Optimised grate furnaces for wood fuels

Modern grate furnaces achieve a high burnout quality in stationary operation if high-quality wood is burned. But increasingly, a greater range of fuels with high ash content and variable humidity are used as a means of increasing the use of wood. This affects the burnout quality and causes increased emissions on nitric oxides (NOX) and particulate matter (PM). These parameters are influenced by the moisture and ash content as well as the fuel size and the heat load of the plants. The researchers optimised the design and operation of the grate to ensure high burnout quality and low nitric oxide emissions even when lower grade wood is burned and during partial load operation.

The share of wood in the Swiss energy supply could be higher if more lower-grade wood were used. But this is only feasible if fuels with a high ash content, higher nitrogen content and high moisture content can be burned reliably and with low emission of pollutants. And if the fuel properties vary, flexible operations are a must. The problem is that today's grate furnaces emit relatively high levels of nitric oxide. If they do not operate at full capacity and if fuels with varying properties are burned simultaneously, there is the additional risk that burnout is not complete, which in turn leads to increased emissions of carbon monoxide, organic compounds and soot.

The project focuses on the construction of a multi-sectoral moving grate furnace with a modular afterburning zone, whereby the individual grate sectors can be separately controlled. The aim is for a homogenous fuel bed in which there are no spots where temperatures are too low nor any areas with high pollutant concentrations. For this purpose, the researchers used low-NOX air staging in the combustion chamber, which they combined with an optimised burnout of solids on the grate. In parallel, they simulated the combustion in a two-part model approach which takes into account, on the one hand, the transformation of the solids on the grate and, on the other hand, the consecutive gas phase reactions in the combustion chamber.

Improved grate furnace technology allows for low-quality solid biomass fuels to be burned while keeping pollutant levels low. This increases the share of renewable energies in the overall energy consumption. At the same time, the optimised plant technology and the newly developed design methods can help to technologically advance the industry and thereby secure jobs in Switzerland.

The project team developed a model for a biomass grate with optimised geometry and operational parameters. A prototype at semi-industrial scale was produced and tested. This multi-sectoral grate makes it possible to achieve near-complete burnout, maintaining high efficiency even with low-grade types of wood fuels. The furnace is functional even with a 30% capacity. The success is due to an optimised air supply system that can be regulated zone by zone to respond to varying conditions.

Clean technologies for wood combustion from 500 kW to 50 MW