Two CHP units with SCR catalysts

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

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

ETW Energi­etech­nik took a step-by-step approach to this: First the used CHP engines with 800 kilo­watts (kW) each were dis­man­tled. At the same place the com­pa­ny ins- talled two new larg­er gas engines with an elec­tri­cal out­put of 1560 and 1200 kilo­watts. These are each con­tain­er CHP units, i.e. the pow­er plants fit into a spe­cial con­tain­er mea­sur­ing 14 by 3.2 by 3 metres.

First of all, there is the con­tain­er CHP „ETW 1560 BG“ with a gas genset MWM TCG 2020 V16 (elec­tri­cal out­put: 1560 kW, ther­mal out­put: 1528 kW, fuel input: 3683 kW). The sec­ond is the con­tain­er CHP „ETW 1200 BG“ with a gas aggre­gate MWM TCG 2020 V12 (elec­tri­cal out­put: 1200 kW, ther­mal out­put: 1153 kW, fuel input: 2804 kW).

Com­pared with the old gensets, the changeover increas­es the elec­tri­cal effi­cien­cy by almost 1.6 per­cent­age points: Where­as the used CHP units had an elec­tri­cal effi­cien­cy of 40.4 per­cent, this now amounts to 42 per­cent. The over­all effi­cien­cy of the plant increas­es slight­ly to 85 percent.

The heat util­i­sa­tion con­cept con­tributes sig­nif­i­cant­ly to this high fig­ure. The waste heat from the block-type ther­mal pow­er sta­tions is used to heat the fer­menters and the oper­at­ing build­ing. The excess waste heat is then made avail­able to the city of Back­nang for dry­ing sewage sludge.

A fur­ther leap in effi­cien­cy is achieved by a spe­cial fea­ture of the plant: fer­men­ta­tion residue exhaust air is added to the com­bus­tion air of the gas engines. Although this exhaust air has too low a methane con­tent (less than 1.75 per­cent) for it to be used direct­ly in a gas engine, the plant is able to use it in a gas engine. How­ev­er, by mix­ing it into the com­bus­tion air, the low methane con­tent is made usable. This has a fur­ther advan­tage: This methane con­tent in the com­bus­tion air does not have to be sup­plied via the bio­gas pipe and can there­fore be saved on the bio­gas side.

„This was the first time that we have equipped an ETW plant in this way“, reports Alexan­der Szabo. The engi­neer is the respon­si­ble sales man­ag­er at ETW Energi­etech­nik. The munic­i­pal waste man­age­ment com­pa­ny hopes that this exchange will enable it to make CHP oper­a­tion more vari­able in the future due to the high­er engine out­put while main­tain­ing the same gas quan­ti­ties dur­ing the day. In addi­tion, the waste man­age­ment com­pa­ny is expect­ing an increase in the amount of elec­tric­i­ty fed into 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 by a gas wash­er-dry­er and then fed into the com­bus­tion air sup­ply of the gas engines. To avoid an ignitable mix­ture in the com­bus­tion air, the fer­men­ta­tion residue exhaust air freed from hydro­gen sul­phide (H2S) is con­tin­u­ous­ly mon­i­tored by means of a gas analysis.

The sys­tem update is sus­tain­able in that two expect­ed changes in leg­is­la­tion in Ger­many are already being fulfilled:

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

It has already been decid­ed that CHP plants from 2023 may not emit more than 100 mil­ligrams of nitro­gen oxides (NOx) per cubic meter. Cur­rent­ly, this upper lim­it is 500 mg/m3. These val­ues apply in each case at a resid­ual oxy­gen con­tent of five per­cent. This is stat­ed in the Ordi­nance on Medi­um-Sized Com­bus­tion, Gas Tur­bine and Inter- nal Com­bus­tion Engine Instal­la­tions, the 44th Fed­er­al Immis­sion Con­trol Ordi­nance (44th BIm­SchV), which was updat­ed in June 2019.

In order to avoid lat­er, cost­ly retro­fitting of the exhaust sys­tem, both cogen­er­a­tion plants have there­fore already been equipped with mod­ern nitro­gen oxide cat­alyt­ic con­vert­ers. The cat­alyt­ic ele­ments are mount­ed on a ceram­ic car­ri­er. This SCR tech­nol­o­gy – SCR stands for „Select­ed Cat­alyt­ic Reduc­tion“ – is the only tech­nol­o­gy for reduc­ing the amount of sick oxides (NOx) in the exhaust gas of the gas engine. The nitro­gen oxi- des in the exhaust gas are com­posed of nitro­gen monox­ide (NO) and nitro­gen diox­ide (NO2).

To reduce nitro­gen oxides, Adblue must be inject­ed into the exhaust sys­tem — this mix­ture has a urea con­tent of 32.5 per­cent. The high exhaust gas tem­per­a­ture con­verts Adblue into ammo­nia. The ammo­nia reacts with the 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 — even with­out urea injec­tion — formalde­hyde in the exhaust gas. Formalde­hyde (CH2O) is con­vert­ed into water and CO2. The com­plete SCR cat­alyt­ic con­vert­er sys­tem includ­ing urea injec­tion will not go into oper­a­tion until 2023. In order to com­ply with the stricter lim­its, only minor retro­fitting is then required due to the mod­ern exhaust gas clean­ing sys­tem (see below).

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. 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.

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

A urea tank, the stain­less steel pip­ing between urea tank and injec­tion, a suc­tion line into the gas engine and a urea dos­ing device are still need­ed. This requires an air com­pres­sor, pres­sure and tem­per­a­ture sen­sors, nitro­gen oxide and oxy­gen sen­sors upstream and down­stream of the cat­alyt­ic con­vert­er. For this pur­pose, the cat­a­lyst hous­ing is com­plete­ly equipped with cat­a­lyst elements.