Equipped with SCR catalysts for 2023, CHP now achieves a total efficiency of 85%.

The waste management company Rems-Murr in Backnang-Neuschöntal in Baden-Württemberg has two new energy-efficient combined heat and power plants in which methane from biogas plants is burned together. From 2023, they will emit five times less nitrous oxide than they do today.

On 23 October 2019, two new combined heat and power (CHP) plants from the municipal waste management company Rems-Murr (AWRM) went into operation in BacknangNeuschöntal. They were built by ETW Energietechnik GmbH from Moers in North Rhine-Westphalia. For the renovation, the waste management company's two old gas engines were replaced after 65,000 hours of operation and more than eight years of operation. This resulted in a jump in efficiency, which was mainly based on the installation of larger, more powerful gas engines as well as more advanced gas engine technology. In addition, fuel savings are achieved by using the residual methane content in the fermentation residue exhaust air.

ETW Energietechnik approached this step by step: First, the used CHP motors using 800 kilowatts (kW) were dismantled. In the same place, the company installed two new larger gas engines with electricity generation of 1560 and 1200 kilowatts. Each of these are CHP units, that is, power plants fit in a special container measuring 14 x 3.2 x 3 meters.

First, there is the container CHP "ETW 1560 HP" with the gas generator MWM TCG 2020 V16 (electrical output: 1560 kW, thermal output: 1528 kW, fuel input: 3683 kW). Secondly, it is the CHP "ETW 1200 HP" container, the gas aggregate of which is MWM TCG 2020 V12 (electrical output: 1200 kW, thermal output: 1153 kW, fuel inlet: 2804 kW).

Compared to older generators, the change increases electrical efficiency by almost 1.6 percentage points: the electrical efficiency of the CHP units used used was 40.4 percent, which now equates to 42 percent. The overall efficiency of the plant increases slightly to 85 percent.

The concept of heat use contributes significantly to this high figure. The waste heat of block-type thermal power plants is used to heat the fermenters and the premises of the enterprise. The excess waste heat is then supplied to the city of Backnang for drying of the sewage sludge.

Another leap in efficiency is achieved by a special feature of the plant: fermentation residue exhaust air is added to the combustion air of gas engines. Although this exhaust air has a methane content (below 1.75 percent) that is too low for use directly in a gas engine, the plant can use it in a gas engine. However, by mixing it into the combustion air, the low methane content is made available. This has another advantage: this methane content in the combustion air does not need to be supplied through the biogas pipe and can therefore be stored on the biogas side.

"This is the first time we have equipped an ETW factory in this way," said Alexander Szabo, Responsible Sales Manager at ETW Eneregietehnik.

The municipal waste management company hopes that this change will make CHP operation more volatile in the future due to higher engine power, while maintaining the same gas quantities throughout the day. In addition, the waste management company expects an increase in the amount of electricity supplied to the public grid.

For the pilot project, the fermentation residue exhaust air, extracted from the liquid fertilizer storage tanks and the sedimentation tank of the biogas plant, is cleaned with a gas scrubber dryer and then fed into the combustion air supply of the gas engines. To prevent an ignitable mixture in the combustion air, the fermentation residue exhaust air released from hydrogen sulfide (H2S) is continuously monitored by means of a gas analysis.

The system update can be sustained due to the fact that two expected changes to the legislation in Germany have already been carried out:

- One of them is the use of residual methane in the fermentation residue exhaust air to prevent greenhouse gas from escaping. This system was designed by the Stuttgart planning company Ingenieurgruppe RUK GmbH.

- It has been decided that CHP facilities in 2023 will emit no more than 100 milligrams of nitrogen oxide (NOx) per cubic meter. Currently, this upper limit is 500 mg / m 3 . These values apply to the residual oxygen content of five percent in each case. This is the 44th edition updated in June 2019. Federal Emission Control Regulations (Article 44). BImSchV) is specified in the Regulation on Medium Combustion, Gas Turbine and Internal Combustion Engine Facilities.

In order to avoid the subsequent costly strengthening of the exhaust system, both cogeneration plants are already equipped with modern nitrogen oxide catalytic converters. Catalytic elements are mounted on a ceramic carrier. This SCR technology – SCR stands for "Selected Catalytic Reduction" – is the only technology needed to reduce the amount of sick oxides (NOx) in the exhaust gas of the gas engine. The nitrogen oxides in the exhaust gas consist of nitrogen monoxide (NO) and nitrogen dioxide (NO2).

Adblue must be injected into the exhaust system to reduce nitrogen oxides - the urea content of this mixture is 32.5 percent. The high exhaust gas temperature converts Adblue into ammonia. Ammonia reacts with nitrogen oxides on the catalytic surface of the catalyst elements with the following reaction formula: 4NO + 4NH3 + O2 4N2 + 6H2O

ETW Energietechnik has already installed some of the SCR catalytic converter elements. These reduce formaldehydes in the exhaust gas even without urea injection. Formaldehyde CH20 is converted into water and CO2. The complete SCR catalytic converter system, including urea injection, will not work until 2023. To comply with stricter limits, only a small renovation is required due to the modern exhaust gas cleaning system.

Following the conclusion of the contract for the new CHP plants, ETW Energietechnik GmbH also signed the contract for the expansion of the waste fermentation plant. This includes the entire process, measurement and control technology. The scope of supply also includes the gas wash drying plant for the fermentation residue exhaust air.

Adaptation for NOx reduction In 2022, ETW Energietechnik will adapt two combined heat and power plants so that they can comply with the limit value of 100 mg/m³ of nitrogen oxide from 2023:

A urea tank, the stainless steel pipe between the urea tank and the injection, a suction line to the gas engine and a urea dosing device are still required. This requires an air compressor, pressure and temperature sensors, nitrous oxide and oxygen sensors up and down of the catalytic converter. For this purpose, the catalyst housing is fully equipped with catalyst elements.