Biography

HenrikKofoed Nielsen is a professor of renewable energy at University of Agder, Norway. He is presently head of research group for Bioenergy and Thermal energy. His research interests include energy crops, biomass sampling, and the interaction between biomass characteristics and applications like combustion, gasification, biochar, etc. Kofoed Nielsen holds a PhD in agricultural engineering from the University of Copenhagen and has been with University of Agder since 1992.

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BIOMASS SAMPLING AND ITS IMPORTANCE FOR MECHANICAL ENGINEERS

HenrikKofoed Nielsen

Professor

Department of Engineering Sciences

University of Agder

Serviceboks 509, N-4898 Grimstad

Norway

Abstract

Biomass conversion into heat, power and biofuels is greatly influenced by the characteristics of the biomass no matter which conversion process is the case. The characterization is very frequently based on small biomass samples for analysis of heating value, volatiles, moisture content, carbon content, ash melting behaviour, lignin, cell structure and biogas potential etc. For ultimate and proximate analysis sample size is often less than one gramme. This is contradictory to the normal size of woody and herbaceous biomass plants or crops, i.e. sampling has to be planned carefully since one characteristic may vary in a stand between plants, shoots and even for different heights of a single shoot or tree. This variation may sometimes explain problems in getting statistically significant results for gasification, combustion etc. The aim of this research was therefore to describe the variation of the characteristics and subsequently propose an appropriate sampling procedure minimising sampling uncertainty.

As case studies, the variation of moisture content, ash content, chemical elements, and ash melting behaviour has been studied for short rotation coppice of willow and to less extent hybrid poplar.

78 shoots of willow were harvested from a willow plantation in Southern Norway by 13 occasions during winter 2011-2012. The moisture content varied up to 19 percentage points (wet basis) within one shoot. Between largest and smallest shoot in a stump the average difference was 3.1 percentage points (wet basis) for whole shoot mean moisture content. The largest shoot was always dryer than the smallest in willow. In hybrid poplar this was opposite for main harvest in March 2012. The ash content varied from clone to clone and with height within one shoot as much as 2 percentage points (dry basis). This was relative large variation as the average whole shoot ash content was around 1.5 % (dry basis).

In willow the concentration of:

  • Ca, Cu, K, Mg, Mn, Na, P and Zn had an in general increasing trend with height
  • Cd had an in general decreasing trend.
  • Si a general flat trend.
  • Al and Fe very variable trends with height.

Al is one of the critical elements for ash melting. The variable trend of Al content was roughly found again in the ash melting temperature.

The case studies showed for willow: 10 cm sections at the breast height (125–135 cm above ground) resembled the mean concentration of the whole stem for most of the elements and moisture. It was difficult to detect a very clear connection between biomass characteristics and gasification performance for several biomass types and two gasification principles.