By Siân Green
Dutch power producers have agreed to reduce carbon dioxide emissions from coal fired power plants by 6 million t per year. Co-firing biomass and waste with coal will help them achieve this target, but much research and development is still required.
On 24 April 2002, Dutch power generators signed an agreement with the government to reduce carbon dioxide emissions from coal fired power plants. The agreement, called the ‘Coal Covenant’, stipulates that carbon dioxide emissions from coal fired plants will be reduced by 6 million t per annum from their current annual level of 20 million t by 2008.
Figure 1. Proven and reviewed types of direct co-combustion in the Netherlands
In order to achieve this, the power generators will increase the efficiency of their plants, and will also replace some of the coal fuel with biomass or waste-derived fuel, i.e. convert existing coal fired power plants into co-firing plants.
Under the Covenant, the investments that the energy companies are required to make will be guaranteed, so that the government will compensate them if there is a sudden change in policy that leaves their investments worthless.
According to Dr. Ton Konings, Senior Consultant with Kema Power Generation and Sustainables, 50 per cent of the CO2 emission reductions required by the Covenant will be met by introducing co-firing of biomass to coal fired power plants. “They will replace coal with sustainable fuels – either biomass but also waste materials – and that will account for around half of the reduction,” says Konings. “The other half will be realised by efficiency improvements in the power plants.”
Figure 2. Indirect co-combustion by upstream gasification at the Amer coal fired power plant
Co-firing technology is not new to the Netherlands. Driven by a variety of financial and policy incentives, power generators in the country have in recent years looked to increase the amount of power generated from sustainable resources, and have examined the feasibility of co-firing biomass and other ‘alternative’ fuels with coal in existing power plants.
In 2000, some 900 kilotonnes of biomass and waste fuels were burned in Dutch coal fired power plants. A number of power plants in the country, including Amer, Borssele and Gelderland, have implemented co-firing technology on either a commercial or demonstration basis. These schemes have met with mixed success, but the fact remains that the generators must now implement co-firing on a widescale basis to meet the terms of the Covenant.
Figure 3. Wood processing and co-firing at Electrabel’s Gelderland 13 power plant
There are, however, other factors driving power generators in the Netherlands to increase the level of renewable generation, including tax incentives and attractive pricing. In Holland, domestic electricity consumers pay a levy on their electricity tariffs. The money raised is then distributed to the generators and distributors according to how much renewable energy they have generated and sold.
Most co-firing experience in the Netherlands is in direct co-combustion, where the secondary fuel, i.e. the biomass or waste, is milled and combusted with the coal, or is blown directly into the coal boiler. According to Konings, this method is used in the Netherlands whenever possible as it is less capital intensive than the alternative of indirect co-combustion.
“The capital investment for direct co-firing is in the order of g500 per installed kW, which is roughly three to four times cheaper than the indirect co-combustion route,” comments Konings. Direct co-firing does have some limitations, however.
Sewage sludge, wood, petroleum coke and paper mill sludge can all be used in a direct co-firing system, so where these fuels are available, the direct method is usually used. However, if the fuel is particularly wet, then the amount that the plant operator is able to use will be limited by the drying capability of the coal mill. The burning of sewage sludge, which has a high ash content, is also limited by the capacity of a plant’s electrostatic precipitator (ESP) equipment. Sewage sludge also contains relatively large amounts of mercury, which also has implications for emission control.
With other fuels – for example petroleum coke – the amount of unburned carbon in the fly ash can increase, especially in older boilers. “In the Netherlands, we have to be able to re-use all the fly ash as a building material,” explains Konings. “But in principle you always use direct co-combustion whenever possible.
Dutch power utilities are studying ways to increase co-firing levels in existing power plants
“When direct co-firing is not possible, for example if the fuel is contaminated with heavy metals and the like, then operators will choose indirect co-firing. This includes parallel co-combustion … or the use of a pre-gasifier.”
According to Konings, there are plans to build a large parallel co-combustion plant based on fluidized bed technology at Rotterdam. It is expected that this plant will burn around 600 000 t/a of waste. A pre-gasification plant has already been constructed by Dutch utility Essent at the Amer plant. This, however, has not been working well and Essent is completely re-engineering the fuel gas treatment plant.
“The fuel gas treatment plant [at Amer] removed most of the ammonia, hydrogen chloride, tar and so on in the gas, cooled it to 40°C and completely dedusted it,” says Konings. “This was not engineered right, and is now being completely re-engineered. They will greatly reduce the amount of treatment and it will become a lot more like the Lahti plant that Foster Wheeler constructed in Finland. The gas will be cooled to 500°C and will then be passed to a hot cyclone for partial de-dusting. The hot fuel gas will then be combusted in the boiler.”
Co-firing biomass or waste in conventional power plant, especially by direct co-firing, can present operators with numerous challenges in terms of fuel handling and operation and maintenance. This can mean an increase in plant running costs.
With direct co-firing, the most common problem operators face is with the coal mills, which can experience an increase in wear and tear if the secondary fuel is ground with the coal. In addition, says Konings, the fineness of the coal can also be affected, leading to an increase in unburned carbon in fly ash.
