Biogas Conversion to Dibutyl ether and Dimethyl ether supporting ETS

1.1 Content

1 Biogas, Biomass Gas, Carbon Dioxide Conversion to Dimethyl Ether and Dibutyl Ether based on Emission Trade System
1.2 Biogas Plant (Structure, plant and properties)
1.3 Biogas (Phases of Gas Production)
1.4 Gas Components Biogas
1.5 Gas – Measurement
1.6 Chemical Reactions in the Dynamic System
1.6.1 Biogas Plant – Dynamic Model
1.6.2 Dynamic Model of the Bioreactor
1.7 Combustion biogas in gas engine
1.7.1 Gas Engine
1.8 Take Over Biogas
1.8.1 Low pressure compressor
1.8.2 Biogas Preparation
1.8.3 Biogas Scrubber
1.8.4 Activated Carbon Filter
1.8.5 Gas Drying
1.9 Deep Gas Filter
1.10 Biogas Conversion to Dimethyl ether (DME)
1.10.1 Dry Reforming
1.10.2 Steam Reforming
1.10.3 Methanol Synthesis und Dehydration
1.10.4 Summary pathways to dimethyl ether
1.10.5 Two-step process toward DME
1.10.6 One-step process toward DME
1.10.7 Biogas conversion to DME
1.11 Dimethyl ether (DME)
1.12 Reduction of Greenhouse gas (GHG)
1.13 Fossil Fuel compared with dimethyl ether
1.14 Dimethyl Ether and Emission Trade System (ETS)
1.14.1 Application to a bus company
1.14.2 Application to transport company (heavy trucks)
1.15 Conclusion
1.16 Symbols and Short Cuts
1.17 Drawing and Pictures
1.18 Table
1.19 References
1.19.1 Companies

1 Biogas, Biomass Gas, Carbon Dioxide Conversion to Dimethyl Ether and Dibutyl Ether based on Emission Trade System

Johann Gruber-Schmidt

Abstract: In this short report we describe the biogas conversion to dimethyl ether. The generation of biogas in a structured biogas plant based on biomass wet biomass. The simple model of a bioreactor helps to build up a simple dynamical model for predicting the biogas generation. In this chapter the gas components of biogas will be described including pollution and additional components beside methane and carbon dioxide. After collecting the biogas gas, the cleaning, the preparation and the conversion to dimethyl ether is the next step. The conversion of biogas to dimethyl ether (DME) is also known as liquefaction of biogas gas, if we use biomass biomass often called biomass to liquid fuel. Additional the necessary and sustainable property of Zero Emission is described and analyzed. The main influence on the ETS ( = emission trade system ) is analyzed and shown.

Keywords: Biogas, Gas Cleaning, Gas Preparation, Methanol Synthesis, Dimethyl Ether, Dibutyl Ether

1.2 Biogas Plant (Structure, plant and properties)

Biogas Plants and Biogas technology today is well-known and standard. The idea behind biogas plants is to build up a large fermentation reactor system similar to the stomach of a cow. [ 25 ]

Whether plastics no inert parts can be used for conversion to biogas gas. The composition of biogas mainly consists of methane and carbon dioxide. The generation of biogas is connected with the existence of water, microbial bacteria and nutrients. The water collected in the biogas plant and returned to the biogas plant is called leachate. This makes sense, because so the nutrients and microbes can be saved.

Modern biogas plants consist of storage systems for the substrate, substrate preparation, where the leachate is collected and the leachate cannot drain into the ground water. The leachate is supported by a leachate piping network into the digester.

The digester, where the fermentation and conversion to biogas takes place, and a storage for the digestate. [ 25 ]

Every section of the biogas plant is divided into structures connected to the gas collection pipes. The gas will be sucked out of the digester. The gas pipes are lead to collectors and the collectors are connected with pipes leading the biogas out of the biogas plant. This marks the limit of the biogas plant. In most cases biogas is burned in gas engine to generate electric power and thermal heat, well-known as combined heat process (CHP).

