Reducing Cost of Drying

TLDR:

• Decreased cost of algae from $1.12/kg to $0.12/kg

There are 3 main cost categories for algae. They are cultivation, dewatering, and drying. In my previous update, I worked on decreasing the cost of drying. This week, I will go through my current hypothesis to decrease the cost of drying.

What is Drying?

Drying is the process of removing moisture through the application of heat. After algae is separated from water (through dewatering) you would dry the algae biomass. From there you can either extract proteins, lipids, and carbohydrates to turn them into valuable add products. Another option is to use an industrial process like pyrolysis to make valuable add products. A value add product is the change in physical state to enhance the value of the product.

Drying is about 20% of total algae cost. The status quo cost of algae biomass is $5/kg, which means your drying cost is about $1.0/kg.

I started this week looking potential methods of turning algae into a valuable product. From an emissions perspective, it was something that can stay in the ground for a long period of time that has some value people would pay for.

What is the Product?

The product that fits the criteria of would be biochar. Biochar can be stored in the ground for thousands of years and is a good alternative to fertilizer. Here is a great paper highlighting all of the benefits from over 20 years of research. The most important benefits are increased resistance to disease and reducing nutrient leaching.

When taking into account other applications of algae like cosmetics, food Additives, Supplements. Many require approval from the Food and Drug Administration (FDA). For initial starting applications, I thought I would avoid products that require FDA approval to speed up the process of deployment. This led me down the pathway of industrial processes like gasification, carbonization, and pyrolysis.

How Can You Make Biochar?

From looking at industrial processes, I found hydrothermal processes and non-hydrothermal processes. On the hydrothermal side, you have hydrothermal gasification, hydrothermal liquefaction, hydrothermal carbonization. On non-hydrothermal side you have gasification, pyrolysis, carbonization, torrefaction. The difference between both type of industrial processes is hydrothermal does not require dry biomass while non-hydrothermal products do. No drying means a cost factor removed.

When looking into the hydrothermal processes and non-hydrothermal processes both had 3 main products: Bio-oil, synthetic gas, and some form charcoal (or char). The only different product wise is hydrothermal processes make hydrochar instead of biochar (with non-hydrothermal processes). The difference between the two is biochar has 20+ years of research on it, while hydrochar is fairly new (because hydrothermal processes are fairly new at scale). From the limited papers that exist on hydrochar there seems to be no yield benefit when you put it in soil. This results in hydrochar not providing revenue for its value, making it not useful. This made me decide that biochar was the product I needed.

I chose to dry the biomass first and then select the best process for producing biochar among with other products like bio-oil and synthetic gas.

What is My Drying Hypothesis?

Similar to my work on reducing the cost of dewatering of algae, I was aiming to have next to 0 operational cost.

Based on that I started to look into solar drying because it was using solar energy to dry the object (which is essentially free). To be clear, I’m not choosing solar drying because it is necessarily the best method of drying. I’m currently optimizing for the fastest information progress that would give me a cost projection cheap enough to start building out an MVP.

What is the History of Solar Drying?

The original form of drying was natural drying or open sun drying, the crops are put on compact earthen floor, mat, concrete, floor, and road in the full sunny days. The biggest problem was because all of your product was exposed to various contaminations—dirt, pest infestation, and loss by birds, and many more. The contamination made you lose product and/or reduce the quality.

Newer forms of solar drying consisting of different greenhouse structures fix this problem through it’s closed system. This means the contaminants are blocked from entering because there is a barrier.

What is the Optimal Solar Drying Design?

As you go through review papers on solar drying you will find there are many different types of solar dryers and designs proposed by researchers.

Based off the techno-economic analysis of 10 different designs and models. All models had payback period (for their capital cost) in under 3 years. This suggests to me regardless of model greenhouses for the use case of drying are not incredibly capital heavy.

Key Differences of Greenhouse Models

For every greenhouse you can make the decision to have passive or forced convection.

Natural convection is the method of air flow that requires no energy input. Forced convection uses a fan to speed up the movement of the air flow, requiring energy input. The difference in outcome is forced convection is about 20-25% faster to dry biomass compared to natural convection.

Natural convection (or the passive mode) works on the principles of thermosyphic effect. The initial energy source is the radiation of the sun or any other heat source. The energy of the sun (in the form of heat) is then captured by a solar collector. Solar collectors generate heat energy from radiation of the sun, whereas solar panels produce electricity from photons. When air or water (which are inside the solar collector) are heated, they gain kinetic energy from the heating source and become excited. As a result, the water and air becomes less dense, expands, and thus rises.

Increasing temperature of air inside a greenhouse makes the water (moisture) inside algae to become less dense to escape algae. Once the water has evaporate it rises to the goes to the top of the greenhouse where it is then pushed out of the closed system through a ventilator.

Forced convection works on the same principle, but includes a fan to speed up the movement of air flow, resulting in faster drying times.

What is the Economic Trade Off for Natural and Forced Convection?

When looking through dozens of different papers I found a study by El-Shiatry et al. He had a solar tunnel dryer that can dry 100–200 kg of food per day under forced convection mode. The dryer was tested for drying of tomatoes, grapes, onions, potatoes, basil and wild marjoram under the metrological conditions of Egypt. The highest electric consumption of 19.5 kW h for drying of 1000 kg of grapes was recorded; while only 4.4 kWh was consumed by radial fan for 200 kg of basil. The consumption of electric energy for operating the dryer in most cases was between 0.1-0.2kwh (about 1-2 cents if you bought the energy from the grid) per kg of dried algae. The overall payback period for capital cost was estimated to be about 2-3 years. Since the energy would come from a solar collector and stored in sand (mentioned later in the update) there would be no operational cost required and minimal capex since you need only a solar collector to cover energy requirement.

An important problem to note is algae has more initial moisture than agricultural crops making algae take more time and energy to dry. In the case that the drying with forced convection is too expensive then natural convection will be used.

Changed To Cost of Algae

With a solar dryer we can remove the entire drying cost of $1/kg. Before this update we our cost was $1.12/kg and now our cost is $1.12/kg or $120/tonne.