The ST100 & MT100 series thermal mass flow meters from FCI have been the market leader in combustion air flow measurement for many years throughout Europe. Reliable & repeatable flow measurement in large secondary air ducts with little or no straight sections is a major challenge for conventional DP based technologies, but the FCI thermal mass meters with multipoint averaging capability have proven themselves time and time again.

Now, with the addition of SIL compliant devices, they are even more applicable to the latest Biomass – waste to energy power stations.

RDF or Refuse Derived Fuel

RDF or refuse derived fuel is a term used to describe commercial and residential waste that has been processed so it can be burnt as a fuel to produce energy. It is as an efficient way to deal with waste that would otherwise be sent to landfill. Such components include non-recyclable plastics, paper cardboard, labels and general packaging. There are clearly huge environmental benefits, as less and less fossil fuel is consumed.

However the combustion process for RDF is very different to traditional fossil fuel boilers. In conventional large scale boilers, the combustion of gas or coal dust is relatively easy to control. The fuel has consistent properties and will combust in a very predictable way. RDF has a very high moisture content, its calorific value is not as consistent and the fuel particle size is variable. A different approach is needed which has led to the use of Fluidised bed combustors.

Fluidised bed technology

Fluidised bed technology is ideally suited to low grade fuels because it forces the RDF solid particles to behave as a fluid, maximising particle contact with the upward flow of pre-heated air driving off the moisture. The RDF is retained on a perforated plate during the combustion cycle & pre-heated hot air passes through the plate from below. Natural gas can be used to initiate combustion at which time the RDF bed heats the boiler water / steam tubes.

 One of the key issues with Fluidised beds is controlling the air flow through the bed. It is critically important that this air flow is evenly spread below the plate and can be reliably controlled. Insufficient air flow to all or some bed compartments may cause incomplete combustion and accumulation of combustible material.

Thermal Mass Flow Meters for Biomass Boilers

Biomass fuel has a high volatile content, which is released during the combustion process as a mixture of explosive gases including methane (CH4), hydrogen (H2). Occasionally, un-combusted, explosive gas mixtures can build up within a biomass boiler’s combustion chamber and flue, which are subsequently ignited and an explosion in some form can occur.

The most important safety related measurements of air flow must be compatible with the SIL target set for the BMS. The ST100A thermal mass flow meter has been classified as a Type B subsystem in accordance to IEC 61508-1 with a hardware failure tolerance of 0. The Probability of Failure on Demand (PFD) meets SIL-1 capability standards.

In addition, the ST100A Thermal mass flow meter is capable of operating up to 450°C with exceptional low flow sensitivity and a variety of communication options including 4-20mA / HART 7 / Modbus / Fieldbus & Profibus. ATEX approvals for Zone 1 / 2 are a standard feature.

Why not take this opportunity to explore more about us, and what we can do for you by visiting our website at

Need to discuss your requirements or seek advice? Then call us today on 01268 526161.