California Energy Commission Agreement Number: EPC-14-045
Progress Report – 10/24/2018
Project Title: Advanced Recycling of MSW
Submitted to: Donald G. Taylor, Taylor Energy
Prepared by: Arun Raju, CE-CERT, UCR
Task 6. Analysis and Evaluations
Aspen Plus modeling of Waste Gasification through Power Generation
Feedstock — The average chemical composition of the MSW feedstock and its energy content is given in Table 1. The energy content of the dry-MSW sample is 7,690 Btu/lb (17.9 MJ/kg) in HHV.
Table 1: Feedstock composition
| Proximate | Volatile Matter | 71.1 |
| Fixed Carbon | 14.6 | |
| Ash | 14.4 | |
| Ultimate | C | 43.4 |
| H | 5.6 | |
| O | 35.5 | |
| N | 0.77 | |
| S | 0.26 |
Process Design –The plant is assumed to be located near a landfill or a waste processing facility and the waste material is composed of both organic and inorganic residues. Cost of MSW gathering, loading and unloading and transportation is included in the analysis.
The power generation plant process diagram is shown in Figure-1. The plant includes a feedstock preprocessing area where wet-MSW is dried and shear-shredded according to the gasifier requirements. The MSW is then gasified in the gasification area to produce a medium/high energy content syngas. The raw syngas is cooled and cleaned to remove contaminants and undesired components in the syngas processing area. The power island converts the syngas into electricity using a combined cycle gas turbine or an internal combustion engine depending on the configuration. The plant size is 500 dry metric tons per day of MSW throughput. Except for the gasifier, all technology components such as the feed pretreatment system, syngas cleanup system, and gas turbine/engine are considered mature, and commercially available. The overall gasification process can be summarized as follows, with the pyrolysis step much faster than the gasification.
Pyrolysis: Feed + Heat (400-1200 ºC) → Coke (char) + Liquids (tar) + gases
Gasification: Feed + Gasifying agent + Heat (700-1400 ºC) → Gases (H2, CO…)
+ Minerals (ash)
Figure-1. Flow diagram of the waste to power conversion facility
The key reactions involved are listed below1[1].
Reaction 1 is the hydrogasification reaction which accounts for most of the methane production. Reactions 5 and 6 are combustion reactions, traditionally employed for generating the required process heat by supplying oxygen or air into the gasifier. Reactions 2, 3 and 4 are the steam gasification reactions.
The following subsections provide information on the specific areas of the conversion facility.
Feedstock Pretreatment (Area 100)
This area contains feedstock size reduction and screening steps as well as drying. The feedstock is first transported from storage pile to a trammel screen where stones and dirt are removed after initial category. The feedstock then enters scalping screening after going through an electromagnet to removal the iron metal in the feedstock. Oversized material is filtered out and crushed into small pieces in a crusher and particle size less than 20-mm is conveyed to storage bin with 12 hr storage capacity. The MSW with an initial moisture content of 40% is dried using the tail gas drying system. The material is continuously fed from the intermediate storage facility to a plug screw feeder. The wet feed then enters the dryer via a disc shredder and is dispersed into an atmosphere of hot exhaust. The exhaust acts as a transport gas for the material through a drying duct where the moisture evaporates via direct heat exchange with hot tail gas from the gas engine. The dried product is separated from the exhaust in a high efficiency cyclone and discharged to the gasifier at approximately15-20% moisture by weight.
Gasification (Area 200)
The gasification area hosts the gasifier and accessary equipment that convert the feedstock into syngas. Dried feedstock is transported to a gasifier elevator from a dry MSW storage bin with a 12-hr storage capacity. The feedstock is then delivered into the gasifier from the surge bin. The gasifier is described in the main section of the report. The feedstock undergoes devolatilization as it flows through the gasifier, producing char, higher hydrocarbons and gaseous products. The air supply ensures the combustion reactions that provide the energy for the devolatilization, drying and gasification reactions. A medium BTU content syngas is produced; the ash is collected and removed in cyclones. The temperature of the gasification reactor is controlled by the amount of air fed to the gasifier, feed rate, and other parameters. Cyclones are used to capture escaping fine particles in the syngas and all the solid residues from storage bin are removed for storage and disposal. The carbon conversion at high temperature (about 1350oC) is almost complete; tars and oils are almost completely converted to CO, CO2, H2 and H2O in the gasifier. The raw syngas is finally cooled down in the heat exchanger before it enters gas cleanup unit.
Syngas Processing (Area 300)
This area includes syngas conditioning and cleanup systems. The raw syngas contains particulate matter and other contaminants including ammonia, chlorine and sulfur species that are cleaned up before delivery to the power island. A water-scrubber is used for gas cooling, which also removes fine fly-ash and trace-tars to limit downstream plugging. The syngas is cooled to 40oC to condense the water followed by contaminant removal. The scrubbing water is recycled at a 90% rate and a dehumifier is used to condense additional water-vapor in the gas phase. At last, filter beds filled with sorbent are used if necessary to meet the fuel specifications for the power generation system.
