Article pubs.acs.org/EF
Experimental Investigation into a Pilot-Scale Entrained-Flow Gasification of Pulverized Coal Using CO2 As Carrier Gas Xiaolei Guo,† Wenxue Lu,‡ Zhenghua Dai,† Haifeng Liu,† Xin Gong,*,† Lei Li,‡ Honglin Zhang,‡ and Baogui Guo‡ †
Key Laboratory of Coal Gasification, Ministry of Education, East China University of Science and Technology, Shanghai, 200237, China ‡ National Engineering Center of Coal Water Slurry Gasification and Coal Chemical, Yancon Lunan Chemical Fertilizer Plant, Tengzhou, Shandong, 277527, China ABSTRACT: Using CO2 as a carrier gas in pneumatic conveying of pulverized coal instead of N2 has been paid more and more attention with the wide application of dry-feed entrained-flow coal gasification technology due to its attractive effects as a reactant. Accordingly, a pilot-scale performance of entrained-flow gasification using CO2 as the carrier gas of pulverized coal was investigated at gasifier temperatures between 1300 and 1400 °C. The differences in solid mass flow rate and flow stability are insignificant in the dense-phase pneumatic conveying between CO2 and N2 carrier provided that the conveying pressure differences and solid velocities in the pipeline are kept constant. Regarding syngas composition in dry basis, there was a desired drop in inert gas (N2) concentration from about 6% to less than 2%, while the CO2 concentration increased by about 4%age points in optimal operation conditions when CO2 carrier substituted for N2. At the same time, gasification profiles in different operation conditions indicated that CO2 carrier can act as an auxiliary gasification agent. Steam as another important gasification agent, however, cannot be completely substituted by CO2 carrier gas.
1. INTRODUCTION The entrained-flow gasification process has become a promising and attractive coal utilization technology in China. This technology has been validated and proven in many commercially operating units including 32 GE,1 15 Shell,2 and 17 OMB (opposed multi burner) licenses.3 The reasons for this include the prospect of increased efficiency and environmental performance including CO2 capture through the use of IGCC in the power industry as well as the fact that the production cost of some bulk chemicals such as ammonia and methanol with coal feedstock is much lower, compared to petroleum or natural gas feedstock, due to their increasing prices. In other words, the growing demand for syngas utilization such as IGCC, hydrogen, synfuel, and chemical industries acts as a strong driving force for syngas production from the entrained-flow coal gasification. Generally, either pulverized coal or coal-water slurry can be employed for entrained-flow gasifier feeding. Some principal advantages of dry feeding over coal-water slurry are the following:4,5 • Lower oxygen and coal consumptions; • Higher concentration of CO+H2 in syngas and carbon conversion; • More coal species available such as those with poor slurry ability. Either N2 or CO2 can be employed as a carrier gas to convey pulverized coal pneumatically due to their inert property. Although N2 is so far the preferred carrier gas due to its availability from air separation unit and lower reactive nature, CO2 is more desirable because of its attractive effects as a gasification agent. Moreover, N2 is not an active compound © 2011 American Chemical Society
when synfuel, methanol, and so on are the downstream products, which means that it would be enriched through recirculation in the synthesis plant and thus reduces its efficiency. So, using CO2 as a carrier gas in pneumatic conveying of pulverized coal instead of N2 has been paid more and more attention with wide application of dry-feed entrainedflow coal gasification technology. Some work focusing on the effects of CO2 carrier gas on entrained-flow coal gasification has been published. Guo et al.4 demonstrated typical syngas compositions in a dry-feed entrained-flow gasification pilot plant using CO2 carrier gas. Based on Gibbs energy minimization principle, Dai et al.6 simulated the effects of operation conditions such as gasification pressure, ratios of oxygen-coal, steam-coal and CO2coal on gasifier performance coupling with more than 99% carbon conversion and found that CO2 carrier was able to act as an auxiliary gasification agent functioning in the similar way as steam addition. However, there is little systematically studied data available in public on the performance of entrained-flow gasification of pulverized coal using CO2 as a carrier gas up to now, especially in a pilot scale. The main objective of this work is to evaluate the performance with respect to dense-phase pneumatic conveying, productivity of CO + H2, and syngas composition using CO2 as a carrier gas in a semi-industrial scale plant of up to 30 tons coal per day. Furthermore, the results are intended to give valuable insights concerning optimization of the process Received: October 7, 2011 Revised: December 26, 2011 Published: December 27, 2011 1063
dx.doi.org/10.1021/ef201528b | Energy Fuels 2012, 26, 1063−1069
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Article
Figure 1. Schematic flow diagram of the pilot-scale plant. the slag solidified in the quench bath is discharged to a slag lock hopper. A thermojunction is employed to monitor gasifier temperature (inner surface temperature of refractory lining) for the purpose of controlling gasifier operation. Based on the ash fusion temperature (flow point) of the feedstock, the gasifier temperature is controlled between 1300 and 1400 °C approximately. That is because lower temperature will inhibit slag-off, while higher temperature damages the refractory lining and thermojunction. Based on some measurement results, the productivity of CO + H2 (Y(CO+H2)) and carbon conversion (ø) can be defined by eq 1 and eq 2 as follows
operation and provide a base for validation and improvement of existing simulations.
