Introduction Underground coal gasification (UCG) technology is critical to coal development in China, which has attracted recent attention. UCG technology can directly convert coal into syngas rich in CH4, H2, CO2, and CO beneath the earth surface, which is appropriate for the employment of coal resources with a low value (i.e., coal resources in large depths or with restricted thickness). UCG technology significantly reduces the economic cost and safety risks of coal mining and improves the overall efficiency of coal utilization. Syngas obtained from UCG technology is used to produce higher-value-added chemicals, such as methanol, ammonia, ethylene, and Fischer-Tropsch synthesis products, and the fuel for heating and power generation. Using syngas produced by UCG technology for power generation is mainly carried out through internal combustion engines. The energy conversion efficiency is limited by the Carnot cycle, usually about 30%, which diverges from the goal of clean and efficient use. Solid oxide fuel cells (SOFCs) convert the chemical energy in syngas fuel directly into electrical energy through electrochemical reactions. The conversion efficiency is considerably high and is not limited by the Carnot cycle. Power generation efficiency of > 60% and combined heat and power (CHP) efficiency of > 90% can be achieved. Therefore, SOFCs can efficiently employ the syngas from UCG, which is important for the clean and efficient development and utilization of coal. Flat-tube SOFCs exhibit high thermal shock resistance and stability under the redox cycle and are capable of being used in harsh environments. This paper was to investigate power generation using flat-tube SOFCs fed with carbon-containing fuels. In addition, the performance and durability of anode-supported flat-tube SOFCs with direct internal reforming of syngas from UCG were also analyzed. Methods The flat-tube Ni/yttria-stabilized zirconia (YSZ) anode-supported cell contains a porous NiO-3YSZ (3% yttria-stabilized zirconia, in mole fraction, ~1 mm thick) anode support, a NiO-8YSZ (8% yttria-stabilized zirconia, in mole fraction, 15-20 μm thick) anode functional layer, a dense 8YSZ electrolyte layer (15-20 μm thick), a gadolinia-doped ceria (GDC) barrier layer (~3 μm thick), and a porous (La0.6, Sr0.4)(Co0.2, Fe0.8)O3) (LSCF)-GDC cathode layer (15-20 μm thick). The cell was indicated as NiO-3YSZ|NiO-8YSZ 8YSZ|GDC|LSCF-GDC. The cell dimensions were 155 mm × 65 mm × 5.5 mm with an active cathode area of 60 cm2. The cell was heated in air at 750 ℃after assembly. The anode was reduced under 0.3 L·min1 H2 prior to the electrochemical tests. H2 or syngas was fed to the anode to evaluate the cell performance. To evaluate the electrochemical cell performance, the open-circuit voltage (OCV), current-voltage (I-V) characteristics, and electrochemical impedance under different conditions were measured by a model VMP3B-20 electrochemical workstation (BioLogic Co., France). The electrochemical impedance spectra (EIS) were recorded under OCV condition in the frequency range of 30 mHz to 30 kHz at an alternating current of 10 mV.Results and discussion The increasing temperature reduces a carbon deposition in SOFC fueled with different syngas. Especially at 750 ℃, the carbon deposition is not favorable for most types of UCG syngas. When the flow rates of H2, CO, CH4, and CO2 are fixed, the composition of N2 does not affect the carbon deposition. Increasing the composition of CO2 mitigates the carbon deposition. The power density of SOFC with H2 at 750 ℃ is 260.3 mW·cm2 at 0.8 V, and the maximum power density is 395.1 mW·cm2. The power density of a cell fueled with syngas (Swan Hills) at 0.8 V is 240.1 mW·cm2, and the maximum power density is 361.9 mW·cm2, reaching 91.6% of that with H2. The power densities of SOFC fueled with other types of UCG syngas are lower than those fueled with syngas (Swan Hills). The stability test of the flat-tube SOFCs fed with syngas (Swan Hills) is conducted at 750 ℃ under 300 mA·cm2. During the test, the composition of CO2 is changed. The flat-tube SOFC can discharge stably for 960 h. When the CO2 content is higher than or equal to 20% (in volume fraction), the degradation rate is less than 2.7% every 100 h in each stage. No sudden performance degradation emerges, indicating that the flat-tube SOFCs can run stably under such conditions. Since the original composition of syngas (Swan Hills) is similar to that of stage 2 (for 112-214 h), a stable power generation from flat-tube SOFCs directly using UCG syngas (Swan Hills) is viable. The CO2 content reduces to 10% at approximately 460 h. The voltage fluctuates sharply at 520 h. Subsequently, when CO2 further decreases to 0, the cell operates stably for another 144 h. However, when N2 flow is stopped at 750 h, the performance deteriorates significantly. After the long-term test, two characteristic peaks of carbon deposition appear in the Raman spectrum, i.e., the D peak (at 1 335-1 350 cm1) and the G peak (at 1 580-1 600 cm1). The result indicates that the carbon deposition is formed on the surfaces of the fuel channels. The SEM images show that after the test at 960 h, a significant Ni particle agglomeration occurs in the active anode layer of the cell fed with syngas (Swan Hills), and the average Ni particle size is increased by 17.0%. The Ni content loss in the active anode layer appears. Especially, the Ni content near the electrolyte layer (0-5μm) decreases from 30.0% (pixel, same below) to 19.2%. Carbon deposition, nickel agglomeration, and nickel particle loss result in a performance degradation.Conclusions The cell performance and durability of flat-tube solid oxide fuel cells by direct internal reforming of syngas from underground coal gasification were investigated. The peak power density of the cell with syngas (Swan Hills) from underground coal gasification was 361.9 mW·cm2 at 750 ℃ (Note: Swan Hills is a coal mine in Canada), which was approximately 91.6% of that with H2. Astable operation with a constant current of 300 mA·cm2 at > 960 h was realized. The ohmic resistance and polarization resistance increased from 0.108 Ωcm2 and 0.485 Ω·cm2 to 0.190 Ω·cm2 and 0.494 Ω·cm2, respectively. The increased rates of the ohmic resistance and polarization were 0.085 Ω·cm2·kh1 and 0.009 Ω·cm2·kh1, respectively. The main degradation mechanisms were the carbon deposition on the anode, Ni percolation loss, and Ni particle agglomeration, detected by Raman spectroscopy and scanning electron microscopy. Elevating CO2 concentration suppressed the carbon deposition, improving the cell stability.