Use of HTC char in Soil Amelioration

NEWAPP is an Eurpean research project focusing on hydrothermal carbonization (HTC) of wet biomass residues.

By means of HTC, wet biomass is converted into carbonaceous solids at relatively high yields in water, with pressure and temperatures at the lower region of liquefaction process. The lack of need for energy-intensive drying before the process opens up new possibilities for waste streams like manures, sewage sludge, municipal solid waste or agricultural waste. These organic waste streams can be used as feedstock for HTC technology, producing hydrochar and carbonaceous liquids, high value products that can be used as fuel, activated carbons for water treatment, soil remediation, carbon sequestration schemes and other applications.

In the year 2011, the EU-27 imported carbon products for a value of 22.666.570.073 €1. At the same time, EU generates yearly 80.000.000 tons of wet biowaste2 that can be effectively recycled to carbon materials by means of HTC. NEWAPP project paves the way to provide economically attractive and environmentally friendly alternatives for the utilization of wet biomass, while strengthening Europe’s competitiveness and reducing resource dependency.

This project is co-funded by the European Commission through the “Research for SME associations” instrument of the 7th Framework program. You can find out more about the organizations involved under the “Partners” section.

About the NEWAPP Project

When the first Europeans explorers returned from their expeditions into the Amazon region of South America to Europe in the 16th century they already reported about wealthy and prospering cultures. Based on archaeological findings, it is estimated that back in the 16th century more than 10 million people lived in the Amazon region. The nourishment of the numerous small settlements in this region was mainly based on agriculture. As the red soils in tropical rainforest conditions, as they occur in the Amazon region, are usually very poor, it is not clear how the nourishment of such a large number of people could be secured, especially at that time. In slash-and-burn practices, which are often used today, the Amazon’s red soils are usually depleted after only 2-3 years and do not provide sufficient agricultural yields any more. Therefore, beginning in the 1950s, various research projects have been conducted to identify the reasons for the great and long-lasting fertility of the Amazon soils.

The soils, which are called Terra Preta (full name: Terra Preta do Indio), which simply means black soil in Portuguese, not only contain higher concentrations of nutrients such as nitrogen, phosphorus, potassium and calcium, but also greater amounts of stable soil organic matter. Their water and nutrient storage capacities as well as their pH are significantly increased and they are rather black instead of red or yellow.

These Terra Preta soils are today believed to be artificial soils, i.e. they were produced by humans over hundreds of years. Residues of everyday life, e.g. charcoal of incompletely combusted wood, bones, seashells or snail shells from food production, human and animal faeces, various types of organic residues and many more were incorporated into the naturally occurring soils which had lost their fertility after just a few years.

The productivity of Terra Preta soils is even today, hundreds of years after their creation, significantly increased compared to surrounding soils. Modern soil amelioration products, which have been produced as a replication of Terra Preta, have proven to increase yields by up to 400 % and to maintain this high fertility for many years.

The increased productivity can be explained as follows: The char that is contained in Terra Preta soils has a very large surface area and a very complex surface structure. Therefore it significantly increases the nutrient and water storage capacities of soils. Nutrients are fixed in a way that they are not washed out from the soil into the groundwater but at the same time they are easily available for plants. The same applies to the water. The char moreover offers a perfect micro-habitat for various soil microorganisms. The improved soil fauna will lead to increased soil microbial activity, e.g. to an improved availability of nutrients or an improved soil structure through earthworm activities. Further advantages of adding char to soils comprise, amongst others, an improved quality of the groundwater, reduced greenhouse gas emissions and a reduced soil acidity.

In times of soil degradation and simultaneously decreasing per-capita availability of soils caused by the intensive cultivation of agricultural areas and the growing world population, Terra Preta soil amelioration products can offer a perfect opportunity to increase the productivity of agriculture and therefore contribute to the sustainable food supply of the world population. They are cheap and easy to produce and can be used almost anywhere.

Today a variety of char-based soil amelioration products are sold by professional and non-professional producers and a number of instructions to produce these products on your own are available in the internet. As a result, Terra Preta is becoming more and more known and actually applied.

However, all the positive effects described and all currently available Terra Preta soil amelioration products were observed on Terra Preta containing charcoal produced in a process called pyrolysis, which can be described as the decomposition of organic materials in anoxic conditions at high temperatures. This is the traditional way how char is produced mainly from wood every day all over the world and used by billions of people mainly for heating and cooking. Since wood is becoming more and more scarce and natural forests are being degraded or destroyed, alternative processes for the production of char using different feedstock have to be developed. One of these new processes could be the Hydrothermal Carbonisation (HTC) of biomass.

A number of studies have been conducted during the last years to evaluate the suitability of HTC char for soil amelioration products. The results, which will be explained in detail in the following chapter, were different but promising. However, most of the studies were very specific and analysed only very few but not all relevant parameters for often very specific types of biomass.

Therefore, the aim of the present study was to close the remaining gaps in knowledge. A comprehensive series of tests was conducted by ttz Bremerhaven / TERRA PRETA GmbH and HTC char from a variety of feedstock produced under different conditions was analysed in order to finally answer the question if HTC char can be a suitable substitute for Pyrolysis char in Terra Preta-like soil amelioration products.