Clean syngas is combusted in an advanced gas turbine based combined-cycle employed for power generation. An alternate option for power generation using a reciprocating gas engine was also evaluated. The exhaust from the gas engine is initially blown to a rotary drier for feedstock drying. The steam in the exhaust is condensed for heat recovery and the tail gas goes to the stack, while the condensed water is pumped into the waste-water treatment system. Power generated is used for plant requirements and for export to the grid for sale.
Process modeling
The process model was developed using the Aspen Plus software. Aspen Plus is a steady-state process simulator that includes extensive thermodynamic data-bases, built-in routines for common unit operations, and the ability to properly handle complex solids including biomass, MSW, and other waste matter. The Aspen simulation is controlled using FORTRAN routines (calculator blocks). For example, when necessary, the various design specifications were modified or fixed to reduce the number of independent variables used in the calculations. The process would automatically adjust those associated variables, i.e., the dependent variables, when the independent input variables were modified by the calculator block or a design specification. This was necessary to reduce unnecessary computing time without loss of overall model accuracy. The major simulation blocks used in the model are listed in Table 2.
Table 2: Major simulation blocks used in the model
| Operation area | Unit operation | Aspen plus model | Specifications |
| A100 | Feedstock screening | Screen | Rigorous simulation of the trommel and scalping screen |
| Feedstock shredding | Crusher | Rigorous simulation of particle size reduction | |
| Rotary dryer | RYield | Rigorous simulation of MSW drying | |
| A200 | Gasifier | RGibbs | Rigorous equilibrium simulation of the product mass distribution based on Gibbs free energy minimization |
| Cyclone | Cyclone | Simplified simulation of ash and char capture | |
| Air blower | Pump | Simplified simulation of gasifier air supply | |
| A300 | Water scrubber | HeatX | Simplified simulation of heat exchange between raw syngas and water |
| Dehumifier | Separation | Simplified simulation of water removal | |
| Filter | Separation | Simplified simulation of dust and contaminants removal | |
| A400 | Gas turbine | Rstoic | Rigorous simulation of syngas combustion |
| Heat recovery | HeatX | Simplified simulation of heat exchanger |
The gasifier is simulated in the model in two blocks: the decomposition and gasification units. These units are based on built-in Aspen reactor blocks and calculate the equilibrium composition in the reactor under the given conditions by means of Gibbs free energy minimization. The model uses the Peng-Robinson equation of state for thermodynamic calculations. The first step is the decomposition of the feedstock, resulting in the production of char from carbon, and gases from the volatile portion. This devolatilization step is followed by the actual gasification, which can be described as the reaction between the carbon in char form and air. The decomposition block converts the non-conventional feedstock such biomass or coal into its basic elements on the basis of yield information using the RYIELD block. The components are then sent to the gasification block (RGIBBS), which calculates the equilibrium product gas composition using the Gibbs free energy minimization approach. The carbon conversion information, feed flow rates and compositions, and the reactor operating conditions are supplied by the user based on existing experimental data. The ash and unreacted char are removed from the reactor in a solids stream and the product gas is subjected to gas cleanup in order to remove contaminants such as sulfur. The power generation system is simulated using a combustion model and heat exchanger units. The model uses of 20-40% excess air with specific operational parameters from commercial systems. A simplified version of the Aspen flow diagram is given in Figure 2.
Fig 2: Aspen Plus flow diagram
Results
The Aspen model was run under a range of process conditions to perform initial sensitivity analysis. The most viable conditions were selected for further optimization with respect to thermal and electric efficiencies. The net energy efficiency was maximized through waste heat recovery and thermal integration of the facility.
The key parameters affecting net plant energy efficiency include the feedstock moisture content, power generation technology option, and thermal integration. The plant energy conversion efficiency ranges from 28% to 47% based on these parameters. The lower range of energy efficiency values correspond to a reciprocating engine configuration with minimal waste heat recovery and little thermal integration. The upper range efficiency corresponds to a combined cycle system with a 20% feed moisture content and optimized waste heat recovery.
The optimal power plant performance data from the optimized model for the 500 TPD MSW (dry basis) throughput facility is given in Table 3. Based on the process simulation results, syngas (CO and H2) volume fraction is 38.5% among the gases in the gasifier outlet with cold gas efficiency of 85.7%. The fraction rises up to 41.6 after steam condensation and the fuel gas goes to the power generation section with energy content of 151.1 Btu/SCF. The overall power generation in the gas engine is 49.1 MW with 46.6 MW export to the grid after the auxiliary loads in the plant. The total plant electricity efficiency (electricity/thermal input) is 45% using a combined cycle power generation system.
