23. Nov 2022
Convert the remaining carbon dioxide in the biogas together with hydrogen completely and efficiently into methane and double the turnover?
We are looking for an innovative biogas plant operator and industrial partner in plant construction. Within the framework of the Gas Research Fund of the Swiss Gas Industry (FOGA) and the Research Fund (FFA), Mems AG, together with the Institute for Environmental and Process Technology (UMTEC) of the University of Eastern Switzerland (OST), has developed a power-to-gas (PtG) plant with a sorption enhanced methanation (SEM) reactor that can completely convert the CO2 in the biogas into methane. The system now wants to prove itself in a biogas plant.
The production of methane from hydrogen and carbon dioxide is based on the following chemical reaction:
CO2 + 4·H2 ⇿ CH4 + 2·H2O
Thus, not only methane is produced during methanation, but also water. Sorption Enhanced Methanation", SEM for short, enables the complete conversion of carbon dioxide with hydrogen to methane by adsorbing the reaction water in a zeolite.
Treatment or drying of the exhaust gas is therefore not necessary. The optimized catalyst allows a full conversion of 100 % even if a CO2/H2 mixture is mixed with more than 70 % of other "ballast gases" such as methane from the biogas. It is therefore not absolutely necessary to separate CO2 beforehand, which is an economic advantage with regard to the investment costs of power-to-gas (PtG) plants. The technology can be used advantageously as an in-line biogas upgrade, in which only H2 is added to biogenic CH4/CO2 mixtures.
The sorption reactor is integrated into a larger system. This system starts with the feed of the reactant gases (CO2, H2) and ends with the feed of the product gas (CH4) into the gas grid, as shown in the diagram below.
In between, there are various system components. Measuring devices and sensors (represented by circles in the diagram) record data for monitoring and controlling the system. The mass flow controllers (MFC) and the valves use the sensor signals to control the system and set it to various process states. A programmable logic controller (PLC) is used to measure and control this system.
The system periodically passes through three phases during operation:
If the reactor is charged stoichiometrically with H2 and CO2, the conversion to methane takes place taking into account the reactor efficiency "x" according to the following equation (0 % ≤ x ≤ 100 %):
4·H2 + CO2 → x·(CH4 + 2·H2O) + (1-x)·(CO2 + 4·H2O)
In a conventional methanation reactor, "x" is about 85 % in the best case, resulting in a considerable amount of unused H2 and CO2.
If the zeolite slowly reaches the end of its water absorption capacity, the reaction efficiency "x" decreases. Then, in addition to methane, water, H2 and CO2 are also present in the product gas, as in conventional methanation. However, the mixture of all these gases has a higher thermal conductivity than pure methane, which is recognised, detected and communicated to the PLC by the gasQS™ measuring devices.
In order to remove the water from the saturated zeolite, it must be flown through with a drying gas. It is best to use a gas that is already present in the process. Methane has the advantage that the drying phase can be monitored with the same measuring device that is used to monitor the methanation phase. Moist methane has a slightly lower thermal conductivity than methane, so the end of the drying phase is reached when the measured thermal conductivity value matches that of methane again. The water that leaves the reactor together with the carrier gas during the drying phase must be separated in the post-processing stage (cold trap, molecular sieve).
Since various reactor and catalyst principles exist on the market, which must, however, be operated at higher pressures, a new and scalable reactor design was developed from additional own funds and in close cooperation with the company Fluitec AG (Neftenbach). While all PtG reactors suffer from hotspots, an innovative heat exchanger design was used here, which has not existed in PtG until now. A first prototype was produced and delivered (Figure 3). The fixed-bed reactor has an empty volume of 10 liters at a length of just under 1 m, but is correspondingly longer and can be produced with any diameter up to 2 or 3 m. This means that heat transfer effects (heat spots) can be avoided. This means that heat transfer effects (hotspots) associated with scaling the reactor are no longer a problem. For cost reasons, the reactor is certified up to 20 bar, but can be used up to 100 bar. Fluitec's know-how in heat exchange in reactor and reaction systems is used to ensure a homogeneous and efficient temperature distribution (methanation = exothermic, drying = endothermic) in sorption operation. In contrast to conventional PtG reactors, the design is characterized by internal, more efficient heat transfers. In this context, a thermal oil heat exchanger system in an industrial design with corresponding communication interfaces was also acquired from the company Regloplas.
As described in the three phases, the states of the reactor can be determined and controlled by the thermal conductivity of the product gas (Figure 4). The gasQS™ static from Mems AG provides reliable measurement results here through its pressure-compensated thermal conductivity measurement over the entire, permissible pressure range up to 15 bar. Each reactor is equipped with a measuring device. Furthermore, the mixing of the reactant gases can also be controlled with one device. The microthermal sensor used in the gasQS static is a mass product, stable over the long term and, unlike chemical sensors, does not need to be recalibrated.
The control of the process is relatively simple and can be described with a three-stage state machine (Figure 4), which maps the three phases of the reactor.