Elsevier

Energy Conversion and Management

Volume 76, December 2013, Pages 581-587
Energy Conversion and Management

Process simulation of oxy-fuel combustion for a 300 MW pulverized coal-fired power plant using Aspen Plus

https://doi.org/10.1016/j.enconman.2013.08.007Get rights and content

Highlights

  • Combustion processes were studied with Aspen Plus for a 300 MW pulverized coal power plant.

  • The amount of each flue gas component in coal-fired processes with air or O2/CO2 as oxidizer was obtained.

  • Differences between the two process models were identified.

  • The influences of operation parameters on the flue gas components were examined.

Abstract

This work focuses on the amounts and components of flue gas for oxy-fuel combustion in a coal-fired power plant (CFPP). The combustion process of pulverized coal in a 300 MW power plant is studied using Aspen Plus software. The amount of each component in flue gas in coal-fired processes with air or O2/CO2 as oxidizer is obtained. The differences between the two processes are identified, and the influences of temperature, excess oxygen ratio and molar fraction of O2/CO2 on the proportions of different components in flue gas are examined by sensitivity analysis. The process simulation results show that replacing atmospheric air by a 21%O2/79%CO2 mixture leads the decrease of the flame temperature from 1789 °C to 1395 °C. The equilibrium amount of NOx declines obviously but the SOx are still at the same level. The mass fraction of CO2 in flue gas increased from 21.3% to 81.5%. The amount of NOx is affected sensitively by the change of temperature and the excess oxygen ratio, but the change of O2/CO2 molar fraction has a little influence to the generation of NOx. With the increasing of O2 concentration, the flame temperature and NOx emission enhance rapidly. When the molar fraction of O2 increases to 30%, the flame temperature is similar and the mass fraction of NOx is about 1/8 of that air atmosphere.

Graphical abstract

This paper studied the combustion processes of pulverized coal in a 300 MW power plant using Aspen Plus software. The amount of each component in flue gas in coal-fired processes with air or O2/CO2 as oxidizer was obtained. The differences between the two processes were identified, and the parameter influences of temperature, excess oxygen ratio and molar fraction of O2/CO2 on the proportions of different components in flue gas were examined by sensitivity analysis.

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Introduction

It is well known that CO2 is the main greenhouse gas. According to the published statistics by the international energy agency (IEA), the amount of global CO2 emissions is 31.6 billion tons in 2011 and over 40% of the total is attributed to the coal combustion [1]. Therefore, it is very important to reduce CO2 emissions from coal-fired power plant (CFPP). Oxy-fuel combustion is considered as one of the major options for CO2 capture for both new and existing CFPPs. Coal is burned with a mixture of oxygen and recycled flue gas to obtain a CO2 rich stream ready for sequestration. At the same time, the NOx emissions can also be reduced, so oxy-fuel combustion is a comprehensive measure in pollution control and a new clean combustion technology [2], [3], [4].

In recent years, lots of investigations on oxy-fuel combustion technology have been carried out relating to the characteristics of coal combustion. Wang et al. [5] studied the combustion of pulverized coal in O2/CO2 as well as in air atmosphere. Predictions using a one-dimensional computer code were compared with the experimental data from tests conducted by Battelle Columbus Laboratories. The results showed that the combustion of pulverized coal could be completed more thoroughly in O2/CO2 atmosphere over a range of CO2-to-O2 molar ratios between 2.23 and 3.65. Kimura et al. [6] studied the pulverized coal combustion in the O2/CO2 using a 1.2 MWt tunnel furnace with a coal feed rate of 150 kg/h. Payne et al. [7] studied the combustion of coal with oxygen and recycled flue gas. The feasibility of this process had been evaluated in a pilot scale test furnace. The results showed that flue gas recycle is a viable means of controlling combustion and heat transfer characteristics, and that recycle conditions exist at which performance changes are minimal from operation with air. Suda et al. [8] studied the flame propagation velocity in O2/CO2 atmosphere. Experiments were carried out in a spherical chamber with inner diameter of 200 mm. Micro-gravity condition was used in order to achieve uniform pulverized coal cloud in the chamber. Flame propagation velocity was measured from the photographic image of the flame front by using high speed camera. The results showed that the flame propagation velocity of pulverized coal cloud in O2/CO2 mixture gas decreased to about 1/3–1/5 of that in N2/O2 gas mixture at the same oxygen concentration.

