Retrofit of Rodenhuize 4 power station: The Max Green and Cold Back-up-projects

Transcription

1 Retrofit of Rodenhuize 4 power station: The Max Green and Cold Back-up-projects Dr. Stefan Hamel, Babcock Borsig Steinmüller GmbH Dr. Christian Storm, Babcock Borsig Steinmüller GmBH Peter Goorden, Project Director, Electrabel N.V. Thomas Bauthier, Contract Manager, Tractebel Engineering s.a. Introduction In December 2009 BBS was awarded the contract for the modification of the Rodenhuize 4 power plant in Belgium. The Owner of the plant is NV Max Green, a joint venture between Electrabel (GDF SUEZ) and Ackermans & van Haaren. The Owner s Engineer for this project was Tractebel Engineering (GDF SUEZ). Rodenhuize 4 was constructed in 1978 and originally designed to fire blast furnace gas (BFG) and heavy fuel oil. In 1989 it was converted to burn pulverized coal instead of heavy fuel oil. Six years later, in 2005, adaptations were made to partly operate the plant with biomass (milled wood pellets). The main original design parameters of the boiler are shown in Table 1. Table 1: Main original design parameters of the Rodenhuize 4 boiler Type Single drum, natural circulation, wall fired boiler with burners in boxer Construction year 1978 Design steam Production 860 t/h Design steam pressure 156 barg Design live steam temp. 540 C Design reheat steam temp. 540 C Design reheat steam pressure 45 barg Design thermal load on coal Furnace dimensions 720 MWth Width: 11,5 m Depth: 11,5 m Height: 44 m BBS was awarded the Max Green and Cold-Backup projects for the modification of the firing system. The aim of the Max Green project was to rise the thermal capacity of the power plant on biomass from 370 MWth (in co-combustion with coal and BFG) to 560 MWth (100% biomass). At the same time, stringent NO x, CO and dust emission standards had to be met. Page 1 / 11

2 The purpose of the Cold-Backup project was to increase the thermal capacity on blast furnace gas from 400 MWth to 560 MWth while also complying with the emission standards. For 90% of its operating time Rodenhuize 4 will burn biomass alone (wood dust from milled fresh wood pellets), for a total power of 560 MWth. During the remaining 10% operating time the power plant will burn BFG (or BFG/BOFG mix) when it assumes the role as a back-up for the new BFG power plant at the nearby steelmill. Configuration of the firing system before and after retrofit The configuration of the firing system before and after the retrofit is shown in Figure 1. Before the retrofit the 16 old-fashioned swirl burners on rows 2 and 3 on so-called sides Gent and Zelzate were operated with biomass. The remaining 8 burners on row 1 were operated with hard coal. In addition, the burners on row 1 and 3 were equipped with standard natural gas burners. The 16 BFG burners used to be on rows 4 and 5 on the so-called sides Desteldonk and Kanaal. In the old configuration it was not possible to burn biomass without coal combustion. The burners were not designed for the boundary conditions of the biomass feeding system. A defined ignition of the biomass flame was not possible. The location of the biomass burners after the retrofit stayed the same. However all 24 burners were converted to low-no x wood dust burners. The biomass burners on row 1 and 3 were equipped with new low-no x natural gas burners. As for the BFG burners, the location of the 6 burners on row 5 was not used anymore. Instead the 6 new low-no x burners were placed on row 2 on the sides Desteldonk and Kanaal. The position of the remaining BFG burners on row 4 remained unchanged. In addition to the modification of the burners an overfire air (OFA) system was installed in the boiler. It consists of eight OFA-nozzles at 28 m. Four nozzles are located on side Gent while the other four are located on the opposite wall. The air for the OFA is taken from the existing secondary air ducts. For this purpose new ducts including dampers and expansion joints were installed. The modifications in the scope of supply of BBS included: Installation of 16 new low-no x dual fuel burners (NG and biomass) Installation of 8 new low-no x single fuel burners (biomass) Installation of 12 new low-no x burners for blast furnace gas Installation of new frequency-controlled primary air fans including ducts and flow measurements. Installation of water lance blowers Installation of additional sootblowers Page 2 / 11

3 Installation of new ducts for combustion air for biomass and BFG, including flow measurements. Installation of new ducts for blast furnace gas and recirculated flue gas, including flow measurements. Installation of new gas preheaters for BFG Installation of a new fan for recirculated flue gas Installation of a new flame-supervision system Installation of an HP-bypass (water/steam side) for temperature control of Eco Installation of new pressure parts on the boiler membrane walls to allow installation of new BFG burners and new OFA ports as well to extend boiler lifetime Page 3 / 11

4 Gent Desteldonk Zelzate Kanaal Gent Desteldonk Zelzate Kanaal Coal + natural gas Wood dust + natural gas Wood dust Blast furnace gas Figure 1: Burner-configuration of the power plant before (top) and after the retrofit (bottom). Page 4 / 11

