I have successfully installed GRUB on couple of Chromebooks, by reflashing its BIOS. Can I do this with my older Android tablets, ie can I reflash Android tablet? All of them run x86 processor.

Maybe.

I guess I'll ask this. Exactly what are they?

You don't tell us what brand/model tablets they are, so what do you think we can tell you??? Android isn't a drop-in replacement...your best bet is to look up your model on XDA developers, and see if it's supported with a different ROM set.

I have couple of Samsung K013s. Also, I have HP Stream 7s tablet (not Android). For Chromebook is a more or/less generic approach; boot into developer mode, using "flashrom" (https://www.flashrom.org/Flashrom) flash with the proper coreboot image (https://www.coreboot.org/) and than install GRUB or other bootloader. The question is; is the same procedure available for Android? My search was unsuccessful and I do not know if I can even run flashrom? Are coreboot images available for Android tablets or I should create my own? I do not even know how to run linux cli on my tablets? I have zero experience with Android OS and I would greatly appreciate some references.

If you want to create your own bootrom for that particular tablet, have at it. Otherwise, you're stuck with whatever the TABLET can support...the processor is only a small part of that puzzle.

XDA Developer site forums would be your best bet, as said. They seem to have several things listed for your particular tablet, but haven't gone through and read them.

I think I had or have the Stream 7. It had a bios that allowed one to use an on screen keyboard at boot.

Since it was the goofy 32 and 64 bit boot deal there are a few web pages on that atom processor and how to get linux on it.

Thank you for help; I have checked XDA Developer site; unfortunately this site is more about using android rather than getting rid of it. I'll try to use "flashrom" software, which can detect many eeprom chips. If it can detect K013 eeprom, then I can try to find image for this tablet. I'll give a feedback when I'll be successful.

Right; again, you need a ROM image for your tablet...which runs Android. You're not going to run Linux on those tablets, period. Flashrom isn't for tablets...did you read about what it is/does, and what hardware it supports??? It's on their website.

You MIGHT be able to upgrade Android past what the manufacturer supports, but even then you may lose support for some devices (camera/mic/sound/etc.), since they're not always 100% complete. Tablets are *NOT* PC's...even some Surface tablets won't be able to be reloaded.

As Newbie, I seldom comment opinion of experts. However, last comment of TBone needs some explanation for other Newbies;
Original question was about re-flashing Android tablet, so Linux OS could be installed and replace the Android OS. This is a hardware question. Further, the discussion meander toward general possibility of replacing Android by the Linux. Android OS is the commercial software with the main objective to serve smart phone devices (tablets are oversized smart phones w/o the call connectivity). It is a narrow purpose software and it is not suitable for many other applications. The Android OS is close source product developed by the single company and the Android forum "XDA Developers" is quite misleading. Since all Android development is done behind the company's walls, this forum should be colled Android user forum because no much development is going outside the company. On the other hand Linux is very universal OS. This OS is employed in high security environment as well as in no security environment. Linux is used from managing super computers to running tiny embedded devices. These are the differences, so what are the similarities? To lower the development cost of Android, the Android people are reusing a Linux kernel with added some proprietary "binary blobs". Also, Android OS uses some Linux essential software, like Busybox. Because Linux kernel and software like Busybox; Android has to partially comply with the POSIX (also known as IEEE 1003) standard. The POSIX provides standard API as well as some basic I/O interface such as display, keyboard access, etc.. Just the kernel and Busybox provide a quite complete Linux environment where other software could be installed easily. So can I replace Android by Linux? In theory YES. In practice maybe; some hardware companies like to control that only their approved software can be loaded into their device. Samsung and Asus are known to be such companies, this is precisely why I have posted my question. I would like to re-purpose Asus k013 tablets to do something else. I have posted the question, counting that someone has experience flashing the original company bios; I was wrong.
In the above context; is it worth to recycle Samsung tablet? The answer is definitely NO; the perfectly capable Samsung device will go directly into TRASH. During the chip shortages and high cost of raw materials; the Samsung, Asus, LG, HP made the business decision to prohibit the recycling of their electronic devices. It is a very shortsighted decision.