Article pubs.acs.org/IECR
Equilibrium Modeling of Sorption-Enhanced Cogasification of Sewage Sludge and Wood for Hydrogen-Rich Gas Production with in Situ Carbon Dioxide Capture Vineet S. Sikarwar,†,# Guozhao Ji,†,# Ming Zhao,*,†,‡ and Yujue Wang*,† †
School of Environment, Tsinghua University, Beijing 100084, China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education, Beijing, 100084, China
‡
S Supporting Information *
ABSTRACT: Sewage sludge disposal is troublesome because of the presence of microbes, toxins, and heavy metals in it. Cogasification of sewage sludge with wood is a promising pathway to dispose of sewage sludge and generate usable syngas, simultaneously. By using a sorbent for in situ sorption of CO2, H2 fraction in the generated syngas can be enhanced considerably. An equilibrium model was developed taking wood and sewage sludge as the model compounds and CaO as the sorbent. This evaluation was performed by employing ASPEN PLUS (V 8.8) software. Principle of Gibbs free-energy minimization was adopted to predict the outlet gas composition and gas yield. The impact of reactor temperature (600 to 900 °C) and sludge content (0 to 100 wt % at 700 °C) in the feedstock on syngas yield and constituents were assessed. With 30 wt % sludge, maximum gas yield was observed as 0.526 kg h−1 at 900 °C while minimum CO2 fraction was achieved at 700 °C. At 700 °C, the highest gas yield of 0.251 kg h−1 was recorded at 50 wt % sludge, whereas minimum CO2 concentration was observed at 30 wt % sludge. The model predictions were in good agreement with the experimental findings. The study reflects that CO2-sorption enhanced gasification of sewage sludge with other biomass such as wood is an attractive option to dispose of sewage sludge in an environmental friendly manner and to generate hydrogen-rich fuel gas.
1. INTRODUCTION
resources. Biomass has been researched for decades to extract usable energy and chemicals via numerous processes. Gasification offers an efficient and feasible route to valorize the biomass waste by usable energy production in an environment friendly manner. Biomass is inherently a carbonneutral source. Deployment of a sorbent such as calcium oxide (CaO) to capture CO2 in situ results in an enhancement in H2 production.4 In addition, it makes the biomass a carbon negative resource.5 The basic idea is the removal of CO2 from the reaction system (R2, R3, R4, and R6 in Table 1) as soon as it is formed, which in turn shifts the equilibrium according to Le Chatelier’s principle, resulting in an increase in H2 yield.1
Climate change because of the rise in temperature due to the greenhouse gas (GHG) effect is a constant source of concern for all countries. GHG emissions occur mainly because of fossil fuel combustion coupled with other anthropogenic activities.1,2 Fossil fuels are the primary source of power generation around the globe and therefore suffer from the threat of extinction. More importantly, they are the cardinal source of CO2 emission (∼41%), which is the key GHG responsible for global warming.3 This calls for a shift from conventional petro-fuels to renewable energy resources such as solar photovoltaic, hydroelectric generation, wind, biomass, etc. 1 Biomass exploitation for power generation has an edge over other renewables as its transportation (for example, Bioliq concept) and storage is easier.1 Moreover, it is less reliant upon climate and location, and amply available unlike other nonconventional © 2017 American Chemical Society
Received: Revised: Accepted: Published: 5993
January 21, 2017 April 19, 2017 April 26, 2017 April 26, 2017 DOI: 10.1021/acs.iecr.7b00306 Ind. Eng. Chem. Res. 2017, 56, 5993−6001
Article
Industrial & Engineering Chemistry Research
