Page 127 - New Trends In Coal Conversion

90 New Trends in Coal Conversion

4.1 Introduction

As an abundant, renewable, and environmentally friendly energy resource, biomass
and its cofiring with coal for heat and power generation has gained increasingly wide-
spread concerns and also advanced remarkable progress and development worldwide.
In the design and optimization of all the biomass cofiring processes and technologies
under different firing conditions, computational fluid dynamics (CFD) modeling has
played and will continue to play a vital role.
This chapter mainly focuses on CFD modeling of coal and biomass cofiring. First,
the basic modules in general cofiring CFD are presented. Then, the specific or new
modeling strategies or issues associated with different combustion technologies or
combustion conditions (e.g., air-fuel or oxy-fuel) are discussed. A balance between
the breadth and depth is made in the chapter, aiming to attain both a good overview
and an in-depth understanding of the modeling strategies and issues for coal and
biomass cofiring.

4.2 An overview of coal and biomass cofiring

Biomass offers carbon-neutral flexible power generation at low capital cost regardless
of weather conditions. Globally, biomass cofiring has been applied in more than 240
power plants, in which 48%, 24%, 19%, and 9% are equipped with suspension-firing,
bubbling fluidized bed (BFB), circulating fluidized bed (CFB), and grate boilers,
respectively (IEA, 2017). The key features, pros, and cons of these coal and biomass
cofiring technologies are summarized in (Yin and Li, 2017). In Europe, current eco-
nomic circumstances also favor a change to biomass cofiring as a partial solution
for power generators (Lavery, 2013), and suspension-firing has witnessed great suc-
cess in cofiring of woody biomass at low thermal shares (Al-Mansour and Zuwala,
2010). In the development of the coal and biomass cofiring technologies, CFD has
proven to be a powerful tool for increased understanding, exploration of unfamiliar
conditions, design, troubleshooting, and optimization of combustion processes (Kitto
and Stultz, 2005; Yin et al., 2008a; Yin and Yan, 2016).

4.3 CFD modeling of coal and biomass cofiring: the basic
and generic modules

For dilute multiphase reacting flows as seen in suspension cofiring systems and in the
freeboard in both grate-fired boilers and fluidized bed combustion boilers, the
EulerianeLagrangian approach is commonly used in CFD modeling. The fluid trans-
port equations are numerically solved in the Eulerian framework, whereas the reacting
fuel particles are tracked in the Lagrangian framework, both of which are closely
coupled to each other. Fig. 4.1 shows the overall structure and the key modules of