Meas. Sci. Technol. 7 (1996) 1687–1706. Printed in the UK REVIEW ARTICLE Mass flow measurement of bulk solids in pneumatic pipelines Yong Yan† School of Science and Technology, University of Teesside, Middlesbrough, Cleveland, TS1 3BA, UK Received 30 May 1996, in final form 1 August 1996, accepted for publication 16 September 1996 Abstract. Many types of techniques for metering the mass flow rate of bulk solids in a pneumatic pipeline have been proposed and developed during the past 20 years. This paper presents a detailed and comprehensive review of the techniques and the current state of knowledge and experience. The techniques are classified under three main categories: direct measurement of solids mass flow rates, measurement of volumetric concentrations of solids and measurement of solids velocity. Future developments and possible trends in this field are also included. Yong Yan has worked ex- tensively on the develop- ment of electrodynamic, ca- pacitive and radiometric sen- sors for mass flow measure- ment of pneumatically con- veyed solids. He started his academic career as a Lec- turer at Tsinghua University, People’s Republic of China, where he earned a BEng in Instrumentation and Control Engineering and a MSc in Ad- vanced Instrumentation. He entered the UK as a Re- search Assistant in 1989 and three years later he received a PhD from the University of Teesside. After a period of postdoctoral research in pulverized fuel flow metering, he was appointed Lecturer in the Division of Measurement and Control at Teesside in 1993 and later became Leader of the Measurement Science and Technology Research Unit. He recently joined the School of Engineering at the University of Greenwich as a Senior Lecturer in Electrical/Electronic Engineering and is currently leading a team of researchers developing new instrumentation and control systems for various industrial applications. 1. Introduction 1.1. Background Pneumatic transportation of bulk solids is becoming increasingly important in many industries. The solid materials that can be transported by pneumatic means range from adipic acid to zircon. Although all pneumatic † Present address: School of Engineering, University of Greenwich, Wellington Street, Woolwich, London SE18 6PF, UK. injection systems exhibit technical similarities irrespective of solid materials, the pulverized coal used in electrical power generation and steel production serves here as an important example to illustrate the significance of mass flow measurement of bulk solids in pneumatic pipelines. Figure 1 shows a block diagram of a typical pneumatic conveyor used in coal-fired power stations. Coal is supplied from the bunker into a pulverizing mill and then conveyed towards the furnace via a number of separate tuyeres (typically 2–4). A coal-fired power station can have up to ten such conveyors feeding pulverized coal with a total number of tuyeres as many as 20 per furnace. The mass flow rate and velocity of pulverized coal in each tuyere are crucial parameters influencing both the operation of pneumatic conveyors and the combustion efficiency of furnaces. Firstly, the coal velocity needs to be maintained around the minimum safe value to achieve optimum conveying conditions. Excessively high particle velocity will cause high energy consumption, severe pipeline wear and particle degradation (this latter can influence the effective calorific value of coal). In contrast, insufficient velocity will cause particle stratification in the pipeline and even pipeline blockage which can result in an explosion. Moreover, the velocity at which pulverized coal is injected into the furnace is an important factor affecting directly the uniformity of combustion flames and ultimately the combustion efficiency. Secondly, the presence of unburned carbon in the ash is a major source leading to low combustion efficiency of coal-fired boilers. To keep this loss to a minimum, the optimal mixing of coal and air fed into a furnace should be achieved by maintaining the continuous, consistent and accurate mass flow rate of coal in each tuyere. 0957-0233/96/121687+20$19.50 c  1996 IOP Publishing Ltd 1687