The emission characteristics of coal oxy-fuel combustion were also studied. Okazaki and Ando [9] and Okawa et al. [10] found that the emission of NOx under O2/CO2 atmosphere is only about 25% of that in air atmosphere. Meng et al. [11] studied the effects of temperature and excess air ratio on NOx formation using a one-dimensional drop-tube furnace. The results showed that, the amount of NOx increases quickly with temperature in air, but very slowly in O2/CO2 atmosphere. The peak value of NOx increases with excess oxygen ratio in both air and O2/CO2 atmospheres. The combustion characteristics of a bituminous coal and an anthracite coal in O2/CO2 atmosphere were studied by Niu et al. [12]. The results showed that, the total conversion of the fuel-N to NO is strongly decreased in O2/CO2 atmosphere, and at 1473 K, the conversion is reduced by 28.99% for the bituminous coal and 22.54% for the anthracite coal, respectively. Sivaji and Sreenivas [13] studied NO formation in oxy-fuel combustion at high pressures showing that the peak NO concentration nearly doubles when the pressure increases from 1 atm to 30 atm. Zheng and Furimsky [14] considered that the SOx emissions are governed by O2 concentration in O2 + CO2 mixture. Pak et al. [15], Li et al. [16] and Salkuyeh and Mofarahi [17] studied the CO2 emission, capture and storage for the oxy-fuel combustion.

The combustion characteristics and emission characteristics of pulverized coal in O2/CO2 atmosphere have been studied by many researchers. However, few studies refer to the amounts and components of flue gas for the oxy-fuel combustion in a CFPP. The differences between air and O2/CO2 atmospheres in a CFPP are also needed to be identified. The purpose of this article is to analyze the components of flue gas in a 300 MW CFPP under the atmospheres of air and O2/CO2 mixture, in which Aspen Plus software is used to simulate the process.

Section snippets

Process description

The process of coal combustion to be modeled is based on the following assumptions: (1) the combustion process is divided into four sequential steps: pulverized coal drying, pyrolysis, burning and flue gas dust removal; (2) all the blocks are in stable operation states, the parameters could not be changed with time; (3) air and pulverized coal are homogeneously mixed in the reactor; (4) in the process of coal pyrolysis, elements of O, H, N and S are vapourised into gas phase, and C element is

Results and discussion

The simulation results are presented in Table 3. The major component of flue gas is CO2 with the highest mass fraction of 94.4% which occurs in stream GASES. In the final outlet of flue gas, the mass fraction of CO2 is 81.5%, H2O and O2 are 14.9% and 2.2%, respectively. Other gas contents in the flue gas are quite low. Due to the high concentration of CO2 content, the flue gas can be compressed directly to realize near zero emission of CO2 to the atmosphere.

Conclusions

The oxy-fuel combustion process in a 300 MW CFPP unit is simulated by the software Aspen Plus, according to the calculation and analysis results, the conclusions are as follows:

  • (1)

    Compared with combustion in the air, the flame temperature under 21%O2/79%CO2 atmosphere decreases by 394 °C, and the mass flow rate of flue gas produced reduces to 1/4 of that in the air. When the molar fraction of O2 increases to 30%, the flame temperature is similar with air atmosphere.

  • (2)

    The generation of NOx is

Acknowledgements

The authors gratefully acknowledge financial supports from the National Natural Science Foundation of China (NSFC, 51176009) and the Fundamental Research Funds for the Central Universities (2013YJS078, 2013YJS069) for this work.

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