2. EXPERIMENTAL SECTION 2.1. Process Overview. Figure 1 shows a schematic flow diagram of the pilot-scale plant with a daily throughput of 30 t coal at gasifier pressure of 1.0−3.0 MPa. First, pulverized coal in an atmospheric bunker is discharged into a called lock hopper. Next, the lock hopper is pressurized by CO2 until the pressure in it is the same as that in a highpressured feed hopper, and then the pulverized coal can be discharged from the lock hopper to the feed hopper. Finally, the pulverized coal in the feed hopper is pneumatically conveyed into a gasifier through a pipeline with I.D. 15 mm and about 20 m long. A solid mass flow rate meter,7 a three-way valve, and a burner on the top of the gasifier are connected by the pipeline. Therefore, with this arrangement, a pneumatic conveying loop is available from the feed hopper to the atmospheric bunker both linked up through a three-way valve to the burner. Oxygen and steam can be added into the gasifier by the burner. The atmospheric bunker with load cells can also be used to calibrate the solid mass flow meter by weighing method. After calibration, a measurement deviation of ±7% (Wt) was achieved within the range of values between 900 and 1300 kg/h for both CO2 and N2 carrier. Consequently, the online mass flow rate of pulverized coal passing through the conveying pipeline was obtained. At the same time, the online flow rates of oxygen and steam added into the gasifier were measured by orifice meters, respectively. Thus, the control for oxygencoal ratio and steam-coal ratio can be easily realized. When gasifier is ready, pulverized coal being pneumatically conveyed in the loop can be fed into the gasifier by the three-way valve. The gasifier, as shown in Figure 1, consists of two parts: gasification chamber in the upper part and quench bath in the lower part. The gasification chamber has an inside diameter of about 900 mm and total height of about 5.0 m, in which pulverized coal experiences pyrolysis, devolatilization, and a series of gasification reactions. One burner is centrally mounted at the top, and refractory lining is employed to protect gasifier shell from exposure to high temperature. Hot syngas leaves the gasification chamber together with molten slag and then is fully water-saturated in the quench bath. Raw syngas containing much fine slug leaves into a dust precipitation system, and
Y(CO + H2) =
φ=
Q RS × (XCO + X H2) Wpc
(1)
[Q RS × (XCO + XCO2) − Q carr .]/ 22.4 × 12 Wpc × Mc × 100%
(2)
where QRS is the standard state volume flow rate of raw syngas on dry basis, Nm3/h; Qcarr. is the standard state volume flow rate of CO2 carrier gas, Nm3/h; Wpc is the mass flow rate of pulverized coal, kg/h; Mc is the weight fraction of carbon element in pulverized coal; and XCO, XH2, and XCO2 are the volume fraction of CO, H2, and CO2 in syngas on dry basis, respectively. 2.2. Coal Analysis. The feedstock is Beisu coal of Shandong province in China. Its properties are summarized in Table 1 and Table 2.
3. RESULTS AND DISCUSSION 3.1. Pneumatic Conveying. The effects of gas type on pneumatic conveying are rarely reported in the literature so far, although CO2 has some specific characteristics such as a higher density and a greater temperature reduction effect in comparison with N2. Geldart et al.8 investigated dense phase pneumatic behaviors of pulverized coal at a pressure of 5.27 MPa using N2 and H2 as a carrier gas, respectively. Based on 1064
dx.doi.org/10.1021/ef201528b | Energy Fuels 2012, 26, 1063−1069
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solid velocity at a feeding pressure difference of 1.0 MPa between the feed hopper and atmospheric bunker. It can be seen that the mass flow rates are almost identical for the two gases at the same solid velocity from about 2.5 to 5.0 m/s, which means that there is not a notable difference in mass flow rate between them on the condition that both the feeding pressure difference and solid velocity remain identical. As shown in Figure 3, online mass flow rates of pulverized coal were recorded to compare their conveying stabilities at a gasifier pressure of 1.0 MPa under the similar conveying
Table 1. Particle Properties of the Pulverized Coal parameter bulk density particle density angle of repose mean particle size D(4,3) particle size distribution
value kg/m3 kg/m3 ° μm wt.% μm
536 1400 50 60.1