Table 3: Power plant performance
| Plant performance | |
| MSW (20% moisture, ton/day) | 625 |
| Air to gasifier (ton/day) | 799 |
| Gasifier operating pressure (psi) | 40 |
| Gasifier exit temperature (oC) | 1200 |
| Gasifier exit gas composition (Vol%) | |
| H2 | 15.94 |
| CO | 22.68 |
| CO2 | 5.69 |
| CH4 | 0.84 |
| H2O | 7.18 |
| N2 | 46.75 |
| Ar | 0.54 |
| Others (C2+, H2S, NO, etc.) | 0.38 |
| Syngas composition to gas engine (Vol%) | |
| H2 | 17.17 |
| CO | 24.43 |
| CO2 | 6.13 |
| CH4 | 0.91 |
| N2 | 50.38 |
| C2+ | 0.41 |
| Ar | 0.58 |
| Cold gas efficiency | 85.7% |
| Syngas energy content (MMBtu/SCF) | 151.1 |
| Power generated | 49.1 |
| Auxiliary load | 2.5 |
| Net power export | 46.6 |
| Plant electric efficiency | 45% |
Life Cycle Assessment
Two of the most important criteria used for the technological evaluation of industrial systems are the total energy consumption and the net emissions of the desired pathway. Conventional methods of evaluation often focus on a limited number of steps in a production pathway and are inadequate in their ability to quantify the “cradle-to-grave” energy use and emissions. LCA models iteratively calculate the energy use and emissions associated with specific pathways using large databases consisting information on various stages of the pathways and some user-specified input values. An LCA of the gasification process for power generation was conducted and the results are given below.
Greenhouse gases. The key GHGs considered by the LCA and their Global Warming Potential (GWP) compared to CO2 are given in the Table below. The GWPs are the 100 year warming potential values published by the Intergovernmental Panel on Climate Change (IPCC) in 2007 and are often referred to as the IPCC 2007 GWPs[2]. The GHG emissions for each pathway are calculated for each GHG and are reported on a carbon dioxide equivalent (CO2e) basis using the GWPs.
Table 4. Global Warming Potentials of the key GHGs
| GHG Name | 100 Year GWP |
| Carbon dioxide (CO2) | 1 |
| Methane (CH4) | 25 |
| Nitrous Oxide (N2O) | 298 |
| Chlorofluorocarbons(CFC-12) | 10,900 |
| Hydrofluorocarbons (HFC-134a) | 1,430 |
Energy use. The categories of energy use are listed below.
- Total and fossil energy used per unit of energy produced for each stage of the power generation process
- Total energy used per MJ of energy produced
- Fossil energy used per MJ of energy produced
- The proportions of types of energy used for each stage of power generation cycle
A number of software packages are available that include extensive databases and ‘pathways’ that can be used to evaluate most of existing technology/pathway options. The Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model is one such model that is widely used in academic studies, especially in the United States. This study is conducted using the CA-GREET 2.0 Tier 2 model (CA-GREET 2017). The CA-GREET model is a modified version of the GREET model consisting of California specific assumptions.
The basic assumptions used in model are listed below:
- Analysis year: 2015
- Feedstock: Baseline-California power mix; Biomass gasification pathway- forest residue
- CAMX grid (California-Mexico grid) mix is regional electricity mix for utility supply
- CA Crude is selected for regional crude oil use
- Natural gas (NG) feedstock is considered as North American (NANG)
- Final product FT Diesel use: passenger car with 24.81 MPGGE
- Process efficiency: Biomass gasification to power: 45%
- Steam/electricity export credits: none
The Well to Pump (WTP) results of the biomass to power life cycle analysis are presented in Table 5. The total and fossil energy use is listed including specific petroleum, coal and natural gas use information. The table shows the emissions of all the major greenhouse gases in CO2 equivalent values. The GHG emission for the baseline case is 105.2 g CO2e/MJ while the GHG emission for the biomass gasification process is 21 g CO2e/MJ. The criteria pollutant emission information is also shown in the table. The results show significant GHG emission reductions compared to the grid mix.
Table 5. Well-to-Pump Energy Consumption and Emissions: MJ or g per MJ of Electricity
| Baseline electricity (CAMX Mix) | Electricity from biomass gasification | |
| Total Energy | 1.08 | 1.47 |
| WTP Efficiency | 48.0% | 40.5% |
| Fossil Fuels | 0.81 | 0.12 |
| Coal | 0.12 | 0.00 |
| Natural Gas | 0.66 | 0.08 |
| Petroleum | 0.03 | 0.04 |
| CO2 (w/ C in VOC & CO) | 97.81 | 12.96 |
| CH4 | 0.263 | 0.039 |
| N2O | 0.003 | 0.024 |
| GHGs | 105.2 | 21.0 |
| VOC: Total | 0.017 | 0.007 |
| CO: Total | 0.107 | 0.040 |
| NOx: Total | 0.148 | 0.051 |
| PM10: Total | 0.015 | 0.014 |
| PM2.5: Total | 0.011 | 0.008 |
| SOx: Total | 0.111 | 0.003 |
| VOC: Urban | 0.003 | 0.001 |
| CO: Urban | 0.026 | 0.007 |
| NOx: Urban | 0.029 | 0.007 |
| PM10: Urban | 0.001 | 0.003 |
| PM2.5: Urban | 0.001 | 0.001 |
| SOx: Urban | 0.002 | 0.001 |
[1] Carbon and Coal Gasification, NATO ASI Series, eds., J.L. Figueiredo and J.A. Moulijn, Martin Nijhoff Publishers, 1986
[2] IPCC 2007, Climate Change 2007: Working Group I: The Physical Science Basis, from https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html.
