Experimental combustion rigs

Throughout this thesis SI units are used. In order to ease comprehension of reported values for a reader who is more familiar to units such as volumetric and mass percentages and parts-per-million, SI units are used with pre-factors keeping the same numeric dimensions, i.e. 10^{−2 m}_m³3, 10^{−2 kg}_kg, and 10^{−6 m}_m³₃. As far as possible, for gas concentrations and volumetric flow rates reported in this thesis information on their wet or dry reference state is given. Also for fuel analyses, the analysis reference state (i.e. raw, dry, dry and ash free) is usually given. In few instances, reporting data from others, such indications are not possible due to a lack of the respective information.

by a balance. The accuracy and stability of this steam generation system was successfully crosschecked versus H₂O concentration measurements with help of an FTIR gas analyzer.

The furnace has a length of 2.5 m and a diameter of 0.2 m. Its electrical heating is divided into five zones that can be operated individually up to temperatures of 1400^◦C. The furnace of BTS-VR offers access for sampling of flue gases, deposits, and ashes and for sorbent injection via a lateral port in a distance of 1.5 m from the burner and via the furnace exit. A sorbent injection probe can be introduced from the furnace exit counter-currently to the gas flow (see also figure 3.6). The flue gas production in all experiments of this thesis was kept constant at about 11.5^m_h³,ST P, wet. At the exit of the furnace, the majority of the produced flue gas (about 9.5 to 10^m_h³,ST P, wet) was extracted into a main flue gas duct, with the remainder being directed to an exhaust system. Since the bottom part of the BTS-VR furnace, with its provisions for introduction of sampling and DSI probes, is difficult to seal gas tight, this overflow of gas

dosing feeder

stack ID fan

bagfilter furnace

bagfilter ash milled fuel

ii) i)

iii) E₁

E₅ E₄ E3

E₂

secondary gas primary gas carrier gas

CO₂ O₂

SO₂ NO H₂O

evap.

evap. Hg

N₂

H₂O

E

electrically heated line

CO₂ O₂ air O2

air CO2

air

Figure 3.1: Schematic of the20 kWcombustion test facility BTS-VR. The locations at which dry sorbents have been injected in DSI experiments are highlighted by dashed arrows and marked with i), ii), and iii).

is required to avoid ambient air from being introduced into the main flue gas duct. This is of particular importance for experiments investigating SO₃ formation and DSI that are included in this thesis. Those experiments rely on reliable gas concentration measurements along the main flue gas duct. The flue gas duct of BTS-VR is electrically trace heated to reduce heat losses and maintain a temperature and residence time profile that is similar to the one of a power plant. Also the facility’s fabric filter is electrically heated to maintain operating temperatures in a range of about 200^◦C. In the experiments included in this thesis, a number of different experimental settings were tested that are introduced in detail in sections 3.3.1, 3.3.4, and 3.3.3.1.

3.1.2 500 kW combustion rig (KSVA)

The 500 kW (thermal) atmospheric, pulverized coal combustion test rig (KSVA, see figure 3.2) can be operated in air and in oxy-fuel combustion mode with flue gas recirculation. In the conducted

air

air dosing

feeder

stack ID fan

bagfilter ESP

FD fan

GPH furnace

ESP

bottom ash GPH ash ESP ash

(E1, E2, E3) bagfilter ash

milled fuel A)

C)

O₂ CO₂ B)

ii) i)

iii)

Figure 3.2:Schematic of the500 kW(thermal) combustion test facility KSVA. The valves A), B), and C) allow to operate the plant in air as well as oxy-fuel mode. The locations at which dry sorbents have been injected in dedicated experiments are highlighted by dashed arrows and marked with i), ii), and iii).

experiments, KSVA was operated at a load of about 300 kW (thermal), which corresponds to fuel feed rates of about 40^kg_h for hard coal and about 60^kg_h for lignite. The KSVA plant is not electrically heated which implies a relatively long start-up and heating phase (approx. 10 to

20 h) to prepare for experiments. Therefore, for all experimental campaigns included in this thesis the facility has been continuously operated in a 24 h-3-shift mode for durations of around 100 h. The plant’s furnace has a total length of approx. 7 m and an internal diameter of 0.8 m (see figure 3.6b). The milled fuel is fed to the top mounted burner pneumatically by air or, in oxy-fuel operation, by CO₂, using a gravimetric feeding system. At the burner, the fuel is mixed with oxidant gas (i.e. air or oxygen enriched recirculated flue gas). The KSVA facility is equipped with an air/oxidant gas pre-heater and an ESP. In oxy-fuel operation, flue gas is recirculated wet after the electrostatic precipitator (ESP), preheated in the oxidant preheater, mixed with O₂from a tank, and supplied to the burner. Certain experiments reported in this thesis (oxy-fuel combustion of lignite L2) were conducted without premixing of O₂and instead, with direct injection of pure O₂via separate nozzles of the burner. There are measuring ports all along the furnace (see figure 3.6b) that can be used for gas composition, temperature, and other measurements and to sample fly ash and deposits from the furnace with dedicated sampling probes. A permanent flue gas sampling and analysis system is installed at the furnace exit for continuous measurement of the flue gas composition (O₂, CO, CO₂, SO₂, and NO_x). H₂O can be measured after the ESP.

3.1.3 30 MW oxy-fuel pilot plant “Schwarze Pumpe”

Several authors previously described Vattenfall’s oxy-fuel pilot plant and its process configura-tion [3, 155, 193, 194]. The pilot plant was operated between 2008 and 2014 and was located in Lusatia (Eastern Germany), next to the 1600 MW (electric) lignite fired power plant “Schwarze Pumpe”. The facility was fired by a single, 30 MW (thermal), top-mounted burner and was equipped with an ASU, flue gas cleaning equipment (i.e. ESP, WFGD), a flue gas condenser, and a CPU. The system could be operated in air and oxy-fuel mode. For oxy-fuel combustion it employed a flue gas recirculation downstream the ESP (as in the KSVA facility) and upstream WFGD (i.e. recycle option a in figure 2.2). This implies that wet sulfur rich flue gas is recircu-lated to the boiler leading to considerably increased SO₂levels in oxy-fuel operation, compared to air firing. The gas for fuel feeding was cleaned and dried recirculated flue gas.

During the tests reported in this thesis the lignite L3 was combusted under oxy-fuel conditions in a mixture of recirculated flue gas and O₂. Fly ash and deposits were sampled from measuring ports in the first and second draft of the plant’s boiler in a distance of approx. 0.5 m from the furnace wall. The boilers first draft has a quadratic cross-sectional area of approx. 16 m²and a height of approx. 9.5 m. Figure 3.3 displays the boiler’s first and second draft schematically, with the locations of sampling ports used in the present study. Samples were taken from level 4 in the first draft and level 8 at the entry of the second draft just before the first convective heat exchanger (SH1). During the sampling campaign reported in this thesis flue gas was sampled just downstream of the ESP and its composition was continuously analyzed. In addition, SO₃ measurements have been conducted at two locations upstream the ESP: Directly before ESP

SH1 SH2 ECO5

sampling ports sampling level 4

sampling level 8

Figure 3.3: Schematic of the first and second draft of the boiler of the oxy-fuel pilot plant

“Schwarze Pumpe” with the ash and deposit sampling ports used in this study. SH1, SH2, and ECO5 indicate different convective heat exchangers placed in the second draft.

at a flue gas temperature of about 180^◦C and in the third draft between the economizer heat exchangers at a gas temperature of approx. 330^◦C.