Understanding the biogas plant as a bioreactor biogenic biomass in combination with water and microbes are needed. Supporting the model of a bioreactor leads to the demand of preparing separating and recycling parts like plastic, metals or sand. In the end the aim should be to have inert material lower than 5%. Under these conditions the biogas plant is nearly a perfect stationary bioreactor with a high efficiency generation over a long-time range biogas gas. [ 24 ]

The biogas plant consists of digester closed with a double flexible membrane to collect the biogas and to provide a high density of the digester to the environment.

Biogas plants from former times are acting as a black box. Measuring the biogas volume flow and the gas composition, and assuming the volume of the biogas plant enables assuming the possible biogas production. Because of the structured design and the definite types of biomass and biogenic biomass composition there is the possibility to control all parts of the biogas plant, to optimize the operation and to operate on a high safety level.

A modern and new biogas plant designed as a bioreactor is very simple in erection, easy in handling and operation. It includes the storage of the substrate, the preparing of the substrate and the microbial fermentation process converting biogenic substrate to biogas gas. The biogas plant is well structured and supported with a piping network.

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Figure 1: Simple structure of a Biogas Plant operating on the basis of wet fermentation as shown in the project Hagenbrunn [ 2 ]

According to the observation and measurements the composition of the substrate used in the biogas plant is given by: (first classification)

Storage for as long time:

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Samples: maize straw, sugar sorghum straw, corn straw, horse manure

Structure: concrete open silo [ 24 ]

Storage for a short time:

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Samples: vegetables, biomass food, potatoes, onions, …

Structure: Tanks, pumpable [ 24 ]

The better the structure of the biogas plant is done, the more we know about the biogenic mass and mass distribution and substrate type used in the digester, the better the gas generation can be controlled and optimized. Under this condition we can reduce the risks for the environment.

Under recycling and separating the municipal biomass into recycling streams and biogenic biomass with a very low content on inert material, the biogas plant is acting like a stationary bioreactor. Inert material can be separated in different material streams again. One advantage is the reduction of the biogas plant volume, the other advantage is the reduction of hazard biomass in the biogas plant and on the economical side the operating costs of an advanced biogas plant are very low. The advanced biogas plant in combination with recycling and separation leads to an optimized gas generation.

1.3 Biogas (Phases of Gas Production)

The typical phases of gas production inside the biogas plant have been studied over long years and the typical phases can be described for the continuous reactor:

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The duration of the different phases can be measured and qualitative empirical described in the time measure “day”: (estimation, depends on the type of substrate and the preparation):

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The gas production of the phase II and IV are the highly productive phases and are in main interests.

1.4 Gas Components Biogas

The gas composition of biogas and the volume flow depends on the different phase of the biogas plant. The design and the calculation for the conversion of biogas can be taken from the phase III and IV. Important is the gas mixture of methane and carbon dioxide, mainly methane.

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Additional additive components produced during the process of biogas gas, which are also sucked out (additives are higher than pollution):

Non-methanogenic organic components 0% up to 0,25%

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Pollution observed and measured in biogas depend on the substrates used:

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Inside the biogas side the operation temperature is varying in the range of 37°C up to 57°C. The lower temperature enables the growing of the microbial bacteria, the higher temperature supports the chemical conversion reactions. The biogas is taken out of the biogas plant with a vacuum compressor, inside the biogas plant a small vacuum pressure (p ~ 25 mbar) is given, after the vacuum compressor a higher pressure (p ~ 1.5 up to 2 bar) is given. Cooling down the biogas gas, the steam changes the phase and is removed as condensate. Therefor a condensate trap has to be used. The gas components methane and carbon dioxide define the lower caloric heat value, the additives and pollution define the cleaning and preparation of biogas gas.

1.5 Gas – Measurement

The gas measurements on the biogas plant are necessary and needed to control the conversion to biogas to optimize the efficiency and the production of biogas gas. The measurements include the concentration of methane, carbon dioxide, and nitrogen. Additional the organic sulfur compounds, the chlorine compounds and fluorine compounds and the halogen organic compounds. The measurement is done continuously.

Beside the main parts of the biogas the pollution is important for the emission to the environment a continuous measurement is needed: sulfur compounds, fluorine hydrogen compounds (HF), chlorine hydrogen compounds (HCL), siloxane compounds (X-Si-X), volatile organic carbon compounds (VOC).

