European Biomass Conference and Exhibition 2018 ...

26th European Biomass Conference and Exhibition, 14-17 May 2018, Copenhagen, Denmark

ASSESSMENT OF A NOVEL BIOMASS-COAL GASIFICATION-BASED SYSTEM FOR HYDROGEN AND POWER CO-GENERATION INTEGRATED WITH CHEMICAL LOOPING TECHNOLOGY Dumitru Cebrucean, Viorica Cebrucean, Ioana Ionel Politehnica University of Timisoara, M. Viteazu 1, 300222 Timisoara, Romania, tel/fax: +40256403670/403669, e-mails: [email protected], [email protected], [email protected]

ABSTRACT: In this paper, a novel gasification-based system integrated with two chemical looping processes, namely, one designed for H2 production using a three-reactor chemical looping hydrogen generation (CLHG) process and other one for O2 production using a two-reactor chemical looping air separation (CLAS) process was modeled and its performance estimated. In addition, the system integrated a hot gas cleaning unit for the removal of syngas contaminants at high temperature. Aspen Plus process simulation software was employed in the study to design and develop a detailed process model. The results show that the proposed system configuration can achieve better performances than conventional plants, capture almost all of the feedstock carbon, and generate low CO2 emissions. Moreover, the system becomes carbon negative when considering the carbon neutrality of the biomass fuel. Keywords: gasification, hydrogen co-generation, chemical looping, CO2 capture, carbon negative.

1

INTRODUCTION

Fossil fuel combustion is the main source of increased CO2 emission concentration in the atmosphere. Compared to pre-industrial levels, its concentration has increased by more than 40% [1]. Among fossil fuels, coal is the most carbon intensive fuel, being responsible for about 45% of the total CO2 emitted. Most of these emissions are produced by coal-fired plants in the energy sector. The CO2 can be reduced by improving the energy efficiency of the plants, using carbon neutral fuels and/or applying carbon capture and storage technologies. Carbon capture and storage is the only technology that can significantly reduce the emissions of CO2 from any fossil fuel power plant. However, capturing CO2 with existing technologies may lead to significant energy and cost penalties [2,3]. One promising technology is based on the chemical looping (CL) process [4,5]. During this process, CO2 is inherently separated from other combustion products, i.e., N2 and O2, and thus no energy is used for its separation. The CL process uses a metal oxide as an oxygen carrier (OC) to transfer oxygen from the combustion air to the fuel. In this way, the direct contact between the fuel and air is avoided. The CL systems can be fueled with gaseous (syngas, natural gas, biogas) as well as solid fuels (coal, biomass), and can be designed to produce a variety of valuable products [5,6]. In this study, the performance of a novel gasificationbased system integrated with two CL technologies, one designed for H2 production and the other one for O2 production, was evaluated.

2

Figure 1: Simplified block diagram of the proposed H2 and power co-generation system, with CO2 capture, based on gasification and chemical looping technologies

PROCESS DESCRIPTION

2.1 Gasification and gas cleaning For gasification, a Shell type gasifier operating at a pressure of 40 bar and temperature of 1450°C was used [7]. This gasifier type can achieve high carbon conversion and high cold gas efficiency [8]. The gasifier was fed with a mixture of 90% coal and 10% biomass on a heat input basis (LHV). The total heat input of 1000 MW was considered. The properties of both fuels are given in Table I [9,10]. As can be seen, in comparison to coal, the biomass fuel selected for this study (untreated wood) has a lower energy value and higher H2O content, but negligible amounts of S, Cl and ash.

The system proposed for investigation in this study, for simultaneous H2 and power co-generation, including CO2 capture, is mainly composed of: (i) a gasification and gas cleaning section in which biomass and coal are co-gasified and the syngas generated is cleaned under high temperature; (ii) a chemical looping hydrogen generation unit for H2 production; (iii) a chemical looping air separation unit for O2 production; (iv) a power generation block for electricity generation; and (v) a product gas conditioning line for CO2/H2 compression and dehydration. Figure 1 shows a simplified diagram of the investigated process configuration.

