Future proofing CHP units at Backnang-Neuschöntal

In October 2019, two new combined heat and power (CHP) plants of the municipal waste management company Rems-Murr (AWRM) went into operation in Backnang-Neuscho¨ntal, Germany. They were built by ETW Energietechnik GmbH, which is based in Moers, North Rhine-Westphalia.

As part of the project, two existing gas engines operated by AWRM at the site were replaced after 65000 operating hours and more than eight years of operation.

The end result is a significant increase in efficiency, deriving essentially from installation of higher capacity gas engines as well as evolutions in gas engine technology. In addition, further fuel savings arise from making use of residual methane in fermentation residue exhaust air at the AWRM facility.

The first step in the project was to dismantle the existing CHP gas engines, which were 800 kW each, and replace them with the new, higher capacity, machines. These are each containerised CHP units, with the power plants installed in 14 m by 3.2 m by 3 m containers.

The new gas genset models are: MWM TCG 2020 V16 (electrical output 1560 kWe, thermal output 1528 kWt, and fuel input 3683 kWt); and MWM TCG 2020 V12 (electrical output 1200 kWe, thermal output 1153 kWt, fuel input 2804 kWt).

Compared with the old gensets, the electrical efficiency of the facility has increased by almost 1.6 percentage points, from 40.4% to 42%, while the overall efficiency of the plant in CHP mode has been raised slightly to 85%.

The waste heat from the gas fuelled gensets is used to heat the fermenters and plant building. The excess waste heat is then made available to the city of Backnang for drying sewage sludge.

A further increase in efficiency is achieved via a special feature of the plant: fermentation residue exhaust gas is added to the combustion air of the gas engines. This exhaust gas has too low a methane content (less than 1.75%) for it to be used directly in a gas engine. However, by mixing it with the combustion air, the low methane content becomes usable, and biogas fuel consumption is reduced.

“This was the first time that we have equipped an ETW plant in this way”, says Alexander Szabo of ETW Energietechnik, employing a system designed by Ingenieurgruppe RUK GmbH of Stuttgart.

AWRM hopes this novel configuration will enable it to make CHP operation more flexible, allowing higher engine output while maintaining the same biogas fuel consumption, and increasing the amount of electricity fed into the public grid.

The fermentation residue exhaust gas extracted from liquid fertiliser storage tanks on the site and the sedimentation tank of the biogas plant is cleaned with a gas washer-dryer before it is fed into the combustion air supply of the gas engines. To avoid having an ignitable mixture in the combustion air, the fermentation residue exhaust gas, once hydrogen sulphide has been removed, is continuously monitored using a gas analysis system.

The upgraded plant is considered to have a viable future in that the requirements of two expected changes in legislation in Germany are already being fulfilled:

• The use of residual methane from the fermentation residue exhaust in the gas engine combustion air avoids greenhouse gas emissions.
• From 2023, CHP plants in Germany may not emit more than 100 mg/m3 of NOx (@ 5% O2). Currently, the upper limit is 500 mg/m3 (@ 5% O2). This is stated in the Ordinance on Medium-Sized Combustion, Gas Turbine and Internal Combustion Engine Installations, 44th BImSchV (Federal Emissions Control Ordinance), which was updated in June 2019.

To avoid later, costly retrofitting of the exhaust system, both cogeneration units are being equipped with SCR (selective catalytic reduction) systems, with the catalytic elements mounted on a ceramic carrier.

Adblue, with a urea content of 32.5%, is injected into the engine exhausts. The high exhaust gas temperatures convert the Adblue into ammonia, and the ammonia reacts with the nitrogen oxides on the catalytic surfaces of the catalyst elements (4NO + 4NH3 + O2 ? 4N2 + 6H2O).

ETW Energietechnik has already installed some of the SCR catalytic converter elements at the Backnang-Neuschontal facility. These reduce – even without urea injection – formaldehyde (CH2O) in the exhaust gas, converting it to water and CO2. The complete SCR catalytic converter systems, including urea injection, will not go into operation until 2023.

Following the award of the contract for the new CHP plants, ETW Energietechnik GmbH was also awarded the contract for the extension of the waste fermentation plant, with a scope of supply that included the gas washing/drying plant for the fermentation residue exhaust.

In 2022, ETW Energietechnik will retrofit the two combined heat and power plants so that they can comply with the 100 mg/m3 limit for nitrogen oxides coming into force from 2023 onwards.

The retrofit will include installation of the urea tank, stainless steel piping between the urea tank and the engine injection points, suction lines into the gas engines and a urea dosing system.