Not all kinds of biomass are as easy to process as clean wood. So-called agricultural waste products, for instance, cause problems for virtually all gasifiers and boilers. ECN researcher Rian Visser, a waste specialist, came up with a technical adjustment to an old installation that makes it possible to break down these agro-wastes: a low temperature zone for gasification and another zone for combustion. “I wanted to prove that a small adjustment could allow an existing installation like the NARGUS to tackle tricky types of biomass too. The results have exceeded all my expectations,” says the jubilant energy technologist.
Environmental goals and, above all, the reduction of CO2 emissions motivate the use of biomass as fuel in stead of fossil fuels. Naturally, the most usable kinds of biomass, such as clean wood, are the most obvious source. But wood is expensive and relatively scarce in the Netherlands, which logically leads us to consider the trickier types of biofuel. “Grass cuttings and coniferous twigs are not currently being used as fuels because they pose too many problems,” says Visser. “Staatsbosbeheer, the state forestry agency of the Netherlands, and Natuurmonumenten, the foremost nature conservation society, have thousands of tonnes of prunings and uncultivated grass waste on their hands. And the agricultural sector also produces a large number of waste streams, although in smaller volumes. Many attempts have been made to process these streams using commercial incinerators, but these attempts never lasted very long as they ran into lots of problems. My assessment was that these difficult biomass streams required a technological adjustment. But I was going to have to prove that first.”
A successful final act for the 30-year-old NARGUS fluidised bed gasifier
Urgency
In normal circumstances, an ECN researcher would want to present a more thorough body of work, research a greater number of variables and repeat an experiment now and then. But in this instance she was spurred on by a factor that required her to finish her work in six weeks time: the old fluidised bed gasifier (taken into use in the early 1980s) was scheduled for demolition to make way for new facilities. So it was now or never. Visser’s strategy was to create two different temperature zones within one reactor by using flue gas recirculation. A 650-700 ̊C zone for the gasification of the biofuel at the bottom, and above that, where the fluidised sand does not reach, a higher temperature (850-900 ̊C) to ensure efficient combustion and to reduce emissions. Visser: “By gasifying at a relatively low temperature in the lower part of the fluidised bed, we hoped to keep the grains of sand from agglomerating. That is what you call it when grains of sand ‘stick together’, bringing the process to a halt and requiring the whole installation to be cleaned. It was a success. We managed to test all five biofuels without agglomeration occurring once!”
Different fuels
Rian Visser’s group has good reason to be jubilant. Not only did the realisation and maintenance of the temperature zones prove to be achievable aims; the excellent flexibility in terms of the range of biofuel use in one installation also exceeded their expectations. “Of the five different types of fuels we tested (clean wood pellets, grass seed chaff, compost overflow (wood with organic matter attached to it), pellets made up of grass, grass seed chaff and leaves, and, finally, 100 per cent grass) four of them could be considered successful. The last one, grass, gasified well and burnt without agglomeration occurring in the sand bed. However, the installation had to be taken out of operation because the bag filters at the back of the reactor became clogged up.” But there is even hope for the tricky grasses, in Visser’s view: “We tried combinations of grass with other fuels, and using these we did manage to keep the process going. Unfortunately, we never got the chance to carry out more extensive testing because we ran out of time.”
While the demolition plans were being drawn up, Visser got to see the results of the analysis of the flue gases in the experiment. This report contains astonishingly good results. The emissions fall below – sometimes even far below – the standards, and the figures for dioxin are lower than have ever been measured before in the NARGUS. This goes to show that the two-zone system works splendidly.
Notice the two fairly stable temperature zones in the NARGUS fluidised bed biomass gasifier: one for gasification (lowest) and one for combustion (highest) of the fuel.
A favourable market for small installations
The research has not come to a halt. Partner HoSt is continuing its research and a few members of her team are helping with the HoSt tests in Tzum, Friesland. Visser does look towards the future with a certain degree of wistfulness: “HoSt is a company, not a research group. I know they will continue the work in a different way than we would do it and that is only to be expected. Perhaps the technology is currently still at too premature a stage for a successful market introduction and it would be a shame to jeopardise that.”
But then she reconsiders: “The main thing is that the research is carried on into the next stage,” she states unreservedly. “I think there is a good market for smaller installations, with a capacity of a few megawatts: large municipalities, Staatsbosbeheer. And in agricultural areas there are many and various amounts of waste materials. This could be used as green fuel to provide local heat and electricity in the short-term.”
Contact
Rian Visser
ECN Biomass, Coal and Environmental Research
Tel.: + 31 22 456 4557
E-mail: Rian Visser
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