news / 2015-03-16
Biomass in coal-fired power stations can reduce carbon emissions
What options are available for the co-combustion of biomass in power plants in ever greater volumes? Researchers at the Hamburg University of Technology are currently exploring this issue in a simulation based on a detailed power plant model in the 800-megawatt power class. The aim is to ensure the rate of internal consumption of power plants remains as low as possible when using regenerative fuel too.
The internal consumption of power plants reduces their degree of efficiency. It is consequently the natural aim of researchers to keep this consumption as low as possible. A further objective, however, is to increase fuel flexibility. This can be achieved, for instance, by adding biomass. It can prove a worthwhile step, as large power stations in comparison to small power plants achieve a potentially higher degree of efficiency. Due to the low heat value and high water content of biomass, however, the mass and volume flow in the co-incineration process is higher. For a higher mass flow, however, and, in particular, when it comes to crushing fibrous wood products in preparation for incineration, hammer mills require much more energy. Avoidance is the keyword in this respect.
One idea is for the biomass to undergo a process of torrefaction prior to crushing. This process involves heating the sustainably produced fuel to between 250 and 300 degrees Celsius using steam extracted from the power plant process with circulating flue gas. The flue gas ensures an almost inert atmosphere, as otherwise the biomass is susceptible to oxidising, resulting in the solid material exhibiting a lower calorific value. This would also entail the risk of the biomass igniting due to exposure to oxygen. Following the torrefaction process, the biomass exhibits both a higher energy density and a higher density in the residual solid material. The structure and consequently the mechanical properties of the biomass are also modified. It becomes much more brittle, reducing the effort expenditure in crushing. This in turn reduces the internal consumption of the power plant, boosting efficiency. Expressed in figures, the energy consumption of hammer mills increases by 21.4 MW per 10 per cent co-combustion without preconditioning. Following drying to a water content of 5 per cent by weight, this figure is 4.5 MW per 10 per cent co-combustion. Following torrefaction, the additional power consumption of hammer mills reduces to 355 kW per 10 per cent co-combustion. This equates to a saving of 98.3 per cent compared to untreated biomass.
Further optimisation: Using heat from flue gas
Researchers have experimented with various degrees of torrefaction. At a specified temperature, this can be influenced by time in the reactor. The degree of torrefaction indicates the percentage of solid material volatilised post-treatment. The highest degree of efficiency was determined by the researchers at a value of 16 per cent, which is why this in future is used in their ongoing observations.
In the simulations, the researchers observed co-combustion rates of between 10 and 50 per cent in relation to the combustion heat output. The investigations revealed an optimal operating point for torrefaction of 300 degrees Celsius. As the time required to pretreat biomass at 250 degrees Celsius is multiplied, reactors would need to be constructed suitably larger for the same material throughput. The investment costs in this regard are beyond the scope of economic viability.
Although the initial approach involves deriving the necessary energy from live steam, the efficiency of the overall process is increased compared to untreated biomass. The researchers hope to investigate in a subsequent step the extent to which the necessary energy can be derived from flue gas. This would allow the potential of torrefaction to be much better utilised, as the live steam would be available entirely for electricity generation. The researchers expect in this connection improved net efficiency of plants.