Inside VMG | September 2014
Tips and Tricks: Estimating Heating Values for Gasification Waste-Derived Feedstocks in VMGSim
By Herbert Loria - VMG Calgary
Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide. This is achieved by reacting the material at high temperatures (>700 C), without combustion and a controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas (synthesis gas or synthetic gas) and is itself a fuel. The power derived from gasification and combustion of the resultant gas is considered to be a source of renewable energy if the gasified compounds were obtained from biomass.
There are a large number of different feedstock types for use in a gasifier, each with different characteristics, including size, shape, bulk density, moisture content, energy content, and chemical composition. Coal and petroleum coke are used as primary feedstocks for many large gasification plants worldwide. Additionally, a variety of biomass and waste-derived feedstocks can be gasified, for instance: wood pellets and chips, waste wood, plastics, aluminum, and Municipal Solid Waste (MSW).
The heating value (lower or higher) is a physical property that is necessary to calculate the ideal gas enthalpy of formation of the coal, which is a parameter of high importance when performing energy calculations around gasification unit operations.
In the case of biomass and waste-derived feedstocks, the heating value (lower or higher) is often unknown and must be estimated if gasification simulations involving this type of feedstock are desired. In order to overcome this problem, a new procedure for the calculation of heating values for biomass and waste-derived feedstocks has been developed and applied in VMGSim. This implementation is based on the procedure 14D1.1 from the American Petroleum Institute (API) Technical Handbook.
Estimation of heating values
The procedure described in this section is applied to characterize a waste-derived feedstock when its lower or heating value is missing. To use this feature, go to the Thermo Model form of VMGSim and select Gasification (2010) as the Thermodynamic Model and then click on the Coal button to access the Coal Characterization form.
Then, add a name to the gasification hypo, type 10,000 as the molecular weight (MW) and select the Estimated option in the Heating Value Type dropdown list.
The next step is to enter the chemical analysis of the waste-derived feedstock in the Input Composition frame.
Once the Input Composition is entered, click on the Characterize button and VMGSim will estimate the Higher and Lower Heating Values for the waste-derived material. The results are shown in their respective boxes from the Coal Characterization form. Finally the gasification compound can be installed in the active property package by clicking on the Install Coal button.
The previous estimation procedure has been validated with experimental data from different types of coals, biomass, and municipal waste.
The following table shows the average absolute deviations (AD) between the experimental and estimated higher heating values (HHV) of different types of solid feestocks from gasification style plants.
|Coal A, BTU/lb||11930||12070||1.2%|
|Coal B, kcal/kg||6630||6710||1.2%|
|Dry wood chips, kcal/kg||4780||4450||6.9%|
|Wet wood chips, kcal/kg||2470||2450||0.8%|
|Coal B and wood chips, kcal/kg||4960||4910||1.0%|
|Municipal waste, kcal/kg||3290||3090||6.1%|
|Tires with steel belts, kcal/kg||7930||7890||0.5%|
|Municipal waste and tires, kcal/kg||3790||3660||3.4%|
The previous table clearly show how the proposed estimation method produces values in close agreement to the experimental data (less than 10% of AD and in some cases less than 1%). In addition, these results show the capability of the procedure to predict acceptable values for solid feestocks of different nature and composition.
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