Elsevier

Fuel

Volume 96, June 2012, Pages 131-140
Fuel

Optimization and efficiency analysis of polygeneration system with coke-oven gas and coal gasified gas by Aspen Plus

https://doi.org/10.1016/j.fuel.2011.12.050Get rights and content

Abstract

Polygeneration system for chemical and power has been regarded as one of promising technologies to use fossil fuel more efficiently and cleanly. A new optimization method for polygeneration system integration has been proposed in this paper. Element utilization and energy utilization were presented as objective functions simultaneously, parameter variations and technology conditions were performed to investigate the influence of each unit specific operation conditions on the performance of the system by Aspen Plus 2006. The treatment of multi-objective values selection is discussed as well. The new optimization method proves to be effective to solve multi-objective optimization problems. It not only shows technologies and operating conditions needed to be improved obviously but also provides detailed changes of each unit performance during optimization process. With 62.3% energy efficiency, 64.8% element conversion efficiency and 56.6% CO2 + CH4 conversion efficiency, the optimized system shows a better performance in considering of the thermodynamic characteristics, element conversion and environment. And CH4/CO2 reforming process is found to be the key to element conversion and greenhouse gas emissions reduction.

Highlights

► The dual-gas sourced polygeneration system is an optimal coal utilization system. ► One unit with optimal performance does not equal the best whole system. ► It is significant to do the entire optimization for polygeneration system. ► Chemical process and power generation system were integrated seamlessly. ► It realized element reasonable transformation and energy cascade utilization.

Introduction

Polygeneration system synergistically integrates chemical production processes with power generation system, and can overcome the defects of high energy consumption in chemical production plant or power plant [1], [2], [3]. To date, a number of polygeneration systems of various configurations have been proposed [4], [5], [6], [7], [8], [9], [10], [11], and several test projects have been funded for polygeneration plants in the world [5], [12]. However, polygeneration system, as a very complicated one that integrates chemical production processes with a power generation system, the overall performance of which is not at its best due to integrated density being not optimal. Polygeneration system integration and optimization, being still in investigation, is the key to system performance improvement. It is very important to find a better methods to optimize the whole process to improve the system integrated density, and to achieve the system best performance.

Polygeneration systems involve various fields of research, including chemistry, power generation, and even economy. In general, polygeneration systems designed by chemical experts are apt to pay more attentions to the chemical conversions being carried out; they are interested in element efficiency improvement. As a result, the system can obtain high yield but increase much energy consumption in the unreacted gas circulation and conversion [7], [9]. While, polygeneration systems designed by the power generation experts tend towards thermal energy conversion [1], [13], [14], [15]. This method can enhance system energy utilization, but ignorance of the chemical process will inevitably lead to the decline in production capacity, the cost increase of chemical products, and even the decrease in benefit of the whole system. In fact, it was more cost-effective to improve the production of chemical products, the cost of which was reduced due to the integration between chemical process and power generation system [3], [4], [16], [17]. Therefore, if we only follow with interest of chemical conversions or energy conversions, and ignore the actual relationship between the chemical production processes and the power generation unit, the entire system cannot achieve the best performance at all.

Hence, the system integration and optimization needs to focus on both chemical conversion and thermal energy conversion synergistically. It is vital to establish an appropriate objective function which cannot only better reflect the energy and element utilization of the system, even economic performance, but also show the interaction effects between element conversion process and energy utilization process in each unit. In fact, as the key factor, the influence between element conversion process and energy conversion process needs to be understood in order to enhance the performance of the polygeneration system [2].

Currently there is no accurate assessment standard to analyze element use in chemical industry. The researches [18], [19], [20], [21] considered that the evaluation of the elements or materials for a particular chemical industry should reflect resource utilization, the economic and environmental impact of the entire production system to a certain extent. Through the element flow analysis, energy saving and resources utilization of the system can be improved, even sustainable development of economy and environment can be achieved. Generally, analysis and evaluation for energy efficiency is based on energy equilibrium that is the first law of thermodynamics. But in consideration of the energy grade difference among different energy forms, the essence of energy utilization of the system cannot be reflected just from the perspective of energy conservation. The concept of exergy combines the first law of thermodynamics with the second, and takes into account the energy quality and its degradation in real processes. The exergy-based analysis and evaluation methods are widely accepted and used in all kinds of energy system [1], [2], [9], [13], [15], [22], [23].

From previous results to determine the optimal operating parameters [17], we have taken the equivalent yield of methanol and conversion of CH4 + CO2 as the objective functions for optimizing the operation parameters in the reforming unit and DME synthesis unit. The production efficiency of chemical products increased a lot after optimization. However, we lacked the analysis of the whole system optimization, energy efficiency and CO2 emission, and effect of the key units on the system. A single unit with optimal performance cannot give full expression to the overall system, and maximum chemical production yield also does not mean optimal energy utilization and CO2 emission performance. Meanwhile, reforming reaction is strongly endothermic, and energy consumption and conversion of the reforming unit directly determine the system energy utilization and other performance.

Therefore, the whole system optimization, energy utilization analysis, and the effect of reforming unit performance and CO2 emission on the system efficiency were extremely necessary. In this paper, element utilization and energy utilization were presented as the objective functions simultaneously, parameter variations and technology conditions were performed to investigate the influence of each key unit specific operation conditions on the chemical conversion and energy utilization process of the whole system. The objective function and optimization process can be expressed by following equations:F(ξ,η)=ξ=i=1nfi(αi,βiγTEC)η=i=1ngi(αi,βi,γTEC)optimizationξopt=i=1nfi(αi,βi,γTEC)optηopt=i=1ngi(αi,βi,γTEC)optFopt(ξ,η)By studying the element use and the energy use of each unit change with the system operating parameters, the best operating parameters and process conditions were found out to improve the whole efficiency of the system.

Section snippets

Modeling and simulation depiction

The system simulations were carried out by using Advanced System for Process Engineering steady state simulation software (Aspen Plus). Aspen Plus is a process modeling software suitable for a variety of steady state modeling applications. Currently, this software was widely applied in simulating cogeneration plants and even polygeneration system, and the very good agreement between the industrial data and those determinants using the ASPEN models was obtained [24], [25], [26], [27], [28].

The reforming reaction temperature

The increase of temperature improves CO2 + CH4 conversion and element utilization of reforming unit, while also increases its energy consumption. When the temperature rises to about 1000 °C, the reforming reaction nearly reaches reaction equilibrium and the composition of syngas keeps constant, causing little change in element utilization of the synthesis unit. Therefore, element utilization of the system increases with temperature firstly to 52% and finally leveled off, which can be seen in Fig. 5

Conclusions

Through the whole system optimization analysis, the important information that a single unit with optimal performance does not equal the whole system with the best performance can be got, it is particularly significant to do the whole optimization for polygeneration system. After optimization, chemical process and power generation system were integrated perfectly and realized element reasonable transformation, as well as energy cascade utilization. The element utilization of system as high as

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21076136), the National Basic Research Program of China (2005CB221207), and Shanxi Returned Scholarship (2010-2).

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