Techno-economic evaluation of biomass drying in moving beds: The effect of drying kinetics on drying costs

Abstract

Drying woody biomass holds the potential to improve the energy efficiency of certain processes, such as in CHP plants. Drying can also be a necessary unit process in several energy conversion processes (e.g. in biomass gasification). Belt dryers are typically used for drying when low temperature air ($\le$100–110 °C) is used. This article aims to produce new knowledge about the influence of the main design parameters on the drying costs of a low temperature belt dryer when three different materials (forest residue, bark, as well as sawdust and soot sludge mixture) are dried using it. The influence is analyzed by changing the following parameters: bed height, air temperature, air velocity and initial/final moisture contents of the material. The study aims to evaluate which of these parameters has an actual effect on drying costs. Results indicate that the lowest costs are achieved with the highest air temperature if the heat price is the same for every air temperature level. However, an optimal bed height depends on the material. Increasing the air velocity does not necessarily decrease the costs. In the sensitivity analysis, to factor in the influence of the temperature on the heat price, the price was changed for every drying air temperature (1, 5, 10 and 15 €/MWh). This analysis showed that the lowest drying costs are achieved by the lowest air temperature in all cases, thus indicating that the price of the heat has a remarkable influence on the economics of drying. Furthermore, the results support the use of low temperature heat sources in drying if they are clearly less expensive than higher temperature heat sources. However, if the prices for lower and higher air temperatures are of the same magnitude, the higher air temperatures are preferable. In general, this paper shows that it is important to pay attention to the main design parameters to optimize total drying costs. For example, if an overly low bed height is used in woodchips or bark drying, the total drying costs might be dozens of per cent higher than in the most economic case.

Keywords:

• Annual drying costs
• annuity method
• belt dryer
• excess heat
• woody biomass

Nomenclature

$A_{\text{dryer}}$	=	cross-sectional area of a continuous-working belt dryer [m2]
$AF$	=	annuity payment factor
$A_{\text{inv}}$	=	annual investment costs [€/a]
$A_{\text{oper}}$	=	annual operational costs [€/a]
$C_{\text{elec}}$	=	price of the electricity [€/MWh]
$C_{\text{heat}}$	=	price of the heat [€/MWh]
CHP	=	Combined Heat and Power plant
$c_{\mathrm{p},\mathrm{d}\mathrm{a}}$	=	specific heat capacity of dry air [kJ/kg °C]
$c_{\mathrm{p},\mathrm{w}}$	=	specific heat capacity of water vapor [kJ/kg °C]
da	=	dry air
db	=	dry basis
$h$	=	enthalpy of humid air [kJ/kgda]
$i$	=	rate of interest [%]
$I_{\text{tot},\text{dryer}}$	=	total investment costs of the dryer [€]
LHV	=	lower heating value
${\dot{m}}_{\text{da}}$	=	mass flow of dry air [kgda/s]
${\dot{m}}_{\text{db}}$	=	mass flow of material [kgdb/s]
MILP	=	Mixed Integer Linear Programming model
$n$	=	economic lifetime [a]
NPV	=	net present value
$P$	=	adjusted annual payment/drying costs [€/a]
$P_{\text{elec}}$	=	electricity consumption of the dryer fans [MW]
$\Delta P_{\mathrm{h}\mathrm{e}+\mathrm{a}\mathrm{d}}$	=	pressure drop over the plate heat exchanger(s) and the air duct system [Pa]
$\Delta P_{\text{material}}$	=	pressure drop over the material bed [Pa]
$\Delta P_{\text{pres}}$	=	total pressure drop over the dryer configuration [Pa]
SNG	=	Synthetic Natural Gas
T	=	air temperature [K]
$t$	=	air temperature [°C]
u	=	moisture content of material [kgH2O/kgdb]
$v$	=	velocity of air before the fan(s) [m/s]
wb	=	wet basis
vol-%	=	volume-%
$x$	=	moisture content of air [kgH2O/kgda]
$z$	=	bed height of material inside the dryer [m]
${\delta }_{\text{da}}$	=	density of dry air [kgda/m3]
${\rho }_{\text{db}}$	=	bulk density of dry material [kgdb/m3]
$\tau$	=	residence time of material inside the dryer [s]
$\gamma$	=	annual operating time of the dryer [h/a]
$\mathrm{\varnothing }$	=	heat consumption of the dryer [MW]
${\eta }_{\text{fan}}$	=	efficiency of the fan(s)

Additional information

Funding

This study has been produced with financial support from the Finnish Foundation for Technology Promotion (Tekniikan edistämissäätiö, TES), Foundation of Heikki and Hilma Honkanen (Heikki ja Hilma Honkasen säätiö) and Doctoral Program in Energy Efficiency and Systems (EES).