Biogas and Dimethyl ether are providing water, fertilizer for an intelligent smart soil

Biogas and Dimethyl ether are providing water, fertilizer for an intelligent smart soil

Johann Gruber-Schmidt

Introduction

Biogas is well known in agriculture and food industry. In Austria we realized a project Hagenbrunn [ 3 ] nearby Vienna/ Austria, a biogas plant running on waste food and liquid biogenic waste. The electric power output is given with 1320 kWele, the thermal heat generation is given with 1800 kW thermal heat realized with warm water (95°C/60°C), the substrate feed is given with 25 000 t/year, and the digestate coming out of the biogas plant is 35 000 t/year. The biogas generation of the different substrates, like waste food, grass, potatoes (e.g.) is done by measurements, testing and calculation and leads to the biogas generated by fermentation in a range of 700 Nm³/h up to 800 Nm³/h, with a methane concentration of 50% up to 65%. The biogas plant Hagenbrunn [ 3 ] has a deep influence on the way of irrigation, the way of fertilizing in agriculture and vine culture. The biogas plant Hagenbrunn [ 3 ] has a deep influence on the regional jobs, the regional companies, it is acting like a knot and center of competence and initiating a lot of spin offs. The main advantage of the biogas plant is using biogenic waste as input substrate and therefor the communities are glad to have a sink using and converting waste food and liquid biogenic waste to biogas. Biogas from the plant is only an intermediate step. In the first realization step biogas is converted to electricity and heat.

In the next realization step the biogas plant was enlarged with a preparation of the digestate, to distilled water, solid particles and fertilizer, and the gasification of biogenic solids and the conversion of syngas from gasification of the biogenic solids, biogas from the biogas plant, and the waste biogas from the closed digestate tank to generate syngas with steam gasification and producing dimethyl ether. Dimethyl ether is stored in two tanks, with a volume of 30 000 liters, and a filling station of mobile movable bottles substituting LPG (liquid propane gas) by Dimethyl ether. Heavy tank trucks are transporting Dimethyl ether to the clients.

The production of dimethyl ether in Hagenbrunn [ 3 ] is given by 800 L/h, using 10 000 t/year biogenic solids, 80 000 m³/h waste gas (CH4=5%, CO2=95%) from the digestate tank of the Biogas plant, and 400 m³/h biogas (CH4=50%, CO2=50%) from the biogas plant. Additional heat from the CHP engine is used for drying the solids biomass to a moisture lower than 10%. The electric power needed for the production of dimethyl ether, for the drying process, and the generation of fertilizer and water from digestate is produced by the gasification plant. Now the Biogas plant now can convert liquid biogenic waste and solid biogenic waste and can so take over the waste from the region.

Biogas Plant

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Figure 1: Simple model of a Biogas Plant (Source: Johann Gruber-Schmidt, 2017)

A biogas plant consists of a substrate storage for at least a six months operation, buffering the different mass flow of substrate during the operation of a year. Additional we have a preparation of the substrates increasing the surface, and to prepare for the fermentation in the digester. Because of the wet process we have to mix the substrate with the fluid circulating in the plant between the digester by a pump (often called central pump). For the biogas process we have to understand biogas processes operating on waste, like food waste, manure, and substrates like corn, maize, beet. The process ends with a digester and the with a storage of the digestate (end product of the fermentation process). In input substrate is defined by the gas production of the fresh mass (measured, tested or calculated by formulas (Brick, Schumann)), the anaerobic fermentation process, and at the end to get back the digestate. The biogas resulting form the digester is collected and burned in a CHP engine to produce electricity and heat. But nobody needs electricity and heat, additional the earn of electricity and heat is very small, the economic situation becomes very bad. To get a feeling about the dimension, area, input, output, efficiency we have: electric power output: P=500 kWele, input feed: 17 500 t/year wet fresh substrate, output 15500 t/a digestate, generated heat Qth= 700 kWth, generated biogas 300 Nm³/h (50% CH4, 50% CO2), the agricultural area needed for supporting the biogas plant with waste is at least needed with 200 ha area. The area needed for the biogas plant itself is about 1 ha. With this correlation we plan and design the water consumption, water storage, fertilizer and soil needed. Additional the digestate resting in a tank is producing waste biogas (5% CH4, 95% CO2) with 40 000m³/year.