The pollution in the biogas observed and measured is given by benzene, toluene, siloxane, sulfur hydrogen, and odor compounds. The pollution is important for the conversion of biogas to electricity and heat and fuel. The quality of the fuel is an important property and has to be reached during the conversion process and it has a deep influence on the costs and the plant components. These measurements are not needed continuously – it is needed discontinuous.

Converting biogas to heat, it will be burned in a combustion chamber, measurements of fluorine, chlorine, sulfur, carbon monoxide, nitrogen oxide are done continuous. Additional measurements have to be done: dioxin, furan, polycyclic aromatic hydrocarbons (PAK), polycyclic halogen aromatic hydrocarbons (HPAK). To reduce the emission to the environment, we have to use exhaust gas cleaning and preparation, in some cases high temperature combustion is needed.

Converting biogas to the liquid fuel dimethyl ether demands a definite purity of the biogas according the ISO 16 618:2013, ASTM 7509-14A und ASTM 7509-14B defines the allowed impurities and pollution in the biogas. [ 31 ]

1.6 Chemical Reactions in the Dynamic System

To understand the production off biogas the main components are listed up in simple chemical reaction equations:

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For the biogas the following components are important: water, organic compounds (cellulose) and carbon. The chemical reactions show the production of carbon dioxide and methane. These simple equations give a first inside look about the processes taking place in the biogas plant. The main problem for every biogas plant is the knowledge of the distribution on the organic and biogenic biomass and the water distribution (we often speak from load in the digester: 2 up to 4 g/l). Water is the main important compound, therefore in all model the solved carbon in water the so called aqueous carbon is needed. This carbon is the main compound for the generation of methane and carbon dioxide. [ 24 ]

According to the measurements and observations some useful design values and construction values can be listed up:

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The caloric heat value (lower) (LHV [kJ/kg; kJ/Nm ³; kWh/kg; kWh/Nm³]) of different fuels compared with each other from [ 20 ]:

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Biogas is also a lean gas. A lean gas is a gas with a very low caloric heat value in the range of 1,5 kWh/Nm ³ up to 6 kWh/Nm ³. It is well known to burn lean gas in a combustion chamber. Another possibility is the conversion to liquid fuel. This development is supported by low costs in the operation of a biogas plant, and low costs for the biogas itself. The costs of the produced biogas are the sum of costs for opening a biogas plant, the leachate piping grid, the gas suction piping grid, the safety components of the plant, the preparing of the recycled leachate. Additional the operating costs like personal, maintenance is added. It makes sense to separate the biomass and to structure the biogas plant, so that the optimum gas generation can be achieved for a long-term rate. The costs of the generated biogas influence the costs for the liquid fuel. At this point we have to mention the Zero Emission property again increasing the efficiency and sustainability by using the generated carbon dioxide as a raw material for the liquid fuel.

1.6.1 Biogas Plant – Dynamic Model

The Biogas model is a very simple but effective model, with a high empiric part. The model is based on a system of differential equations and predicts the methane generation:

The model is a result of practical experience. The idea is to make an assumption of the DDOC (decomposable organic carbon) based on the measurements, the observations of existing biogas plants. At least the measurements of the concentration of the biogas and the volume flow of the last operation year have to be taken into account. Under this condition two data sets have to be combined the actual operating year and the operation year ago. With this simple model the unknown structure of biomass distribution and biomass types can be neglected. The result helps to gain first hints for the production of biogas and the running time of the biogas plant.

Beside this simple model there have been developed a lot of models but all models suffering on the lack of data and information of the biogas plant and structure lead to results with high uncertainty.

The result is: if the biomass is prepared and separated and the structure of the biogas plant is designed according to the biomass type, the biogas plant will act like a large stationary bioreactor with large volume, a small surface and the gas generation can be optimized. Influencing the water content, the microbial distribution, the nutrient and the biogenic biomass type, the gas generation will lead to high volume flow with high methane concentration.