5 Modifications on the biomass burners Figure 2 shows a view of one of the modified biomass burners. The BBS biomass burner consists of 4 concentric tubes for core air, primary air/biomass, secondary air I and secondary air II. The wood dust coming from the hammer mills is mixed with primary air in the wood dust injection piece. The primary air is needed in order to have sufficient flow velocity to transport the wood dust through the burner. Due to the abrasive nature of the wood dust, all surfaces of the burner that are in contact with it are clad with abrasion resistant material. The biomass burners on level 1 and 3 are outfitted with natural gas burners which are located inside the core air tube. The NG burners are mainly used for startup and shut down or when switching between different fuels. The combustion air is supplied through the cross sections for secondary air I and II. Dampers in the secondary air duct are used to control the airflow to each burner which is measured by venturi nozzles upstream of the control dampers. The swirl blades in the secondary air I cross section can be moved to adjust the swirl of the combustion air in order to optimize the combustion for the individual types of fuel. The burner is equipped with a flame stabilizer at the end of the primary air tube which creates ideal conditions for stable ignition of the wood dust. The burner throats on the primary and secondary air I tubes deflect the combustion air away from the wood dust. Mixing of the combustion air with the flame thus takes place further downstream at a later stage. Secondary air I and II are not present during pyrolysis and ignition which results in a zone with an understoichiometric atmosphere. These are favorable conditions for low NO x emissions. Page 5 / 11

6 Figure 2: View of a modified biomass burner Page 6 / 11

7 Processing and handling of the wood pellets The fresh wood pellets can originate from different regions, i.e. Canada, Scandinavia, Russia, etc. and therefore have different characteristics. The ranges of the most important fuel properties are shown in Table 2. The first two columns in the table show the minimum and maximum values of the fuel composition. In the third column a typical composition of a fuel that has been fired is shown. Table 2: Fuel properties Min. Max. Typical Proximate Analysis LHV MJ/kg ar ,6 Moisture % ar ,7 Ash % dry 0 5 0,97 Volatiles % dry 65 83,5 Ultimate Analysis C % dry ,1 H % dry ,22 N % dry 0 0,5 0,13 S % dry 0 0,1 0,02 O % dry Diff. Diff. 42,6 The wood pellets are transported to Rodenhuize power plant by ship and stocked in a roofed storage area. From there they are transported to three silos by conveyor belt. They are subsequently transported to hammer mills where the pellets are milled into wood dust. Between the hammer mills and the boiler, the wood dust is forwarded to the burners by pneumatic transportation (dense phase conveying). Each burner is fed by an individual line which is equipped with a roots blower. One hammer mill supplies two burners with wood dust. Right before the impact bend of the biomass burner there is the so called wood dust injection piece where the wood dust is mixed with primary air before entering the burner. The primary air is needed in order to adapt the flow velocity in the wood dust cross section of the burner and the temperature of the mixture composed by primary air, conveying air and wood dust. Page 7 / 11

8 Design of the firing system BBS made extensive use of CFD-modelling during the design phase of the firing system. The geometry of the whole boiler including all the heating surfaces was created. Different geometrical and process parameters were varied in the simulations in order to find the best possible design. Figure 3 shows the meshed geometry of the boiler. OFA Biomass row 3 Biomass row 2 BFG row 4 BFG row 2 Biomass row 1 Figure 3: CFD-model of the boiler Page 8 / 11

9 The three-dimensional furnace model consists of more than 13 Mio.cells. The biomass burner alone is modeled with cells. The results of the simulations include for example velocity and temperature distribution, heat flux on furnace walls and concentrations of CO, NO x and O 2. An example of a graphical representation of the CFD results is presented in Figure 4 which shows the velocity distribution for the biomass burners on level 2. Figure 4: Temperature profile for the burners on row 2. Page 9 / 11

10 With the help of the CFD simulations the biomass burners were optimized with respect to emissions and stability. Predicted NO x -emissions in dependence of the fuel nitrogen content are shown in Figure Predicted and measured NOx (mg/m 3 i.n. 6 % O2) ,1 0,2 0,3 0,4 0,5 0,6 Fuel N-content (Mass %, dry) Predicted Measured Figure 5: Predicted and measured NO x -emissions in dependence of fuel nitrogen content Operational experience Emissions After installation and erection of the new equipment, commissioning started in spring Right from the beginning it was no problem to reach the contractually defined emission limits for NO x and CO. At full load of about 200 MW el, NO x was below 230 mg/nm 6% O 2 dry, while CO was usually below 30 mg/nm 6% O 2 dry. The nitrogen content of the fuel was in the range values of 0,12 % (as received). The level of unburned carbon in the fly ash was usually significantly below 5 %. Slagging, fouling and corrosion Page 10 / 11

11 Operational experience from various power plants has shown that in general the co-firing of biomass with coal can lead to severe problems in a boiler, for example high temperature corrosion on superheater tubes, accelerated deactivation of SCR-catalysts and corrosion and slagging on the furnace walls. The amount of biomass that is allowed to be co-fired depends on many factors, mainly the chemical and physical properties of the fuel, the properties of the main fuel, the design of the furnace, the position of the burners and the operating conditions. Straw which is comparatively high in chlorine, alkali metals and sulfur is a more critical fuel than fresh wood without bark with its comparatively low content of chlorine, sulfur and heavy metals. In the case of Rodenhuize 4 power station there is no more co-combustion after the retrofit, but combustion of 100 % of biomass equivalent to 560 MWth; even combustion up to app. 640 MWth. biomass equivalent to app. 225 MWe net output has been demonstrated. During commissioning no excessive corrosion, slagging and fouling could be observed in the boiler. This can be attributed to the high quality of the fuel and the favorable geometry of the furnace, i.e. large volume for combustion. Page 11 / 11