Not all types of wood-waste burn the same way. The burning characteristics depend on the composition of the material. In the UK woodworking industry, wood-waste composition can range from fine dust to solid chips generated by wood chippers.
Those who use industrial biomass boilers to heat their factories and facilities are subject to strict regulations. These regulations are expected to evolve with lower emission thresholds. Notably, stringent emission limits currently apply to systems with over 90 kilograms per hour of wood consumption, while those below this threshold are not subject to the same restrictions.
Over the last 40 years, Ranheat has developed a range of combustion systems, from simple retorts to complex step and moving grates. These systems are designed to accommodate the diverse array of wood-waste the company encounters.
The simplest ‘fixed’ retorts are refractory-lined and have no moving parts. Regardless of the system chosen, proper air distribution within the combustion system is crucial.
During combustion, primary air is necessary to initiate the burning of wood-waste, releasing wood gases in the process. These gases mix with secondary air at higher levels, igniting and burning at elevated temperatures.
Besides considering the production of these secondary combustion gases, we must also examine their composition. This composition is influenced by the chemical reactions amongst the various components of the combustion process. Carbon and nitrogen are present in the fuel, while nitrogen and oxygen are present in the air. As the combustion process progresses, complex chemical reactions occur, producing various compounds or gases.
When analysing flue gases, the main constituents are as follows. Through Environmental Permitting Note 5/1 (18), the government sets limits on these analytes. The table below, taken from the guidance note, outlines the limits that must be met.
Air primarily consists of 78% nitrogen, 20.9% oxygen, and a very small percentage of other gases.
During an annual testing of a combustion system as part of a Part B permit, an independent laboratory takes tests of all the analytes mentioned above. Ranheat follows some simple rules known as the “3 Ts”.
These rules refer to Temperature, Time, and Turbulence. It’s essential that the air and gases mixing in a hot environment are sufficiently hot, remain in the combustion zone for an adequate duration (often termed residence time), and are thoroughly mixed together.
By adjusting the design of the combustion system and controlling primary and secondary air, you can influence the chemical composition of the flue gases.
Through experience, research, and development, Ranheat has arrived at optimal designs for our extensive range of retorts and grates. By precisely controlling the introduction of air, both primary air through the burning material and secondary air above the combustion level, the production of nitrogen oxides (NOx) and carbon monoxide (CO) can be controlled and adjusted.
Particulate matter, or ‘grit,’ is managed through mechanical separation rather than chemical reaction. We employ ceramic filter filtration systems for this purpose.
Ranheat utilises a system of removable retorts and grates, allowing the combustion system to slide into the boiler base, typically air-cooled and brick-lined. The cooling air doubles as combustion air, enhancing boiler efficiency through preheating.
Furthermore, Ranheat employs refractory arches to provide radiant heat above the burning fuel charge.
Picture showing the firebrick lined boiler foundation and the cast iron moving grate elements, the stoker inlet (fuel inlet), and the refractory arch over the grate.
Typical test results from a Ranheat Part B assessment, featuring a ceramic filter, demonstrate remarkably low levels of particulate matter and nitrogen oxides.
Originally, combustion engineers primarily hailed from mechanical engineering backgrounds. However, with the increasing complexity of chemical reactions in the combustion zone, there’s a growing demand for expertise in chemical engineering.
Ranheat continuously enhances grate and retort designs. We utilise flue gas recirculation systems, mixing a portion of flue gases (with 9% oxygen) back with the combustion air (with 21% oxygen). This process reduces the oxygen content in the primary combustion air, thereby minimising NOx production in both the grate and primary combustion.
No matter which Ranheat system you choose or any wood-fired installation that uses wood-waste, compliance with current regulations is vital.
Legal obligations include obtaining planning permission, chimney height approval and permits to burn wood wastes legally. This may involve obtaining an exemption from the Environment Agency for up to 50 kilograms per hour, or acquiring a permit for a Site Waste Incineration Plan (SWIP) or a full Part B permit.
At Ranheat, we offer free assistance in obtaining these permits and permissions, ensuring your compliance every step of the way.
For further information on all types and sizes of industrial wood-burning equipment from 75kW upwards, do not hesitate to get in touch with Ranheat at 01604 750005 or email sales@ranheat.com. You can also visit www.ranheat.com. We also manufacture spares and offer service and repair for other makes of wood-burning boilers and heaters.
MATT LOCAL