1.6.2 Dynamic Model of the Bioreactor [ 22 ]

A dynamic model for the calculation of the generation of biogas is based on the continuity equation, the energy equation, and the growing equation, well known in the classical model of Monod. Substrate S, Product X and the saturation constant K, decay rate m:

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As described before the solid carbon in biomass, organic biomass and biogenic biomass is solved as aqueous carbon in water. This liquidated carbon is important for the generation of methane and carbon dioxide: continuity on the solid carbon mass, and the chemical reaction equation for the aqueous carbon are given:

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Continuity:

Solid organic carbon:

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Continuity:

Liquid organic carbon:

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Dynamic biogas model:

Acidogenic Biomass:

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Methanogenic Biomass:

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Acetate:

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Methane:

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Carbon Dioxide:

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This simple dynamic model gives a first inside in the generation of methane and carbon dioxide.

The biogenic part describing the microbial and the growing equation are given by the following parameters: Ks, Kd, m und Y, describing the microbial fermentation processes.

The other part of the model describes the conversion of the solid carbon Cs in solved aqueous organic carbon Caq. The aqueous carbon Caq is converted into acidogenic carbon Cxa and in acetate Cac. The carbon Cac is converted into Cxm methanogenic carbon. The generation of methane CCH4 is done from Cxm. The generation of Cco2 is based on Cxm and Cxa. This is the simple flow diagram described by the set of differential equations. [ 24 ]

In the following part some basic simple chemical reaction is listed up which can occur during the anaerobe fermentation. The process starts with cellulose. The cellulose is converted into glucose. Glucose is converted into carboxylic acids (acetic, propionic, butyric). The acetic acids are the basis for methane and carbon dioxide generation, additional from hydrogen and carbon dioxide methane and water are generated. These simple reactions show a simple system of chemical reactions inside the biogas plant.

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The physical Process describing the diffusion process inside the biogas plant from the solid parts to the gaseous and liquid parts, and inside the gaseous and liquid parts are in reality not known. For every simulation we need the distribution of the solid parts and the sizes and the material types. In normal we know nothing about these parameters. Because of the lack of information on the distribution of the solid parts, also the heat conduction inside the biogas plant is not known. Only measurements of the temperature give an average temperature inside the local biogas plant structure. Diffusion and heat conduction cannot be simulated in detail because of the lack of data and information. [ 24[ 25 ]

Based on the equation of the simple dynamical model measurement in the temperature distribution, measurements in the gas volume flow and gas components mixture, enables to use the evaluation model according to the VDI 2180: 2015-1. [ 22 ]. With the model equation the measured parameters can be evaluated, whether they are confident or no, whether the model predicts in the right way. To start the evaluation the following empirical parameter can be used to start the simulation:

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Table 1: Table of measured and empirical values of the bio kinetic parameters for the generation of methane and acetogens. (Source: [ 24 ])

The advantage in using a set of nonlinear time dependent differential equation is the set of parameters and variables, which can be compared with the measurements data done on plant. Now we see the advantage of a simple set of equations compared with an empirical assumption and measurement of the biogas plant. The general form of the above equations is given by

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If the set of equations possesses a solution we can write

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Linear System by evaluating the Jacobian Matrix F:

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Now we can apply the idea of solving a nonlinear set of equations with measurement values and probabilistic deviation values. We combine the stationary nonlinear set of equations and the boundary conditions (BC):

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The measurement leads to the deviation of the exact solution x and the measurement µ:

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In the probabilistic theory we have the expected value E(x) and the variance Var(x) and the covariance of the values x: Cov(x):

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With this basis we can now calculate the confidential values and detect the failures and fault in the measurements by solving a minimizing problem with Lagrange factor: [ 22]

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The solution of the minimizing problem is given by the Lagrange factor l, deviation vector a, the Covariance matrix S v

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The empirical Covariance matrix S x is calculated from the measurements and the boundary conditions. Applying the described method from above, we can calculate the deviation vector a, which shows the new variable values and parameter values and the changing of the values during the operation. [ 22 ]

A detailed description is given in the VDI 2048-1:2015. It looks very complicated, but in practical it is very east to be applied at every dynamical system, and it enables to verify the reliability of the measurements and the assumed values in the dynamical system and process. [ 22 ]

If we compare the empirical model Biogas Plant with the simple set of differential equation shown above has the advantage to combine measurements with physical and chemical models, we additional observe that the empirical model can be applied to nearly all biogas plant. For biogas plants modelled by a bioreactor, the coupled set of dynamic equations can be applied.

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