Dimethyl ether

Dimethyl ether is well known. It is the simplest ether consisting of two carbon atoms, one oxygen atom and six hydrogen atom, molar mass MZ~ 46 g/mol [ 11 ]. It is certificated by ISO 16 681: 2013 by the IDA. At a pressure of 6 bar Dimethyl ether is in liquid phase at an environment temperature of 25°C [ 11 ]. The simplest ether is synthetic and can produced with two pathways: the production in two steps in the first step over the intermediate product methanol ( methanol synthesis ) Abbildung in dieser Leseprobe nicht enthalten and in the second step over dehydration ( water removal ) Abbildung in dieser Leseprobe nicht enthalten, or in the direct synthesis in one step Abbildung in dieser Leseprobe nicht enthalten. The difference between both chemical processes is the energy (heat generated and needed) the resulting mass flows generated by the processes. The caloric heat value of dimethyl ether is given caloric combustion enthalpy Hc=1460 kJ/mol [ 11 ], the formation standard formation enthalpy Hf=184 kJ/mol [11]. The combustion of dimethyl ether in diesel engines leads to nearly no soot, dust, a reduction of carbon monoxide and nitrogen oxide [ 10 ].

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Figure 2: Biogas Plant and Syngas Reformer (Source: Johann Gruber-Schmidt, 2017)

The convertion of biogas to dimethyl ether can be done in two pathways. One pathway is steam reforming of biogas, which leads to a snygas consisting of carbon monoxide and hydrogen and carbon dioxide (SR = steam reforming reaction )Abbildung in dieser Leseprobe nicht enthalten, or in combination with more steam ( WGS = Water gas shift reaction) to carbon dioxide and hydrogenAbbildung in dieser Leseprobe nicht enthalten. The second pathway is dry reforming of biogas to carbon monoxide and hydrogenAbbildung in dieser Leseprobe nicht enthalten [ 1 ]. In both processes we need heat, generated from the biogas itselfAbbildung in dieser Leseprobe nicht enthalten. The gained syngas is then converted to dimethyl ether. The standard process of convertion of syngas to dimethyl ether consist of a gas compression up to 50 bar till to 100 bar, the reduction of carbon dioxide by cooling down the syngas and condensation of carbon dioxide and storing in a tank, the convertion of the syngas to methanol in one reactor, with recycling of the unconverted syngas and condensation of water and methanol mixture [ 1 ]. The condensate mixture is separated with distillation into process water and methanol. The methanol is dehydrated to dimethyl ether and the condensate mixture of dimethylether, water is separated with distillation into process water and dimethyl ether.Abbildung in dieser Leseprobe nicht enthalten [ 1 ]

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Figure 3: Biogas Cleaning and Preparation and Syngas Reformer (Source: Johann Gruber-Schmidt, 2017)

Combination of processes

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Figure 4: Biomass Plant and Syngas Reformer (Source: Johann Gruber-Schmidt, 2017)

To increase the efficiency of a Biogas plant the biogas plant is combined with a gasification plant. Such as gasification plant is realized in the project Traismauer in Austria [ 2 ]. The gasification plant converts solid biomass to weak gas mainly consisting of CO=20%, H2=23%, CH4=1%, CxHy=3%, Rest CO2, with a heat caloric value Hu~ 2.2 kJ/m³, tar < 5 mg/Nm³, dust < 5 mg/Nm³ which is used to generate with a CHP engine (combined heat process) electricity and heat. Syngas from the gasification and char coal are converted with steam gasification to syngas with a steam reformer operating at nearly environment pressure and a temperature of 800°C up to 1000°C. The syngas composition is given by CO = 30%, H2 = 30%, CH4 = 1%, CxHy =3% and the rest is CO2, with a heat caloric value Hu~ 2.8 kJ/m³.

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Figure 5: Biogas Plant combined with a gasification plant and syngas reformer (Source: Johann Gruber-Schmidt, 2017)

In the project Hagenbrunn [ 3 ] we combined the biogas plant with a gasification plant as done in the project Traismauer [ 2 ]. We call this gasification the bottom cycle producing weak gas with a caloric heat value Hu ~ 2.2 kWh/Nm³. To generate the needed heat for steam (hv = 2560 kJ/kg) and to superheat steam up to 800°C till 1000°C, we need a combustion chamber, in which the weak gas of the gasification plant is burned with oxygen to carbon dioxide and steam. Additional we use the biogas from the biogas plant in the steam gasifier and fine milled biomass mixed with char coal from the biomass gasifier. Abbildung in dieser Leseprobe nicht enthalten, Abbildung in dieser Leseprobe nicht enthalten.

A very interesting property is given by using carbon dioxide in steam gasification Abbildung in dieser Leseprobe nicht enthalten. The syngas has now a composition by CO = 40%, H2 = 40%, CH4 = 1%, CxHy =3% and the rest is CO2 with a heat caloric value Hu~ 3.5 kJ/m³, tar < 0.5 mg/Nm³, dust < 1 mg/Nm³. This syngas is converted to dimethyl ether. From the CHP plant we generate the electric energy and heat needed in the enlarged biogas plant to run all processes.

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