699


Table I: Fuel properties

Composition (wt% db) Carbon Hydrogen Oxygen Nitrogen Chlorine Sulfur Ash Moisture (ar) LHV (MJ/kg ar)

Coal

Biomass

71.7 5.1 7.7 1.4 0.3 2.8 10.9 11.1 26.17

49.5 6.0 43.0 0.3 0.04 0.05 1.1 20.0 14.25

sorbent is regenerated by means of a pressurized air stream containing low O2 concentration to avoid the formation of ZnSO4 [15]. From DES, after solids separation, the syngas is passed through a hot gas filter and then expanded to a required pressure before entering the fuel reactor of the chemical looping hydrogen generation unit. The exhaust gas stream from REG, being at high pressure and temperature, is first expanded in a turbine and then sent to a wet limestone-based flue gas desulfurization (FGD) system for SO2 removal. After exiting the FGD, the gases are vented into the atmosphere. 2.2 Chemical looping hydrogen generation (CLHG) The CLHG unit, shown in Figure 2 (adapted from [13]), consists of a fuel reactor (FR) for complete conversion of syngas by means of an OC, a steam reactor (SR) for H2 production from steam after reacting with reduced OC, and an air reactor (AR) for complete regeneration of OC with air. Among OCs, Fe-based ones are the most suitable to perform the process achieving high conversion efficiencies for both the syngas and steam [16-18].

Before feeding into the gasifier, coal is dried to a moisture content of 5 wt% [9]. Biomass is passed through a torrefaction process, operating at 260°C, during which its moisture content reduces to 3 wt% and energy density improves [10]. After pre-treating and grinding, biomass and coal are mixed and fed into the gasifier by means of a transport medium. In this work, a fraction of the captured CO2 product stream is used for this purpose. The reason of choosing CO2 instead of N2, as usually considered in dry-feed gasification systems [8], is to avoid the presence of non-condensable N2 in the captured CO2 product. Several studies investigating gasification-coupled chemical looping systems showed that the use of N2 for solid fuel transport can significantly contaminate the final CO2 product stream making it unsuitable for transport and storage [11-13]. In the gasifier, the fuel mix is gasified with O2 and steam producing a gas mainly composed of CO and H2 (>85 mol% CO+H2). The oxidant (O2 purity of 96.3 mol% the rest being water) is produced using an innovative chemical looping process specifically designed for air separation (discussed in Section 2.3). The oxidant product is then compressed to 1.2 times the gasifier pressure before entering the gasifier. After compression and water condensation, the concentration of O2 in the stream at the gasifier inlet increases to 99.8 mol%. This purity is significantly higher than that usually produced by conventional cryogenic-based air separation units and used in IGCC applications [9,14]. The hot syngas leaving the gasifier needs to be cooled to a required temperature before treating for contaminants. First, the syngas is quenched to 900°C by means of some syngas recycled from downstream. After quenching, the syngas is cooled in a heat exchanger to about 525°C. It is then split in two streams with one sent directly to the gas cleaning unit while the other one is further cooled in an additional heat exchanger to about 350°C and used as a quench gas in the process. The heat generated from syngas cooling is recovered by producing high pressure steam. The syngas, after cooling, is cleaned in a so-called hot gas cleaning unit in which particulates, sulfur and other contaminants are removed at high temperature. Here, in this study we considered that after particulates removal using, e.g., a ceramic filter, the syngas is sent to a hot gas desulfurization (HGD) unit for the removal of H2S and COS by means of a zinc-based sorbent. The HGD unit is comprised of two reactors, one for syngas desulfurization (DES) and the other one for sorbent regeneration (REG). During the desulfurization process, at 550°C, both the COS and H2S are almost completely removed by reacting with ZnO. After that, the loaded sorbent (ZnS) is sent to REG where at 750°C the

Figure 2: Schematic of the Fe-based CLHG unit The clean syngas stream, after leaving the HGD unit, is fed into the FR where it is fully oxidized to CO2 and H2O by means of Fe2O3 metal oxide. The FR is designed as a moving bed reactor with a countercurrent gas-solid flow patter. The solid particles are introduced from the top of the reactor while the gas stream from the bottom of the reactor. The reactor operating conditions are as follows, temperature of 900°C and pressure of 30 bar. After fuel oxidation, the reduced OC, in the form of Fe and FeO particles, exits the FR bottom and transferred to the SR for H2 production via the steam-iron reaction. The SR is also designed as a countercurrent moving bed reactor and operates at 750°C and 30 bar. Under these conditions, the steam is reduced while the OC is partially oxidized exiting the reactor mainly in the form of Fe3O4. These particles are further sent to the AR where they are fully regenerated to Fe2O3 by means of

