and reaction kinetics of coal under gasification conditions and coal-bed methane under reforming or partial gasification conditions; 2) Investigation of the possible cooperative effects among the reactions of the co-gasification process; 3) Determination of the optimum gas-solid contact model and a sound reactor design; mass and heat balance.
A modified lab-scale fluidized bed gasification demonstration unit with ID of 2.5 inches and an enlarged portion of 4 inches was used to conduct all major experiments on gasification of selected coal samples and also co-gasification of coal and coal-bed methane for the process demonstration. The process chemistry, fundamental mechanisms, and reaction kinetics were investigated in a one inch OD test rig. Test results indicated that:
► For all tested coals, co-gasification of coal and simulated methane could generate synthesis gas with the H2/CO ratio at nearly 2.0, by which chemicals production through Fischer-Tropsch (F-T) synthesis could be achieved. Carbon conversion efficiencies of KY bit-2 and methane conversion efficiency could be achieved at above 80%, especially in the oxygen-blown mode. Another KY bit-1 shows lower carbon conversion efficiency due to its lower gasification reactivity. It could be improved by extending its residence time at the elevated temperature in the gasifier, such as the high temperature Circulating Fluidized Bed Gasifier (CFBG). The gasification selectivity of coal and methane conversion (the cold gas efficiency in the sum of H2 and CO) could be achieved at above 80% in the oxygen-blown mode. Two low rank U.S. coals show better reactivity than Kentucky coals, but lower gas yield and gas quality because of higher moisture and/or ash content in these low rank coals. The development of high temperature (less than ash melting temperature) Circulating Fluidized Bed Gasification process (CFBG) seems imperative for all Kentucky coals in the near future.
► Thermodynamics calculation predicts that the methane conversion could be achieved by almost 100% and selectivity (the sum of H2 and CO) by almost 100% at a temperature range of 900-1050 oC. However, the H2/CO ratio is below 1.5 if all possible reactions on CH4 reforming and partial oxidation occur. The followed-up kinetics study in the one inch testing rig presents the possibility of CH4 reforming and partial oxidation with a pretty favorable H2/CO ratio, which is greater than 5. The higher H2/CO in CH4 reforming and partial oxidation process means less CH4 in mass needed to adjust the H2/CO ratio in the co-gasification process of coal and coal-bed methane, which is efficient. Ash failed to be a good candidate of catalyst on CH4 reforming and partial oxidation because of its very low specific surface area available for proceeding of catalyst reactions. However, coal chars present very promising catalytic performance on CH4 reforming and partial oxidation because of their larger specific surface area. In this study no other constitutes in coal fly ash or special surface properties of coal chars were correlated with the enhanced CH4 conversion efficiency. It seems that the specific surface area is only variable in controlling CH4 conversion efficiency. Another important conclusion in 1” testing rig educated us on the enhanced contact mode between coal char and methane. In the followed-up tests in the 2.5” gasification demonstration unit, addition of methane is below in the dense phase but above the area where the oxygen concentration is enriched so that the higher CH4 conversion efficiency and selectivity could be achieved in this study.
► Mercury and potentially solid residues, which are both generated in the gasification process could be enacted by EPA rules because they are dangerous to our environment. In this study, a conceptive development of Hg capture by char residues from gasification process was pursued. Tests indicated that gasification char residue could be re-activated by steam to generate effective Hg adsorbent with its developed pore structure. Most promisingly, all activated char residues, which were derived from Kentucky coals, generally showed enhanced Hg capture capability and even better than char residues from low rank coal. It was likely that the sulfur species on the surface of activated char residue enhanced Hg capture by strong bonding between sulfur and Hg. Low rank coal is generally used to prepare good adsorbent because of its activation reactivity resulting in the larger specific surface area with less production cost. Char residues, which were derived from gasification process of Kentucky coals, could be good candidates for preparation of Hg adsorbents.