Lately, cogasification of SS and biomass has gained much attention on account of some inherent benefits.3,13,14 Sludge contains high moisture and ash content, whereas biomass (such as wood) contains high volatiles with low ash and water content. Therefore, cogasifying them compensates each other’s weaknesses.15 Furthermore, SS moisture can be utilized as the gasifying agent. In addition, increase in syngas yield and reduction in undesirable gases are other advantages of cogasification.16 Therefore, it can be assumed that cogasification of biomass and sludge followed by syngas cleaning and conditioning would deliver syngas which may be suitable for basic applications such as heat and electricity generation and advanced applications such as IC (internal combustion) engines, gas turbines, and fuel cells. However, further research is required in this direction. Operational variables such as temperature, pressure, flow, etc. should be adjusted inside the gasification vessel to get the most efficient performance. Optimal conditions are assessed by experimental work which requires time and money. Mathematical modeling has emerged as a partial solution to this problem.17 Models can be developed to gauge a diverse range of operating conditions in a cost and time effective manner and provides quantitative and qualitative data for real-life processes.18−20 Moreover, it is a vital aid in testing several feedstock materials and their behavior in different types of reactors without actually building them. Generally, gasification modeling is carried out via two approaches, namely, thermodynamic (equilibrium) or kinetic (rate based).21 Thermodynamic models help in computing syngas characteristics for given operating conditions for a specific gasifier. While developing an equilibrium model, it is supposed that chemical interactions are taking place for an infinite time.22 Practically, it has been found that while the results reflect the system potential, they can vary considerably from real-life scenarios, thus necessitating a more accurate approach.1 Although kinetic models are more accurate vis-à-vis thermodynamic models, equilibrium models convey a quick idea of the limits of operation and are less computationally intensive in nature. Stoichiometric and nonstoichiometric are the two pathways to generate an equilibrium model.23 The former employs selected independent reactions and equilibrium
Table 1. Significant Chemical Reactions in CO2-Sorption Enhanced Biomass Gasification1,6 equation number
reaction name/type
R1 R2
water gas-I water gas-II
R3 R4
water gas shift methane reforming Boudouard oxidation-I oxidation-II methanation-I carbonation calcination methanation-II
R5 R6 R7 R8 R9 R10 R11
chemical equation C + H2O ⇌ CO + H2 C + 2H2O ⇌ CO2 + 2H2 CO + H2O ⇌ CO2 + H2 CH4 + 2H2O ⇌ CO2 + 4H2 C + CO2 ⇌ 2CO C + O2 ⇌ CO2 2C + O2 ⇌ 2CO C + 2H2 ⇌ CH4 CaO + CO2 ⇌ CaCO3 CaCO3 ⇌ CaO + CO2 2CO + 2H2 → CH4 + CO2
ΔHo25 (kJ mol−1) +131.0 +90.1 −41.2 +206.0 +172.0 −394.0 −111.0 −72.8 −178.9 +178.9 −247.0
Carbonation (R9 in Table 1) takes place at lower temperatures (600 to 750 °C) and captures CO2 to form CaCO3 with heat release. This heat is utilized to drive many endothermic chemical interactions during gasification. At higher temperatures, calcination (R10 in Table 1) takes place to regenerate the sorbent along with a release of concentrated stream of CO2.6 Captured CO2 may be sequestered or may be employed as raw material for further chemical synthesis. The conventional biomass gasification process and the advantages of CO2sorption enhanced gasification over the former are explored amply and can be found elsewhere.1,2,5,6 Sewage sludge (SS) is the solid waste generated from municipal and industrial wastewater treatment plants. The rise in population coupled with urbanization has resulted in a drastic increase in SS generation. The composition of SS is complex due to the presence of microorganisms, biodegradable and nonbiodegradable organics, heavy metals, etc.7,8 Appropriate disposal of SS is necessary to prevent soil, air, and water pollution at varied levels. Suitable pretreatment followed by available cost-effective disposal is paramount to ensure minimal environmental impact.9−11 Usually, sewage sludge is either used as fertilizer or disposed in land-fills. It is also treated by combustion and incineration.12
Figure 1. Concept of CO2-sorption enhanced cogasification of wood and sludge. 5994
DOI: 10.1021/acs.iecr.7b00306 Ind. Eng. Chem. Res. 2017, 56, 5993−6001
Article
Industrial & Engineering Chemistry Research Table 2. Analysis of Wood and Sludge14 wood
sludge
proximate
ultimate
proximate
ultimate
matter
(wt %)
element
dry basis (wt %)
matter
(wt %)
element
dry basis (wt %)
fixed carbon volatile matter moisture ash
15.8 83.1 8.6 1.1
C H O N S
49.97 7.91 40.6 0.36 0.06
fixed carbon volatile matter moisture ash
15.9 15.6 76 68.5
C H O N S
12.99 2.54 16.3 2.37