Slagging around the burners can also be a problem, and has been seen in power plants in Germany, says Konings. “In a co-firing demonstration in the Amsterdam power plant, [slagging] was under control,” notes Konings. “They noticed some increase in slagging, but they could handle it.”
“Slagging is one of the properties that is difficult to predict,” says Konings. “We think that up to a certain threshold co-firing percentage, no problems occur, but above that point, an avalanche effect occurs. For example, if you have ash that becomes sticky, up to a certain point it will not influence your operations, but above a certain percentage it will suddenly aggravate problems.”
In its work on biomass firing and co-firing, Kema uses modelling techniques to help predict the behaviour of different fuels in boilers. This helps plant operators select the optimum fuel mix to maximize efficiency while controlling emissions and ash, and minimizing maintenance needs.
“We use computational fluid dynamics (CFD) modelling to predict temperatures in the boiler. We then process that data with some chemical equilibrium packages and software in which you can calculate complex gas-solid-liquid thermodynamic equilibria. From this we can determine the ash melting point, and we can do this for the ash from the fuel that is being co-combusted and from the coal ash, as well as from mixtures of the two.”
Such analysis is a complex process, and it is necessary to create several different scenarios for one plant and to carry out sensitivity analysis for each scenario. “You will not have complete thermodynamic equilibrium in a boiler,” says Konings, “So sensitivity analysis is needed.” The results, says Konings, are usually accurate. “We have good experience with these programs.”
Of the power plants in the Netherlands that have implemented direct co-firing, most have replaced no more than five per cent of the coal (by mass) with alternative fuels. A few have gone up to ten per cent. However, in order to meet their target CO2 emission reductions, they will need to replace more coal, and do it across all of their coal fired power plants.
“To meet their agreement with the government, [the generators] will have to replace 15 per cent of coal with renewable fuels,” states Konings. In order to achieve this, each generator will have to look at all different kinds of fuels to work out which works best without causing operational problems. In addition, it is likely that a shift from direct to indirect co-firing will have to take place.
The only indirect co-fired power plant in the Netherlands is Essent’s Amer plant, which experienced problems during commissioning and with the fuel gas treatment system. The co-fired Amer unit is a 90 MWth plant which uses a pre-gasifier to gasify waste wood. The owner attempted to get the plant started for about a year, but experienced a number problems, for example with the biomass feeding system. Once these issues were solved, the syngas cooler was found to be too small, and it was decided that the whole fuel gas clean up system should be re-engineered.
“It never really got through hot commissioning,” says Konings of the Amer pre-gasification plant, “and they are now re-engineering the fuel gas treatment system.”
Much of Kema’s work in co-firing is therefore now concentrated on how indirect co-firing can be implemented in a wide scale in the Netherlands. Fly ash quality is a major focus in this respect, and this, says Konings, is one area where the power plant operators do not feel to be in competition with each other. “We get quite a lot of research in this [fly ash] area where the Dutch power producers are trying to work together to solve the problem and get it all in a legal framework,” notes Konings.
“When you are increasing the amount of fuel that you are co-firing, you are changing the properties of the fly ash and bottom ash,” explain Konings. “When you change those properties, you can encounter problems with legislation.”
In the Netherlands, the use of fly ash from coal fired power plants is governed by specific legislation. To accommodate power generators who have started to co-fire alternative fuels, the legislation has been altered to accommodate fly ash from power plants that are co-firing up to ten per cent secondary fuels by weight.
However, generators are now looking to co-fire up to 20 per cent alternative fuels and this warrants research into the impact this will have on fly ash composition. “We have obtained fly ash from 20 per cent co-firing from our 1 MWth test rig. We make concrete samples and test them for a variety of properties,” says Konings.
Other important areas of technology investigation at the moment include the heat distribution point in the boiler and ignition of fuels.
Another key area of research is into how co-fired power plant will operate in a liberalized market where availability and load following capability are important. “The number one interest of the power plants in the liberalized market is the availability of the power plant and the load following ability,” states Konings. “Obviously delivering the electricity when it’s most wanted is of prime importance to the economics of the plant.”
For indirect co-firing plants, the availability of the co-firing section may not be a limiting factor for the availability of the whole power plant. So if the section treating and firing the secondary fuel shuts down, the main coal fired plant should be able to ramp up to full load on its main fuel.
The challenge ahead
So with the Covenant in place, many operators are researching their options and are already looking to increase co-firing levels. Essent, which is the largest coal fired operator in the Netherlands with two 600 MW units and one 400 MW plant, has made plans to replace coal with biomass. For the last 12 months, Kema has been carrying out desk studies with Essent of its co-firing options, while Essent itself has been conducting large-scale demonstrations.
Electrabel in the Netherlands has been co-firing alternative fuels at its Gelderland 13 unit in Nijmegen since 1995, and has plans to expand this scheme. However, it has been experiencing permitting problems and will have to put its expansion plans – involving a tripling of biomass firing – on hold for at least a year. The plant’s environmental permit was rejected by provincial authorities when it was ruled that the secondary fuel being fired (waste wood) was classified as a waste material. Co-firing activities at the plant have therefore ceased until a new permit is issued.
The contaminated wood, although it accounts for less than three per cent of the wood being used in the plant, contains arsenic, chromium and other substances. The European Council therefore ruled that the plant’s permit was not valid as it did not cover the co-combustion of waste material.