700


pressurized air. The AR operates at a temperature not exceeding 1200°C and pressure of 30 bar. The outlet stream from the AR is passed through a cyclone where the solids are separated and then recycled back to the FR, thus completing the cycle. After solids separation, the gaseous stream (depleted air) is first expanded in a turbine before sending to a heat recovery unit. A fraction of the depleted air stream, after heat recovery, is withdrawn and used for fuel drying purposes. The other two gas streams leaving the process, namely, the hot exhaust gas (CO2/H2O) coming out from the FR and that (H2/H2O) from the SR, after heat recovery, are sent to a conditioning unit where most of the water is removed through condensation. Both, the CO2 and H2 product streams are then sent to the compression unit. A small fraction of the CO2 stream is extracted from the captured product and used as a solid fuel transport gas to the gasifier.

2.4 Power generation As mentioned, the hot gas streams exiting the CLHG unit, namely, the CO2 containing stream from the FR, H2 stream from the SR, and the depleted air stream from the AR after expansion are all sent to a heat recovery steam generator (HRSG). The high pressure, high temperature steam generated in the HRSG, and that resulted from the syngas cooling, is sent to a steam turbine system for electricity generation. In addition, some power is also produced by air and syngas expanders.

2.3 Chemical looping air separation (CLAS) In this study, the O2 needed for gasification is produced using a chemical looping air separation process, which is more energy efficient than conventional cryogenic-based air separation technology [6]. Using a two reactor configuration, one for oxidation (OR) and other one for reduction (RR), and a suitable OC circulating between them, under certain operating conditions a high purity O2 stream can be generated. One of the OCs that proved to be suitable for O2 production in the CLAS process, and selected for this work, is that based on Mn metal oxide. As shown in Figure 3, the OC in the form of Mn2O3 enters the RR where it releases O2 in the presence of an inert gaseous medium. Steam is used here to reduce the equilibrium partial pressure of O2 in the reactor, and thus promoting its release from the OC. The RR operates at atmospheric pressure and temperature of around 855°C. The exhaust gas from the RR, mixture of O2 and steam, is then passed through a condenser to obtain almost pure O2 stream (96.3 mol%). This stream is further compressed before finally using in the gasifier. The solid stream exiting the RR, i.e., Mn3O4, is sent to the OR where air is fed into the reactor for metal oxide regeneration. The OR operates at a higher temperature than RR. It was assumed in this study that the heat required for the endothermic reduction reaction is provided by hot solids from the OR and steam introduced into the reactor, and thus, no external energy is needed.

3

2.5 CO2 and H2 compression The CO2 and H2 product streams generated within the process, after water removal, are pressurized using multistage compressors with inter-cooling. The final delivery pressure for the CO2 product is set to 110 bar [14] while that for the H2 product is 60 bar [5].

MODELING AND SIMULATION

Aspen Plus simulation program was employed in the present study for modeling and assessing the performance of the proposed system configuration. Two property methods were selected: PR-BM for processes involving gas/solids and STEAM-TA for water/steam flows. For three unconventional components included in the process, i.e., coal, biomass and ash, HCOALGEN and DCOALIGT models were used to calculate the enthalpy and density, respectively. The components list was further completed with solids (e.g., C, ZnO, Mn2O3, Fe2O3, etc) and conventional type elements (e.g., CO, CO2, H2, H2O, etc). Main unit operation blocks used in developing the process flowsheet were as follows: RGibbs were used to model the gasifier, CLAS and CLHG reactors; RYield blocks were necessary for the decomposition of both solid fuels; RStoic reactors for coal drying and to model the HGD reactors; Compr to model compressors and turbines; MCompr were used as multi-stage compressors; and Heater for cooling and heating purposes. The assumptions used for the modeling of the CLAS and CLHG processes are summarized in Table II. In Ref. [7] additional data required for designing of the whole system can be found. Table II: Key modeling assumptions Parameter CLAS: OC RR operating temperature (°C) OR operating temperature (°C) Operating pressure (bar) Steam/OC in RR (kg/kg) Air/OC in OR (kg/kg) CLHG: OC FR operating temperature (°C) SR operating temperature (°C) AR operating temperature (°C) Operating pressure (bar) Excess OC in FR (%) Excess air in AR (%)

Figure 3: Schematic of the Mn-based CLAS unit

Value Mn2O3 ~855